Controlled Source Audio-magnetotelluric Method Research Articles

Mapping of buried faults using the 2D modelling of far-field controlled source radiomagnetotelluric data

Controlled source radiomagnetotellurics (CSRMT) is a relatively new geophysical method for near-surface applications. A rectangular signal with base frequencies between 0.1 and 150 kHz is injected through a grounded electric dipole which is used as a transmitter. Electric and magnetic field components are observed at these frequencies and at their subharmonics, usually in the far-field zone so that apparent resistivities and impedance phases can be obtained in a broad frequency range between 1 and 1000 kHz. Inline or broadside configuration can be used for measurements. Similar to the controlled source audiomagnetotelluric method, tensor measurements are also possible when locating two transmitters perpendicular to each other. A scalar CSRMT survey was carried out on the buried faults in the Vuoksa region, 110 km north of St. Petersburg to test the applicability of this method to the mapping of near-surface faults. A 700 m electric dipole with base frequencies of 0.5, 11.3, 30 and 105 kHz was used as a transmitter. Smooth apparent resistivity and phase values as a function of frequency from 1 kHz to 1 MHz were observed in the far-field zone for the inline configuration at 57 stations using a station distance of 20 m. Electric fields observed in the direction of the transmitter were perpendicular to the assumed strike direction of the buried faults so that they could be associated with the TM mode. The observed apparent resistivity and phase TM mode data were interpreted using the 2D inversion algorithm, and a good data fitting could be obtained. The resistivity structure beneath the survey area (down to a depth of 80 m) could be derived and the buried faults could be mapped successfully. In addition to the CSRMT observations, a conventional radiomagnetotelluric (RMT) survey was also carried out on the same profile. An excellent correlation of the observed RMT and CSRMT transfer functions and 2D conductivity models was achieved.

Combination of Geophysical Methods to Support Urban Geological Mapping

Urban geological mapping is a key to assist management of new developed areas, conversion of current urban areas or assessment of urban geological hazards. Geophysics can have a pivotal role to yield subsurface information in urban areas provided that geophysical methods are capable of dealing with challenges related to these scenarios (e.g., low signal-to-noise ratio or special logistical arrangements). With this principal aim, a specific methodology is developed to characterize lithological changes, to image fault zones and to delineate basin geometry in the urban areas. The process uses the combination of passive and active techniques as complementary data: controlled source audio-magnetotelluric method (CSAMT), magnetotelluric method (MT), microtremor H/V analysis and ambient noise array measurements to overcome the limitations of traditional geophysical methodology. This study is focused in Girona and Salt surrounding areas (NE of Spain) where some uncertainties in subsurface knowledge (maps of bedrock depth and the isopach maps of thickness of quaternary sediments) need to be resolved to carry out the 1:5000 urban geological mapping. These parameters can be estimated using this proposed methodology. (1) Acoustic impedance contrast between Neogene sediments and Paleogene or Paleozoic bedrock is detected with microtremor H/V analysis that provides the soil resonance frequency. The minimum value obtained is 0.4 Hz in Salt city, and the maximum value is the 9.5 Hz in Girona city. The result of this first method is a fast scanner of the geometry of basement. (2) Ambient noise array constrains the bedrock depth using the measurements of shear-wave velocity of soft soil. (3) Finally, the electrical resistivity models contribute with a good description of lithological changes and fault imaging. The conductive materials (1–100 Ωm) are associated with Neogene Basin composed by unconsolidated detrital sediments; medium resistive materials (100–400 Ωm) correspond to Paleogene, and resistive materials (600–1,000 Ωm) are related with complex basement, granite of Paleozoic. The Neogene basin-basement boundary is constrained between surface and 500 m depth, approximately. The new geophysical methodology presented is an optimized and fast tool to refine geological mapping by adding 2D information to traditional geological data and improving the knowledge of subsoil.

Audiomagnetotelluric characterization of range-front faults, Snake Range, Nevada

Two controlled-source audio-magnetotelluric (CSAMT) profiles were collected on the eastern flank of the Snake Range in eastern Nevada across geologically complex terrain to investigate the suspected presence of faults along the range front. The location of the range-bounding faults is not easily determined on geologic grounds because of the presence of extensive young sedimentary cover and overall geologic complexity. Characterization of the presence, location, and structural style of the range-front faults is critical to assessment of connectivity of groundwater aquifers near the mountain front and in adjacent alluvial basins. A total of 48 CSAMT soundings were recorded along two lines that were chosen to maximize subsurface geologic information. Two generations of faults were interpreted based on the CSAMT data: an older, low-angle fault that is cut by a younger, more steeply dipping fault. Lack of deep boreholes in the region required that the subsurface interpretation rely on analogy from surface outcrops within and adjacent to the study area. The success of the CSAMT method as applied in this study hinged on near-ideal collection conditions, the relatively high contrast in electrical resistivity provided by the rock types involved, and well-developed geologic conceptual models of the region.