Audiomagnetotelluric Soundings Research Articles

Application of integrated geophysical techniques in geothermal exploration in Binhai County, Jiangsu Province

AbstractIntegrated geophysical technology is a necessary and effective means for geothermal exploration. However, integration of geophysical technology for large‐scale surveys with those for geothermal reservoir localization is still in development. This study used the controlled source audio‐frequency magnetotelluric method technology for large‐scale exploration to obtain underground electrical structure information and micromotion detection technology to obtain underground wave velocity structure information. The combination of two detection technologies was used for local identification of geothermal reservoirs. Further, auxiliary correction and inversion constraint were implemented through the audio magnetotelluric sounding technology for maximum authenticity restoration of the near‐ and transition‐field data. Through these technology improvements, a geothermal geological model was established for the Binhai County of Jiangsu Province in China and potential geothermal well locations were identified. On this basis, a geothermal well was drilled nearly 3000 m deep, with a daily water volume of over 2000 m3/day and a geothermal water temperature of 51°C at the well head. It is found that predictions using the above integrated geophysical exploration technology are in good agreement with the well geological formation data. This integrated geophysical technology can be effectively applied for geothermal exploration with high precision and reliability.

Geophysical reconnaissance of the western basin of the Plaine du Nord aquifer, Haiti

A geophysical reconnaissance was conducted to support aquifer characterization and evaluate groundwater potential in the western portion of the Plaine du Nord aquifer to provision the growing water demands of the city of Cap-Haitien in the Republic of Haiti. Hybrid-source audio-magnetotelluric (HSAMT) soundings and gravity measurements were collected to support the Government of Haiti in sourcing additional water supplies for the city. The Plaine du Nord aquifer is poorly characterized considering it is one of Haiti’s largest aquifers, few drilling logs are available, and only one borehole has investigated the deeper extents of the aquifer. HSAMT supports depth profiling of subsurface resistivity, which supports interpretations of geological strata and groundwater conditions. Gravity measurements allow for complementary mapping of Earth density anomalies and contrasts, which assist in interpreting aquifer thicknesses or depth to bedrock. Pairing HSAMT and gravity methods permitted both an estimation of aquifer thickness and relative changes in sediment characteristics. Gravity results allowed for the mapping of aquifer thickness across the study area, with interpreted thickness greater than 300 m in the coastal zone. HSAMT results cross correlated with available well records and enabled the delineation of zones with increased groundwater potential. The results of the combined methods indicate groundwater storage potential of freshwater in deeper alluvium and emphasize the importance of strengthening resource characterization, modelling and monitoring to guide sustainable resource development and management.

Genetic Analysis of Geothermal Resources in Deep-Seated Fault Area in Tonghe County, Northeast China and Implications of Geothermal Exploration

Northeast China is an area with high energy consumption and high carbon emissions, and the utilization of geothermal resources can effectively overcome these problems. However, there are few geothermal manifestations in Northeast China and no systematic method for geothermal exploration at present, which hinders the utilization of geothermal resources. Here, a systematic analysis, including hydrochemistry, petrology, isotopes, controlled source audio magnetotelluric sounding, drilling, and temperature curve of two boreholes was carried out to investigate the genesis of geothermal resources in Tonghe County, Northeast China, along the Yilan-Yitong lithospheric fault (YYF). We found that the geothermal water is alkaline Na-HCO3 type water, is of local meteoric origin, and is recharged from the hilly area with an elevation of ~280 m around the study area. We established a geothermal water circulation path model: (1) cold water infiltrated along the YYF to a depth of 2–3 km, (2) cold water was heated by mantle heat, and (3) hot water was stored in sandstone/siltstone, forming a sandstone geothermal reservoir with a temperature of ~70 ℃. These results have important guiding significance for the scientific exploration of geothermal resources in Northeast China.

