Audio-frequency Magnetotelluric Method Research Articles

An integrated approach for sewage diversion: Case of the Huayuan mine, Hunan Province, China

Environment protection is a core priority for many governments in this century. Most environmental problems have diverse causes: emission of greenhouse gases from fossil fuels, resource depletion, or intense mining activities such as the Huayuan manganese mine. The positioning of mining factories and water treatment stations impacts the surrounding groundwater reservoir. As the mine expands, the environmental impact also increases, and the previous plan based on monitoring wastewater leakage has become inappropriate. Therefore, to solve this issue, a new study is required to understand the lateral resistivity distribution underground and to define a new station location for water treatment and divert the sewage to that station. In this study, the audio-frequency magnetotelluric method is used. Surveys of two long lines that cross the mining area to its boundaries are carried out. Data are robustly processed and inverted. Based on the inverted models, in addition to the geologic information, drilling inspections, and solid waste distributions map, the integrated interpretation develops two sites on the top of impermeable layers, which constitute a buffer point between the unsafe (high concentration of pollutants) and the safe zones in the northwestern part of the mine. From the resistivity distribution combined with the water quality analysis, a relationship between the fault structures reveals an interconnected conductive zone (CZ) in the southeastern part. As the main channels for water circulating underground, these CZs delineate the main groundwater reservoir with a clastic aquifer layer. However, close to factories, the water from faults contains solid wastes, thereby making the groundwater in that zone nonpotable, unlike the safety zone located in the northwestern part. This workflow can become a field guide to improve the environment of mines and the deployment of hydrogeologic drilling in a safe area.

Research on 3D Time-Lapse Electric Field Inversion Algorithm for Controlled Source Audio-Frequency Magnetotelluric Method

The controlled source audio-frequency magnetotelluric method (CSAMT) stands out for its economic efficiency and widespread application in geophysical monitoring. However, the separate inversion of time-lapse monitoring data encounters challenges in comparing and identifying abnormal changes due to variations in data fitting. Furthermore, the utilization of a method akin to Cagniard apparent resistivity for inversion necessitates the simultaneous observation of at least two components of the electromagnetic field, making it unsuitable for extensive three-dimensional observations. This paper proposes a 3D time-lapse electric field inversion algorithm for CSAMT, addressing the complexities in geophysical monitoring. The algorithm introduces two regularization factors and defines an objective function with both temporal and spatial constraints. Synthetic testing reveals the stability of the 3D time-lapse electric field inversion algorithm, demonstrating its effectiveness in delineating underground variations. This solution resolves the challenges posed by the independent inversion of time-lapse monitoring data.

The comparative analysis of the Ex and Hz fields sensitivity generated by electric dipole sources

Abstract In the controlled source audio-frequency magnetotelluric method, orthogonal electric and magnetic fields are commonly measured to determine the Cagniard apparent resistivity. However, in the near-field zone, the Cagniard resistivity is severely distorted, which is unrelated to underground structures. The Ex and Hz amplitudes in a homogeneous half-space monotonically vary in resistivity, and a numerical algorithm could achieve high-precision apparent resistivity without distortion for all frequencies. On this basis, the main focus of this investigation is on the comparative analysis of the sensitivity for the Exfield, Hzfield, and Cagniard apparent resistivity to conductive and resistivity targets via synthetic models. The achieved results confirm that the Ex field could exhibit a more enhanced sensitivity for the resistive objects, whereas the Hz field could more effectively identify the conductive target. Besides, the static effect often distorts the electromagnetic data, which rigorously influences their application. The influence of the static effect on both the Exand Hzfields is also examined in detail. The apparent resistivity based on the Exfield and Cagniard apparent resistivity is significantly affected by the static effect, which can mask deep anomalous blocks. However, the apparent resistivity based on the Hz field is almost unaffected by the static effect. Finally, a more efficient observation approach is provided for both the insulating and conductive targets.

