Controlled Source Audio-frequency Magnetotelluric Method Research Articles

Estimation of rock core indices for development of underground infrastructure using non-invasive geophysical methods

Rock mass characterization offers the basis of the stability criterion for the planning and advancement of civil engineering projects. Rock core index (RCI) is one of the key geotechnical parameters adopted in rock mechanics and rock engineering. RCI provides risk assessment regarding the success criteria of engineering design. Consequently, an accurate estimate of such parameters is a challenging task in the context of credibility, budget, and time. However, the traditional estimation of geomechanical parameters needs lots of drilling tests at some selected points. The point-scale data often cause more ambiguity and less authenticity in engineering design. Besides, the conventional approaches are invasive, uneconomical, and laborious and cannot investigate the complete project site. We offer an indirect technique for the estimate of RCI using empirical relationships between drilling and geophysical data, which addresses the drawbacks of the conventional methods. Geophysical techniques offer the subsurface volumetric data and are faster, more affordable, and easier to use. In this study, we employ a non-invasive CSAMT (controlled-source audio-frequency magnetotellurics) method for the first time to quickly assess 2D and 3D RCI models. The suggested approach assesses the intricate geological subsurface at a depth of 1 km in order to produce a more precise and complete image of the quality of the rock mass across a wide area. These findings are crucial for improving our understanding of the intricate geological conditions, estimating the likelihood of failure early on, and supporting the safe, stable, and cost-effective construction of deep underground engineering structures. In regions where there is a deficiency of mechanical drilling data, our approach decreases the gaps between an appropriate geotechnical model and insufficient data, yields more objective indices, and serves as a guide for more correct engineering structure design.

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.

3-D Inversion of the Data-Normalized Time-Lapse Controlled-Source Audio-Frequency Magnetotelluric Method

3-D Inversion of the Data-Normalized Time-Lapse Controlled-Source Audio-Frequency Magnetotelluric Method

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.

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.

PyCSAMT: An alternative Python toolbox for groundwater exploration using controlled source audio-frequency magnetotelluric

Controlled source audio-frequency magnetotelluric (CSAMT) is one of the frequently used geophysical methods in various exploration problems such as fault mapping and groundwater exploration around 1 km depth. CSAMT is commonly used in combination with other geophysical methods to emphasize underground structures. Although it seems to be a good alternative, such a survey is expensive and requires more logistics which is not profitable for companies with low operating budgets. In addition, despite the combination of the CSAMT method with other geophysical methods, several boreholes drilled for groundwater exploration have failed due to the inaccurate locations to emphasize the fractures zones. Therefore, we developed the Python toolbox for CSAMT to solve this problem. The toolbox encompasses all the standard data processing as well as the modeling and implements a novel approach called pseudostratigraphic to forecast the strata log at each station. The advantage of this technique is twofold. First, it predicts the strata log at each station by demarcating the fracture zone, and secondly, it estimates the layer thicknesses with less margin error useful before the drilling operations. The approach was tested in the Xingning area, Hunan province, China. As a result, the model predicted with the proposed approach is similar to the mechanical borehole drilled after the geophysical survey with thickness errors of less than six meters. Furthermore, the technique would reduce the rate of unsuccessful drilling and be beneficial for geophysical and drilling companies to save money.

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.

Optimal Transceiver Distance of Controlled-source Audio-frequency Magnetotelluric Sounding Method

Currently, the use of Cagniard apparent resistivity and phase is still the main method and means to process and interpret controlled-source audio-frequency magnetotelluric (CSAMT) sounding data. The CSAMT data must meet the conditions in the far zone to use the magnetotelluric (MT) sounding inversion interpretation method. The conditions in the far zone are directly related to transceiver distance; Hence, the distance measured by the CSAMT method is directly related to the reliability and credibility of the data. In this study, based on the formation wave and ground wave of the analytical expression for the electric field in a uniform half-space condition, the response laws of the formation wave and the ground wave are analyzed. After rigorous mathematical derivation, the formula for determining the optimal transceiver distance of the CSAMT is obtained, and the steps for using this formula in practical application are given. The actual field application verifies the rationality of the derived formula. This study provides a reference for the design of a CSAMT field source, and effectively guides the determination of CSAMT transceiver distance in the field.

