Audio-magnetotelluric Studies Research Articles

Boat-towed radio-magnetotelluric and controlled source audio-magnetotelluric study to resolve fracture zones at Äspö Hard Rock Laboratory site, Sweden

SUMMARY Boat-towed radio-magnetotelluric (RMT) measurements using signals between 14 and 250 kHz have attracted increasing attention in the near-surface applications for shallow water and archipelago areas. A few large-scale underground infrastructure projects, such as the Stockholm bypass in Sweden, are planned to pass underneath such water zones. However, in cases with high water salinity, RMT signals have a penetration depth of a few metres and do not reach the geological structures of interest in the underlying sediments and bedrock. To overcome this problem, controlled source signals at lower frequencies of 1.25 to 12.5 kHz can be utilized to improve the penetration depth and to enhance the resolution for modelling deeper underwater structures. Joint utilization of boat-towed RMT and controlled source audio-magnetotellurics (CSAMT) was tested for the first time at the Äspö Hard Rock Laboratory (HRL) site in south-eastern Sweden to demonstrate acquisition efficiency and improved resolution to model fracture zones along a 600-m long profile. Pronounced galvanic distortion effects observed in 1-D inversion models of the CSAMT data as well as the predominantly 2-D geological structures at this site motivated usage of 2-D inversion. Two standard academic inversion codes, EMILIA and MARE2DEM, were used to invert the RMT and CSAMT data. EMILIA, an object-oriented Gauss–Newton inversion code with modules for 2-D finite difference and 1-D semi-analytical solutions, was used to invert the RMT and CSAMT data separately and jointly under the plane-wave approximation for 2-D models. MARE2DEM, a Gauss–Newton inversion code for controlled source electromagnetic 2.5-D finite element solution, was modified to allow for inversions of RMT and CSAMT data accounting for source effects. Results of EMILIA and MARE2DEM reveal the previously known fracture zones in the models. The 2-D joint inversions of RMT and CSAMT data carried out with EMILIA and MARE2DEM show clear improvement compared with 2-D single inversions, especially in imaging uncertain fracture zones analysed in a previous study. Our results show that boat-towed RMT and CSAMT data acquisition systems can be utilized for detailed 2-D or 3-D surveys to characterize near-surface structures underneath shallow water areas. Potential future applications may include geo-engineering, geohazard investigations and mineral exploration.

Audio-magnetotelluric investigation of sulfide mineralization in Proterozoic–Archean greenstone belts of Eastern Indian Craton

Greenstone belts are well known for gold occurrences at different regions of the world. The Dhanjori basin in the eastern Singhbhum region shows major characteristics of a rifted greenstone belt. Initially, we conducted 14 audio-magnetotelluric (AMT) measurements for a profile of $$\sim $$ 20 km in the frequency range of 1 kHz to 10 Hz over this rather complex geologic environment covering Dhanjori Volcanics (DhV) and Kolhan Group (KG). Subsequently, gravity and magnetic surveys were also conducted over this AMT profile. The purpose of the survey was to identify and map conductive features and to relate them to metallogeny of the area along with the mapping of the basement of Dhanjori basin. The strike analysis showed $$\hbox {N30}^{\circ }\hbox {W}$$ strike for DhV for all the frequencies and for sites over KG domain in the frequency range of 100–10 Hz, but for KG domain, the obtained strike in 1 kHz to 100 Hz is $$\hbox {N45}^{\circ }\hbox {E}$$ . As the combination of transverse electric (TE), transverse magnetic (TM) and tipper (Tzy) can recover the electrical signature in complex geological environment, we discuss the conductivity model obtained from TE+TM+Tzy only. The inversion was carried for the regional profile with 14 sites and for 7 sites over KG domain. Conductivity model shows two well resolved conductors, one each in KG and Quartz Pebble Conglomerate Dhanjori (QPCD) domains respectively showing common linked concordant features between these regional and KG profiles. The conductors are interpreted as sulfide mineralization linked with QPCD group of rocks which may host gold. These conductors are also horizontally disposed due to the intrusive younger Mayurbhanj Granite. These intrusives correlate well with the gravity modeling as well. The thickness of the Dhanjori basin at the central is about 3.0 km, similar to that from gravity modeling. The conductivity model also indicates the presence of shallow conductors, but could not be resolved due to lack of high frequency data. However, the results from the close-by drill site indicate the presence of shallow sulfide mineralization hosting gold. The deep level conductors delineated from AMT studies are associated with gravity high and low magnetic. ICP-AES results of Dhanjori samples show significant concentration of gold $$\sim $$ 5.0 g/t, which is of economic consideration. Thus, it can be inferred that the conductors have evidences of sulfide mineralization which host gold.