Deep Structure of Epithermal Deposits in Youxi Area: Insights from CSAMT and Dual-Frequency IP Data

Epithermal deposits represent a significant category of gold occurrences, with their subsurface structure playing a key role in reserve assessments. Fujian Province, characterized by extensive Mesozoic volcanic activities, stands out as a noteworthy region for shallow hydrothermal mineralization in China. This paper focus on the Youxi area within Fujian Province, employing the dual-frequency induced polarization method (DFIP) and controlled-source audio-frequency magnetotelluric method (CSAMT) to investigate the target ore. The DFIP results revealed predominant northeast-oriented zones with high polarizability and notable apparent resistivity. The CSAMT data were inverted using the SCS2D software. Two-dimensional resistivity profiles reveal a three-layer electrical structure, comprising subsurface banded rhyolites influenced by fault zones, intermediate-low resistivity sandstone layers, and deep-seated high-resistivity conglomerates. The resistivity gradient zones and highly polarizable locations align closely with known local faults. We interpreted these resistivity gradient zones as prospective target areas for mineralization, a hypothesis subsequently validated by drilling results. Combining geochemical analyses of epithermal gold deposits with the electrical resistivity structure, we propose an explanatory model for the mechanism of the formation of epithermal gold–silver deposits in the Youxi area. The magmatic hydrothermal fluids ascended along the fault, underwent convection-driven interaction with meteoric waters, and subsequently metasomatized the host rocks. This integrated approach provides valuable insights into the geological processes governing epithermal gold–silver deposit formation in the Youxi region.

Coal Mine Goaf Interpretation: Survey, Passive Electromagnetic Methods and Case Study

Coal mine goaf detection remains confronted with the lack of fast, effective and low-cost exploration means, especially for the accurate prediction of mining threats primarily caused by hydraulic infiltration. The rapid popularization of passive electromagnetic methods has contributed greatly to improving the interpretation effects of different types of goafs. This paper, firstly, summarizes the pros and cons of various exploration methods in goaf detection. Then, the feasibility of goaf detection using novel passive electromagnetic methods (e.g., the super low frequency alternating magnetic component method (SLF) and audio frequency magnetotelluric method (AMT)) is proposed and further discussed. With well-designed geo-electrical goaf models, the theoretical results demonstrate that the semi-quantitative interpretation of SLF responses can be directly used for the delineation of the target layer in the estimated depth range. In contrast, 3D inversion provides more information about conductive targets with the appropriate initial model selection. Then, shallow, low-resistive targets can be more accurately allocated in the inversion maps. Moreover, the real data interpretation results from study areas demonstrate that the SLF method can utilize the magnetic component responses to effectively identify the fault structures, and indirectly contributes to judge the goaf collapse locations in favor of describing the potential distribution of fracture water infiltration. Combined with the three-dimensional (3D) resistivity inversion of AMT data, the low-resistive water-rich areas within the depth of 400 m were revealed. The inverted depth distributions are basically consistent with those of the water-filled goafs and surrounding layers, which were also confirmed by known logging data. The detailed delineations of water-control fracture zones can be inferred to relate to aquifers in some mining areas; this can reveal potential collapses that require successive mining planning. In specific working faces, goaf risks have been handled in advance by strengthening the continuous monitoring of the water level and water inflow. The above verification has laid a theoretical and practical foundation for passive electromagnetic interpretation methods for effectively predicting collapse-type risks or hydraulic threats in coal mine goafs.

Study on Application of Comprehensive Geophysical Prospecting Method in Urban Geological Survey—Taking Concealed Bedrock Detection as an Example in Dingcheng District, Changde City, Hunan Province, China

In order to ascertain the concealed bedrock and its spatial distribution in an urban low-resistance coverage area of a typical lacustrine basin in Hunan Province, a multi-method comprehensive experimental study was carried out in Dingcheng District, Changde City where there are multiple sets of strata and fault structures. In this study, the wide-area electromagnetic method and microtremor survey were utilized on the basis of traditional methods, including the high-density resistivity method and controlled-source audio-frequency magnetotelluric method, to infer the concealed Cambrian limestone, fault structure, and vertical distribution of strata and the results were verified by drilling. The results indicate that the wide-area electromagnetic method is effective to explore the bedrock and concealed structure in urban geological survey. The microtremor method has an obvious effect on the detection of the Cretaceous and Quaternary silty strata within 100 m. The study may provide references for similar projects in this area.

