Controlled-source Audio-frequency Magnetotelluric Data Research Articles

Mineralization based on CSAMT data: A case study on the Gaoloushan section of Yangshan gold belt

AbstractYangshan gold belt (YSGB) in Gansu Province is a super‐large orogenic gold deposit located in the middle section of the Mianxian‐Lueyang arcuate suture zone (Mian‐Lue suture zone). The YSGB is mainly divided into the Gejiaowan, Anba, and Gaoloushan mine sections. The ore veins are strictly controlled by strong strain zones in the fracture zone and secondary fractures superimposed on the inherited activity and are vein‐like in general and lenticular in localities. For a long time, there have been different understandings regarding its ore‐controlling factors and mineral genesis. By utilizing controlled‐source audio‐frequency magnetotellurics (CSAMT) technology to conduct electrical characteristic studies, the deep electrical structural characteristics of the Gaoloushan section are identified, and a tectonically‐controlled geoelectric model of the deposit is established, which provides geophysical evidence for the view that the Yangshan gold mine is a tectonically‐controlled low‐temperature hydrothermal deposit in altered rock, guiding the arrangement of exploration work and identifying prospective mining areas, and thereby providing a geophysical exploration paradigm for deep exploration of large orogenic gold deposits.

Deep Learning Based Noise Identification for CSAMT Data Processing

Controlled-source audio-frequency magnetotellurics (CSAMT) has been seriously affected by strong electromagnetic interferences including large-scale drift, durative outbreak interference, and impulsive outliers. To improve the efficiency of noise reduction, a deep learning strategy was proposed to identify the type of noise interference, so as to select the appropriate noise reduction method. First, a CSAMT time series simulation algorithm was developed based on current decomposition and one-dimensional (1D) forward modeling. Three kinds of noise interferences were also generated by simulation and randomly added to the pure signals. A total of 30000 groups of simulated noisy electromagnetic signals were generated. Together with 210 sets of practically measured data, these samples were used to train a long-short-term memory network (LSTM) noise classifier. Then, three targeted de-noising algorithms were adopted to separate the three interferences in the CSAMT time series according to the identifications. The test results by simulated data showed that the identification accuracy of LSTM for noise interferences can reach more than 95%. Finally, the noise identification and suppression methods were applied to a practical CSAMT dataset and the effect was further verified.

HYBRID PARTICLE SWARM OPTIMIZATION AND GREY WOLF OPTIMIZER ALGORITHM FOR CONTROLLED SOURCE AUDIO-FREQUENCY MAGNETOTELLURICS (CSAMT) ONE-DIMENSIONAL INVERSION MODELLING

The Controlled Source Audio-frequency Magnetotellurics (CSAMT) is a geophysical method utilizing artificial electromagnetic signal source to estimate subsurface resistivity structures. One-dimensional (1D) inversion modelling of CSAMT data is non-linear and the solution can be estimated by using global optimization algorithms. Particle Swarm Optimization (PSO) and Grey Wolf Optimizer (GWO) are well-known population-based algorithms having relatively simple mathematical formulation and implementation. Hybridization of PSO and GWO algorithms (called hybrid PSO-GWO) can improve the convergence capability to the global solution. This study applied the hybrid PSO-GWO algorithm for 1D CSAMT inversion modelling. Tests were conducted with synthetic CSAMT data associated with 3-layer, 4-layer and 5-layer earth models to determine the performance of the algorithm. The results show that the hybrid PSO-GWO algorithm has a good performance in obtaining the minimum misfit compared to the original PSO and GWO algorithms. The hybrid PSO-GWO algorithm was also applied to invert CSAMT field data for gold mineralization exploration in the Cibaliung area, Banten Province, Indonesia. The algorithm was able to reconstruct the resistivity model very well which is confirmed by the results from inversion of the data using standard 2D MT inversion software. The model also agrees well with the geological information of the study area.

