| 1. |
- Dahlin, Torleif, et al.
(författare)
-
Measuring techniques in induced polarisation imaging
- 2002
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Ingår i: Journal of Applied Geophysics. - 0926-9851. ; 50:3, s. 279-298
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Tidskriftsartikel (refereegranskat)abstract
- Multi-electrode geoelectrical imaging has become very popular and is used for many different purposes. For some of these, the inclusion of IP data would be desirable as it would allow the interpreter to distinguish between, e.g. sand formations with saltwater infiltration and clay formations or help delineate landfills. However, present-day IP measuring techniques require the use of nonpolarisable potential electrodes and special wire layout and are thus cumbersome and expensive. In this paper, we suggest making IP measurements with multi-electrode cables and just one set of steel electrodes. The polarisation potentials on the potential electrodes are corrected for by subtracting the polarisation potential measured when no primary current and no IP signal are present. Test measurements indicate that the polarisation potentials vary slowly and that the correction procedure is feasible. At two sites in southern Sweden, we have compared measurements with only stainless steel electrodes and measurements with both stainless steel and Pb-PbCl nonpolarisable electrodes using one or two sets of multicore cables, respectively. Almost no difference between the two data sets was observed. At one site, the charge-up effect on the potential electrodes was not important, while at the other site, the correction procedure was crucial. Though only two sites have been studied so far, it seems that time-domain IP imaging measurements can be taken with only steel electrodes and ordinary multicore cables. Coupling in the multicore cables has not presented any problems at the investigated sites where grounding resistances were moderate, making the coupling effect small. High grounding resistance sites have not yet been investigated. (C) 2002 Published by Elsevier Science B.V.
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| 2. |
- Dahlin, Torleif, et al.
(författare)
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Resolution of 2D Wenner resistivity imaging as assessed by numerical modelling
- 1998
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Ingår i: Journal of Applied Geophysics. - 0926-9851. ; 38:4, s. 237-249
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Tidskriftsartikel (refereegranskat)abstract
- Modelling of 2D resistivity imaging was done in order to understand the principle resolution of the technique in different geological situations, and for assessing the behaviour of the interpretation methods under controlled circumstances. The Wenner array was used throughout. The results show that the 1D approximation only provides reasonable results in environments with very gradual lateral resistivity changes, otherwise the result may be strongly misleading. Inversion using the 2D quasi-Newton technique results in adequate resolution of the structures in moderately complex environments, but the Gauss–Newton method holds a significant advantage in some complicated cases. The data density can also be of crucial importance for the resolution capability, notably of narrow structures.
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| 3. |
- Loke, MH, et al.
(författare)
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A comparison of the Gauss-Newton and quasi-Newton methods in resistivity imaging inversion
- 2002
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Ingår i: Journal of Applied Geophysics. - 0926-9851. ; 49:3, s. 149-162
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Tidskriftsartikel (refereegranskat)abstract
- The smoothness-constrained least-squares method is widely used for two-dimensional (2D) and three-dimensional (3D) inversion of apparent resistivity data sets. The Gauss-Newton method that recalculates the Jacobian matrix of partial derivatives for all iterations is commonly used to solve the least-squares equation. The quasi-Newton method has also been used to reduce the computer time. In this method. the Jacobian matrix for a homogeneous earth model is used for the first iteration, and the Jacobian matrices for subsequent iterations are estimated by an updating technique. Since the Gauss-Newton method uses the exact partial derivatives, it should require fewer iterations to converge. However, for many data sets, the quasi-Newton method can be significantly faster than the Gauss-Newton method. The effectiveness of a third method that is a combination of the Gauss-Newton and quasi-Newton methods is also examined. In this combined inversion method, the partial derivatives are directly recalculated for the first two or three iterations, and then estimated by a quasi-Newton updating technique for the later iterations. The three different inversion methods are tested with a number of synthetic and field data sets. In areas with moderate (less than 10:1) subsurface resistivity contrasts, the inversion models obtained by the three methods are similar. In areas with large resistivity contrasts, the Gauss-Newton method gives significantly more accurate results than the quasi-Newton method. However, even for large resistivity contrasts, the differences in the models obtained by the Gauss-New-ton method and the combined inversion method are small. As the combined inversion method is faster than the Gauss-Newton method, it represents a satisfactory compromise between speed and accuracy for many data sets.
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| 4. |
- Chapellier, D., et al.
(författare)
-
Preface
- 1994
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Ingår i: Journal of Applied Geophysics. - : Elsevier BV. - 0926-9851 .- 1879-1859. ; 31:1-4, s. 7-
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)
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| 5. |
- Frenje, L, et al.
(författare)
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Scattering attenuation: 2-D and 3-D finite difference simulations vs. theory
- 2000
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Ingår i: Journal of Applied Geophysics. - : Elsevier. - 0926-9851 .- 1879-1859. ; 44:1, s. 33-46
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Tidskriftsartikel (refereegranskat)abstract
- Scattering of seismic waves is studied by producing synthetic vertical seismic profiling (VSP) seismograms with 2-D and 3-D finite difference modelling in random media. The random models used are Gaussian and band-limited self-similar, or fractal random media. The modelling is performed acoustically, but we believe that, considering the geometry of this study, the results obtained will hold for the elastic case as well.Properties of the random media are discussed, in particular the difference between discrete and continuous media, and the importance of this difference. We show that when using the band-limited Von Karman correlation function when generating the random medium, the size of the model should be greater than 2πa, where a is the correlation distance, and the grid spacing should be less then a. If not, the medium will not have the proper characteristics.Analytical expressions for scattering attenuation, derived from single scattering theory, can be used to estimate scattering Q from borehole velocity logs, if it is known what minimum scattering angle, θmin, to use. θmin, the minimum angle energy, must be scattered to be regarded as not contributing to the propagating wave. We estimate θmin by comparing Q values estimated from our synthetic VSP seismograms with the analytical expressions. The comparison also shows when the assumption of single scattering is valid. Previous studies in 2-D give a θmin of ∼30°. In this paper, we make a comparison for both the 2-D and 3-D cases, and show that the Q estimate is highly sensitive to how the analysis is done. We show that single scattering theory agrees well with finite difference simulations for self-similar media with low Hurst numbers, but with a somewhat lower θmin of 10–20°. This holds for a range of correlation lengths, a, including the case of infinite, or absence of, a. For Gaussian and exponential media, simulations and theory agree as well with θmin of 10–20°, but only for ka<5, where k is the wave number of the source. For ka>5, simulations and theory diverge, and single scattering theory cannot explain the amplitude attenuation observed in the scattering simulations for these types of media, indicating that it may be difficult to estimate the fractal properties of a medium from seismic data alone.With the difficulties of characterizing the scattering medium, and to estimate the scattering attenuation in the simple case of synthetic data with pure scattering, we conclude that it may be difficult to separate scattering and intrinsic attenuation from real data.
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| 6. |
- Autio, U., et al.
(författare)
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Combining electromagnetic measurements in the Mygdonian sedimentary basin, Greece
- 2016
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Ingår i: Journal of Applied Geophysics. - : Elsevier BV. - 0926-9851 .- 1879-1859. ; 135, s. 261-269
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Tidskriftsartikel (refereegranskat)abstract
- We present a novel approach where time-domain electromagnetic (TEM) data are transformed and subsequently used in two-dimensional (2-D) magnetotelluric inversion of the determinant of the impedance tensor. The main idea is to integrate TEM with magnetotelluric (MT) data to produce subsurface electrical resistivity models. Specifically, we show that 2-D MT data inversion of the determinant of the impedance tensor supported by inclusion of TEM–MT-transformed data has superior resolution at the near surface and at the same time static shift afflicting the MT data can be addressed. Thus, the approach allows for practical express integration of TEM data with MT measurements as opposed to a full combined 3-D inversion, which requires significant resources. The approach is successfully applied in the Mygdonian sedimentary basin located in Northern Greece. In addition to TEM and MT data, also controlled source — and radiomagnetotelluric data are available from the Mygdonian basin, which have been subjected to 2-D analysis previously. We have extended the analysis to a full 3-D inversion using ModEM code. All obtained models are analysed and are in a good agreement.
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| 7. |
- Basir, Hadi Mandavi, et al.
(författare)
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Reverse time migration by Krylov subspace reduced order modeling
- 2018
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Ingår i: Journal of Applied Geophysics. - : Elsevier BV. - 0926-9851 .- 1879-1859. ; 151, s. 298-308
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Tidskriftsartikel (refereegranskat)abstract
- Imaging is a key step in seismic data processing. To date, a myriad of advanced pre-stack depth migration approaches have been developed; however, reverse time migration (RTM) is still considered as the high-end imaging algorithm. The main limitations associated with the performance cost of reverse time migration are the intensive computation of the forward and backward simulations, time consumption, and memory allocation related to imaging condition. Based on the reduced order modeling, we proposed an algorithm, which can be adapted to all the aforementioned factors. Our proposed method benefit from Krylov subspaces method to compute certain mode shapes of the velocity model computed by as an orthogonal base of reduced order modeling. Reverse time migration by reduced order modeling is helpful concerning the highly parallel computation and strongly reduces the memory requirement of reverse time migration. The synthetic model results showed that suggested method can decrease the computational costs of reverse time migration by several orders of magnitudes, compared with reverse time migration by finite element method.
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| 8. |
- Bastani, Mehrdad, et al.