Review and Challenges in the Geophysical Mapping of Coal Mine Water Structure

Generally, the water-bearing structures of coal mines mainly include coal seam, roof, coal seam, floor, and goaf, while the main water-conducting structures include faults, collapsed columns, and collapsed goaf areas. The most commonly used methods for the detection of the above structures include the seismic method, high-density electrical method, controlled source audio-frequency magnetotelluric method, and transient electromagnetic method. Theoretically, the seismic methods have a higher resolution, which can be used to determine the targets’ geometry, but unable to determine whether the target is filled with water, while the electromagnetic methods are capable of this, although with lower resolution. Therefore, it is necessary to adopt the comprehensive geophysical prospecting in the actual field measurement of the coal mine water to guarantee the detection accuracy. Based on this, in the passage, first, we introduced the characteristics of the water-bearing and water-conducting structures and then analyzed the detection results of different methods by examples. Finally, we pointed out that, first, it is essential to develop the mine or airborne methods for the sake of convenience; and second, it is time that we adopt three-dimensional detection technology, multisource and multiresolution detection technology, and electromagnetic big data technology for high accuracy.

Geothermal Resource Exploration in Magmatic Rock Areas Using a Comprehensive Geophysical Method

Geothermal resources have significant development and usage potential. It is critical to conduct geological investigation of geothermal resources prior to mining, so as to deepen our knowledge and comprehension of geothermal resources. Ground water is heated by magmatic rocks and geothermal resources can be created in magmatic rock areas. However, their communication is weak, and the depth of burial is typically great. It is difficult for traditional geophysical methods, such as induced polarization method, to achieve useful exploration depths, and they have low accuracy. In this article, a comprehensive geophysical method, based on the controlled source audio-frequency magnetotelluric method (CSAMT) and transient electromagnetic method (TEM), is applied to geothermal exploration in a magmatic rock area. This method compensates for the shortcomings of a single method and achieves a good exploration effect, thereby providing a reliable geological foundation for further development and utilization of geothermal resources.

Hydrogeological prospecting in the Da Qaidam area of the Qaidam Basin using the audio-frequency magnetotelluric method

The Qaidam Basin has very little precipitation but large surface evaporation. Besides, the groundwater resources are unevenly distributed, which gradually decrease from east to west. Therefore, reasonable exploration and management of the groundwater resources are very important in this area. This study performs a hydrogeological survey in this area using the audio-frequency magnetotelluric method (AMT). We establish a priori model using the hydrogeological records and geophysical data. Then, 226 AMT sites are arranged to collect data. Using the three-dimensional (3-D) inversion technology, we obtain a reliable electrical structure of the underground area within 3000 m deep of the Qaidam Basin. On the basis of the 3-D inversion results and the priori model, we analyze the distribution and features of the underground aquifers and investigate the hydrological characteristics, including the quaternary strata, aquifers, paleochannels, deep geological structure and groundwater migration characteristics. This research may provide scientific references for the underground water exploration and management in the Da Qaidam area.

Imaging underground electric structure over a potential HLRW disposal site

With fast development of Chinese economy, a large amount of garbage have been produced. Especially, high level radioactive waste (HLRW) bring dangers to the security of human being. Finding a suitable site to dispose HLRW is a key management to government. A potential HLRW disposal site has been chosen in west north of China at first, however, it is necessary to understand whether there exist electrical structures inside the rock mass, whether the rock mass is intact or not. We applied the controlled source audio-frequency magnetotelluric method (CSAMT) to confirm that. Three-dimensional numerical modeling of finite difference method was performed for typical structure in the study area, and response characteristics were analyzed to confirm which actual frequency range can be chosen and what structure dimension can be detected. Then we laid out 14 parallel survey lines to carry out data acquisition. By analyzing typical sounding curves, combined with drilling information and the inversion results, we suggest that the rock is intact and suitable for building a repository, which is a good recommendation for government department.

Investigation of geological structures with a view to HLRW disposal, as revealed through 3D inversion of aeromagnetic and gravity data and the results of CSAMT exploration

The Alxa area in Inner Mongolia has been selected as a possible site for geological disposal of high-level radioactive waste (HLRW). Based on results of a previous study on crustal stability, the Tamusu rock mass has been chosen as the target. To determine the geological structure of this rock mass, aeromagnetic and gravity data are collected and inverted. Three-dimensional (3D) inversion horizontal slices show that the internal density of the rock mass and the distribution of magnetic properties are not uniform, with fractures and fragmentation being present. To confirm this result, the controlled source audio-frequency magnetotelluric method (CSAMT) was applied to explore the geological structures, the typical CSAMT sounding curve was analyzed, and the response characteristics of the geological structure and surrounding rock are distinguished. The original data were processed and interpreted in combination with data from surface geology and drilling and logging data. It is found that the CSAMT results were consistent with those from 3D inversion of the gravity and magnetic data, confirming the existence of fractures and fragmentation in the exploration area.