Structural setting of the Zoujiashan-Julong’an region, Xiangshan volcanic basin, China, interpreted from modern CSAMT data

The Zoujiashan-Julong’an area is situated on the western side of the Xiangshan volcanic basin. The basin represents the central part of the Jiangxi Province and it is well endowed with uranium deposits. Ongoing brownfield exploration requires a better understanding of the deep-seated geological structures of the region so as to discover additional, but concealed, deposits at depth. Aiming at elucidating the deep structural and geological framework, 14 parallel controlled-source audio-frequency magnetotelluric (CSAMT) survey lines were conducted to document the relationships between lithological contacts, faults and uranium mineralization with a penetration depth of about 2 kms. By integrating the interpreted geological profiles with geological surface maps and drilling data, a 3D geological model was constructed by using GOCAD software. The 3D model illustrated the major lithological units, faults and concealed uranium mineralization, respectively. The obtained data show that: (1) The Ehuling Formation as well as the underlying metamorphic basement displays a medium-to-high resistivity, whereas the Daguding Formation has a low-resistivity. The unconformity surface between volcanic rocks and the metamorphic basement is represented by a distinct low-resistivity zone which is locally intersected by faults. The fault structure in the CSAMT inversed sections appears as a low resistivity zone or a high-low resistivity variation gradient zone cutting through different interfaces. (2) As is documented by the 3D geological model, the favorable sites for uranium mineralization are distributed in the depressions of the metamorphic rock basement, at the boundary of volcanic rock formations, and in the transitional zones between the depressions and uplifts, where the thickness of the strata increases (or decreases) and the composite faults exist. (3) The location and spatial distribution of the unexposed structures delineated by the 2D resistivity inversion results are consistent with the geological mapping results. The delineated interface between porphyritic lava and rhyodacite is basically consistent with the drilling data. The CSAMT detection results can provide reliable deep geological structure information for 3D geological mapping and deep metallogenic prediction in Xiangshan area.

Seismic structure-constrained inversion of CSAMT data for detecting karst caves

Karst cave is a sort of special and buried geological structure that was widely developed in the Permo-Carboniferous coal accumulation area of North China. It brings karst collapse and safety hazard in the mining industry. In this study, we propose a seismic structure-constrained inversion of controlled source audio-frequency magnetotelluric (CSAMT) data on a detailed survey and detection of karst caves. Instead of constrained by seismic impedance, the method in this study directly takes the seismic imaging results as structural constraints, which is different from the cross-gradient technique used by conventional structural constraints. First, the seismic migration section is divided according to the CSAMT inversion grid and applied pixel extraction for each grid. Clustering is carried out according to the structural information interpreted by the seismic migration section and the average pixel value of each cluster is calculated. Then the clustered results were used in the seismic structure-constrained inversion of CSAMT data based on cross-gradient technique. After that, as a karst cave model developed in limestone was established, the study compares the structure-constrained inversions with different clustering strategies shows a much more precision of karst cave detection than the method only applies CSAMT data. Moreover, experimental verification is provided in this study, which is for the detection of a suspected karst collapse column from Shandong Province, China. The comparison results further show that the structure-constrained inversion method proposed in this paper is applicable and may effectively improve the locating accuracy of karst caves.

Imaging acidic contaminants in a confined aquifer using electromagnetic geophysical method constrained by hydrochemical data

Acid in-situ uranium leach mining is an effective, popular technology for extracting uranium without large uranium mill tailings. However, acid in-situ leaching causes extremely high concentrations of acidic contaminants in aquifers. To represent the migration and range of acidic contaminant pollutants in confined aquifers, this study applied the controlled source audio-frequency magnetotellurics (CSAMT) method to a uranium mining area located in the Yili Basin, Xinjiang, northwest China. We collected ten CSAMT profiles from 364 sites with an average site interval of 20 m. The electrical resistivity model of the uranium mining area was obtained via two-dimensional inversion using CSAMT data, with 23 frequencies, ranging from 8192 Hz to 4 Hz. Combined with geological, geochemical, and well log data, this resistivity model was successfully used to identify the scale and position of acid contaminant plumes beneath the uranium mining area, which appear as electrically well-defined features that closely coincide with the distribution of sulfate and uranium ions in groundwater. With uranium mining activities ceasing, the scale of acid contaminant plumes has been reduced, owing to the natural attenuation of the aquifer or the contaminants well confined in the mining site. The low-permeability sandstone surrounding the aquifer was imaged as a resistive feature, which confines the contaminant plumes to the uranium mining area and slows the downstream migration of contaminants. According to the modified Archie's law, the porosity of the sandstone bearing uranium beneath the injection well zone increases, owing to acidification, while the porosity of the sandstone beneath the production well zone decreases, owing to pore plugging.