(författare)
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CSRMT measurements in the frequency range 1-250 kHz to map a normal fault in the Volvi Basin, Greece
- 2011
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Ingår i: Journal of Applied Geophysics. - : Elsevier BV. - 0926-9851 .- 1879-1859. ; 75:2, s. 180-195
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Tidskriftsartikel (refereegranskat)abstract
- In order to gain a better understanding of the geometry of surface faults, five Controlled Source/Radio Magnetotelluric (CSRMT) profiles were measured across the Volvi basin, 45 km northeast of the city of Thessaloniki in Greece. The data were collected in two frequency ranges: a) 1–12.5 kHz using a remotely controlled double horizontal magnetic dipole transmitter (CSAMT measurements), and b) 15–250 kHz using the signal from distant radio transmitters (RMT measurements). The transition from the RMT band to the CSAMT band was smooth and continuous allowing us to combine both datasets for plane-wave modeling. The surface geology shows a predominantly 2D structure, and therefore we planned the survey into profiles perpendicular to the geological strike. We have used a 2D interpretation tool to model the data in TE, TM, TE + TM and determinant modes. Using a 4% error floor on the impedance, 2D resistivity models from inversion of the determinant data provide lower RMS data fits (4.2 and 1.2 for resistivity and phase, respectively) compared to the combined TE + TM data (4.4, 2.8, overall resistivity and phase, respectively). 2D inversion of the measured tensor data shows a sharp change in the depth to the top of resistive gneiss–schist basement that is overlain by a less resistive overburden at southern basin flanks. The change in depth to the bedrock is clearly seen in all 2D models along the measured profiles suggesting the existence of normal faults with strike directions of NE–SW to E–W. The 2D electrical resistivity models suggest that the bedrock deepens towards south-west. The resistivity models are also compared with the existing borehole information in the area and show a reasonable correlation. For example the sharp change of depth to the bedrock towards the center of the basin as seen in the resistivity models are also confirmed by the borehole data.
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| 9. |
- Beiki, Majid
(författare)
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TSVD analysis of Euler deconvolution to improve estimating magnetic source parameters : An example from the Asele area, Sweden
- 2013
-
Ingår i: Journal of Applied Geophysics. - : Elsevier BV. - 0926-9851 .- 1879-1859. ; 90, s. 82-91
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Tidskriftsartikel (refereegranskat)abstract
- In this paper, I introduce a new approach based on truncated singular value decomposition (TSVD) analysis for improving implementation of grid-based Euler deconvolution with constraints of quasi 2D magnetic sources. I will show that by using TSVD analysis of the gradient matrix of magnetic field anomaly (reduced to pole) for data points located within a square window centered at the maximum of the analytic signal amplitude, we are able to estimate the strike direction and dip angle of 2D structures from the acquired eigenvectors. It is also shown that implementation of the standard grid-based Euler deconvolution can be considerably improved by solving the Euler's homogeneity equation for source location and structural index, simultaneously, using the TSVD method. The dimensionality of the magnetic anomalies can be indicated from the ratio between the smallest and intermediate eigenvalues acquired from the TSVD analysis of the gradient matrix. For 2D magnetic sources, the uncertainty of the estimated source location and structural index is significantly reduced by truncating the smallest eigenvalue. Application of the method is demonstrated on an aeromagnetic data set from the Asele area in Sweden. The geology of this area is dominated by several dike swarms. For these dolerite dikes, the introduced method has provided useful information of strike directions and dip angles in addition to the estimated source location and structural index.
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| 10. |
- Beka, Thomas I., et al.
(författare)
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Integrated electromagnetic data investigation of a Mesozoic CO2 storage target reservoir-cap-rock succession, Svalbard
- 2017
-
Ingår i: Journal of Applied Geophysics. - : Elsevier. - 0926-9851 .- 1879-1859. ; 136, s. 417-430
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Tidskriftsartikel (refereegranskat)abstract
- Recently acquired time-domain electromagnetic (TEM) and magnetotelluric (MT) data sets are utilized in the first electromagnetic (EM) characterization of a geological CO2 storage target site in Adventdalen, Arctic Norway. Combining the two EM data sets enabled to resolve the electrical resistivity structure of the target site better than either of the methods alone. 2D inverting the MT data in the audio period interval (0.003–1 s) with supporting input derived from the TEM data (0.01–10 ms) provided a geologically meaningful resistivity model that included information not previously evident from existing seismic and borehole data. The ca. 1.8 × 1 km 2D resistivity model displays a laterally constrained highly conductive anomaly (ca. 10 Ω m) at about 400–500 m depth, where reflectors of a parallel seismic section are concealed and core samples indicate a highly fractured décollement zone formed during Paleogene compression. The base of the permafrost is imaged at ca. 200 m depth. Synthetic inversion tests, however, suggest that this may be exaggerated by tens of meters, due to a thin conductive layer present approximately between 10 and 25 m depth. The resistivity model does not give indication for a fluid pathway we can connect to leakage, in line with water injection and leak-off tests in the reservoir and cap-rock, both of which indicate a sealing shale-dominated cap-rock separating an over-pressured compartment above the sealing shale from a severely under-pressured reservoir interval. The results we present indicate the advantage of integrating EM exploration techniques in a CO2 reservoir-cap-rock study to obtain a more complete picture.
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| 11. |
- Bergmann, Peter, et al.
(författare)
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Time-lapse processing of 2D seismic profiles with testing of static correction methods at the CO(2) injection site Ketzin (Germany)
- 2011
-
Ingår i: Journal of Applied Geophysics. - : Elsevier BV. - 0926-9851 .- 1879-1859. ; 75:1, s. 124-139
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Tidskriftsartikel (refereegranskat)abstract
- The Ketzin project provides an experimental pilot test site for the geological storage of CO(2). Seismic monitoring of the Ketzin site comprises 2D and 3D time-lapse experiments with baseline experiments in 2005. The first repeat 2D survey was acquired in 2009 after 22 kt of CO(2) had been injected into the Stuttgart Formation at approximately 630 m depth. Main objectives of the 2D seismic surveys were the imaging of geological structures, detection of injected CO(2), and comparison with the 3D surveys. Time-lapse processing highlighted the importance of detailed static corrections to account for travel time delays, which are attributed to different near-surface velocities during the survey periods. Compensation for these delays has been performed using both pre-stack static corrections and post-stack static corrections. The pre-stack method decomposes the travel time delays of baseline and repeat datasets in a surface consistent manner, while the latter cross-aligns baseline and repeat stacked sections along a reference horizon. Application of the static corrections improves the S/N ratio of the time-lapse sections significantly. Based on our results, it is recommended to apply a combination of both corrections when time-lapse processing faces considerable near-surface velocity changes. Processing of the datasets demonstrates that the decomposed solution of the pre-stack static corrections can be used for interpretation of changes in near-surface velocities. In particular, the long-wavelength part of the solution indicates an increase in soil moisture or a shallower groundwater table in the repeat survey. Comparison with the processing results of 2D and 3D surveys shows that both image the subsurface, but with local variations which are mainly associated to differences in the acquisition geometry and source types used. Interpretation of baseline and repeat stacks shows that no CO(2) related time-lapse signature is observable where the 2D lines allow monitoring of the reservoir. This finding is consistent with the time-lapse results of the 3D surveys, which show an increase in reflection amplitude centered around the injection well. To further investigate any potential CO(2) signature, an amplitude versus offset (AVO) analysis was performed. The time-lapse analysis of the AVO does not indicate the presence of CO(2), as expected, but shows signs of a pressure response in the repeat data.
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| 12. |
- Brodic, Bojan, et al.
(författare)
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Multicomponent broadband digital-based seismic landstreamer for near-surface applications
- 2015
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Ingår i: Journal of Applied Geophysics. - : Elsevier BV. - 0926-9851 .- 1879-1859. ; 123, s. 227-241
-
Tidskriftsartikel (refereegranskat)abstract
- During the last few decades there has been an increased demand for infrastructure, along with a greater awareness of environmental issues in the construction industry. These factors have contributed to an increased interest in using seismic methods for near surface characterization, particularly in urban environments. Seismic sensors not affected by anthropogenic electromagnetic noise are therefore important, as well as an acquisition system that is easy to deploy, move and non-invasive. To address some of these challenges, a multicomponent broadband MEMS (micro-electro mechanical system) based landstreamer system was developed. The landstreamer system is fully digital, therefore it is less sensitive to electrical or electromagnetic noise. Crosstalk, leakage and tilting tests show that the system is superior to its predecessors. The broadband nature of the sensors (theoretically 0–800 Hz), 3C (three-component) recording and the close spacing of the sensors enable high-resolution imaging. The current streamer configuration consists of 20 sensors 4 m apart and 80 sensors 2 m apart. The streamer can easily be combined with wireless recorders for simultaneous data acquisition. In this study, we present results from testing of the streamer with various sources, such as a shear wave vibrator and different types of impact sources. MEMS-sensors and their high sensitivity allowed recording clear reflections that were not observed with coil-based sensors. A complementary test was also carried out at a planned access ramp for an urban tunnel where potential poor quality rocks had been identified by drilling. First-break traveltime tomography showed that these poor quality rocks correlate with low velocity zones. The presented landstreamer system has great potential for characterizing the subsurface in noisy environments.
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| 13. |
- CARLSTEN, S, et al.