Mechanism and Prediction of Geothermal Resources Controlled by Neotectonics in Mountainous Areas: A Case Study of Southeastern Zhangjiakou City, China

The geothermal resources in mountainous areas are usually controlled by neotectonic faults. To minimize the risk in site selection for geothermal drilling, the controlling mechanism must be identified. Based on the neotectonic control theory for geothermal resources, the occurrence of the geothermal resources in the mountainous area in southeastern Zhangjiakou city was investigated. The investigation was divided into three stages. Firstly, field investigation for neotectonics was carried out to characterize the kinematics and the stress state of the faults, including fault occurrences, tectonic stages, and paleostress analysis, and in total 19 neotectonic faults were confirmed. Based on Riedel shear model and present stress field, their conductivity for geothermal water was inferred; secondly, geophysical and geochemical surveys were applied to identify the stress state and characterize the occurrence of the potential water conductive faults. The results showed that the combined use of magnetotelluric sounding (MT), controllable source audio magnetotelluric sounding (CSAMT), and 2D seismologic survey is effective in delineating fault occurrence, strata distribution, and water conductivity within 1,000 m depth, while measurement of 222Rn activity in soil gas across the neotectonic faults is effective to investigate the groundwater conductivity of the faults, and all the neotectonic faults in the study area striking clockwise from N5°E to N25°W are considered to be water conductive; thus, the areas at the intersection zones of neotectonic faults in the basin in groundwater runoff and discharge regions are promising targets for geothermal exploration. This conceptual model was verified by the geothermal fields already discovered in the study area and proved to be reasonable, and then a potential drilling site was predicted and proved to be successful. It is suggested that this workflow for geothermal exploration is suitable for Zhangjiakou city and may also be applied to other mountainous areas.

ПРИМЕНЕНИЕ МЕТОДА МИКРОСЕЙСМИЧЕСКОГО ЗОНДИРОВАНИЯ ДЛЯ ИЗУЧЕНИЯ СТРОЕНИЯ ОСАДОЧНЫХ БАССЕЙНОВ (НА ПРИМЕРЕ ЕВРЕЙСКОЙ АВТОНОМНОЙ ОБЛАСТИ)

The authors present the results of the microseismic sounding method application to the study of the earth’s crust structure in the western part of the Middle Amur sedimentary basin. It has been revealed a good correspondence between the built sections, results of seismic profiling and geoelectric section, according to the data of audio-magnetotelluric sounding. The researchers have clarified the boundaries of the Bashmaksky, Preobrazhenovsky, and Samaro-Ditursky grabens.

Geoelectric heterogeneities of the Kerch iron ore basin

The three-dimensional geoelectric model of the Earth’s crust and upper mantle of the Kerch Peninsula has been built for the first time based on the results of experimental observations of the Earth’s low-frequency electromagnetic field, carried out in 2007—2013 by the Institutes of the National Academy of Sciences of Ukraine. Its physical and geological interpretation and detailing of the near-surface part were carried out according to the data of the audiomagnetotelluric sounding method to study the deep structure of the Kerch iron ore basin. To the east of the Korsak-Feodosiya fault along the southern part of the Indolo-Kuban trough (in the north of the South Kerch and almost under the entire North Kerch zones), a low-resistance anomaly (ρ=1 Ohm∙m) was found at depths from 2.5 km to 12 km about 20 km wide. Its eastern part is located in the consolidated Earth’s crust and is galvanically connected with surface sedimentary strata, while the western part is completely in sedimentary deposits. The anomaly covers the territory of the Kerch iron ore basin and occurrences of mud volcanism. The characteristics of the upper part of the layered section of the Kerch Peninsula in the interval of the first hundreds of meters were obtained from the results of one-dimensional inversion of the audiomagnetotelluric sounding data (frequency range 8—4000 Hz). It is shown that the first 15 m of the section, corresponding to Quaternary deposits, have resistivity values up to 1 Ohm∙m. Below, in the Neogene sediments, the electrical resistance increases to values of 5 Ohm∙m and more. Both horizontally and vertically, the distribution of resistivity values has a variable character, manifesting as a thin-layered structure with low resistivity values. Possibly, such areas have a direct connection with the channel for transporting hummock material and gases. A connection is assumed between the low-resistivity thin-layered near-surface areas, a deep anomaly of electrical conductivity in the upper part of the Earth’s crust, and the likely high electrical conductivity of rocks at the depths of the upper mantle with iron ore deposits, as well as the manifestation of mud volcanism. The heterogeneity of the crustal and mantle highly conductive layers may indicate a high permeability of the contact zones for deep fluids.

