| 1. |
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| 2. |
- Ji, Yuntao, et al.
(author)
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Characterization of pore structure and strain localization in Majella limestone by X-ray computed tomography and digital image correlation
- 2015
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In: Geophysical Journal International. - : Oxford University Press (OUP). - 0956-540X .- 1365-246X. ; 200:2, s. 699-717
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Journal article (peer-reviewed)abstract
- Standard techniques for computed tomography imaging are not directly applicable to a carbonate rock because of the geometric complexity of its pore space. In this study, we first characterized the pore structure in Majella limestone with 30 per cent porosity. Microtomography data acquired on this rock was partitioned into three distinct domains: macropores, solid grains, and an intermediate domain made up of voxels of solid embedded with micropores below the resolution. A morphological analysis of the microtomography images shows that in Majella limestone both the solid and intermediate domains are interconnected in a manner similar to that reported previously in a less porous limestone. We however show that the macroporosity in Majella limestone is fundamentally different, in that it has a percolative backbone which may contribute significantly to its permeability. We then applied for the first time 3-D-volumetric digital image correlation (DIC) to characterize the mode of mechanical failure in this limestone. Samples were triaxially deformed over a wide range of confining pressures. Tomography imaging was performed on these samples before and after deformation. Inelastic compaction was observed at all tested pressures associated with both brittle and ductile behaviors. Our DIC analysis reveals the structure of compacting shear bands in Majella limestone deformed in the transitional regime. It also indicates an increase of geometric complexity with increasing confinement-from a planar shear band, to a curvilinear band, and ultimately to a diffuse multiplicity of bands, before shear localization is inhibited as the failure mode completes the transition to delocalized cataclastic flow.
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| 3. |
- Joud, Mehdi S. Shafiei, et al.
(author)
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Use of GRACE Data to Detect the Present Land Uplift Rate in Fennoscandia
- 2017
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In: Geophysical Journal International. - : Oxford University Press (OUP). - 0956-540X .- 1365-246X. ; 209:2, s. 909-922
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Journal article (peer-reviewed)abstract
- After more than 13 years of GRACE monthly data, the determined secular trend of gravity field variation can be used to study the regions of glacial isostatic adjustment (GIA). Here we focus on Fennoscandia where long-term terrestrial and high-quality GPS data are available, and we study the monthly GRACE data from three analysis centres. We present a new approximate formula to convert the secular trend of the GRACE gravity change to the land uplift rate without making assumptions of the ice load history. The question is whether the GRACE-derived land uplift rate by our method is related to GIA. A suitable post-processing method for the GRACE data is selected based on weighted RMS differences with the GPS data. The study reveals that none of the assumed periodic changes of the GRACE gravity field is significant in the estimation of the secular trend, and they can, therefore, be neglected. Finally, the GRACE-derived land uplift rates are obtained using the selected post-processing method, and they are compared with GPS land uplift rate data. The GPS stations with significant differences were marked using a statistical significance test. The smallest RMS difference (1.0 mm/a) was obtained by using GRACE data from the University of Texas.
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| 4. |
- Eggertsson, Gunnar, et al.
(author)
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Earthquake or blast? : Classification of local-distance seismic events in Sweden using fully connected neural networks
- 2024
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In: Geophysical Journal International. - : Oxford University Press. - 0956-540X .- 1365-246X. ; 236:3, s. 1728-1742
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Journal article (peer-reviewed)abstract
- Distinguishing between different types of seismic events is a task typically performed manually by expert analysts and can thus be both time and resource expensive. Analysts at the Swedish National Seismic Network (SNSN) use four different event types in the routine analysis: natural (tectonic) earthquakes, blasts (e.g. from mines, quarries and construction) and two different types of mining-induced events associated with large, underground mines. In order to aid manual event classification and to classify automatic event definitions, we have used fully connected neural networks to implement classification models which distinguish between the four event types. For each event, we bandpass filter the waveform data in 20 narrow-frequency bands before dividing each component into four non-overlapping time windows, corresponding to the P phase, P coda, S phase and S coda. In each window, we compute the root-mean-square amplitude and the resulting array of amplitudes is then used as the neural network inputs. We compare results achieved using a station-specific approach, where individual models are trained for each seismic station, to a regional approach where a single model is trained for the whole study area. An extension of the models, which distinguishes spurious phase associations from real seismic events in automatic event definitions, has also been implemented. When applying our models to evaluation data distinguishing between earthquakes and blasts, we achieve an accuracy of about 98 per cent for automatic events and 99 per cent for manually analysed events. In areas located close to large underground mines, where all four event types are observed, the corresponding accuracy is about 90 and 96 per cent, respectively. The accuracy when distinguishing spurious events from real seismic events is about 95 per cent. We find that the majority of erroneous classifications can be traced back to uncertainties in automatic phase picks and location estimates. The models are already in use at the SNSN, both for preliminary type predictions of automatic events and for reviewing manually analysed events.