(författare)
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RADAR TECHNIQUES FOR INDICATING INTERNAL EROSION IN EMBANKMENT DAMS
- 1995
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Ingår i: JOURNAL OF APPLIED GEOPHYSICS. - : ELSEVIER SCIENCE BV. - 0926-9851. ; 33:1-3, s. 143-156
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- For improved dam safety in Sweden, radar techniques have been tested for detecting anomalous zones in the central core of embankment dams and the level of the core crest. Anomalies in the core may indicate a continuing process and a greater risk of intern
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| 14. |
- Chaki, Soumi, et al.
(författare)
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A probabilistic neural network (PNN) based framework for lithology classification using seismic attributes
- 2022
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Ingår i: Journal of Applied Geophysics. - : Elsevier. - 0926-9851 .- 1879-1859. ; 199
-
Tidskriftsartikel (refereegranskat)abstract
- This paper proposes a Probabilistic Neural Network (PNN) based framework for classification of lithology from a number of seismic attributes. The PNN has been the natural choice for classification in several research areas due to its insensitivities towards outliers and higher computational speed compared to multilayer perceptron (MLP) networks. Initially, the lithology is labelled into four classes such as sand, shaly sand, sandy shale, and shale through thorough analysis of multiple well logs by a proficient geologist. The seismic attributes and well logs pertaining to twelve closely spaced boreholes from a western onshore hydrocarbon field in India are used in this study. The performance of the designed framework consisting of preprocessing, classification, and lithological maps generation stages is compared with existing supervised classifiers in terms of classification accuracy, sensitivity, and specificity and the results are reported. The selection of appropriate parameters associated with individual classifier and importance of individual seismic predictors are also investigated. Finally, lithology maps indicating the different classes are produced using the tuned parameters of PNN over the study area. This framework would be of immense help to geologists along with other geological measures to estimate the probability of the presence of hydrocarbon in a large study area.
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| 15. |
- Cheraghi, Saeid, et al.
(författare)
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Scaling behavior and the effects of heterogeneity on shallow seismic imaging of mineral deposits : a case study from Brunswick No. 6 mining area, Canada
- 2013
-
Ingår i: Journal of Applied Geophysics. - : Elsevier. - 0926-9851 .- 1879-1859. ; 90, s. 1-18
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Tidskriftsartikel (refereegranskat)abstract
- We have studied the scaling behavior of compressional-wave velocity and density logs from an exploration borehole that extends down to about 700 m depth in the Brunswick No. 6 mining area, Bathurst Mining Camp, Canada. Using statistical methods, vertical and horizontal scale lengths of heterogeneity were estimated. Vertical scale length estimates from the velocity, density and calculated acoustic impedance are 14 m, 33 m, and about 20 m, respectively. Although the estimated scale length for the acoustic impedance implies a weak scattering environment, elastic finite difference modeling of seismic wave propagation in 2D heterogeneous media demonstrates that even this weak scattering medium can mask seismic signals from small, but yet economically feasible, massive sulphide deposits. Further analysis of the synthetic seismic data suggests that in the presence of heterogeneity, lenticular-shaped targets may only exhibit incomplete diffraction signals whereby the down-dip tails of these diffractions are mainly visible on the stacked sections. Therefore, identification of orebody generated diffractions is much easier on the unmigrated stacked sections than on migrated stacked sections. The numerical seismic modeling in 2D heterogeneous media indicates that in the presence of large horizontal, but small vertical scale lengths (structural anisotropy), identification of massive sulphide deposits is possible, but their delineation at depth requires detailed velocity modeling and processing algorithms which can handle the anisotropy.
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| 16. |
- Dahlin, Torleif, et al.
(författare)
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Negative apparent chargeability in time-domain induced polarisation data
- 2015
-
Ingår i: Journal of Applied Geophysics. - : Elsevier BV. - 0926-9851. ; 123, s. 322-332
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Tidskriftsartikel (refereegranskat)abstract
- It appears to be relatively common to assume that negative apparent chargeability data in time-domain induced polarisation (IP) surveying is a sign of bad data quality. Negative IP data can however occur as a consequence of the distribution of chargeable zones in the ground, which is well documented in literature. A general mechanism behind negative IP data is proposed as follows; if the chargeable zones are mainly located in zones of negative sensitivity, and there is low or no chargeability in the positive sensitivity volumes in the investigated volume, it will result in negative apparent chargeability. Numerical modelling confirms that the phenomenon will typically occur for longer electrode separation if the chargeability is concentrated in a thin layer at the surface only, but that other distributions of the chargeable bodies can also cause negative IP data. Different electrode arrays differ in tendency to produce negative IP data, where dipole-dipole and pole-dipole arrays are more prone to generate negative data than nested arrays in the modelled examples. In addition to the relative location of the chargeable zone the resistivity is important for its impact on the apparent chargeability. Field data recorded in connection with the 3rd International IP Workshop on Ile d’Oleron in April 2014 confirm that negative apparent chargeability can be caused by a thin chargeable layer at the surface. The abundant negative IP data can be explained by an inverted model with low residuals, in which the chargeability is concentrated in a thin layer with modest chargeability close to the surface. Removing the data with negative apparent chargeability before inversion results in apparently poor resolution of the bottom layer and artefacts that are not present in the inversion results from the original data set. The results clearly demonstrate that negative apparent chargeability data can be a result of the distribution of chargeable zones in relation to the sensitivity distribution, and that such data should not be edited away on a routine basis since they contain important information.
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| 17. |
- Danielsen, Berit Ensted, et al.
(författare)
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Numerical modelling of resolution and sensitivity of ERT in horizontal boreholes
- 2010
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Ingår i: Journal of Applied Geophysics. - : Elsevier BV. - 0926-9851. ; 70:3, s. 245-254
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Tidskriftsartikel (refereegranskat)abstract
- Resistivity in horizontal boreholes can give useful detailed information about the geological conditions for construction in rock, i.e. in front of a tunnel bore machine. This paper is an attempt to identify a suitable methodology for an effective measuring routine for this type of geophysical measurements under actual construction site conditions. Prior to any measurements numerical modelling was done in order to evaluate the resolution of different electrode arrays. Four different arrays were tested; dipole-pole, cross-hole dipole-dipole, cross-hole pole-tripole and multiple gradient array. Additionally the resolution of a combination of cross-hole dipole-dipole and multiple gradient was assessed. The 2D sensitivity patterns for various arrangements of the cross-hole dipole-dipole and multiple gradient array were examined. The sensitivity towards inaccurate borehole geometry and the influence of water in the boreholes were also investigated. Based on the model study the cross-hole dipole-dipole array, multiple gradient array and a combination of these were found to give the best result and therefore were used for test measurements in horizontal boreholes. The boreholes were 28.5 m long and drilled 6.5 m apart. Prototypes of semi-rigid borehole cables made it possible to insert multi electrode cables in an efficient way, allowing fast measurement routines. These measurements were then studied to determine their accuracy and applicability. The results showed a high resistivity rock mass at the site. A transition from high resistivity to slightly lower resistivity coincides well with a change in lithology from gneiss-granite to gneiss. It is likely that the shotcrete on the tunnel wall is seen as a low resistivity zone. The measurements are a valuable tool, but further development of the cables and streamlining of measuring routines have to be performed before the resistivity tomography can be used routinely in pilot holes during construction in rocks. (C) 2010 Elsevier B.V. All rights reserved.
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| 18. |
- Dehghannejad, Mahdieh, et al.
(författare)
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Reflection seismic imaging of the upper crust in the Kristineberg mining area, northern Sweden
- 2010
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Ingår i: Journal of Applied Geophysics. - : Elsevier BV. - 0926-9851 .- 1879-1859. ; 71:4, s. 125-136
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Tidskriftsartikel (refereegranskat)abstract
- The Kristineberg mining area is located in the western part of the Palaeoproterozoic Skellefte Ore District, one of the most important mining districts in Europe. As a part of a 3D geologic modeling project, two new reflection seismic profiles were acquired with a total length of about 20 km. One profile (HR), parallel to previous seismic profiles, was acquired using a 10 m receiver and source interval and crosses the steeply dipping structures of the Kristineberg mine. The other profile (Profile 2) runs perpendicular to all existing profiles in the area. Although the structural geology is complex, the processed seismic data reveal a series of steeply dipping to sub-horizontal reflections, some of which reach the surface and allow correlation with surface geology. Our general interpretation of the seismic images is that the Kristineberg mine and associated mineral horizon are located in the northern part of a series of steeply south dipping structures. Overall, main structures plunge to the west at about 30 degrees-40 degrees. Cross-dip analysis and reflection modeling were carried out to obtain the 3D orientation of the main reflections and to provide insight into the possible contribution of out-of-the-plane reflections. This helped, for example, to obtain the 3D geometry of a deep reflection that was previously interpreted as structural basement to volcanic rocks. The new reflection seismic profiles have improved our understanding of shallow geological structures in the area and in conjunction with recently acquired potential field data, magnetotelluric data and geological observations will help to refine previous 3D geologic modeling interpretations that were aimed at larger scale structures.
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| 19. |
- Ehsan, Siddique Akhtar, et al.