Research on Pseudo-2D Joint Inversion of TEM and CSAMT Based on Well Log Constraint

The transient electromagnetic method (TEM) and controlled-source audio-frequency magnetotellurics method (CSAMT) are commonly used in detecting water abundance of rock formation and faults in coal mines. However, these methods show low accuracy, given the multiplicity of their inversion results, especially for areas with minor differences in lithology and electrical property. To improve the accuracy of electromagnetic exploration, a pseudo-2D joint inversion is performed. The objective function of this pseudo-2D joint inversion is established, and the joint inversion process is constrained by resistivity logging data. Afterward, the symmetric successive over-relaxation (SSOR) is used to realize the pseudo-2D joint inversion calculation of TEM and CSAMT with well log constraint. The effectiveness of joint inversion is verified by combining synthetic and field data. Results show that the pseudo-2D joint inversion results of TEM and CSAMT with well log constraint correspond to the actual geological situation. Compared with either TEM or CSAMT, joint inversion has a significantly better capability of reflecting water abundance in rock formation and faults.

Characterization of Geological Structures under Thick Quaternary Formations with CSAMT Method in Taiyuan City, Northern China

A controlled-source audio-frequency magnetotelluric (CSAMT) survey was used to detect geological structures beneath the thick quaternary formation in Taiyuan, Shanxi Province, northern China. Two main CSAMT survey lines with 182 survey sites were recorded. A two-dimensional (2D) inversion technique was used to interpret the CSAMT data. The inversion results suggested that: 1) there are four main buried faults named F1, F2, F3, and F4 with dip angles about 65° across the survey line from west to east, the fault displacements of these faults are about 230 m, 180 m, 220 m and 200 m, respectively; 2) the depth of the bedrocks decrease from 1600 to 500 m along the survey lines; and 3) from top to bottom, there are four major layers in the survey area that include the upper layer with the resistivity less than 40 ohm-m represents unconsolidated sediments in the Quaternary formation, a second layer with the resistivity range from 40 to 120 ohm-m represents mudstone and sandstone, a third layer with the resistivity range from 120 to 280 ohm-m represents coal measure strata in the Permian and Carboniferous and a bottom layer with the resistivity higher than 280 Ω·m represents limestone in Ordovician. The CSAMT method is an effective technique for exploring buried fault of several hundred meters deep in metropolitan environment.

Application of the CSAMT Method to Pb–Zn Mineral Deposits: A Case Study in Jianshui, China

The electromagnetic (EM) method is commonly used in mineral exploration due to the method’s sensitivity to conductive targets. Controlled source audio-frequency magnetotellurics (CSAMT) is developed from magnetotelluric (MT) method with an artificial EM source to improve the signal amplitude. It has been used for mineral exploration for many years. In this study, we performed a case study of the CSAMT application for the Eagles-Nest lead–zinc (Pb–Zn) ore deposits in Jianshui, China. The Eagles-Nest deposit is located in southwest in China in forest-covered complex terrain, making it difficult to acquire the geophysical data. Based on the previous dual-frequency induced polarization (IP) results, we designed four profiles for the CSAMT data acquisition. After data processing and inversion, we mapped the subsurface resistivity distribution. From the CSAMT results, we inferred the location of the ore body, which was verified by the drilling wells. The Pb–Zn ore body was found at a depth between 373.70 m to 407.35 m in the well.

Linear source CSAMT response simulation in the 2D anisotropic formation with topography

Electrical anisotropy and topography are important factors affecting the distribution of electromagnetic fields. Therefore, it is necessary to study the response of controlled source electromagnetic fields to electrical anisotropic topography. In this study, a two-dimensional anisotropic geological model with topography was designed and the finite element method was used to calculate the electromagnetic field response of a Controlled Source Audio-frequency Magnetotelluric (CSAMT) line source. It was found that not only were the apparent resistivity and phase curves sensitive to anisotropic characteristics, but topography also exerted a significant influence on electromagnetic field distribution. Moreover, upper convex topography had a greater influence on electromagnetic response than lower concave topography. Therefore, topographic correction or the use of anisotropic models for data processing and interpretation should be considered in areas with extreme topography or prominent anisotropy. Results from the present study may help guide field applications of the CSAMT method and improve the theoretical and practical utility of the controlled source electromagnetic method.