Earth’s Electromagnetic Environment

The natural spectrum of electromagnetic variations surrounding Earth extends across an enormous frequency range and is controlled by diverse physical processes. Electromagnetic (EM) induction studies make use of external field variations with frequencies ranging from the solar cycle which has been used for geomagnetic depth sounding through the 10\(^{-4}\)–10\(^4\) Hz frequency band widely used for magnetotelluric and audio-magnetotelluric studies. Above 10\(^4\) Hz, the EM spectrum is dominated by man-made signals. This review emphasizes electromagnetic sources at \(\sim\)1 Hz and higher, describing major differences in physical origin and structure of short- and long-period signals. The essential role of Earth’s internal magnetic field in defining the magnetosphere through its interactions with the solar wind and interplanetary magnetic field is briefly outlined. At its lower boundary, the magnetosphere is engaged in two-way interactions with the underlying ionosphere and neutral atmosphere. Extremely low-frequency (3 Hz–3 kHz) electromagnetic signals are generated in the form of sferics, lightning, and whistlers which can extend to frequencies as high as the VLF range (3–30 kHz).The roughly spherical dielectric cavity bounded by the ground and the ionosphere produces the Schumann resonance at around 8 Hz and its harmonics. A transverse resonance also occurs at 1.7–2.0 kHz arising from reflection off the variable height lower boundary of the ionosphere and exhibiting line splitting due to three-dimensional structure. Ground and satellite observations are discussed in the light of their contributions to understanding the global electric circuit and for EM induction studies.

The potential of audiomagnetotellurics in the study of geothermal fields: A case study from the northern segment of the La Candelaria Range, northwestern Argentina

Despite its reduced penetration depth, audiomagnetotelluric (AMT) studies can be used to determine a broad range of features related to little studied geothermal fields. This technique requires a stepwise interpretation of results taking into consideration diverse information (e.g. topographic, hydrological, geological and/or structural data) to constrain the characteristics of the study area. In this work, an AMT study was performed at the hot springs in the northern segment of the La Candelaria Range in order to characterize the area at depth. Geometric aspects of the shallow subsurface were determined based on the dimensional and distortion analysis of the impedance tensors. Also, the correlation between structural features and regional strikes allowed us to define two geoelectric domains, useful to determine the controls on fluid circulation. The subsurface resistivity distribution was determined through 1D and 2D models. The patterns of the 1D models were compared with the morpho-structure of the range. Shallow and deep conductive zones were defined and a possible shallow geothermal system scheme proposed. A strong correlation was found between the AMT results and the geological framework of the region, showing the relevance of using AMT in geothermal areas during the early stages of subsurface prospecting.

Audiomagnetotelluric studies to trace the hydrological system of thermal fluid flow of Bakreswar Hot Spring, Eastern India: A case history

An audiomagnetotelluric (AMT) study has been carried out in the Bakreswar Hot Spring (BHS) area of eastern India to locate the geothermal source in the vicinity of BHS. Phase-tensor analysis of the AMT data shows that the region is broadly 2D. Rapid relaxation inversion (RRI) for both transverse-electric (TE) and transverse-magnetic (TM) modes has been carried out to obtain resistivity images of the subsurface. AMT results show that the north-south fault close to Bakreswar is a shallow feature, not deeper than [Formula: see text], and thus cannot act as a heat source. The subsurface formation below the fault zone is highly resistive up to a great depth, indicating the absence of a heat source and geothermal reservoir in the vicinity of the BHS. AMT results indicate that the location of the geothermal reservoir is deep and lies beyond the profiles of measurement in the northwestern side of the Bakreswar Hot Spring.