(författare)
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Re-processing and interpretation of 2D seismic data from the Kristineberg mining area, northern Sweden
- 2012
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Ingår i: Journal of Applied Geophysics. - : Elsevier BV. - 0926-9851 .- 1879-1859. ; 80, s. 43-55
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Tidskriftsartikel (refereegranskat)abstract
- The Kristineberg mining area in the western part of the Skellefte ore district, northern Sweden, contains the largest massive sulphide deposit in the district. In 2003, two parallel seismic lines, Profiles 1 and 5, each about 25 km long and about 8 km apart were acquired in the Kristineberg area. The initial processing results were successful in imaging the large-scale structures of the area down to 12 km of the crust, but resulted in relatively poor seismic image near the mine. In this paper, we re-processed the seismic data along Profile 1 that crosses the mine. The main objective was to improve the seismic section near the mine for further correlation with new seismic data recently acquired in the area. The crooked-line acquisition geometry, very low fold coverage of less than 17, complex geology and sparse outcrops in the area made the data re-processing and interpretation challenging. Despite these challenges, significant improvement is observed in the seismic data, in terms of event continuity and resolution. Refraction static corrections allowed high frequencies to be retained, which improved the seismic section. The refraction static solution was manually checked and adjusted at every iteration to avoid unstable solutions. 3D visualization of the re-processed data with other seismic profiles recently acquired in the area allowed the seismic reflections to be correlated. The majority of the reflections are interpreted to originate from either fault zones or lithological contacts. A very shallow reflection correlates well with the location of the Kristineberg mineralized horizon.
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| 20. |
- Eshagh, Mehdi, Professor, 1977-, et al.
(författare)
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A strategy to calibrate errors of Earth gravity models
- 2014
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Ingår i: Journal of Applied Geophysics. - : Elsevier BV. - 0926-9851 .- 1879-1859. ; 103:April, s. 215-220
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Tidskriftsartikel (refereegranskat)abstract
- In this paper, three independent Earth gravity models (EGMs) ofGO_CONS_GCF_2_TIM_R4, AIUB-GRACE03S and ULux_CHAMP2013s are combined to degree and order 120. The geoid models of these EGMs are computed and compared with the Global Positioning System (GPS) and levelling data over Fennoscandia. We found that the simple mean of these geoid models is closer to the GPS/levelling data than their weighted mean. This means that errors of the EGMs are not properly estimated as they are used in the weighted mean solution. We develop a method based on solving a nonlinear condition adjustment model to calibrate the errors so that the result of weighted mean becomes the same as that of the simple mean. Numerical results show slight changes in the errors of GRACE03S but large ones in those of GO_CONS_GCF_2_TIM_R4 and ULux_CHAMP2013s. Furthermore, the weighted mean solution considering the calibrated errors and some additional constraints is better than GOCO03S to degree and order 120 over Fennoscandia.
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| 21. |
- Eshagh, Mehdi
(författare)
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A strategy towards an EGM08-based Fennoscandian geoid model
- 2012
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Ingår i: Journal of Applied Geophysics. - : Elsevier BV. - 0926-9851 .- 1879-1859. ; 87, s. 53-59
-
Tidskriftsartikel (refereegranskat)abstract
- Today, the recent global Earth's gravity model, EGM08, is successfully utilised for different purposes in geosciences. Here, EGM08 is used to compute a geoid model for Fennoscandia and since it is restricted to degree and order 2160, the higher frequencies of the geoid, or the truncation bias, is recovered directly from terrestrial gravity anomalies using a simple formula. The total topographic and atmospheric effects are computed and added to the derived geoid as well. A very simple EGM08-based non-integral geoid estimator is developed and applied for computing the geoid of Fennoscandia. The outcome of the estimator is compared with the Global Positioning System (GPS)/levelling data of Sweden, Denmark, Finland and Norway. Numerical results show the successful performance of the presented estimator as the geoid become closer to GPS/levelling data than the one computed solely with EGM08. This study will show that considering the truncation bias of EGM08 will reduce the root mean square error (RMSE) of the differences between the geoid and GPS/levelling data by about 1.3 cm and the additive topographic and atmospheric corrections by 1 cm further. It is shown that the correlations among the data have no significant influence on the estimated geoid.
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| 22. |
- Eshagh, Mehdi, 1977-, et al.
(författare)
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Determination of sub-lithospheric stress due to mantle convection using GOCE gradiometric data over Iran
- 2015
-
Ingår i: Journal of Applied Geophysics. - : Elsevier BV. - 0926-9851 .- 1879-1859. ; 122, s. 11-17
-
Tidskriftsartikel (refereegranskat)abstract
- Sub-lithospheric stress due to mantle convection can be determined from gravimetric data based on Runcorn’s theory. In this paper, the satellite gradiometric data of the recent European satellite mission, the Gravity field and steady-state Ocean Circulation Explorer (GOCE) is used to determine the sub- lithospheric stress locally in Iran. The method of S function (SF) with numerical differentiation is developed further and an integral equation connecting satellite gradiometric data to SF is presented. The integral equation will be used to invert the real gradiometric data of GOCE to recover the SF. Later on, the sub-lithospheric shear stresses, which are the northwards and eastwards derivatives of the SF, are computed numerically. Our numerical results show that the mean square error of the recovered SF is smaller than the values of the SF meaning that the recovery process is successful. Also, the recovered stress has a good agreement with the tectonic boundaries and active seismic points of the world stress map (WSM) database. This stress reaches amplitude of 100 MPa in the territory.
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| 23. |
- Eshagh, Mehdi, et al.
(författare)
-
Local error calibration of EGM08 geoid using GNSS/levelling data
- 2016
-
Ingår i: Journal of Applied Geophysics. - : Elsevier BV. - 0926-9851 .- 1879-1859. ; 130, s. 209-217
-
Tidskriftsartikel (refereegranskat)abstract
- The geoid error, computed from EGM08, is unrealistically large due to the continuation of the spherical harmonic coefficient errors down to the surface of the reference ellipsoid. In this study, we try to calibrate such an error by the differences between the EGM08 and GNSS/levelling geoids over Fennoscandia. We use the variance component estimation procedure through combined adjustments of the geoid and GNSS/levelling heights using corrector surfaces of 4-, 5- and 7-parameter. We also develop a simple iterative method to calibrate the geoid error from the a posteriori variance factor and the errors of GNSS/levelling geoid. Our numerical investigations show that performing the separate adjustment and variance component estimation for each country with a two component stochastic model is more successful than performing it in the whole area with a five-component model. The number of GNSS/levelling data over Sweden and Norway are much larger than those in Denmark and Finland. This causes that the corrector surfaces are fitted better in these countries and consequently the estimated errors for the geoid become larger than what they should be in the others. Based on a 7-parameter corrector surface model, the average error of the EGM08 geoid becomes 12,17, 51 and 34 mm, in Sweden, Denmark, Norway and Finland, respectively. If the two-component stochastic model is used in a combined adjustment over Fennoscandia this average error will be 48 mm.
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| 24. |
- Eshagh, Mehdi, Professor, 1977-
(författare)
-
Numerical aspects of EGM08-based geoid computations in Fennoscandia regarding the applied reference surface and error propagation
- 2013
-
Ingår i: Journal of Applied Geophysics. - : Elsevier BV. - 0926-9851 .- 1879-1859. ; 96, s. 28-32
-
Tidskriftsartikel (refereegranskat)abstract
- So far the recent Earth's gravity model, EGM08, has been successfully applied for different geophysical and geodetic purposes. In this paper, we show that the computation of geoid and gravity anomaly on the reference ellipsoid is of essential importance but error propagation of EGM08 on this surface is not successful due to downward continuation of the errors. Also we illustrate that some artefacts appear in the computed geoid and gravity anomaly to lower degree and order than 2190. This means that the role of higher degree harmonics than 2160 is to remove these artefacts from the results. Consequently, EGM08 must be always used to degree and order 2190 to avoid the numerical problems. © 2013 Elsevier B.V.
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| 25. |
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| 26. |
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| 27. |
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| 28. |
- Harmankaya, Utku, et al.
(författare)
-
Locating scatterers while drilling using seismic noise due to TBM
- 2018
-
Ingår i: Journal of Applied Geophysics. - : Elsevier BV. - 0926-9851 .- 1879-1859. ; 152, s. 86-99
-
Tidskriftsartikel (refereegranskat)abstract
- Unexpected geological structures can cause safety and economic risks during underground excavation. Therefore, predicting possible geological threats while drilling a tunnel is important for operational safety and for preventing expensive standstills. Subsurface information for tunneling is provided by exploratory wells and by surface geological and geophysical investigations, which are limited by location and resolution, respectively. For detailed information about the structures ahead of the tunnel face, geophysical methods are applied during the tunnel-drilling activity.We present a method inspired by seismic interferometry and ambient-noise correlation that can be used for detecting scatterers, such as boulders and cavities, ahead of a tunnel while drilling. A similar method has been proposed for active-source seismic data and validated using laboratory and field data. Here, we propose to utilize the seismic noise generated by a Tunnel Boring Machine (TBM), and recorded at the surface. We explain our method at the hand of data from finite-difference modelling of noise-source wave propagation in a medium where scatterers are present. Using the modelled noise records, we apply cross-correlation to obtain correlation gathers. After isolating the scattered arrivals in these gathers, we cross-correlate again and invert for the correlated traveltime to locate scatterers. We show the potential of the method for locating the scatterers while drilling using noise records due to TBM.
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| 29. |
- Hellman, Kristofer, et al.
