| 1. |
- Shan, Chunling, 1986-, et al.
(författare)
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Integration of controlled-source and radio-magnetotellurics, electrical resistivity tomography and reflection seismics to delineate 3D structures of a quick-clay landslide site in southwest Sweden
- 2016
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Ingår i: Geophysics. - 0016-8033 .- 1942-2156. ; 81:1, s. B13-B29
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- Three-dimensional radio magnetotelluric (RMT), controlled-source radio magnetotelluric (CSRMT), electrical resistivity tomography (ERT), and high-resolution reflection seismic data were acquired at a quick-clay landslide site close to the Göta River in southwest Sweden. These data were collected with the main objectives of evaluating the performance of each method and delineating different subsurface materials down to the bedrock, which in turn control a peculiar and hazardous retrogressive-type landslide. These materials consist of mainly marine clays, quick clays, sand to gravel and the crystalline bedrock. The RMT, CSRMT and ERT data were inverted separately using 3D inversion schemes and the independent results from 3D processing of reflection seismic data together with the existing geotechnical data were used for their interpretations. The 3D CSRMT resistivity model shows the highest depth penetration and well resolves the geometry of the bedrock. The RMT and ERT models, however, show better resolution closer to the surface because of their denser data coverage compared with the CSRMT data. Considerable correlations in resolving a coarse-grained layer and the bedrock were observed between the CSRMT model and the 3D reflection seismic data. Because an Occam-type regularization was used in the inversion, the lateral and depth extent of the quick clays cannot accurately be determined just by using the resistivity models. The depth to the top of the resistive crystalline bedrock is to some extent uncertain. Comparisons with the borehole data suggest that the resistivity models are poor in resolving the sharp boundary between the quick clays and their underlying more resistive coarse-grained materials. The upper boundary of the more conductive marine clays underlain by the coarse-grain materials was well modeled by the CSRMT data. This study illustrates the potentials of 3D geophysical data especially when different types are collected and particularly for better site characterizations.
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| 2. |
- 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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| 3. |
- Bastani, Mehrdad, et al.
(författare)
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2D joint inversion of RMT and ERT data versus individual 3D inversion of full tensor RMT data : An example from the Trecate site in Italy
- 2012
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Ingår i: Geophysics. - 0016-8033 .- 1942-2156. ; 77:4, s. WB233-WB243
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Tidskriftsartikel (refereegranskat)abstract
- Tensor radiomagnetotelluric (RMT) and electrical resistivity tomography (ERT) data were acquired along 10 parallel lines to image electrical resistivity of the vadose and the saturated zone in an area near Trecate, 45 km west of Milan in Italy. In 1994, the area was exposed to an oil contamination caused by a tank explosion and has since been subject to monitoring and remediation programs. For the first time, we have examined a 3D inversion of full tensor RMT data and have compared the results with 2D joint inversion of RMT and ERT data. First, a synthetic 3D resistivity model with similar variations close to those measured at the Trecate site was generated for the comparison. The synthetic tests showed that resistivity models from 2D joint inversion of ERT and RMT data contain more details closer to the surface compared to the models from the 3D inversion of tensor RMT data. High resistivity structures are better resolved by the 2D joint inversion, whereas the more conductive features are better recovered by the 3D inversion. In the next step, the ERT and RMT data collected in the Trecate site were modeled with the same approaches used in the synthetic modeling. Using the measured tensor RMT data, it was possible to carry out full 3D inversion to study the underlying geology. Comparison between the resistivity models from both inversions with the lithological data from the existing boreholes, resistivity models from the inversion of crosshole resistivity data, and water content models from magnetic resonance soundings measurements showed that the electrical resistivity, depth to the top and thickness of the water saturated zone is modeled more accurately With the 3D inversion.
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| 4. |
- Bastani, Mehrdad, 1960-, et al.
(författare)
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3D magnetic susceptibility model of a deep iron-oxide apatite-bearing orebody incorporating borehole data in Blötberget, Sweden
- 2019
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Konferensbidrag (refereegranskat)abstract
- We have processed and modelled in 3D available airborne magnetic data flown over the Blötberget iron-oxide deposit in central Sweden. 3D magnetic susceptibility models were then compared to the existing borehole information, downhole logging data and ground geological observations. Use of such information in the 3D inversion of the magnetic field data in the form of a reference model has considerably improved the 3D model in resolving the deeper parts of the iron-oxide orebody as well as the geometry of nearby geological structures. Unconstrained inversion results, however, do not resolve the true dip and depth extension of the iron-oxide orebody.
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| 5. |
- Bastani, Mehrdad, et al.
