Controlled Source Audio-frequency Magnetotellurics Research Articles

Distributed wide-field electromagnetic method for coal mining goaf detection in a complex urban environment: A case study in Jinan, China

Urban expansion has resulted in many proposed projects located over coal mining zones, which has highlighted the importance of detecting the spatial scope and water abundance of goafs before the commencement of construction work. Although electromagnetic (EM) methods have proven effective for goaf detection, their applications in intensely noisy and urbanized environments remain limited. To address this challenge, we evaluate an investigation of a coal mining goaf in Jinan, China, using the distributed wide-field EM (DWFEM) method. A third-order [Formula: see text] sequence pseudorandom signal with 39 survey frequencies is transmitted to achieve long-time data acquisition at each station. Unlike the controlled-source audio-frequency magnetotellurics method, the DWFEM records only the electric field [Formula: see text] component. The synthetic model tests and field data demonstrate the consistency of the [Formula: see text] apparent resistivity and the Cagniard resistivity in the far field. The long-time acquisition and 1C recording greatly improve data quality and exploration efficiency. We also use an all-angle resistivity calculation formula and an electrode layout method parallel to the wire source to obtain electrical connections in different directions. The DWFEM inversion results are obtained using the 1D Gauss-Newton iterative method under a plane-wave assumption. By interpolating the data from different measurement stations, we image resistivity depth profiles and obtain 3D subsurface electrical data for the subsurface from 0 to 1000 m. We interpret the obtained profiles with geologic and mining information, revealing two significant water-enriched goaf areas. Validation is performed using seismic data and drill cores. The results significantly enhance our understanding of the characteristics of the coal mine under our project and highlight the applicability of the DWFEM for detecting goafs in complex urban environments.

3D Inversion of the Data-normalized Time-lapse Controlled Source Audio-frequency Magnetotellurics Method

3D Inversion of the Data-normalized Time-lapse Controlled Source Audio-frequency Magnetotellurics Method

A novel approach to address source overprint and shadow effects in controlled-source audio-frequency magnetotelluric exploration

Source overprint and shadow effects can significantly affect controlled-source audio-frequency magnetotelluric (CSAMT) data and mask subsurface geologic structures; however, few solutions have been presented. Based on the traditional observation approach, we develop a new approach to solve this problem. An additional transmitter and measurement zone are introduced in the novel approach to constrain the anomalous body causing source overprint or shadow effects in the inversion. To test this approach, six experimental models are designed. Three-dimensional CSAMT responses on the six synthetic models have been simulated, and the results indicate that source overprint and shadow effects have a strong influence on the data. Strong source overprint and shadow effects are found at low frequencies. The distribution of the source overprint or shadow distortions is affected by the shape of the anomalies below the transmitter or between the transmitter and receiver. We evaluate our approach by comparing the inversion results obtained with the new and traditional approaches based on the six synthetic data sets. The results indicate that the new approach not only effectively resolves the anomalies causing source overprint or shadow distortions but also yields more accurate target results in terms of resistivity value, structure geometry, and position than the conventional scheme. The inversion of field data from Yanqing, China, further verifies the effectiveness of our approach. Compared with the traditional inversion, the new scheme inversion obtains results that are more consistent with the rock type revealed by a drillhole.

Evaluation of rock mass units using a non-invasive geophysical approach

Thorough and accurate assessment of rock mass units is important for development of engineering infrastructures and groundwater resources assessments. Rock mass units are widely evaluated by reliable geomechanical parameters namely rock quality designation (RQD) and rock core index (RCI). Conventionally, these parameters are acquired via an extensive number of geotechnical tests. Such tests, however, suffer efficiency for data coverage, cost, equipment and topographic constrictions, and hence cause ambiguity in geological models for a detailed evaluation of rock mass integrity. Conversely, geophysical surveys offer fast, more user-friendly, less invasive, more cost-effective and less time-consuming approach for geological investigations. The past research confirms a useful link between geophysical and geotechnical parameters. But, none of the past studies provides a suitable and generalized relation between these parameters which can reduce geotechnical model uncertainty mostly caused by inadequate data and subsurface heterogeneity. This paper proposes a meaningful and feasible method to obtain geomechanical parameters using a certain number of drillings and geophysical data of four different sites. Based on electrical resistivity obtained from electrical resistivity tomography (ERT) and controlled-source audio-frequency magneto telluric (CSAMT), this research provides the general and adaptable formulas for geotechnical parameter estimation and reduces geological model uncertainty for more detailed 2D/3D imaging of RQD and RCI covering the whole sites where even no drilling data exists. Thus, the investigated sites are assessed laterally and vertically along each geophysical profile via distinct value ranges of geological parameters for a thorough and reliable evaluation of rock mass units in highly heterogeneous setting. Our research reduces the ambiguity caused by structural heterogeneities and scarce data, fills the gap between inadequate well tests and the true geological models, and gives new insights into the rock mass units for proper engineering design and groundwater exploitation.