(författare)
-
Structurally coupled inversion of ERT and refraction seismic data combined with cluster-based model integration
- 2017
-
Ingår i: Journal of Applied Geophysics. - : Elsevier BV. - 0926-9851. ; 143, s. 169-181
-
Tidskriftsartikel (refereegranskat)abstract
- Electrical resistivity tomography (ERT) and refraction seismics are among the most frequently used geophysical methods for site-investigations and the combined results can be very helpful to fill in the gaps between the point measurements made by traditional geotechnical methods such as Cone Penetration Test (CPT), core-drilling and geophysical borehole logging. The interpretation of the results from a geophysical investigation constituting a single method often yields ambiguous results. Hence, an approach utilizing multiple techniques is often necessary. To facilitate interpretation of such a combined dataset, we propose a more controlled and objective approach and present a method for a structurally coupled inversion of 2D electrical resistivity and refraction seismic data using unstructured meshes. Mean shift clustering is used to combine the two images and to compare the separate and coupled inversion methodologies. Two synthetic examples are used to demonstrate the method, and a field-data example is included as a proof of concept. In all cases a significant improvement by the coupling is visible. The methodology can be used as a tool for improved data interpretation and for obtaining a more comprehensive and complete picture of the subsurface by combining geophysical methods.
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| 30. |
- Hussain, Matloob, et al.
(författare)
-
Changes in gravitational parameters inferred from time-variable GRACE data- A case study for October 2005 Kashmir Earthquake
- 2016
-
Ingår i: Journal of Applied Geophysics. - Amsterdam : Elsevier BV. - 0926-9851 .- 1879-1859. ; 132, s. 174-183
-
Tidskriftsartikel (refereegranskat)abstract
- The earth's gravity changes are attributed to the redistribution of masses within and/or on the surface of the earth, which are due to the frictional sliding, tensile cracking and/or cataclastic flow of rocks along the faults and detectable by earthquake events. Inversely, the gravity changes are useful to describe the earthquake seismicity over the active orogenic belts. The time variable gravimetric data are hardly available to the public domain. However, Gravity Recovery and Climatic Experiment (GRACE) is the only satellite mission dedicated to model the variation of the gravity field and an available source to the science community. Here, we have tried to envisage gravity changes in terms of gravity anomaly (Δg), geoid (N) and the gravity gradients over the Indo-Pak plate with emphasis upon Kashmir earthquake of October 2005. For this purpose, we engaged the spherical harmonic coefficients of monthly gravity solutions from the GRACE satellite mission, which have good coverage over the entire globe with unprecedented accuracy. We have analysed numerically the solutions after removing the hydrological signals, during August to November 2005, in terms of corresponding monthly differentials of gravity anomaly, geoid and the gradients. The regional structures like Main Mantle Thrust (MMT), Main Karakoram Thrust (MKT), Herat and Chaman faults are in closed association with topography and with gravity parameters from the GRACE gravimetry and EGM2008 model. The monthly differentials of these quantities indicate the stress accumulation in the northeast direction in the study area. Our numerical results show that the horizontal gravity gradients seem to be in good agreement with tectonic boundaries and differentials of the gravitational elements are subtle to the redistribution of rock masses and topography caused by 2005 Kashmir earthquake. Moreover, the gradients are rather more helpful for extracting the coseismic gravity signatures caused by seismicity over the area. Higher positive values of gravity components having higher terrain elevations are more vulnerable to the seismicity and lower risk of diastrophism otherwise.
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| 31. |
- Ismail, Nazli, 1971-, et al.
(författare)
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Investigation of groundwater resources using controlled-source radio magnetotellurics (CSRMT) in glacial deposits in Heby, Sweden
- 2011
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Ingår i: Journal of Applied Geophysics. - : Elsevier BV. - 0926-9851 .- 1879-1859. ; 73:1, s. 74-83
-
Tidskriftsartikel (refereegranskat)abstract
- We have combined tensor radio magnetotelluric- (RMT, 15-250. kHz) and controlled source tensor magnetotelluric (CSTMT, 1-12. kHz) data for the mapping of aquifers in gravel formations lying in between crystalline bedrock and clay rich sediments in the Heby area some 40. km west of Uppsala in Sweden. The estimated transfer functions, the impedance tensor and the tipper vector generally satisfy 1D or 2D necessary conditions except for the lowest CSTMT frequencies where near field effects become more dominant.The data measured from 8 profiles were inverted with the Rebocc code of Siripunvaraporn and Egbert (2000) assuming plane wave conditions. This meant that only 12 frequencies in the range of 4-180kHz could be used. The four lowest frequencies of CSTMT in the range of 1-2.8kHz were excluded because of source effects. Data from all profiles were inverted with a starting model of 100Ω-m and a relative error floor of 0.02 on apparent resistivity, corresponding to less than 1° on phase. Tipper vectors are generally small except when source effects become dominant in the lowest frequencies of CSTMT and were therefore not used for inversion. Comparing with models derived from vertical electrical soundings, refraction and reflection seismic data as well as ground truth from exploration wells assessed the reliability of the deep part of the models. Furthermore we carried out a non-linear resolution analysis to better quantify the depth extent of the aquifers.The inverted models from the Heby area show well the thickness variations of glacial deposits overlying crystalline bedrock. Generally, the upper 20. m of the models have resistivities below 40 Ω-m, taken to represent clay rich formations. Below the clay layer resistivities increase to about 40-400 Ω-m, interpreted to represent sand/gravel formations with a maximum thickness of about 40. m and a width of several hundred metres. This is a potential aquifer that extends in approximately N-S direction for some kilometres.
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| 32. |
- Ivandic, Monika, et al.
(författare)
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Time-lapse analysis of sparse 3D seismic data from the CO2 storage pilot site at Ketzin, Germany
- 2012
-
Ingår i: Journal of Applied Geophysics. - : Elsevier BV. - 0926-9851 .- 1879-1859. ; 84, s. 14-28
-
Tidskriftsartikel (refereegranskat)abstract
- The Ketzin pilot site is the first European on-shore CO2 storage project site. Injection started in June 2008 and until the latest repeat survey in February 2011 around 45,000 t of CO2 had been injected into a saline aquifer at approximately 630 m depth. Seismic monitoring methods that have been applied at the Ketzin site comprise baseline and repeat observations at various scales. We present here time-lapse results from sparse 3D seismic data measurements at Ketzin, which were acquired to link downhole surveys with conventional 3D surface seismic surveys. The results are consistent with the 3D seismic time-lapse studies over the injection site and show that the sparse 3D geometry can be used to map the CO2 in the reservoir at a significantly lower effort than the conventional 3D surveying. The last repeat survey indicates preferential migration of the CO2 to the west. There are no indications of migration into the caprock on either of the repeat surveys.
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| 33. |
- Jensen, Mai-Britt Mose, et al.
(författare)
-
A correlation of AMS and rock quality in crystalline and sedimentary rocks in the Kiirunavaara hanging wall
- 2012
-
Ingår i: Journal of Applied Geophysics. - : Elsevier BV. - 0926-9851 .- 1879-1859. ; 86, s. 54-69
-
Tidskriftsartikel (refereegranskat)abstract
- A study of the magnetic fabric and rock quality has been performed on crystalline and sedimentary rocks from the hanging wall of the Kiirunavaara iron ore deposit, northern Sweden. The rock samples were taken from 12 outcrops and three oriented drillcores and consist of quartzite, siltstone, conglomerate, porphyritic basalts, quartz-bearing porphyry and mylonite.Anisotropy of magnetic susceptibility (AMS) was used to determine the texture of the rocks by defining the shape of the magnetic susceptibility ellipsoids, the orientation of the magnetic foliation planes and the degree of magnetic anisotropy. AMS data from the outcrops as well as data from the uppermost part of two of the drillcores was used to produce a surface contour map which indicates that both the dip of the magnetic foliation plane and the degree of magnetic anisotropy decreases towards the east. These two parameters thus generally follow the major structural features of the Kiruna region.Two classical rock mechanical parameters, RQD (Rock Quality Designation) and F/m (fractures pr. meter), were used to quantify the amount of discontinuities and fractures in the drill cores. RQD and F/m was subsequently correlated with the AMS data, and a correlation was observed, with the correlation coefficient r varying between 0.6142-0.7557. The correlation was limited to just one of the drillcores however it could be traced across rock type boundaries. The correlation indicates that AMS may be used as an indicator tool of the rock mechanical properties in both crystalline and sedimentary rocks.
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| 34. |
- Johansson, Sara, et al.
(författare)
-
Influence of non-aqueous phase liquid configuration on induced polarization parameters: Conceptual models applied to a time-domain field case study
- 2015
-
Ingår i: Journal of Applied Geophysics. - : Elsevier BV. - 0926-9851. ; 123, s. 295-309
-
Tidskriftsartikel (refereegranskat)abstract
- Resistivity and induced polarization (IP) measurements on soil contaminated with non-aqueous phase liquids (NAPLs) show a great variety in results in previous research. Several laboratory studies have suggested that the presence of NAPLs in soil samples generally decrease the magnitude of the IP-effect, while others have indicated the opposite. A number of conceptual models have been proposed suggesting that NAPLs can alter the pore space in different ways, e.g. by coating the grain surfaces and thus inhibiting grain polarization, or by changing the pore throat size and thus affecting the membrane polarization mechanism. The main aim of this paper is to review previously published conceptual models and to introduce some new concepts of possible residual NAPL configurations in the pore space. Time domain induced polarization measurements were performed at a NAPL contaminated field site, and the data were inverted using the Constant Phase Angle (CPA) model and the Cole–Cole model respectively. No significant phase anomalies were observed in the source area of the contamination when the CPA inverted profiles were compared with soil sampling results of free-phase contaminant concentrations. However, relatively strong phase and normalized phase anomalies appeared next to the source area, where residual free-phase presence could be expected according to the chemical data. We conclude that depending on the NAPL configuration, different spectral IP responses can be expected. In previous research, the NAPL configurations in different samples or field sites are often unknown, and this may to some extent explain why different results have been achieved by different authors. In our field case, we believe that the NAPL forms a more or less continuous phase in the pore space of the source zone leading to an absence of IP anomalies. The increase in phase and normalized phase angle observed next to the source zone is interpreted as a degradation zone. The ongoing biodegradation may have led to a fractionation of the continuous NAPL in the outer part of the original source zone, leading to residual presence of isolated NAPL droplets in the soil pores. With such NAPL configurations, an increased polarization can be expected according to the electrochemical- and membrane polarization mechanisms. More research is needed to confirm the effects of different NAPL configuration on spectral IP parameters.