Subsurface temperature estimation in a geothermal field based on audio-frequency magnetotelluric data

In geothermal exploration, the subsurface temperature distribution is an important parameter since it is closely related to the existing geothermal system. However, subsurface temperature information can only be obtained from boreholes, requiring a considerable cost and a limited number. A new geothermometry technique based on the relationship between temperature and resistivity has been proposed to overcome this difficulty. In order to enhance the reliability of this method, the subsurface heat condition in a different geological and geographical location was reconstructed. The temperature up to some depth beneath the surface could be estimated incorporating the back-propagation neural network using resistivity determined by high-resolution audio-magnetotelluric soundings. The temperature profiles resulting from the neural network fit with those observed from nearby boreholes, providing a satisfying evaluation result of the model’s predictive powers. Joint interpretation was then carried out following the one-dimensional resistivity inversion result and temperature cross-section. Both resistivity and temperature anomalies show an excellent agreement, so that subsurface anomaly such as altered layers, reservoir zone, and some possible faults was detected. With an appropriate training strategy, this technique can help to significantly reduce the costs at estimating the subsurface temperature.

Geophysical research at the Revda intersection of the Serov-Mauk regional fault

Research aim is to study the characteristic features of geophysical fields over the main geological structures in the zone of influence of the Serov-Mauk regional fault in the Middle Urals. Methodology. Electromagnetic studies included an express version of audiomagnetotelluric sounding (AMT) with a broadband OMAR-2m receiver (Institute of Geophysics UB RAS, Ekaterinburg). Office processing is based on obtaining frequency spectra of impedance using fast Fourier transform, and their transformation into deep sections of electrophysical parameters of the medium. Magnetic prospecting was carried out using GEM GSM-19T proton magnetometer (GEM Systems, Canada). Gamma-field survey was carried out with a survey radiometer SRP-68-01 (Electron, Zhovti Vody). Results. Based on observation processing results, high-quality sections of electrical resistivity and effective longitudinal conductivity were constructed on the parametric profile, as well as graphs of magnetic and radiation fields. The studies revealed features of change in the electrophysical parameters and potential fields over various geological structures of the near-contact fault zone. Summary. The signs of the main geologic features border lines were identified by changes in physical properties. The lithological and tectonic boundaries have been identified of a complex rock assemblage adjacent to the regional fault according to the characteristic anomalies of geophysical parameters. Geophysical survey results comply with the real geological conditions of the study area.

Geophysical surveys in the area of the Krylatovskaya water well

Relevance and purpose of the work. Surface sources of clean water are increasingly depleted. In this regard, the role of underground sources of drinking and domestic water supply is increasing. Geophysical exploration methods can be an e ective tool for fnding groundwater. The tasks of the research included determining the possibilities of magnetic prospecting in the areal search for promising zones, and identifying the features of electromagnetic fields during audiomagnetotelluric and magnetovariational soundings in the area of an operating water well. Methodology. The magnetic field was surveyed with an MMP-203 proton magnetometer (Geologorazvedka plant, Leningrad). Electrical exploration included vertical electrical sounding (VES) with ERA equipment (GNPP Geologorazvedka, St. Petersburg) and recording of audio magnetotelluric fields with OMAR-2 equipment (IGF UB RAS, Ekaterinburg). Results. The result of the magnetic survey was a map of the anomalous magnetic field of the study area, where the water-abundant groundwater zone of the weathering crust can be distinguished by structural features. The zones of tectonic faults, promising for the presence of fractured-vein waters, are confedently distinguished by positive linear anomalies of the magnetic field. The characteristic features of electromagnetic fields during audiomagnetotelluric and magnetovariational soundings in the area of distribution of fractured waters have been clarified. The most revealing behavior is the behavior of the real and imaginary quadratures of the magnetovariational tipper, characterized by minimum values and zero crossing, respectively. According to the results of electromagnetic sounding, an increase in the thickness of the weathering crust with a decrease in resistance is recorded in the area of the water intake well, which is evidence of its water cut. Conclusions. The significance of magnetic prospecting for clarifying the structural and geological structure of the territory when searching for water-saturated zones has been determined. The high e£ciency of electromagnetic soundings on direct and alternating current was confirmed to determine the nature, depth and power of the identiŽed structures. Traditionally studied, in the search and exploration of groundwater, geophysical felds are replenished with new electromagnetic parameters. These include the module and quadratures of the audio range magnetovariational tipper. The research results indicate that these parameters are anomalous in relation to aquifers, which allows us to give reasonable recommendations on the location of drilling production wells.