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.

Case study: joint seismic reflection and CSAMT data interpretation for mineral explorations in Fujian, China

The Southwestern area of Fujian Province in China contains a major metallogenic belt. There have been three major tectonic movements since the Late Paleozoic, and the thrusts here are the main mineral deposits. These occurred between the Early Carboniferous and Late Permian eras. Due to dramatic undulations in the surface and associated complex underground structures, a single geophysical method cannot provide reliable imaging results. This is largely due to difficulties in data acquisition and processing. Seismic exploration provides one example. Using this method, raw shot gathers have a low signal-to-noise ratio because of the undulations in the topography. Furthermore, strong lateral velocity variation makes the migration process exceedingly difficult. This means that interpretations of the resulting reflection seismic profile give rise to uncertainty. For this reason, two-dimensional reflection seismology and controlled source audio-frequency magnetotellurics (CSAMT) electromagnetic sounding techniques were performed. After initially completing conventional processing on the seismic reflection data, subsequent pre-stack depth migration (PSDM) over the rugged topography yielded a much better image. It was able to identify thrust faults and magma intrusions. Then, using CSAMT, inversion was conducted with the same topography as the seismic reflection. By combining the migrated seismic profile, inverted resistivity profile, and borehole data, a vertical geological model was constructed. This proved the existence of an overlapped thrust, and it also indicated the possible presence of deeper mineralization.

Three-dimensional inversion of controlled-source audio-frequency magnetotelluric data based on unstructured finite-element method

We propose a new 3D inversion scheme to invert the near- and transition-zone data of CSAMT with topography accurately. In this new method, the earth was discretized into unstructured tetrahedra to fit the ragged topography and the vector finite-element method was adopted to obtain precise responses and good sensitivity. To simulate the attitude and shape of the transmitter, we divided a long-grounded transmitter into dipoles and integrated these dipoles to obtain good responses in the near- and transition-field zones. Next, we designed an L2 norm-based objective functional and applied a standard quasi-Newton method as the optimization method to solve the inverse problem and guarantee steady convergence. We tested our 3D inversion method first on synthetic data and then on a field dataset acquired from select sites near Changbai Mountain, China. In both tests, the new inversion algorithm achieved excellent fitting between the predicted and observed data, even in near- and transition-field zones, and the inversion results agreed well with the true model. These findings reveal that the proposed algorithm is effective for 3D inversion of CSAMT data.

An application of CSAMT for detecting weak geological structures near the deeply buried long tunnel of the Shijiazhuang-Taiyuan passenger railway line in the Taihang Mountains

The Shijiazhuang-Taiyuan (ShiTai) high-speed railway line is designed to cross the Taihang Mountains. The line will include the construction of a 27.85 km long tunnel with a maximum depth of 450 m. The Taihang Mountain Tunnel is the key to the successful construction of the railway. The geological structure of the tunnel is highly complex and the lithology changes dramatically throughout. Fault zones, karst, and groundwater ingress are the main geological engineering problems for the tunnel construction. However, conventional geophysical methods are not effective for detecting these geological structures in a region with such a complex terrain. Therefore, the controlled source audio-frequency magnetotellurics (CSAMT) method was used to effectively characterize the resistivity distribution of the underground rock. This technique is highly sensitive to low resistivity geological structures, such as fault zones, and it is effective for studying the geological engineering conditions of the tunnel. Various corrections, designed denoising, and inversion methods are used in CSAMT data processing to obtain the resistivity profile of the entire tunnel. Based on the resistivity results and existing core information, the underground lithological distribution is inferred. The location of the fault zone is determined and the distribution of dangerous geological bodies is delineated. The interpretation of the CSAMT results is verified by boreholes and related geological information. These results are important for understanding the subsurface geological engineering conditions and have provided stability, safety, and cost evaluation support for the construction of a long deeply buried tunnel in Taihang Mountains.

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.