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| 5. |
- Johansson, Sara, et al.
(author)
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An alternative methodology for the analysis of electrical resistivity data from a soil gas study
- 2011
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In: Geophysical Journal International. - 0956-540X. ; 186:2, s. 632-640
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Journal article (peer-reviewed)abstract
- The aim of this paper is to present an alternative method for the analysis of resistivity data. The methodology was developed during a study to evaluate if electrical resistivity can be used as a tool for analysing subsurface gas dynamics and gas emissions from landfills. The main assumption of this study was that variations in time of resistivity data correspond to variations in the relative amount of gas and water in the soil pores. Field measurements of electrical resistivity, static chamber gas flux and weather data were collected at a landfill in Helsingborg, Sweden. The resistivity survey arrangement consisted of nine lines each with 21 electrodes in an investigation area of 16 x20 m. The ABEM Lund Imaging System provided vertical and horizontal resistivity profiles every second hour. The data were inverted in Res3Dinv using L-1-norm-based optimization method with a standard least-squares formulation. Each horizontal soil layer was then represented as a linear interpolated raster model. Different areas underneath the gas flux measurement points were defined in the resistivity model of the uppermost soil layer, and the vertical extension of the zones could be followed at greater depths in deeper layer models. The average resistivity values of the defined areas were calculated and plotted on a time axis, to provide graphs of the variation in resistivity with time in a specific section of the ground. Residual variation of resistivity was calculated by subtracting the resistivity variations caused by the diurnal temperature variations from the measured resistivity data. The resulting residual resistivity graphs were compared with field data of soil moisture, precipitation, soil temperature and methane flux. The results of the study were qualitative, but promising indications of relationships between electrical resistivity and variations in the relative amount of gas and water in the soil pores were found. Even though more research and better data quality is necessary for verification of the results presented here, we conclude that this alternative methodology of working with resistivity data seems to be a valuable and flexible tool for this application.
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| 6. |
- Johansson, Sara, et al.
(author)
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Spectral induced polarization of limestones: time domain field data, frequency domain laboratory data and physicochemical rock properties
- 2020
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In: Geophysical Journal International. - : Oxford University Press (OUP). - 0956-540X .- 1365-246X. ; 220:2, s. 928-950
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Journal article (peer-reviewed)abstract
- With advances in data acquisition and processing methods, spectral inversion of time domain (TD) induced polarization (IP) data is becoming more common. Geological interpretation of inverted spectral parameters, for instance Cole–Cole parameters, often relies on results from systematic laboratory measurements. These are most often carried out with frequency domain (FD) systems on sandstone samples. However, the two different methods of measuring the spectral IP response differ in both measurement technique and scale. One of the main objectives of this study is, thus, to perform a direct comparison of inverted spectral parameters from TD IP field data with FD IP spectra from laboratory measurements. To achieve this, field measurements were carried out before a ∼50-m-long rock core was drilled down along one of the measurement lines. Solid parts of the core were vacuum-sealed in plastic bags to preserve the natural groundwater in the samples, after which the core samples were measured with FD spectral IP in laboratory. The results showed that the inverted Cole–Cole parameters closest to the borehole were comparable to the IP spectra measured at the core samples, despite differences in measurement techniques and scale. The field site chosen for the investigation was a limestone succession spanning over a well-known lithological boundary (the Cretaceous–Palaeogene boundary). Little is known in previous research about varying spectral IP responses in limestones, and an additional objective of this study was, therefore, to investigate possible sources of these variations in the laboratory. The IP spectra were interpreted in light of measured lithological and physicochemical properties. The carbonate texture differed strongly across the Cretaceous–Palaeogene boundary from fine-grained calcareous mudstone (Cretaceous) to more well-lithified and coarse-grained wackestone and packstone (Palaeogene). Both laboratory and field spectral IP results showed that these differences cause a large shift in measured bulk conductivity across the boundary. Furthermore, carbonate mound structures with limestone grains consisting mainly of cylindrical bryozoan fragments could be identified in the inverted Cole–Cole parameters as anomalies with high relaxation times. A general conclusion of this work is that limestones can give rise to a wide range of spectral responses. The carbonate texture and the dominant shape of the fossil grains seem to have important control over the electrical properties of the material. A main conclusion is that the inverted Cole–Cole parameters from the field scale TD IP tomography were comparable to the magnitude and shape of FD IP spectra at low frequencies. This opens up large interpretational possibilities, as the comprehensive knowledge about relationships between lithological properties and IP spectra from laboratory research can be used for field data interpretation.