Processing and Interpreting the Data of Deep Frequency Sounding in a Complex with Audiomagnetotelluric Measurements (Murman-2018 Experiment)

This paper aims to study the deep electrical conductivity of the Murmansk block (Fennoscandian Shield) using the controlled source audio-frequency magnetotelluric (CSAMT) and audiomagnetotelluric (AMT) methods. One of the most important tasks in deep sounding studies implying the joint application of the AMT and CSAMT methods is to take into account the influence of surface conductors on the measurement results (static shift). To solve this problem, we develop a methodology for processing and interpreting the measurement data of the CSAMT and AMT system using the results of the deep sounding experiment conducted in the frequency range from 1 to 500 Hz performed in 2018 (Murman-2018) as an example. Measurements with the controlled source in the form of two mutually orthogonal horizontal electric dipoles are taken at distances of 12 to 105 km. The generator-measuring complex consisted of the VMTU-10 measuring station (manufactured by OOO VEGA, St. Petersburg) and the Energy-4 generator (developed at the Kola Science Center, Russian Academy of Sciences, Apatity). Using the synchronous time series of the field components at the observation points and the current in the supply dipole, the power spectra of the autocorrelation and cross-correlation functions of the recorded values are calculated based on the fast Fourier transform (FFT). The obtained spectral characteristics are used (i) to determine the amplitudes of the source field’s components and the phase shifts between them, and (ii) to evaluate the components of the AMT field’s impedance tensor. The values of the apparent resistivity are calculated using the amplitudes of the horizontal components of the electromagnetic field and the impedance ratios. The obtained measurement results are corrected for the static shift. For this, the correction coefficients are calculated from the ratio between the apparent resistivity values for the horizontal component of the magnetic field and the apparent resistivity values inferred from the impedance and electrical field values. The data for the cases of axial and equatorial arrays, together with the data of the AMT and remote sounding, made it possible to establish a high degree of lateral homogeneity of the deep electrical section and to expand the frequency range of the obtained curves of the apparent resistivity and impedance phase. A one-dimensional interpretation is made to obtain estimates of the distribution of the deep resistivity.

Study of the technogenesis of the Degtyarsky mine by audio-magnetotelluric express sounding

The audio-magnetotelluric express sounding was performed at four sections crossing the mine field of the currently not functioning Degtyarsky mine. Field measurements were carried out by a universal broadband receiver “OMAR-2m” with active electromagnetic field sensors developed at the Institute of Geophysics UB RAS. Based on the obtained data, deep sections of the electrophysical parameters of the medium – apparent resistivity and effective longitudinal conductivity – are drawn. The nature of the geoelectric structure of the section allows mapping of the major lithochemical contamination plume and identifying the tectonic disturbance zones that drain aggressive mine waters. The mine waters of the Degtyarsky mine are a source of dangerous technogenic pollution. Despite the neutralization of surface runoff, underground routes of acidic water migration occur along tectonic cracks, primarily in the zone of the regional Serovsko-Mauksky fault. Tectonic zones in the mine area contain contaminated fissure-vein water, which is transited at a depth of 70 to over 200 m. Discharging ascending springs of such waters can be located at a great distance from controlled hydrological objects and pollute sources of drinking and household water supply. Urban development in the western and eastern parts of Degtyarsk does not fall within the distribution zone of polluted water. The southern part of the city is located beyond the watershed of the mine water flow area, but a danger of local contamination by tectonic disturbance zones remains possible. The worst environmental situation is observed in the northern outskirts of Degtyarsk, which falls into the area of heavy pollution of underground and surface waters. Besides, acidic fumes from the flooded Kolchedanny quarry can affect the health of city residents when emitted to the atmosphere.