Metallogenic model of the Wulong gold district, China, and associated assessment of exploration criteria based on multi-scale geoscience datasets

The combined use of geological and geophysical modeling is effective for delineating subsurface exploration criteria and conducting quantitative analyses to determine potential mineral targets. In this paper, total horizontal derivative, multi-scale edge detection (worms), and bi-dimensional empirical mode decomposition (BEMD) of district-scale gravity and magnetic data were applied to extract geophysical information reflecting deep-seated structures and concealed intrusions. In addition, deposit-scale 3D modeling and controlled-source audio-frequency magnetotelluric data were used to identify potential ore-controlling structures. The results of our integrated multi-scale geological and geophysical studies suggest that the Early Cretaceous intrusions and gold deposits in the Wulong gold district, which is located on the Liaodong Peninsula, China, at the NE margin of the North China Craton (NCC), are spatially and genetically associated with a network of NE- and NW- striking faults. Gravity and magnetic modeling effectively identified buried Early Cretaceous intrusions and NW- striking faults at depths from zero to five kilometers. In summary, the combined use of multiple-source and multi-scale geophysical information and multi-scale 3D geological modeling facilitated delineation of the metallogenic model and provided exploration criteria for determining potential exploration targets in the study area.

Global optimization of controlled source audio-frequency magnetotelluric data with an improved artificial bee colony algorithm

The inversion of controlled source audio-frequency magnetotelluric (CSAMT) data is a complex nonlinear problem. A linearization of this problem is easily trapped in local minima and the complexity of the artificial source makes CSAMT data interpretation more difficult than that of magnetotelluric (MT) data. This paper presents an improved artificial bee colony (ABC) algorithm for the 2.5D inversion of CSAMT data. New initialization and generation strategies are proposed to improve the optimization achieved by the original ABC algorithm. The global optimization of CSAMT by the improved ABC algorithm is realized based on 2.5D forward modeling theory and is used in the inversion of a complex model of water-bearing anomalous bodies in sandstone. Results show that the algorithm can accurately recover the resistivity and spatial distribution of strata and anomalous bodies. The survey data for a suspected collapse column in Shandong Province are also processed using the proposed method, and inversion via the algorithm accurately shows the water abundance of the suspected collapse column. Thus, the results of the theoretical modeling and practical data indicate that the improved ABC algorithm is effective for analyzing CSAMT data. Moreover, this algorithm improves the interpretational accuracy and resolution of CSAMT data.

Adaptation of Zohdy’s Method for Controlled-Source Audio-frequency Magnetotelluric (CSAMT) Data Interpretation with Layered Model

Surveys with Controlled-Source Audio-frequency Magnetotellurics (CSAMT) are often performed in scalar mode for practical considerations. In such case, CSAMT data are obtained from electric and magnetic fields oriented along and perpendicular to the traverse line, respectively. With closely spaced sounding stations along a profile, layered or 1D model can be considered as sufficeint to obtain an overall view of the subsurface resistivity distribution in the study area. Zohdy’s method that was used to infer resistivity variation with depth from Vertical Electrical Sounding (VES) data is extended for CSAMT data interpretation. An initial model is determined from apparent resistivity sounding data using skin depth principle or its alternative, e.g. Bostick transform. With the number of layers equals to the number of data (in frequency or period), the discrepancy of calculated from observed data can be used to modify iteratively the layer parameters (resistivity and thickness). Preliminary tests using synthetic data associated with simple synthetic models showed satisfactory results. On-going study is still underway to assess smooth resistivity variation with depth and to correlate models from stations along a profile to obtain a quasi-2D resistivity model. Application for CSAMT data from a geothermal prospect is also sought.

Subsurface structure of Tangkuban Parahu area derived from CSAMT and gravity investigation

An integrated Controlled-source audio frequency magnetotelluric (CSAMT) and gravity investigations across the Tangkuban Parahu area has been carried out. The Tangkuban Parahu is located in Bandung, West Java, one of a volcano area. The purpose of the investigations was to locate the subsurface structures of the area that may correlate to a geothermal reservoir in unknown geothermal potential area. The CSAMT survey has been conducted in the area using transmitter locatedin 3-5 km from the survey area. The objective of the survey was to determine a subsurface resistivity structures. On the other hand, the gravity survey was carried out in the same sites with the CSAMT sites. The results of the integrated interpretation of the CSAMT and gravity data show that the subsurface of the area has a correlation with a geothermal reservoir structure. It is also confirmed by several surface manifestations in the Ciater area, the north of the Tangkuban Parahu volcano.