Generation of synthetic wide-band electromagnetic time series

The estimation of the earth transfer functions in MT prospecting method poses the greatest difficulty. As in the seismic prospecting method this task requires the development of advanced processing techniques. In order to assess the performance of each technique, controlled synthetic data and different noise types, which simulate the observed signals, are required. This paper presents a procedure to generate a wide-band noise-free electromagnetic field to be used both for magnetotelluric and audio-magnetotelluric studies. Furthermore, an effort was made to extend the simulation procedures to generally stratified and simple inhomogeneous earth structures. The discrete-time magnetic field values are generated through the inverse Fourier transform of a continuous amplitude spectrum and a sampling procedure. The electric field time series are obtained by the convolution of the magnetic field time series, calculated in the interested frequency band, with a non-causal impedance impulse response. Polarized fields, which are important when inhomogeneous media are considered, are also generated.

An audiomagnetotelluric study of the Leona Batholith, Senegal

Scalar audiomagnetotelluric surveys over the frequency range 8–5000 Hz were conducted across the Leona batholith (northwest Senegal) along two traverses to map resistivity contrasts associated with the Senegal basin margins. The apparent resistivity data from 12 sites, measured in perpendicular directions, ranged from nearly isotropic to anisotropic with decreasing frequency. Modelling indicates a major change in structure at depth with a pronounced vertical contact. Resistivitie exceeding 20,000 ohm·m are obtained for a dome of igneous nature, contrasting with values of about 2000 ohm·m from the Senegal basin sediments. A conductive layer (50–200 m thick), overlying the more resistive layer, is associated with a fractured contractmetamorphic aureole to the intrusive body.

Audiomagnetotelluric study in northeast region of Paraná Basin, South America

We conducted natural‐source audiomagnetotelluric (AMT) soundings at 31 stations along a 215 km profile in the Paraná Basin (South America) across the previously postulated Araxá‐Rio Grande alignment. Our objective was to study the resistive structures of the upper layers of the sedimentary sequence; our results made it possible to map the surficial layer of sediments of the Bauru group, a layer that reaches thicknesses of up to 250 m. We concluded that AMT soundings have reasonably good potential for groundwater exploration in the region. The varying thicknesses of Bauru group sediments and the irregularities of their contact surface with resistive volcanic rocks of the Serra Geral formation along the profile were not known previously. Our AMT profile crossed an important positive gravity anomaly located near the Rio Grande (a natural boundary between the states of São Paulo and Minas Gerais, in Brazil) which has not been fully interpreted. Five sites for deeper MT soundings, which may throw light on possible causes of the gravity anomaly, were selected based on the conclusions from the AMT survey.

Regional audiomagnetotelluric study of the Questa Caldera, New Mexico

Audiomagnetotelluric (AMT) soundings at Questa caldera were made in the course of a mineral resource study of the Columbine‐Hondo and Latir Peak Wilderness areas, New Mexico, in the fall of 1980 and 1981. This regional electrical survey covered an area of approximately 518 km2 and consisted of 92 AMT stations at an average spacing of approximately 3 km. An AMT apparent resistivity map and one‐dimensional models delineate several major lithologic and structural features. A large north‐south resistivity high is interpreted to outline a largely concealed batholith 20 km long, 2–6 km wide, and >4 km thick. Two high‐resistivity east‐west trends coincide with the mapped margins of the Questa caldera, indicating the electrically contrasting rocks at its boundary. Apparent resistivity lows coincide with areas of hydrothermal alteration known to contain stockwork molybdenum deposits.

Audio-magnetotelluric study of a structure with a reverse fault.

Audio-magnetotelluric study of a structure with a reverse fault.