Effect of Karst Geomorphology on the Sedimentary Mineralization and Geochemical Distribution of Bauxite: An Example from the Xiaoyuan Area in Qingzhen, Guizhou Province

Bauxite, in central Guizhou, is predominantly karst bauxite, but there is insufficient research on the effect of karst paleogeomorphology on bauxite development. Xiaoyuan bauxite is also a karst bauxite, and high- and low-iron bauxite deposits exist in the study area. This study conducts geological modeling of karst bauxite using controlled-source audio-frequency magnetotelluric (CSAMT) data and drill core data. The effects of karst paleogeomorphology on bauxite deposition and mineralization are evaluated by assessing karst paleogeomorphology, conducting a mineralogical analysis of drill cores at different locations, and determining the geochemical distribution characteristics of the elements in the horizontal and vertical directions. Combined with previous research results, we propose two metallogenic processes of high-iron and low-iron bauxite. The findings are significant for understanding the mechanism of bauxite formation.

Integrated application of gravity, aeromagnetic, and electromagnetic methods in exploring the Ganzi geothermal field, Sichuan Province, China

The Ganzi geothermal field is located in the Songpan-Ganzi orogenic belt in Sichuan Province. Many hot springs are exposed along the Yalahe valley in Ganzi geothermal field, which is a favorable area for high-temperature geothermal resource exploration. However, the geological model of heat exchange, the regional structure controlling hydrothermal convection and the development model of geothermal reservoirs are still unclear. Therefore, further studies are necessary to meet the geothermal exploration requirements in the middle and deep strata of this geothermal field. In this study, a geological model of the geothermal system of Ganzi geothermal field is proposed. We are convinced that there exists a hydrothermal convection system in the Ganzi geothermal field, the heat transfer of which is accomplished through deep-rooted major faults. Therefore, the identification of deep-rooted major faults and the description of geothermal reservoirs are the research objects of the integrated geophysical methods. The main factors controlling the geothermal reservoirs in the deep-rooted Xianshuihe major fault and Yalahe fault zones are analyzed by using gravity, aeromagnetic, and electromagnetic methods and techniques. The analysis results of regional gravity and aeromagnetic anomalies show that the Xianshuihe major fault has produced obvious gravity and aeromagnetic anomalies on the surface, and thus the position and strike of this fault can be accurately predicted by inversion of the aeromagnetic anomalies. Geothermal reservoirs show low-resistivity anomalies in the electromagnetic profile. The inversion results of the controlled source audio-frequency magnetotelluric (CSAMT) data show that geothermal reservoirs are mainly developed along the Yalahe valley, and the west side of the valley is more favorable for geothermal exploration. This study is of guiding significance to the efficient exploitation and utilization of the Ganzi geothermal field.

Anatomy of the convective geothermal system from geophysical and hydrochemical data: A case study from the Changshou geothermal field, South China

There are abundant low-medium temperature geothermal resources in southern China; however, a lack of research into their properties restricts the utilization of the geothermal energy. To expand the geothermal energy potential in southern China, it is vital to clarify the nonvolcanic geothermal system mechanisms. This study aimed to elucidate the mechanisms for the Changshou geothermal field, as a representative structurally controlled convective geothermal system, by analyzing the geophysical and hydrochemical data. The surface-to-subsurface (400 m depth) electrical resistivity structure of the geothermal field was obtained by 2D inversion of two intersecting controlled-source audio-frequency magnetotellurics and electrical resistivity tomography geophysical profiles. We identified the cap rock, reservoir, and fluid pathways of the geothermal system and verified them via drilling results. The cap was composed of Quaternary sediments and Cretaceous sandstone with moderate resistivity (10–50 Ωm), and the geothermal reservoir and fluid pathways were composed of dolomites and fracture zones with low resistivity (less than 5 Ωm), respectively. The hydrochemical results showed that the geothermal water was of the type HCO3·SO4−Ca·Mg, with low values of total dissolved solids resulting from shallow groundwater mixing. Isotope analyses indicated that the geothermal water was derived from precipitation in the Dahong Mountains, southeast of the geothermal field. The average temperature of the geothermal reservoir, as estimated by chalcedony and K-Mg geothermometers, was 53°C. Based on the geothermal gradient calculation, the circulation depth of the groundwater was 1500 m, with the circulation process driven by heat flows originating deep within the earth. The Changshou geothermal field was identified as a potential source of energy for residential heating in winter, owing to the large flow (2700 m3/d) and high permeability (24.03 m/d) of the geothermal reservoir.

Exploration of geothermal resources using comprehensive electromagnetic method

Geothermal resource is a kind of crucial clean, renewable energy with high potential development and application. However, it involves great exploiting risks since it buried deep beneath the crust of the Earth. The location and depth of geothermal resources can be determined using the geophysical methods. Controlled source audio-frequency magnetotellurics (CSAMT) and transient electromagnetic method (TEM) have relatively high efficiency and robustness. In this paper, a simplified geothermal system resistivity model was established, which allowed TEM and CSAMT to differentiate target faults with various characteristics (resistivity, dip angle and width). Through a comprehensive electromagnetic exploration in Huairen County, Shanxi Province, China as an example, the performance of TEM and CSAMT in determining geothermal resources was evaluated. The results showed that both methods could examine the geoelectrical structures from various perspectives, and the effectiveness of comprehensive electromagnetic method for geothermal resources was presented.