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| 7. |
- Ju, Ma, et al.
(author)
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Moment tensor inversion with three-dimensional sensor configuration of mining induced seismicity (Kiruna mine, Sweden)
- 2018
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In: Geophysical Journal International. - : Oxford University Press. - 0956-540X .- 1365-246X. ; 213:3, s. 2147-2160
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Journal article (peer-reviewed)abstract
- Mining induced seismicity is an undesired consequence of mining operations, which poses significant hazard to miners and infrastructures and requires an accurate analysis of the rupture process. Seismic moment tensors of mining-induced events help to understand the nature of mining-induced seismicity by providing information about the relationship between the mining, stress redistribution and instabilities in the rock mass. In this work, we adapt and test a waveform-based inversion method on high frequency data recorded by a dense underground seismic system in one of the largest underground mines in the world (Kiruna mine, Sweden). A stable algorithm for moment tensor inversion for comparatively small mining induced earthquakes, resolving both the double-couple and full moment tensor with high frequency data, is very challenging. Moreover, the application to underground mining system requires accounting for the 3-D geometry of the monitoring system. We construct a Green's function database using a homogeneous velocity model, but assuming a 3-D distribution of potential sources and receivers. We first perform a set of moment tensor inversions using synthetic data to test the effects of different factors on moment tensor inversion stability and source parameters accuracy, including the network spatial coverage, the number of sensors and the signal-tonoise ratio. The influence of the accuracy of the input source parameters on the inversion results is also tested. Those tests show that an accurate selection of the inversion parameters allows resolving the moment tensor also in the presence of realistic seismic noise conditions. Finally, the moment tensor inversion methodology is applied to eight events chosen from mining block #33/34 at Kiruna mine. Source parameters including scalar moment, magnitude, double-couple, compensated linear vector dipole and isotropic contributions as well as the strike, dip and rake configurations of the double-couple term were obtained. The orientations of the nodal planes of the double-couple component in most cases vary from NNW to NNE with a dip along the ore body or in the opposite direction.
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| 8. |
- Martin, Tina, et al.
(author)
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Desaturation effects of pyrite–sand mixtures on induced polarization signals
- 2022
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In: Geophysical Journal International. - : Oxford University Press (OUP). - 0956-540X .- 1365-246X. ; 228:1, s. 275-290
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Journal article (peer-reviewed)abstract
- Induced polarization (IP) is an acknowledged method in ore exploration and can be applied to evaluate the metal content in dumps containing the residues of ore processing facilities. Existing models explain the relationships between ore content and grain size of the ore particles with IP parameters. However, the models assume full water saturation of the ore containing samples, which is often not the case in field conditions at dump sites. Hence, our study investigates the effect of desaturation on the resulting IP signal. We used six different sand–pyrite mixtures with varying amount and grain sizes of the pyrite particles. Evaporative drying desaturated the samples. Complex conductivity spectra were recorded in the frequency range between 0.02 and 1000 Hz at certain saturation levels. The resulting spectra indicate an decrease of the conductivity amplitude with progressing desaturation. This effect agrees with the second empirical Archie equation. The saturation exponent of the conductivity amplitude shows values slightly larger than one. The measured spectra were processed by a Debye decomposition. We observe a nearly constant total chargeability during desaturation. This finding is in agreement with existing models that relate the total chargeability to the metal content in the sample. However, the mean relaxation time decreases remarkably during the drying process, whereas the normalized relaxation time, which considers the ratio between the mean relaxation time and the resistivity of the embedding material, does not indicate any dependence on water saturation. This behaviour contradicts existing models that predict a decreasing relaxation time with increasing water salinity, which results from evaporative drying. In order to explain the observed effects, we propose a conceptional model that compares a mixture of pyrite particles in an embedding material (sand, water and air) with an electrical RC circuit. The pyrite grains behave as small condensers that are charged and discharged via the conductive background material. According to this simple physical model, the relaxation time is proportional to the resistivity of the embedding material. A resistivity increase while desaturation causes an increase of relaxation time as observed in our experiments. This conceptional model is in good agreement with other experiments that change the resistivity in the background material by varying water salinity or clay content. The capacitive behaviour of the dispersed particles is characterized by the normalized relaxation time that depends on the grain size and the volume content of the pyrite particles.