The Application of Cross-Correlation Algorithm in CSAMT Received Data Processing

Controlled source audio-frequency magnetotelluric method (CSAMT) is an effective frequency domain detecting method in metallic ore exploration. In view of the weak response of a deep target signal and the serious measuring environmental noise,this paper proposes a method to deal with the noise of the CSAMT data based on the theory of cross-correlation algorithm. Emission signal current waveform recorder is designed to record signal parameters.According to cross-correlation technology, this paper uses the characteristics that correlation between emission and received signals is strong, yet the correlation between emission signals and random noise is weak, to deal with emission signals and received signals by cross-correlation ,in order to filter out random noise and other jamming signals.Field exploration test data processing contrast results show that the method can suppress noise signal, and improve the CSAMT measuring accuracy.

Geophysical exploration for a long deep tunnel to divert water from the Yangtze to the Yellow River, China

To identify problematic geological structures which would be encountered when driving tunnels in the high mountainous area of South West China, a joint geophysical and engineering geological study was made. The tunnels will form part of a project to divert water from the Yangtze to the Yellow Rivers. A controlled source audio-frequency magnetotelluric method (CSAMT) was carried out in 2004 between the Ma-ke and Jia-qu rivers in high, steep terrain. The paper discusses the method of data collection, processing, and analysis for the West route. The faults/fractures identified will need to be taken into account in the design and construction of the tunnel.

Underground Structure of Mud Volcanoes in Tokamachi City, Niigata Prefecture Determined by Electromagnetic Exploration, and Geographical and Geological Surveys

In Tokamachi City, Niigata Prefecture, where Tertiary sedimentary rocks are distributed, excavation of the Mt. Nabetachiyama tunnel took a long time to complete due to the presence of a swelling mudstone zone. This swelling zone is distributed at a depth of 180m under the mud volcanoes. Geological and geographical surveys as well as electromagnetic surveys applying the Controlled Source Audio-frequency Magneto-Telluric Method (CSAMT), were carried out to clarify the geological structure under the mud volcanoes and the relationship between geographical features and the swelling zone. The laser scanner survey revealed that the geological structure around the topographic depression is intensely disturbed and the geological survey showed that the mud volcanoes and eruption points of groundwater and natural gas are located mainly along the outside edge of the geologically disturbed zone. This suggests that the area was active in the past due to the uplift and eruption of groundwater. The extremely low resistivity zone (ELR) at a depth of 400m with a diameter of 500m was detected below the geologically disturbed zone by the CSAMT survey. The ELR is estimated to correspond to a mud chamber filled with saline groundwater and mud, based on laboratory tests measuring the electrical properties of several rock samples obtained from the survey area. The low resistivity zone (LR) was also detected; it continues from the ELR to the mud volcano at the ground surface, indicating that saline groundwater, mud, and natural gas may be ascending via a concentric path to the mud volcanoes. On the basis of all the results obtained, it is interpreted that the swelling zone in the tunnel corresponds to the path of mobilized mud and saline groundwater in the fractured mudstone with abnormal pore water pressure.