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| 35. |
- Johansson, Sara, et al.
(författare)
-
Spectral analysis of time domain induced polarization waveforms
- 2020
-
Ingår i: Journal of Applied Geophysics. - : Elsevier BV. - 0926-9851. ; 177
-
Tidskriftsartikel (refereegranskat)abstract
- Spectral induced polarization is a well-established geophysical method that can be measured in both time- and frequency domain. Time- and frequency domain methods are theoretically equivalent via the Fourier transform. However, data acquisition parameters such as the sampling frequency and pulse length affect the actual spectral content of real time domain waveforms. Comparative studies showing the practical equivalence of time- and frequency domain measurements are few and have been carried out using various approaches. Some of these studies concluded that it is not possible to resolve wide band spectral information with time domain waveforms. The few studies available presenting actual spectral analysis of time domain signals were carried out several decades ago on field data with limited data acquisition- and/or analysis methods. There is, therefore, a need to re-evaluate the possible spectral range of time domain induced polarization data considering modern instrument development during the last decades. In addition, there is a need to verify the physical importance of factors such as spectral energy distribution and measurement noise, which previously have been assumed to inherently limit the spectral content of time domain waveforms. In this study, the discrete Fourier transform is applied to analyze the spectral content of synthetic time domain waveforms with different data acquisition parameters and noise. The results are presented as the frequency domain parameters amplitude and phase and compared to the corresponding frequency domain Debye model. To study the effect of noise, a spectral analysis of synthetic full waveform data contaminated with Gaussian noise is also provided, where the noise level were guided by an example of measured field data. Finally, laboratory TD IP measurements were performed on a test circuit, transformed to the frequency domain and compared to the theoretical response of the circuit in terms of the FD IP parameters amplitude and phase. The results illustrate that spectral information contained in the time domain waveform is, as expected, limited by the fundamental frequency of the square wave at low frequencies and the sampling frequency at high frequencies. Variation of the relaxation time shows that the ability to correctly resolve high frequency relaxations is also dependent on the sampling frequency. Furthermore, the result show that high frequency noise might distort the frequency domain representation. However, this noise can be filtered away prior to the spectral analysis, which significantly improves the resolution in frequency domain. The results of this study is in contradiction to earlier studies stating that the spectral range of time domain waveforms is inherently narrow due to the domination of the fundamental frequency over the higher harmonics. The main reason is that the relative spectral energy of different harmonics is of no importance in time domain measurements; in contrast to frequency domain measurements, the total polarization of all length scales are excited simultaneously. The results and discussion in this study are expected to contribute to an improved comprehension of the spectral range that can be achieved with time domain induced polarization data collected with modern instruments.
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| 36. |
- Juhlin, Christopher, 1956-, et al.
(författare)
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Reflection seismic imaging of the end-glacial Pärvie Fault system, northern Sweden
- 2010
-
Ingår i: Journal of Applied Geophysics. - : Elsevier BV. - 0926-9851 .- 1879-1859. ; 70:4, s. 307-316
-
Tidskriftsartikel (refereegranskat)abstract
- Reflection seismic data were acquired along a c. 23 km long profile over the Pärvie Fault system with a nominal receiver and source spacing of 20 m. An hydraulic breaking hammer was used as a source, generating signals with a penetration depth of about 5–6 km. Steeply dipping reflections from the end-glacial faults are observed, as well as sub-horizontal reflections. The location and orientation of the reflections from the faults agree well with surface geological observations of fault geometries. Reflections from a potential fourth end-glacial fault is observed further to the east along the profile. The more sub-horizontal reflections may originate from gabbroic bodies within the granitic basement or from deeper lying greenstones. Our results indicate that the end-glacial faults dip at moderate to steep dips down to at least 2–3 km depth, and possibly continue at this dip to depths of 6 km. This result has significant implications for determining the state of stress required to activate the faults in the past and in the future.
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| 37. |
- Karimpouli, Sadegh, et al.
(författare)
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Understanding the fracture role on hydrocarbon accumulation and distribution using seismic data : A case study on a carbonate reservoir from Iran
- 2013
-
Ingår i: Journal of Applied Geophysics. - : Elsevier BV. - 0926-9851 .- 1879-1859. ; 96, s. 98-106
-
Tidskriftsartikel (refereegranskat)abstract
- The South Pars, the largest gas field in the world, is located in the Persian Gulf. Structurally, the field is part of the Qatar-South Pars arch which is a regional anticline considered as a basement-cored structure with long lasting passive folding induced by salt withdrawal. The gas-bearing reservoir belongs to Kangan and Dalan formations dominated by carbonate rocks. The fracture role is still unknown in gas accumulation and distribution in this reservoir. In this paper, the Scattering Index (SI) and the semblance methods based on scattered waves and diffraction signal studies, respectively, were used to delineate the fracture locations. To find the relation between fractures and gas distribution, desired facies containing the gas, were defined and predicted using a method based on Bayesian facies estimation. The analysis and combination of these results suggest that preference of fractures and/or fractured zones are negligible (about 1% of the total volume studied in this paper) and, therefore, it is hard to conceive that they play an important role in this reservoir. Moreover, fractures have no considerable role in gas distribution (less than 30%). It can be concluded from this study that sedimentary processes such as digenetic, primary porosities and secondary porosities are responsible for the gas accumulation and distribution in this reservoir.
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| 38. |
- Kazemeini, Sayed Hesammoddin, 1973-, et al.
(författare)
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Monitoring CO2 response on surface seismic data; a rock physics and seismic modeling feasibility study at the CO2 sequestration site, Ketzin, Germany
- 2010
-
Ingår i: Journal of Applied Geophysics. - : Elsevier BV. - 0926-9851 .- 1879-1859. ; 71:4, s. 109-124
-
Tidskriftsartikel (refereegranskat)abstract
- An important component of any CO2 sequestration project is seismic monitoring for tracking changes in subsurface physical properties such as velocity and density. Reservoir conditions and CO2 injection quantities govern whether such changes may be observable as a function of time. Here we investigate surface seismic response to CO2 injection at the Ketzin site, the first European onshore CO2 sequestration pilot study dealing with research on geological storage of CO2. First, a rock-physics model was built to evaluate the effect of injected CO2 on the seismic velocity. On the basis of this model, the seismic response for different CO2 injection geometries and saturation was studied using 1D elastic modeling and 2D acoustic finite difference modeling. Rock-physics models show that CO2 injected in a gaseous state, rather than in a supercritical state, will have a more pronounced effect on seismic velocity, resulting in a stronger CO2 response. However, reservoir heterogeneity and seismic resolution, as well as random and coherent seismic noise, are negative factors that need to be considered in a seismic monitoring program. In spite of these potential difficulties, our seismic modeling results indicate that the CO2 seismic response should be strong enough to allow tracking on surface seismic data. Amplitude-related attributes (i.e., acoustic impedance versus Poisson's ratio cross-plots) and time-shift measurements are shown to be suitable methods for CO2 monitoring.
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| 39. |
- Khalili, M., et al.
(författare)
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Monitoring of water volume in a porous reservoir using seismic data : A 3D simulation study
- 2024
-
Ingår i: Journal of Applied Geophysics. - : Elsevier BV. - 0926-9851 .- 1879-1859. ; 229
-
Tidskriftsartikel (refereegranskat)abstract
- A potential framework to estimate the volume of water stored in a porous storage reservoir from seismic data is neural networks. In this study, the man-made groundwater reservoir is modeled as a coupled poroviscoelastic-viscoelastic medium, and the underlying wave propagation problem is solved using a three-dimensional discontinuous Galerkin method coupled with an Adams-Bashforth time stepping scheme. The wave problem solver is used to generate databases for the neural network-based machine learning model to estimate the water volume. In the numerical examples, we investigate a deconvolution-based approach to normalize the effect from the source wavelet in addition to the network's tolerance for noise levels. We also apply the SHapley Additive exPlanations method to obtain greater insight into which part of the input data contributes the most to the water volume estimation. The numerical results demonstrate the capacity of the fully connected neural network to estimate the amount of water stored in the porous storage reservoir.
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| 40. |
- Lei, Qinghua
(författare)
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Impact of fracture normal and shear stiffnesses on the scattering attenuation of P and S waves in a naturally fractured rock
- 2022
-
Ingår i: Journal of Applied Geophysics. - : Elsevier. - 0926-9851 .- 1879-1859. ; 206
-
Tidskriftsartikel (refereegranskat)abstract
- In this paper, numerical simulations are conducted to study elastic wave transport, scattering, and attenuation in a naturally fractured rock associated with length-correlated fracture normal and shear stiffnesses. The model represents the pattern of a real fracture outcrop in an explicit fashion based on the discrete fracture network approach and computes the dynamical interaction between waves and fractures based on the displacement discontinuity method. A broad spectrum of geologically relevant fracture stiffness values are explored to analyse the impact of fracture normal and shear stiffness components on the wavefield evolution. It is observed that when the fracture normal and shear stiffnesses are both high, the wavefield is a propagative mode dominated by a forward ballistic transport. With the reduction of fracture normal and/or shear stiffnesses, the wavefield becomes diffusive characterised by the emergence and dominance of coda waves. If the fracture stiffnesses are very low, waves become trapped entering the so-called localisation regime associated with an absence of effective transport as well as a profound attenuation. Our results show that the scattering attenuation of S waves tends to be greater than that of P waves in the propagation and diffusion regimes, but becomes similar in the localisation regime. The research findings of this paper have important implications for understanding and predicting the seismic wave attenuation behaviour in naturally fractured rocks for various geophysical applications.