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| 9. |
- Martin, Tina, et al.
(author)
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Spectral induced polarization: frequency domain versus time domain laboratory data
- 2021
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In: Geophysical Journal International. - : Oxford University Press (OUP). - 0956-540X .- 1365-246X. ; 225, s. 1982-2000
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Journal article (peer-reviewed)abstract
- Spectral information obtained from induced polarization (IP) measurements can be used in a variety of applications and is often gathered in frequency domain (FD) at the laboratory scale. In contrast, field IP measurements are mostly done in time domain (TD). Theoretically, the spectral content from both domains should be similar. In practice, they are often different, mainly due to instrumental restrictions as well as the limited time and frequency range ofmeasurements. Therefore, a possibility of transition between both domains, in particular for the comparison of laboratory FD IP data and field TD IP results, would be very favourable. To compare both domains, we conducted laboratory IP experiments in both TD and FD.We started with three numerical models and measurements at a test circuit, followed by several investigations for different wood and sandstone samples. Our results demonstrate that the differential polarizability (DP), which is calculated from the TD decay curves, can be compared very well with the phase of the complex electrical resistivity. Thus, DP can be used for a first visual comparison of FD and TD data, which also enables a fast discriminationbetween different samples. Furthermore, to compare both domains qualitatively, we calculated the relaxation time distribution (RTD) for all data. The results are mostly in agreement between both domains, however, depending on the TD data quality. It is striking that the DP and RTD results are in better agreement for higher data quality in TD. Nevertheless, we demonstrate that IP laboratory measurements can be carried out in both TD and FD with almost equivalentresults. The RTD enables a good comparability of FD IP laboratory data with TD IP field data.
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| 10. |
- Nivorlis, Aristeidis, et al.
(author)
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Temporal filtering and time-lapse inversion of geoelectrical data for long-term monitoring with application to a chlorinated hydrocarbon contaminated site
- 2022
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In: Geophysical Journal International. - : Oxford University Press (OUP). - 0956-540X .- 1365-246X. ; 228:3, s. 1648-1664
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Journal article (peer-reviewed)abstract
- We present a solution for long-term direct current resistivity and time-domain induced polarization (DCIP) monitoring, which consists of a monitoring system and the associated software that automates the data collection and processing. This paper describes the acquisition system that is used for remote data collection and then introduces the routines that have been developed for pre-processing of the monitoring data set. The collected data set is pre-processed using digital signal processing algorithms for outlier detection and removal; the resulting data set is then used for the inversion procedure. The suggested processing workflow is tested against a simulated time-lapse experiment and then applied to field data. The results from the simulation show that the suggested approach is very efficient for detecting changes in the subsurface; however, there are some limitations when no a priori information is used. Furthermore, the mean weekly data sets that are generated from the daily collected data can resolve low-frequency changes, making the approach a good option for monitoring experiments where slow changes occur (i.e. leachates in landfills, internal erosion in dams, bioremediation). The workflow is then used to process a large data set containing 20 months of daily monitoring data from a field site where a pilot test of in situ bioremediation is taking place. Based on the time-series analysis of the inverted data sets, we can detect two portions of the ground that show different geophysical properties and that coincide with the locations where the different fluids were injected. The approach that we used in this paper provides consistency in the data processing and has the possibility to be applied to further real-time geophysical monitoring in the future.
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