(författare)
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Boat-towed radio-magnetotellurics : A new technique and case study from the city of Stockholm
- 2015
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Ingår i: Geophysics. - 0016-8033 .- 1942-2156. ; 80:6, s. B193-B202
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Tidskriftsartikel (refereegranskat)abstract
- We have developed a new data acquisition system and technique to measure the radio magnetotelluric (RMT) signals from distant radio transmitters with the objective of mapping and modeling electric resistivity structures below a river or lake. The acquisition system is towed by a boat; therefore, we call the technique boat-towed RMT. The data acquisition is fast with a production rate of approximately 1 km/hr using a nominal sampling spacing of 10–15 m. Given the ample number of radio transmitters available in most parts of the world, the method can be used for near-surface studies of various targets. We have developed boat-towed RMT measurements on Lake Mälaren near the city of Stockholm in Sweden to determine the feasibility of the method. Approximately 15 km of RMT data were collected during three days above a planned 60-m-deep bypass tunnel with the goal of providing information on the bedrock depth and possible weak zones within the bedrock. The measured resistivity and phase data were of high quality with errors on the order of a few percent. The resistivity models from 2D inversion of the data showed a good correlation with available geologic data in resolving bedrock depth and also resistivity layering within the lake. Resistivity maps derived from the dense 2D models suggested a northeast–southwest-striking low-resistivity zone at less than a 30-m depth. The zone likely represents fractured crystalline bedrock. The boat-towed RMT technique is well suited for water bodies with moderate electric resistivity such as in brackish and freshwater environments.
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| 6. |
- Bastani, Mehrdad, et al.
(författare)
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Boat-towed RMT Measurements on the Water Surface over the Äspö Hard Rock Tunnel in Sweden
- 2016
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Konferensbidrag (refereegranskat)abstract
- We carried out 3 km of radio magnetotelluric measurements in three hours on the water surface using a new boat-towed platform at the Borholm Fjord over the Äspö Tunnel in Sweden. The new platform was stable and use of DGPS receiver at the location of electromagnetic sensors facilitated faster data acquisition. The data were inverted in 2D and the resulting resistivity models revealed detailed information about the geological structures below the water. The estimated water depths were compared to the on-site echo sounder data that were collected simultaneously. We observed very close match between the two. The very low resistivity overburden (< 1 Ohmm) thickens towards the south-eastern part of the area with thicknesses > 10 m. After incorporating the RMT data that were acquired on the land at the northern part of the area a low resistivity zone that dips towards the northwest was resolved in the resistivity model. We interpret this zone as a major weak/fracture zone that is observed at depth inside the Äspö Tunnel. The low resistivity of water and overburden has strongly limited the depth penetration of the method and we suggest use of methods with deeper penetration such as controlled source magnetotelluric.
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| 7. |
- Bastani, Mehrdad, et al.
(författare)
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Combined use of controlled-source and radio-magnetotelluric methods for near surface studies
- 2015
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Konferensbidrag (refereegranskat)abstract
- Combination of the controlled source and radio magnetotelluric, the so-called CSRMT, method has been used in modelling near surface electrical resistivity structures with the main aim of studying mineralization and engineering issues. In the controlled source mode, we used a remotely controlled double horizontal magnetic dipole that transmits signal in the frequency range 1-12.5 kHz. The signal sources for the RMT method are the distant radio transmitters in the frequency range 14-250 kHz. Three near surface case studies are presented in this work. In the first case, conductive hydrothermal stockwork copper deposits in an area in Iran were modelled in 2D and the results were compared to the existing borehole information. In the second case located in Greece using the CSRMT method we could model the location and geometry of a subvertical fault that was covered by conductive quaternary sediments. In the third case from Sweden, 3D resistivity models from the inversion of CSRMT data were used to determine presence and boundaries of quick-clay formation and a coarse-grained layer that is in turn responsible for a peculiar retrogressive landslide type in the study area. The three case studies have proven that a combination of controlled source and radio magnetotelluric methods has a considerable potential in addressing near surface problems. Comparison between the existing borehole data and the resistivity models from the inversion of the CSRMT data in each study area revealed reasonable correlation in delineating target structures especially depth to the good conductors. However, special attention needs to be given when using the CSMT data, especially at lower frequencies where they may be affected by the near-field phenomenon.