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| 41. |
- Levendal, Tegan, et al.
(författare)
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Investigation of an Ordovician carbonate mound beneath Gotland, Sweden, using 3D seismic and well data
- 2019
-
Ingår i: Journal of Applied Geophysics. - : Elsevier BV. - 0926-9851 .- 1879-1859. ; 162, s. 22-34
-
Tidskriftsartikel (refereegranskat)abstract
- The Swedish island of Gotland is located within the Baltic Basin. During the Late Ordovician the region around Gotland was part of a shallow epicratonic basin in the southern subtropics. In these warm-water environments algae flourished, diverse reefs developed close to the coastline and further outboard carbonate mounds developed. These mounds formed rigid high relief structures surrounded by fine-grained siliciclastics and marls and can be detected on seismic images as isolated concave upwards features. The sedimentary succession beneath Gotland was intensely investigated in the 1970s and 1980s for its hydrocarbon potential, and subsequently, oil was commercially produced from reservoirs within Ordovician mounds. In 1981, a 3D seismic survey was conducted by Horizon Exploration Ltd. over the Fardume mound on northern Gotland. To date no results from these 3D data have been published in scientific literature.The region of Gotland aims to produce 100% of its energy from renewable sources and currently much of Gotland's electricity is provided by wind turbines. Due to the intermittent nature of wind power, one solution to regulate the supply of electricity from wind energy is Compressed Air Energy Storage (CAES).In this study, we convert the 3D seismic survey acquired over the Fardume mound from scanned TIFF images to SEGY format. These data are then utilized together with well data to gain a better knowledge of the geological structure of the mound and to examine its reservoir characteristics and potential for CAES. To date, carbonate mounds on Gotland have mainly been reported in the scientific literature using well data. This 3D seismic survey, therefore, provides a rare opportunity to better characterize and investigate the structure of one of the carbonate mounds on Gotland.
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| 42. |
- Loke, M. H., et al.
(författare)
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The use of asymmetric time constraints in 4-D ERT inversion
- 2022
-
Ingår i: Journal of Applied Geophysics. - : Elsevier BV. - 0926-9851. ; 197
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Tidskriftsartikel (refereegranskat)abstract
- Time-lapse resistivity surveys are commonly used to monitor temporal changes in the subsurface. In certain cases, it is known from other information that the resistivity will only decrease or increase with time. The 4-D resistivity smoothness-constrained inversion method reduces artifacts due to noise by including a temporal roughness filter constraint that ensures the temporal changes vary in a smooth manner. A least-squares optimization method is used to find a solution by attempting to locate the minimum of an objective function that consists of the data misfit and model (spatial and temporal) roughness. In some cases, the 4-D time-lapse inverse models show an increase in the resistivity with time in parts of the subsurface where it is only expected to decrease (or vice versa). We compare two methods, the barrier function and transformation methods, that attempt to minimize or eliminate these artifacts. We incorporate the barrier function constraint into the 4-D inverse method by using a modified difference matrix as a temporal roughness filter. The barrier function constraint includes an additional term that increases the objective function value greatly if the model values cross the allowed thresholds. This greatly minimizes the artifacts but does not completely eliminate them. It has the advantage that there are minimal changes in the objective function in regions of model space that are not close to the imposed thresholds. The method of transformations changes the model parameter such that the additional positivity or negativity constraints are implicitly included in the transformed model parameter. It has the advantage that it can completely eliminate the artifacts. However, it modifies the entire objective function which could be a disadvantage in some cases. We also explore a combination of the two methods, using the barrier function method to generate an initial model that minimizes the artifacts followed by the transformation method. This hybrid technique completely removes the residual artifacts left by the barrier method, and produces an inverse model which is closer to the true model for a synthetic data set. We also describe a post-inversion modification of the L-curve method to determine the optimum model that takes into account the non-linear nature of the inverse problem and the forward modelling method. The technique gave an estimate of the noise level for a field data set and produced a model which is consistent with independent hydrological measurements at the test site.
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| 43. |
- Malehmir, Alireza, et al.
(författare)
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An investigation of the effects of the choice of stacking velocities on residual statics for hardrock reflection seismic processing
- 2010
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Ingår i: Journal of Applied Geophysics. - : Elsevier BV. - 0926-9851 .- 1879-1859. ; 72:1, s. 28-38
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Tidskriftsartikel (refereegranskat)abstract
- Crystalline rocks are typically deformed, folded and altered and contain interfaces that have varying dips. These interfaces are often short in length and sometimes can be represented as diffractors. Imaging of reflections from these interfaces, which are often of low acoustic impedance contrast, poses a challenge in reflection seismic data processing. An important step in the imaging process is the calculation of residual statics. In this study we investigate the effects of the choice of stacking velocities on the estimation of surface-consistent residual statics in the crystalline environment on a synthetic data set and two real data examples, one 2D data set and a 3D one. On the synthetic data set, residual statics were estimated using three different approaches. In the first approach (normal move out or NMO approach), residual statics were estimated using an NMO velocity function that focuses on steeply dipping reflections and uses velocities ranging from 5000 to 9000 m/s. In the second approach (dip move out or DMO approach), residual statics were estimated using the true media velocity or using an updated velocity function after the implementation of DMO corrections. For the third approach, single reflections were focused on for the residual statics estimation. Analysis of the resulting stacked sections, as well as the estimated residual statics, shows that when short steeply dipping reflections are present, the DMO approach allows better imaging of diffractions and gently dipping reflections. High-stacking velocities in the NMO approach produce more coherent noise, which negatively influences the estimation of residual statics. The best estimation of residual statics can be obtained if the calculation focuses in a narrow window along sub-horizontal or gently dipping reflections. Such reflections will not always be present in real data and the synthetic test shows that focusing on steeply dipping reflections within a narrow window, in the absence of horizontal or gently dipping reflections, provides superior results compared with a wide search window. Application of these approaches on real seismic data acquired over crystalline rocks demonstrates that an improper estimation of surface-consistent residual static corrections can result in the loss of short gently dipping reflections and diffractions that may be an indication of mineral deposits.
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| 44. |
- Malehmir, Alireza, et al.
(författare)
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High-resolution reflection seismic investigations of quick-clay and associated formations at a landslide scar in southwest Sweden
- 2013
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Ingår i: Journal of Applied Geophysics. - : Elsevier BV. - 0926-9851 .- 1879-1859. ; 92, s. 84-102
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Tidskriftsartikel (refereegranskat)abstract
- We present high-resolution reflection seismic data from four lines (total 1.9 km) that cross a quick-clay landslide scar located close to the shore of the Gota River in southwest Sweden, and compare the results with geotechnical data from boreholes. The seismic data allow the imaging of bedrock topography and normally to weakly consolidated sediments to a subsurface depth of about 100 m. Different types of seismic sources, including sledgehammer, accelerated weight-drop and dynamite were utilized and compared with each other. Analysis of their power spectra suggests that weight-drop and dynamite have higher frequency content and energy than the sledgehammer, which makes these two sources suitable also for waveform tomography and surface-wave data analysis. The shallowest non-bedrock reflector is observed at about 10-20 m below the surface, it overlays the bedrock, and is interpreted to originate from the contact between clay formations above and a coarse-grained layer below. The coarse-grained layer appears to be spatially linked to the presence of quick-clays. It is a regional scale formation, laterally heterogeneous, which deepens to the west of the study area and correlates well with the available geotechnical data. Continuity of the coarse-grained layer becomes obscured by the landslide scar. There may be a link between the coarse-grained layer and landslides in the study area, although this possibility requires further hydrogeological and geotechnical investigations. Reflectors from the top of the bedrock suggest a depression zone with its deepest point below the landslide scar and a bowl-shaped structure in the northern portion of one of the seismic lines.
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| 45. |
- Malmgren, Lars, et al.
(författare)
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The excavation disturbed zone (EDZ) at Kiirunavaara mine, Sweden - by seismic measurements
- 2007
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Ingår i: Journal of Applied Geophysics. - : Elsevier BV. - 0926-9851 .- 1879-1859. ; 61:1, s. 1-15
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Tidskriftsartikel (refereegranskat)abstract
- The presence of an excavation disturbed zone (EDZ) around an excavation boundary can significantly affect the overall performance of the excavation and the general safety of men and equipment. Hence, it has been an important subject of research in various rock excavation projects. The EDZ is generally defined as the rock zone beyond the excavation boundary where the physical, mechanical and hydraulic properties of the rock have been significantly affected due to the excavation and redistribution of stresses. For LKAB's Kiirunavaara underground iron ore mine in Sweden, the understanding of the EDZ is essential for optimal design of rock support. With this main objective an EDZ investigation was conducted at the mine using seismic measurement techniques. Cross-hole seismics and spectral analyses of surfaces waves (SASW) were the main techniques used. Borehole Image Processing Systems (BIDS) complemented the seismic measurements. The results show that an EDZ with a thickness of 0.5-1.0 m existed behind the boundaries of the mining drifts being investigated. The magnitude of the Young's modulus of this zone was 50% to 90% of that of the undisturbed rock.
