Radiomagnetotelluric Methods Research Articles

Geophysical Methods and their Applications in Dam Safety Monitoring

The use of geophysical methods in dam sites investigations and safety monitory has proved their good value and versatility in many earthfill dam sites as early as the 1920s. In the following years great development has occurred in the methods, application procedures and tools used. They may be considered today as good ways for carrying out observation tasks on existing dams in non-intrusive and much faster and cheaper ways than the traditional geotechnical methods. It is possible using them to discover anomalies in the dam body or its foundation at an early stage and allowing quick intervention repair works. These methods seek to register and present variations in the basic geotechnical material properties in dams such as; bulk density, moisture content, elasticity, mechanical properties of rocks, electrical resistivity and mineralogy and magnetic properties and so forth. Such variations can indicate increasing seepage flow, progression in cracks’ sizes, formation of voids, caverns and other instability manifestations. Depending on how any investigation is carried out and the targeted anomaly, there is now selection of these methods such as: Electromagnetic Profiling (EM), Electrical Resistivity Tomography (ERT), SelfPotential (SP), Ground Penetration Radar (GPR), variety of Seismic Methods (SM) which can be applied using such equipment as in Seismic refraction, Seismic Reflection, Multi Analysis of Rayleigh surface waves (MASW) instruments, or using Refraction Micrometer (ReMi), macro-gravity method, and Cross-Hole Seismic Tomography. In addition, Temperature Measurements and other less used methods can be used like Microgravity measurement, Magnetic Profiling and Radio Magnetotelluric methods. An attempt is made here to cover the details of these methods, their advantages and limitations and to prove their usefulness in many dam sites all over the world. One observed issue is their adaptability to embankment dams more than to concrete dams and their popularity for checking seepage related problems and material changes within dam bodies and their foundations such as formation of voids and sinkholes.

On the study of industrial waste sites on the Karelian Isthmus/Russia using the RMT and CSRMT methods

Radiomagnetotelluric (RMT) and Controlled Source Radiomagnetotelluric (CSRMT) measurements were carried out on landfills of industrial wastes located on the Karelian Isthmus/Russia. The RMT and CSRMT data were interpreted using the 2D inversion techniques, and the 2D conductivity structures of the investigated industrial waste sites were derived. The RMT method is based on measurements of electromagnetic fields of remote radio transmitters in a frequency range of 10–1000 kHz or in a frequency range of 1–1000 kHz by using a controlled source (CSRMT method). A landfill in the settlement Morozova near St. Petersburg was formed as a result of a long-term warehousing of toxic wastes by the sulfuric acid manufacture. Soils and ground waters were polluted with salts of heavy metals. In the adjacent to the landfill territory, investigations were realized by the RMT method and a polluted zone was determined. This zone is traced up to the coast of the Ladoga Lake and the Neva River. It constitutes a potential danger because the Neva River is used for the water supply of St. Petersburg. Another landfill of debris in a former sandy career is located in the Vyborg area. The high conductivity and a relative large thickness of the wastes did not allow us to map the bottom of the landfill by the RMT method using the frequency range of 10–1000 kHz. Therefore, CSRMT measurements with an extended frequency range up to 1 kHz were carried out on this landfill, and the 2D inversion of the CSRMT data clearly showed the lateral and vertical extension of it.

Combined use of controlled-source and radio-magnetotelluric methods for near surface studies

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.