Geoelectric structure of the Tar Fault Zone in the Middle Urals according to audiomagnetotelluric soundings

Currently, the reduction of reserves and forecast resources of ore minerals due to the development of previously discovered deposits continues. The reproduction of reserves is possible due to additional exploration of the flanks and deep horizons of known ore objects. To study the geoelectric structure of the Ural ore-bearing structures, audiomagnetotelluric soundings were carried out along sub-latitudinal profiles crossing the zone of the Degtyarsky fault near the largest pyrite deposit in the Middle Urals. Field observations were performed using the OMAR-2 universal broadband equipment developed at the Institute of Geophysics, Ural Branch of the Russian Academy of Sciences for electrometric studies of urbanized territories. The applied hardware-software measuring complex is designed to effectively suppress industrial noise with additional methods for digital filtering of network harmonics. The cameral processing of audiomagnetotelluric data consisted in transforming the obtained frequency sounding curves into deep sections of geoelectric parameters using an original transformation algorithm. Based on the materials of the work, lithological and tectonic boundaries of a complex rock complex adjacent to the Degtyarsky regional fault are identified. Electric prospecting methods confidently trace the surface of the bedrock base, and weakened zones are also traced. Studies have shown a good correlation of geoelectric sections with real geological conditions. Good identification of the fault-bearing and ore-controlling structures is noted, which makes it possible to identify areas of mineralization that are promising for mineralization that do not reach the surface. According to the results of the research, places of possible concentration of copper and nickel-cobalt mineralization are identified.

Geophysical survey at the southern end of the Degtyarsky pyrite deposit

Research aim is to study features peculiar to geophysical fields over the main geological features of the southern part of the Degtyarsky pyrite deposit at the Middle Urals. Methodology. Electromagnetic survey included symmetrical electric profiling (SEP) with ERA-MAX equipment (ERA Research and Production Enterprise, St. Petersburg) and rapid audio-magnetotelluric sounding (ATMS) with OMAR-2m wideband receiver (Institute of Geophysics UB RAS, Ekaterinburg). Magnetic survey was carried out with the help of the proton procession magnetometer GSM-19T (GEM Systems, Canada), gamma field survey was carried out with a survey meter SRP-68-01 (Electron, Zhovti Vody). Results. According to the results of observational analysis, high-quality sections of electrophysical parameters of the environment were constructed at the parametric profile together with the charts of other geophysical fields. The studies have shown significant changes of resistivity and potential fields anomalies over various geological features of the ore field. Summary. Geophysical indications of border line of the main geologic features have been determined. Excessive electrical conductivity of ore control tectonic structures and radiation anomaly have been revealed in the promises of the ore body, which can serve a prospecting criterion for a similar ore body. Geophysical results are well within the existing geological data on the Degtyarsky mine.