Using the CSAMT method to predict deep mineralisation of copper and molybdenum: a case study of the Zhongxingtun area in Inner Mongolia, China

ABSTRACTThe Zhongxingtun area in Inner Mongolia, China, hosts an important copper polymetallic ore field. Surface copper and molybdenum–copper orebodies have been found, without deep drilling. A new round of prospecting is focused on finding deep orebodies. To identify these copper and molybdenum orebodies and the deep structure, the controlled-source audio frequency magnetotelluric (CSAMT) method was implemented in the Zhongxingtun area with a V8 multi-function electrical prospecting system using the continuous profile method with a network density of 100 m × 20 m in the western region and 50 m × 20 m in the east. These measurement densities are different from that used in most other areas, and a dense network is mainly utilised due to the need for mineral exploration. The borehole positions were determined by a comprehensive analysis of the two-dimensional imaging from the collected data, the geological features of the study area, and the physical characteristics of the bedrock. The drilling proved that molybdenum orebodies exist in the deeper regions where the mineralisation is characterised by the predominance of molybdenum followed by copper. The results show that through the analysis of the apparent resistivity obtained by the CSAMT method in the Zhongxingtun area, drilling hole positions are determined, and a deep mineralised body is discovered, showing that the CSAMT method is effective. This method is expected to be widely used in prospecting for copper polymetallic ore deposits.

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.

COMPARISON AND ANALYSIS OF THE EFFECTIVE OBSERVATION AREA OF THE TENSOR CSAMT BY NUMERICAL MODELING

AbstractAccording to the response formulas of the scalar CSAMT (controlled source audio‐frequency magnetotelluric) method, its programs can be designed in several manners with different sources such as a vertical double dipole source and a three‐dipole source of varied geometries. Through numerical simulation, this study obtains the value distributions of X‐ and Y‐direction components of the resultant electromagnetic (EM) field, which permit to further analyze the effective observational area. The results show that the distribution of crosslike source exhibits scattered weak responses, representing a smaller effective observational zone. Meanwhile, its requirement to the terrain condition of the working site is very harsh, thus cannot work well in mountainous areas with high topographic relief. Based on the principle of the rotating three‐dipole device, we propose a new device composed of multiple dipole sources for the CSAMT which can facilitate integrated measurement of tensors of the EM field. Compared with the cross‐like source, this approach can realize measurement throughout the whole area without weak‐value zones. Moreover, this new device is easy to deploy in various terrains. The results of this study provide a technical basis for further engineering production of tensor measurement.

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.

Discovery of a Major Coal Deposit in China with the Use of a Modified CSAMT Method

Conventional use of the controlled-source audio-frequency magneto-telluric (CSAMT) method is based on calculating the apparent or Cagniard resistivity from the amplitude ratio of the horizontal electric and magnetic field components. However, direct comparison between these two components shows that the electric field is more sensitive to the underground medium resistivity than its magnetic counterpart. Thus, use of the electric component only should provide adequate information about the electric properties of the subsurface. The measurements typically belong to the far-field zone, but show a non-dipolar characteristic because of the source. In this paper, we therefore propose a simplified CSAMT technique based on measuring the electric field component only. As part of this new formulation, a related theoretical model for the electric field component accounting for the non-dipolar nature of the transmitter antenna is introduced. This is accompanied with a new apparent resistivity definition, including a procedure to transform it into pseudo-phase data, thus removing the static shifts.The potential of this modified CSAMT method is demonstrated using a field case from the Shanxi province in China. Until recently, it has been thought that no coal deposits exist in this region. However, application of the single-component CSAMT technique as advocated for here, revealed a major coal deposit, which was verified later by drilling.

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.