Joint Inversion of RMT and TEM Data for Mapping Active Faults in Northern Greece

Northern Greece is an area with one of the most seismically active region in Europe. Several earthquakes occurred during 20th century. The largest earthquake in recent time with magnitude of 6.5 happened in June 1978. The objective of the presented study is to investigate local geological structure and the top of the basement of the Volvi Basin, Northern Greece. The geophysical surveys are carried out using electromagnetic methods, Radiomagnetotelluric (RMT) and Transient electromagnetic (TEM). The investigations are limited to near surface studies with shallow depth (< 200 m). It is expected that the complex geological structure and the vertical distribution of resistivity layers can be defined by RMT and TEM methods. Joint inversion can increase the number of important model parameters and it can also decrease the ambiguity of the model. The result of joint inversion was in agreement with assumption as the RMT data are resolved the surface layer at depth down to 40 m, and TEM data is small enough to resolve the surface structure (z < 10 m), however it can resolve the deeper structure down to 200 m of depth, depending on the resistivity structure. The joint and sequential models can both represent 1-D single inversion of RMT and TEM data. The model parameters are well resolved in joint inversion in comparison with the single RMT and TEM inversions. Hence, the joint inversion of RMT and TEM data will produce good resolution of model parameters in shallow and deeper parts of fault structure.

Geophysical assessment and geotechnical investigation of quick‐clay landslides – a Swedish case study

We present a preliminary assessment of the potential utility of various geophysical measurements carried out over a quick‐clay landslide site in south‐west Sweden. The multidisciplinary approach includes active P‐ and S‐wave seismic investigations, including 2D and 3D reflection and refraction surveys, passive single and 3C seismic surveys, electrical resistivity tomography and electromagnetic surveys including controlled‐source and radio‐magnetotellurics, ground‐penetrating radar and potential field studies. The P‐wave and particularly S‐wave reflection seismic data show a high‐resolution image of bedrock topography and the stratigraphy of a 100 m thick sequence of sediments that lies on top, which include lightly consolidated quick‐clays. Of particular interest is the identification of a layer of relatively coarse‐grained material between 10–20 m below the ground surface. Geotechnical investigations indicate that most but not all quick‐clays at the site are located above this layer. Further studies are required to determine the importance of their relationship and whether the coarse‐grained layer may have had a role in triggering quick‐clay landslides in the region. Geoelectrical and electromagnetic methods provide high‐resolution images of the unconsolidated subsurface and particularly the normal and leached clays. Radio‐magnetotelluric methods proved valuable near the river where traditional geoelectrical methods failed to provide sufficient depth coverage. The study shows that geophysical data are able to image major subsurface structures associated with quick‐clay landslides.

Groundwater contamination in the Roorkee area, India: 2D joint inversion of radiomagnetotelluric and direct current resistivity data

Abstract The impact of sewage irrigation and groundwater contamination were investigated near Roorkee in north India using the Direct Current Resistivity (DCR) method and the Radiomagnetotelluric (RMT) method. Intensive field measurements were carried out in the vicinity of a waste disposal site, which was extensively irrigated with sewage water. For comparison a profile was investigated on a reference site, where no contamination was expected. In addition to conventional 1D and 2D inversion, the measured data sets were interpreted using a 2D joint inversion algorithm. The inversion results from the data obtained from the sewage irrigated site indicate a decrease of resistivity up to 75% in comparison with the reference site. The depth range from 5 to 15 m is identified as a shallow unconfined aquifer and the decreased resistivities are ascribed as the influence of contamination. Furthermore, a systematic increase in the resistivities of the shallow unconfined aquifer is detected as we move away from the waste disposal site. The advantages of both, the DCR and RMT methods, are quantitatively integrated by the 2D joint inversion of both data sets and lead to a joint model, which explains both data sets.

Delineating hydrothermal stockwork copper deposits using controlled-source and radio-magnetotelluric methods: A case study from northeast Iran