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| 8. |
- Bastani, Mehrdad
(författare)
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EnviroMT : A new controlled source/radio magnetotoelluric system
- 2001
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In the frame of an EU project, a new electromagnetic prospecting instrument was designed and constructed for environmental and engineering applications. The instrument works in the frequency domain in the band 1-250 kHz, whereby the collected data can be used to study the variation of ground resistivity from the surface to a maximum depth of few hundred meters. The system is operational in two modes, the Radio Magnetotelluric (RMT) and the Controlled Source Tensor Magnetotelluric (CSTMT). The RMT method makes use of the signals generated by powerful distant radio transmitters operating in the frequency range 14-250 kHz. The CSTMT technique, with a remotely controllable double magnetic dipole source covering the band 1-100 kHz, is utilized when deeper targets are aimed at or low resistivity terrains are studied. With the aid of user-friendly database software the collected data can be visualized and modeled in1D, directly in the field. In November 1998, the first test survey at the Collendoorn dumpsite in the Netherlands was conducted to map the vertical boundaries and lateral extensions of the pollution plume along four RMT profiles located east of the dumpsite. The results of 1D inversion of the data correlated well with the bore-hole logs, revealing that the estimated resistivities and depths to the top of the pollution plume were realistic. The pollution plume appears to be extending more in the northern part. Another test was carried out north of Uppsala, at Skediga in Sweden, using both RMT and CSTMT techniques, to study the morphology of a clay-sandy aquifer. The depth of the resistive crystalline bedrock as estimated by 1D and 2D inversion of the EnviroMT data is in good agreement with bore-hole data. To the east, towards River Fyris the conductive clay lenses become thicker.
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| 9. |
- Bastani, Mehrdad, et al.
(författare)
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Radio-magnetotelluric and controlled-source magnetotelluric surveys on a frozen lake : Opportunities for urban applications in Nordic countries
- 2022
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Ingår i: Near Surface Geophysics. - : John Wiley & Sons. - 1569-4445 .- 1873-0604. ; 20:1, s. 30-45
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Tidskriftsartikel (refereegranskat)abstract
- In a novel approach, we have carried out controlled-source and radio-magnetotelluric measurements in the frequency range of 2-250 kHz on a frozen lake located over a planned major multi-lane underground road tunnel near the city of Stockholm. The aim was to gain a better understanding of the resistivity variations above and, potentially, within the crystalline bedrock. Previous studies on the lake water using the boat-towed radio-magnetotelluric technique at the higher end of the frequency band lacked resolution at depth and could not provide conclusive information about bedrock level and potential fracture systems within the bedrock. Taking advantage of Nordic winters, we measured four profiles on the frozen lake complementing the previously acquired boat-towed radio-magnetotelluric data utilizing a double horizontal magnetic dipole transmitter that generated signals down to 1 kHz. The new resistivity models, incorporating the lower frequency data, show improvements and deeper penetrations based on a combined analysis of penetration depth, data misfits and sensitivity studies. The resistivity models also show better correlation with the available high-resolution shallow water seismic reflection data and the geological observations. A potential fracture system within the bedrock can also be inferred better in the new models. The idea of running similar surveys on frozen lakes can be further exploited in similar conditions in countries such as Sweden, where approximately 7% of the land is covered by freshwater bodies and poorly explored for infrastructure planning projects.
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| 10. |
- Beiki, Majid, 1977-
(författare)
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New Techniques for Estimation of Source Parameters : Applications to Airborne Gravity and Pseudo-Gravity Gradient Tensors
- 2011
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Gravity gradient tensor (GGT) data contains the second derivatives of the Earth’s gravitational potential in three orthogonal directions. GGT data can be measured either using land, airborne, marine or space platforms. In the last two decades, the applications of GGT data in hydrocarbon exploration, mineral exploration and structural geology have increased considerably. This work focuses on developing new interpretation techniques for GGT data as well as pseudo-gravity gradient tensor (PGGT) derived from measured magnetic field. The applications of developed methods are demonstrated on a GGT data set from the Vredefort impact structure, South Africa and a magnetic data set from the Särna area, west central Sweden. The eigenvectors of the symmetric GGT can be used to estimate the position of the causative body as well as its strike direction. For a given measurement point, the eigenvector corresponding to the maximum eigenvalue points approximately toward the center of mass of the source body. For quasi 2D structures, the strike direction of the source can be estimated from the direction of the eigenvectors corresponding to the smallest eigenvalues. The same properties of GGT are valid for the pseudo-gravity gradient tensor (PGGT) derived from magnetic field data assuming that the magnetization direction is known. The analytic signal concept is applied to GGT data in three dimensions. Three analytic signal functions are introduced along x-, y- and z-directions which are called directional analytic signals. The directional analytic signals are homogenous and satisfy Euler’s homogeneity equation. Euler deconvolution of directional analytic signals can be used to locate causative bodies. The structural index of the gravity field is automatically identified from solving three Euler equations derived from the GGT for a set of data points located within a square window with adjustable size. For 2D causative bodies with geometry striking in the y-direction, the measured gxz and gzz components of GGT can be jointly inverted for estimating the parameters of infinite dike and geological contact models. Once the strike direction of 2D causative body is estimated, the measured components can be transformed into the strike coordinate system. The GGT data within a set of square windows for both infinite dike and geological contact models are deconvolved and the best model is chosen based on the smallest data fit error.
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