CSAMT Static Shift Recognition and Correction Using Radon Transformation

The presence of shallow conductive heterogeneities can cause static shift in controlled source audio-frequency magnetotellurics (CSAMT) apparent resistivity sounding curves. This is observed as a shift along the apparent resistivity axis in double logarithmic coordinates. Such effect can cause difficulties in CSAMT data interpretation. In this letter, we established a new method to identify and correct the CSAMT static shift based on the high-resolution Radon transformation. We took advantage of the property that the static shift of apparent resistivity curves behaves as a point in the Radon domain. We presented 3-D synthetic study to demonstrate that the static shift can be effectively removed from the apparent resistivity curve. However, due to the low resolution and precision, the traditional Radon transform can generate “scissor-tail” and then, the static shift may not be completely removed. We proposed to use a new high-resolution Radon transform by improving the regularization matrix using the least squares inversion. Our numerical simulation shows that by using this high-resolution Radon transform, the static shift converges accurately to a point without showing the “scissor-tail.” In the application to field CSAMT data, the high-resolution Radon transform method was able to correct the static shift effectively; thus, it can improve the precision and accuracy in data interpretation.

Three-dimensional inversion of CSAMT data in the presence of topography

3D controlled-source audio frequency magnetotelluric (CSAMT) responses can be distorted strongly by topography and should be accounted for in data inversion and interpretation. In this paper we present a scheme to incorporate topographic distortions into the inversion instead of correcting them. This approach has been verified by comparing the modelling results with 2D FEM CSAMT solutions and synthetic inversion examples. Compared with the responses generated from a half-space model with flat surface, it is found that not only the topography in the survey area but also that at the source position may strongly distort the CSAMT responses. The field example indicates that results with topography are much better than those without considering topography to map the distribution of coal seam underground, which also illustrates the effectiveness of our approach.

The effects of 3D topography on controlled-source audio-frequency magnetotelluric responses

We have investigated the 3D topographic effects on controlled-source audio-frequency magnetotelluric data. Two 3D topographic models are considered: a trapezoidal-hill model and a trapezoidal-valley model. Different responses are generated, including the amplitude of the electric field, the amplitude of the magnetic field, the apparent resistivity, and phase data. The responses distorted by the 3D topography are simulated for the source located next to and on the hill/valley. Our study indicates that all electric field, magnetic field, apparent resistivity, and phase data are influenced by 3D topography, but to different extents. These topographic effects depend on the transmission-receiver-topography geometry, the transmission frequency, earth resistivity, and the roughness of the surface. The effects in the near-field generated by topography in the survey area are quite different from those in the far-field because of the existence of the source. Compared with those in the far-field zone, the magnetic field and phase data in the near-field zone are less distorted, but more distortions can be found on the electric field and apparent resistivity data over the hill and valley models. Our results also indicate that not only can the 3D topography in the receiver area lead to strong distortions, but also that at the source position can lead to strong distortions. We concluded our study by quantifying the roughness with which the topographic distortion can be ignored, setting the accepted data distortion to a maximum of 10%.

Divergence correction schemes in finite difference method for 3D tensor CSAMT in axial anisotropic media

Resistivity anisotropy and full-tensor controlled-source audio-frequency magnetotellurics (CSAMT) have gradually become hot research topics. However, much of the current anisotropy research for tensor CSAMT only focuses on the one-dimensional (1D) solution. As the subsurface is rarely 1D, it is necessary to study three-dimensional (3D) model response. The staggered-grid finite difference method is an effective simulation method for 3D electromagnetic forward modelling. Previous studies have suggested using the divergence correction to constrain the iterative process when using a staggered-grid finite difference model so as to accelerate the 3D forward speed and enhance the computational accuracy. However, the traditional divergence correction method was developed assuming an isotropic medium. This paper improves the traditional isotropic divergence correction method and derivation process to meet the tensor CSAMT requirements for anisotropy using the volume integral of the divergence equation. This method is more intuitive, enabling a simple derivation of a discrete equation and then calculation of coefficients related to the anisotropic divergence correction equation. We validate the result of our 3D computational results by comparing them to the results computed using an anisotropic, controlled-source 2.5D program. The 3D resistivity anisotropy model allows us to evaluate the consequences of using the divergence correction at different frequencies and for two orthogonal finite length sources. Our results show that the divergence correction plays an important role in 3D tensor CSAMT resistivity anisotropy research and offers a solid foundation for inversion of CSAMT data collected over an anisotropic body.