Radio- and controlled-source-tensor magnetotelluric (RMT and CSTMT) methods are used to target hydrothermal veins of copper mineralization. The data were acquired along six east-west- and three north-south-trending profiles, covering an area of about [Formula: see text]. The tensor RMT data were collected in the [Formula: see text] frequency band. A double horizontal magnetic dipole transmitter in the [Formula: see text] frequency range allowed us to constrain the deeper parts of the resistivity models better. To obtain optimum field parameters, ground magnetic profiling was conducted prior to the RMT and CSTMT surveys. Although the study area (in Iran) is remote, a number of radio transmitters with acceptable signal-to-noise ratio were utilized. The 2D inversion of RMT data led to unstable resistivity models with large datamisfits. Thus, the RMT data were used to complement and analyze the near-surface resistivity anomalies observed in the 2D CSTMT models. Analyses of strike and dimensionality from the CSTMT data suggests that the low-resistivity structures are mainly three dimensional; therefore, 2D inversion of determinant data is chosen. Independent 2D inversion models of the determinant CSTMT data along crossing profiles are in good agreement. Known copper mineralization is imaged well in the CSTMT models. The thinning of the conductive overburden correlates very well with magnetic highs, indicating the bedrock is resistive and magnetic. In this sense, the magnetic and electromagnetic fields complement each other. Analysis of the 2D resistivity models indicates the volcanic rock deepens at the center of the study area. This zone is associated with a magnetic low and therefore is recommended for detailed exploration work.

Aquifer characterization in the Ouarzazate Basin (Morocco): A contribution by TEM and RMT data

ABSTRACTIn this case study we present the first application of the radio magnetotelluric (RMT) technique in Africa. We applied the RMT and transient electromagnetic (TEM) methods to study an aquifer of the Ouarzazate Basin between the High Atlas and the Anti Atlas in Morocco. The main objective of the survey was to constrain the geometry of the near surface sediment layer by mapping the aquitard, an underlying clay layer.We collected RMT data in the frequency range of 16 kHz to 207 kHz and TEM data in the time range of 4 μs to 1 ms along an 8 km long profile in a desert area crossing an oasis region. After 2D inversion of the RMT data and 1D inversion of the TEM data, we carried out 1D joint inversion to enhance the resolution of the subsurface resistivity structure.From the inversions we derived the resistivity distribution down to a depth of about 100 m and identified a relatively resistive layer as the aquifer sediments, overlying a more conductive clay layer. Inversion statistics indicate that the depth of the clay layer is well resolved. Compared with the lithological logs of drillings located at distances of 150 m to 3 km from the profile, the depth of the clay layer beneath the aquifer in the south of the geophysical section agrees to ± 3 m. Furthermore, we observed a lateral resistivity contrast close to the surface separating the desert area and the oasis, due to differences in humidity.

Comparison between VLF and RMT methods: A combined tool for mapping conductivity changes in the sedimentary cover

We compare the resolution power of single frequency very low frequency (VLF) electromagnetic data (real and imaginary parts of the tipper) and multi-frequency RadioMagnetoTelluric (RMT) data in delineating conductive structures typical for the sedimentary cover over crystalline basement in Scandinavia. Using VLF field data from five parallel profiles reveals that the estimated models have responses that fit the observed data well, and the models show an overall agreement with more detailed models derived from broadband RMT data. It is suggested that VLF data be used as a fast mapping tool to fill in the gaps between profiles along which more detailed RMT measurements are made. A generic model with conductive clay lenses and sandy formations over crystalline basement is used to generate synthetic data for the two cases. Using regularized inversion the corresponding estimated models clearly shows the strength and the weakness of both methods. Being inductive methods, they both have difficulties in clearly resolving the depth transition from conductive to resistive units. Especially the single frequency VLF data can be interpreted with very smooth models at depth. However, both methods resolve very well the lateral boundaries of the clay lenses and the RMT data also constrain the thickness of the clays quite well compared with the VLF models, which are less distinct at depth. Single frequency scalar VLF data emphasize those conductive structures that have dominant strikes in the direction of the transmitter. Multi-frequency VLF (tensor VLF) measurements provide the tipper vector which depends upon the underlying conductivity structure only. Real conductivity structures have significant 3D components which can be delineated easily by tensor VLF measurements. We propose that new VLF instrumentation be developed with this in mind.