The Murman-2018 experiment on remote sensing in order to study the boundary of “impenetrability” at the transition between the brittle and plastic states of the crystalline earth's crust

The article describes the experiment “Murman-2018” on remote electromagnetic sounding in combination with frequency and audio magnetotelluric soundings. The current from 29 kW power car-generator was fed into the ground using two mutually orthogonal grounded electric dipoles 1.6 and 1.9 km long. The measurements were carried out along three traces with maximum distances from the source up to 105 km in the mode of frequency sounding (in the range of 4-1000 Hz) and up to 56 km in the mode of remote sensing at direct current. The data processing was performed in the spectral mode and in the accumulation mode. The results of the experiment made it possible to quantify for the first time in the scientific literature the position of the boundary of a sharp increase in rock resistance at a depth of 10-15 km. The nature of the established boundary (the boundary of the “impenetrability” for direct current) is associated with the transition of the properties of the rocks of the Earth's crust from the fragile state in the upper crust to a plastic state at depths of 10-15 km and more. In foreign literature, this boundary is defined as the BDT-zone (brittle-ductile transition zone).

The Murman-2018 Experiment on Remote Sensing in Order to Study the “Impenetrability” Boundary at the Transition between Brittle and Plastic States of the Crystalline Earth’s Crust

This paper describes the Murman-2018 experiment on remote electromagnetic sounding in combination with frequency and audio magnetotelluric soundings. The experimental results made it possible to quantify for the first time in the scientific literature the position of the boundary of a sharp increase in rock resistivity at a depth of 10–15 km. The nature of the established boundary (the “impenetrability” boundary for direct current) is related to the transition of rock properties in the Earth’s crust from the fragile state in the upper crust to a plastic state at a depth of 10–15 km or more.

ОСОБЕННОСТИ СИГНАЛА АМТ В МЕРТВОМ ЧАСТОТНОМ ДИАПАЗОНЕ НА ЧУКОТКЕ (ДАЛЬНИЙ ВОСТОК РОССИИ)

The article analyzes the amplitude spectra of audio magnetotelluric sounding (AMTs) data. Particular attention is focused on the frequency range from 1 to 5 kHz, which is called dead band. We analyzed the data of base stations used in the fieldwork during the summer and autumn seasons in 2013, 2014, and 2017. The area of work is located in the Chukotka Autonomous Area beyond the Arctic Circle. Previous researchers noted that a reliable signal in the dead band can only be obtained at nighttime. The authors of the article found that in Chukotka region in the daytime against the minimum signal within the dead band there is a local maximum at a frequency of 2.4 kHz. When registering a field for more than 3 hours during daytime, in most cases, it is possible to restore the frequencies of 2.2 and 2.6 kHz. These frequencies are reliable benchmarks, allowing in some cases to restore the AMT curve using the correlation between amplitude and phase. We have proposed ways to improve data quality in the dead band when measured during the daytime.

Audio-magnetotelluric investigation of Bakreswar Geothermal Province, Eastern India

Bakreswar Geothermal Province (BGP) lies over the Archaean gneisses and schists formations. The heat flow and geothermal gradient of BGP are \(\sim \)230 mW/m\(^{2}\) and \(95{^{\circ }}\hbox {C}/\hbox {km}\), respectively. The Bakreswar hot spring (BHS) (\(23{^{\circ }}52^\prime 30^{\prime \prime }\hbox {N}\); \(87{^{\circ }} 22^{\prime }30^{\prime \prime }\hbox {E}\)) is located over NNW–SSE trending fault. It lies in the eastern flank of BGP and is connected by Sone–Narmada–Tapti (SONATA) and deep-seated ONGC fault systems. Earlier studies indicated that the geothermal fluid flowed towards the same path as that of deep ONGC fault. In view of this, 24 audio-magnetotelluric soundings in the frequency range 10 kHz–10 Hz were conducted at one end of the deep ONGC fault, which is about 20 km northwest of BHS. The data analysis indicated that the data are internally consistent. Analysis of dimensionality and strike are indicative of complex conductivity structures. The 3D inversion of full impedance was performed to obtain a conductivity model of the study area and resulted in root mean square (RMS) for sites close to unity. The results have been discussed by extracting cross-section along 3 to 7 profiles (four of the profiles are perpendicular to strike, whereas three profiles were oblique to the strike direction and are perpendicular to east–west trending ONGC faults). Several low conducting zones are observed in the profiles drawn parallel and perpendicular to the strike. These common conductors show consistent model features but modified resistivity and dimension. Depth slices from 200 to 1400 m suggest that the conductors in the northern part of the study area are deeper than the conductors in the south. The study area is dominated by several shallow conductors which coincide with several fractures present in the study area. The conductors are surrounded by a resistive feature that can act as an impermeable bed and probably restricts the flow of water within the zone. The four major conductors along the oblique profiles (profiles perpendicular to deep ONGC fault) coincide with deep ONGC fault, through which probably the geothermal fluid flows.