갯벌 지역의 하부퇴적층에 대한 AMT 탐사의 적용 가능성 평가

The marine seismic prospecting using a research vessel in the shallow sea near the coastal area has certain limits according to the water depth and survey environment. Also, for the electrical resistivity survey at seashore area, one may need a specially designed high-voltage source to penetrate the very conductive surface layer. Therefore, we have conducted a feasibility study on the application of magnetotelluric method (MT), a passive geophysical method, on investigating of shallow marine environment geology. Our study involves both theoretical modeling and field survey at the tidal flat area which represent the very shallow marine environment. We have applied the audio-frequency magnetotelluric (AMT) method to the intertidal deposits of Gunhung Bay, west coast of Korea, and analysed the field data both qualitatively and quantitatively to investigate the morphology and sedimentary stratigraphy of the tidal flat. The inversion of AMT data well reveals the upper sedimentary layer of Holocene intertidal sediments having a range of 13-20 m thickness and the erosional patterns at the unconformable contact boundary. However, the AMT inversion results tend to overestimate the depth of basement (30-50 m) when compared with the seismic section (27-33 m). Since MT responses are not significantly sensitive to the resistivity of middle layer or the depth of basement, the AMT inversion result for basement may have to be adjusted using the comparison with other geophysical information like seismic section or logging data if possible. But, the AMT method can be an effective alternative choice for investigating the seashore area to get important basic informations such as the depositional environment of the tidal flat, sea-water intrusion and the basement structure near the sea shore.

Various-scale electromagnetic investigations of high-salinity zones in a coastal plain

Three different-scale electromagnetic (EM) measurements have been performed in the Kujukuri coastal plain, southeast Japan, to investigate the distribution of saline groundwater. The three techniques were audio-frequency magnetotelluric (AMT), transient electromagnetic (TEM), and small loop-loop EM measurements. The resistivity sections estimated from these data sets reveal three independent resistivity distributions extending to different depths. The AMT method reveals a regional-scale resistivity distribution across the plain to a maximum depth of approximately [Formula: see text] and the existence of deep conductive zones, which are inferred to be associated with fossil seawater trapped in a Pleistocene formation. The TEM results show a medium-scale resistivity distribution to depths of approximately [Formula: see text], in which two shallow conductive zones are recognized. It is concluded that these features are caused by present seawater intrusion and high-salinity salt-marsh deposits formed during sporadic marine regressions. The small loop-loop EM method provided a shallow resistivity profile that highlights the conductive salt-marsh deposits and resistive sandy ridges. Although these resistivity sections correspond to different depth ranges, the overlapping portions of the sections are very consistent with one another. These EM methods are useful in detecting and interpreting important resistivity features. Taking the geologic evolution of the coastal plains into consideration is crucial when interpreting resistivity profiles such as these, and our results suggest that the presence of fossil seawater is an important factor controlling resistivity at a variety of depths.

Controlled Source Audiofrequency Magnetotelluric method applied in urban area of Central Osaka, Japan

A detailed study was carried out to find out the most suitable time for electromagnetic sounding in order to apply the CSAMT method in the urban area of Central Osaka. The preliminary data collected during the daytime were highly noise­ contaminated, which couldn't be attenuated even on increasing the sampling time. Because of this reason, all the soundings were obtained during the nighttime when the electromagnetic noises were drastically reduced. The changes in layer resistivities with depth, as obtained from the 1-D inversion of the observed data, correspond to the different reflectors as seen in the seismic section. It is found that the vicinity of Uemachi Fault, in the western part of the study area, is characterised by high conductivity.

Magnetotelluric Sounding of Permafrost

The audio-frequency magnetotelluric method was used to sound a permafrost region in the Mackenzie delta in the Northwest Territories. A simple two-layer model consisting of a high electrical resistivity layer overlying less resistive material gave interpreted depths in agreement with those determined by drilling. The summer active layer was transparent even at high sounding frequencies.

Induction methods in prospecting for hot water

It is commonly observed that in areas where abnormally heated water is rising to the surface, the electrical conductivity of the ground is significantly higher than in adjacent rocks. This has been the basis for the use of direct-current electrical prospecting methods in the search for an evaluation of geothermal power prospects. However, it can be argued theoretically that induction electromagnetic prospecting methods should be more effective in searching for conductive zones than direct current methods. We have used four electromagnetic methods in the evaluation of geothermal prospects: the audio-frequency magneto-telluric method, the two-coil profiling method, the two-coil frequency-domain sounding method, and the coil/wire time-domain sounding method. The audio-frequency magneto-telluric method is a very simple means for reconnaissance to depths of a few hundred meters. The two-loop profiling method provides high resolution for conductive zones within the first 20 or 30 meters of the surface, and may be used in detailing the surface expression of hyperthermal manifestations. The two-loop sounding method may be used to measure conductivities to a depth of 1 or 2 kilometers, even when the surface resistivity is too high to allow the use of direct-current methods. The wire/coil method may be used to map the source of anomalous heat into the lower crust and upper mantle.