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| 46. |
- Martin, Tina, et al.
(författare)
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Classification of slag material by spectral induced polarization laboratory and field measurements
- 2021
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Ingår i: Journal of Applied Geophysics. - : Elsevier BV. - 0926-9851. ; 194
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Tidskriftsartikel (refereegranskat)abstract
- Historical slag dumps are of increasing interest due to economic, environmental or archaeological reasons. Geophysical investigations can help accessing the potential reuse of slag material to recover metallic raw material or for the estimation of the hazard potential of the buried slag material due to dissolution occurrence.In our study, we have investigated various slag material in the laboratory with the spectral induced polarization (SIP) method, obtained from different historical slag dumps, located in the Harz Mountains, Germany. We also present SIP results from field measurements at a historical slag dump where most of the slag samples reveal high amounts of iron, zinc, silica, and barium.Our results reveal a discrimination between three different slag grades (low, medium, high) by using the imaginary conductivity σ″ at a medium frequency (1–10 Hz) in both laboratory and field. Furthermore, additional information is obtained by a classification based on the spectral polarization behaviour and considering the field frequency range (0.1 Hz – 100 Hz). Five different types of spectra (ascending, descending, constant, maximum and minimum type) can be discriminated and recognized in the laboratory and in distinct areas of the slag dump. Even though a direct comparison between the laboratory and field results still needs to be proven, the buried slag material can be differentiated from the surrounding material by the polarization magnitude.
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| 47. |
- Martin, Tina, et al.
(författare)
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Evaluation of spectral induced polarization field measurements in time and frequency domain
- 2020
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Ingår i: Journal of Applied Geophysics. - : Elsevier BV. - 0926-9851. ; 180
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Tidskriftsartikel (refereegranskat)abstract
- Spectral induced polarization (SIP) measurements have been demonstrated to correlate with important parame-ters in hydrogeological and environmental investigations. Although SIP measurements were often collected in thefrequency domain (FDIP), recent developments have demonstrated the capabilities to solve for the frequency-de-pendence of the complex conductivity through measurements collected in the time domain (TDIP). Therefore,the aim of our field investigations is a comparison of the measured frequency-dependence at a broad frequencyrange resolved through FDIP and TDIP. In contrast to previous studies, we conducted measurements with dif-ferent instruments and measuring technologies for both FDIP and TDIP. This allows for investigating the robust-ness of different measurements and assessing various sources of errors, for the assessment of the advantages anddrawbacks from different measuring techniques. Our results demonstrate that data collected through differentinstruments are consistent. Apparent resistivity measurements as well as the inversion results revealed quantita-tively the same values for all instruments. The measurements of the IP effect are also comparable, particularlyFDIP readings in the low frequencies (< 10 Hz) revealed to be quantitatively the same for different instruments.TDIP measurements are consistent for data collected with both devices. As expected, the spatial distribution ofthe values is also consistent for low frequency data (in FDIP) and late times measurements in TDIP (> 0.1 s).However, data quality for higher frequencies in FDIP (i.e., early times in TDIP) show larger variations, whichreflects the differences between the instruments to deal with the electromagnetic contamination of the IP data.Concluded in general, the different instruments and measuring techniques can provide consistent responses forvarying signal-to-noise ratio and measuring configurations.
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| 48. |
- Mehta, Suman, 1988-, et al.
(författare)
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Resolution and sensitivity of boat-towed RMT data to delineate fracture zones - Example of the Stockholm bypass multi-lane tunnel
- 2017
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Ingår i: Journal of Applied Geophysics. - : ELSEVIER SCIENCE BV. - 0926-9851 .- 1879-1859. ; 139, s. 131-143
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Tidskriftsartikel (refereegranskat)abstract
- The resolution and sensitivity of water-borne boat-towed multi-frequency radio-magnetotelluric (RMT) data for delineating zones of weaknesses in bedrock are examined in this study. 2D modeling of RMT data along 40 profiles in joint transverse electric (TE) and transverse magnetic (TM) as well as determinant mode was used for this purpose. The RMT data were acquired over two water passages from the Lake Malaren near the city of Stockholm where one of the largest underground infrastructure projects, a multi-lane tunnel, in Europe is currently being developed. Comparison with available borehole coring, refraction seismic and bathymetric data was used to scrutinize the RMT resistivity models. A low-resistivity zone observed in the middle of all the profiles is suggested to be from fracture/fault zones striking in the same direction as the water passages. Drilling observations confirm the presence of brittle structures in the bedrock, which manifest themselves as zones of low-resistivity and low-velocity in the RMT and refraction seismic data, respectively. Nevertheless, RMT is an inductive electromagnetic method hence the presence of conductive lake sediments may shield detecting the underlying fractured bedrock. The loss of resolution at depth implies that the structures within the bedrock under the lake sediments cannot reliably be delineated. To support this, a synthetic data analysis was carried out providing useful information on how to improve and plan the lake measurements for future studies. Synthetic modeling results for example suggested that frequencies as low as 3 kHz would be required to reliably resolve the bedrock and fracture zone within it in the study area. The modeling further illustrated the advantage of a fresh water layer that acts as a near-surface homogeneous medium eliminating the static shift effects. While boat-towed RMT data provided substantial information about the subsurface geology, the acquisition system should be upgraded to enable controlled-source data acquisition to increase the penetration depth and to overcome the shortcomings of using only radio-frequencies.
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| 49. |
- Muhamad, Harbe Anwar, et al.
(författare)
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Analysis of borehole geophysical data from the Mora area of the Siljan Ring impact structure, central Sweden
- 2015
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Ingår i: Journal of Applied Geophysics. - : Elsevier BV. - 0926-9851 .- 1879-1859. ; 115, s. 183-196
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Tidskriftsartikel (refereegranskat)abstract
- The Siljan impact structure is the largest known impact structure in Europe, the result of a Late Devonian meteorite impact (380.9 +/- 4.6 Ma). It is outlined mainly by a ring of lakes and Palaeozoic sedimentary rocks of Ordovician to Devonian age. The Palaeozoic successions are generally poorly exposed, but often well preserved with clear stratigraphy. At some locations they are strongly tectonised with sharply inclined or nearly overturned packages of crystalline basement and/or sediments. Down-hole logging data were acquired in the western part of the Siljan impact structure to determine some of the physical properties of the Palaeozoic sedimentary rocks in the area. Boreholes Mora 001 (356 m logged depth), Vattumyra Production (420 m logged depth), Mora VM 2 (94 m logged depth) and Mobillyft (437 m logged depth) were logged for temperature, sonic velocity and electrical resistivity. Logging data were compared to the lithology in the Mora 001 core, which had been mapped in detail, and information from other cores in the area. Good agreement between the natural gamma log and the core lithology was found. The sonic log shows a marked difference in velocity for the more clastic Silurian succession compared to the Ordovician succession and the Precambrian basement. A synthetic seismogram shows that a high amplitude reflection is expected at the Silurian-Ordovician boundary, raising some questions concerning interpretation of a seismic profile located about 6-7 km north of the study area. Correlation of the borehole logs shows that the thickness of the Silurian succession varies rapidly in the area and that its composition differs over distances of less than 1 km. These rapid variations suggest that the study area may be located in a megablock zone that was highly influenced by the impact Caledonian tectonics and changing depositional environments may also play a role in explaining the present-day borehole lithologies. Even though the boreholes are relatively far from the seismic profile and the geology is complex, the new data confirm that the Silurian has significant thickness along parts of the seismic profile. Potentially, the Silurian can be up to 450 m thick on parts of the profile. Further geophysical investigations in the area, including seismic surveying and gravity measurements, may help in mapping the complex structures away from the boreholes and discriminating between possible geological models.
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| 50. |
- Muhamad, Harbe, et al.
(författare)
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Integrated interpretation of geophysical data of the Paleozoic structure in the northwestern part of the Siljan Ring impact crater, central Sweden
- 2018
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Ingår i: Journal of Applied Geophysics. - : Elsevier BV. - 0926-9851 .- 1879-1859. ; 148, s. 201-215
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Tidskriftsartikel (refereegranskat)abstract
- The Siljan Ring impact structure is the largest known impact structure in Europe and is Late Devonian in age. It contains a central uplift that is about 20-30 km in diameter and is surrounded by a ring-shaped depression. The Siljan area is one of the few areas in Sweden where the Paleozoic sequence has not been completely eroded, making it an important location for investigation of the geological and tectonic history of Baltica during the Paleozoic. The Paleozoic strata in this area also provide insight into the complex deformation processes associated with the impact. In this study we focus on the northwestern part of the Siljan Ring, close to the town of Orsa, with the main objective of characterizing the subsurface Paleozoic succession and uppermost Precambrian crystalline rocks along a series of seismic reflection profiles, some of which have not previously been published. We combine these seismic data with gravity and magnetic data and seismic traveltime tomography results to produce an integrated interpretation of the subsurface in the area. Our interpretation shows that the Paleozoic sequence in this area is of a relatively constant thickness, with a total thickness typically between 300 and 500 m. Faulting appears to be predominantly extensional, which we interpret to have occurred during the modification stage of the impact. Furthermore, based on the geophysical data in this area, we interpret that the impact related deformation to differ in magnitude and style from other parts of the Siljan Ring.
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