3D Inversion of Audio Magnetotelluric Data for Sulphide Mineralization over Dhanjori Basin

Quartz-pebble conglomerate (QPC) in the Dhanjori basin, a rifted greenstone belt, is known for its gold mineralization. Audio-magnetotelluric (AMT) soundings have been acquired for 32 sites spread in grid fashion for frequency 10kHz to 10 Hz along four parallel profiles. Due to noise conditions in the data only 26 sites have been used for further studies. The site spacing between adjacent AMT sites were ~250 m. The grid spreads over QPCD and Mayurbhang Granite (MG) of rocks. The dimensionality and the directionality analysis hinted towards a complex structure which tempted to perform a 3D inversion of AMT data for full impedance data. To obtain the optimum value of initial uniform half space resistivity, 3D inversion of off-diagonal component of inversion data was performed with various values of resistivity. The optimized value of resistivity was then used to perform 3D inversion for full impedance component with different set of error floor for the impedance component. The final 3D model showed the presence of a continuous conducting feature all along the three profile. Whereas, the southernmost profile showed three well developed conductors. The model corroborates well with electrical resistivity tomography (ERT) sections and GSI borehole which lies within the same geological domain. The borehole intercepted sulphide mineralization with varying concentration from the near surface to ~ 100m. The near surface conducting heterogeneity thus is attributed to sulphide mineralization.

Application of Transient Electromagnetic and Audio-Magnetotelluric Methods for Imaging the Monte Real Aquifer in Portugal

Fifty transient electromagnetic (TEM) and 15 audio-magnetotelluric (AMT) soundings were carried out in the west central part of Portugal over an area of approximately 40 km2 to characterize the geological formations associated with the Monte Real aquifer in order to (1) assess the spatial extent of the aquifer, (2) detect the zones of higher water productivity and (3) evaluate the aquifer salinization risk. The dimensionality and strike direction of the AMT profiles were studied using phase tensor analysis and the AMT data was inverted using a two-dimensional (2D) nonlinear conjugate-gradient algorithm. In addition, the TEM soundings were inverted using a one-dimensional (1D) Levenberg–Marquardt technique in order to obtain 1D layered-earth resistivity models. The TEM results reveal that the aquifer is not uniform in the study area; a more resistive zone is evident in the north and northeast of the investigated area where we expect to have higher productivity. On the other hand, the AMT models suggest that the aquifer basement is probably located at a depth of more than 800 m. Based on the results of this investigation, no traces of saltwater intrusion were found for the Monte Real aquifer. Therefore, we assume that the risk of saltwater intrusion is small in the study area.

Structure of the Northwestern Boundary of the Middle Amur Sedimentary Basin (Russian Far East) from Audiomagnetotelluric Sounding Data

This paper reports the data on the structure of the northwestern boundary of the Middle Amur sedimentary basin which were obtained after resumption of audiomagnetotelluric soundings. The geoelectric sections for two profiles across the basin strike are constructed, the sedimentary cover and basement structures are studied, and their electrical properties are determined. The compared data of the earlier and present studies show low-resistivity zones of 50–150 Ohm m beneath high-resistivity rocks of 500–1000 Ohm m in the northwestern part of the Ul’dura–Churki uplift at a 2–4 km depth, which are absent in the southwestern part of the uplift. This fact is related to strike-slip–overthrust processes due to graben formation in the area of junction with the Jiamusi–Bureya Massif in the Kur fault zone (YYilan–Yitong branch of the Tan-Lu fault zone) and also to global left-lateral strike-slip displacements and volcanic activity.