Comprehensive Evaluation of Goaf Range in a Coal Mine with a Complex Terrain through CSAMT and an Activated-Carbon Method for Radon Measurement

Underground goaves were left in many mining areas due to the continuous exploitation of coal resources. These mining areas seriously affect the production safety of the mines and the safety of life and property of the surrounding residents. Enormous safety hazards will be generated if the goaf range is not accurately controlled. In this study, we proposed a method for the detection of goaves in coal mines with a complex terrain by combining controlled source audio-frequency magnetotellurics (CSAMT) and an activated-carbon method for radon measurement. The disadvantage of failing to interpret goaf depth for the activated-carbon method for radon measurement was compensated by the advantage of the capability of goaf-depth sounding for CSAMT. Subsequently, the reference for CSAMT data was provided by the immunity of the activated-carbon method for radon measurement to the influences of terrain, earth electricity, and EMF. On this basis, the proposed method was employed to detect the goaf of Houjiagou Coal Mine in Liulin County, China, and obtained reliable detection results. The feasibility of the comprehensive geophysical prospecting method in the complex terrain was verified and it provides a new reference for the detection method of goaves with other conditions.

Prospecting for ophiolite-type chromite deposit in Sartohay, West Junggar (NW China): Constraints from geological and geophysical data

Chromite is a critical mineral that is of great significance for the development of metallurgical, chemical, and other industries. In order to study the deep ore prospecting methods of ophiolite-type chromitite, we have carried out plane-profile geological and mineral surveys, as well as high-precision gravity, magnetic, and controlled source audio-frequency magnetotelluric (CSAMT) measurements in the Sartohay, West Junggar. Based on the analysis, interpretation, and inference of multi-source geological information, a three-dimensional (3D) geological model of the typical mining area in Sartohay District was established. The results show that Sartohay ophiolite was occurred in the form of rootless tectonic blocks and generally displayed deformation and metamorphism (e.g., oblique thrust, transverse extrusion, and lateral strike slip). Sartohay ophiolite is in tectonic contact with surrounding strata. Several northwest dipping high-angle thrust faults are developed at the contact sites and inside the ophiolite rock mass, showing a thrust imbricated structure. Meanwhile, ophiolite in the modeling area is a NW-trending monoclinic rock mass. The base surface of the ophiolite is obliquely distributed in an “N-like” shape, and the chromitite bearing lithofacies zones also have a NW-trending distribution. Geophysical data reveal that the studied mining area is characterized by a layered electrical structure in the vertical direction. The upper layer displays a high-resistivity agglomerate and a low-resistivity band distributed alternately, while the lower layer is a complete high-resistivity block. Combined with the deposit characteristics in Sartohay, it is considered that the morphological occurrence, geological structure, lithology, and lithofacies of ophiolite control the spatial distribution of related chromitites. The dunite-harzburgite lithofacies belts, highly strain zones, and strong tectonic alteration belts of the ophiolite rock mass are significant sites for the chromite ore bodies/groups occurrence. This study provides approaches for researching on the production laws of ophiolite-type chromitite and supplys a prospecting pattern for deep concealed orebodies.

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.

An Asymmetric Beamforming Method Based on Arithmetic Phase Difference Weighting in CSAMT

Traditional controlled-source audio-frequency magnetotellurics (CSAMT) radiates symmetric beams using a grounded symmetric dipole (GSD). Only a tiny fraction of radiant energy is taken advantage of during the far-field (Ff) observation due to the low directivity of the GSD. In order to enhance the signal-to-noise ratio (SNR) during the Ff observation, it is necessary to reduce the transceiving distance (TD) or increase the transmitting power (TP), but both methods will cause many problems. Further, when using the tensor method for observation, GSDs in two vertical directions will be employed to radiate energy, and then a series of problems will occur such as an asymmetry of the SNR in two vertical directions if the geological conditions under the two GSDs vary widely. An arithmetic phase difference (APd) weighting asymmetric beamforming method (ABFM) in CSAMT is proposed in this paper, which uses a GSD array instead of a single GSD, and a signal with APd is transmitted to control the wavefront for beam steering. A significant enhancement (about 3 dB) of the SNR will occur by collecting the radiant energy in the region of concern (RoC) using ABFM. The analysis and simulation results demonstrate that under the premise of the same TD and TP, the ABFM has obvious advantages in improving energy utilization in CSAMT. In other words, the APd-weighted ABFM can deal with a complex noise environment in the field better than the traditional method.

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.

Comparative study of a novel arithmetic amplitude-phase weighted beamforming method in vector and tensor CSAMT

Abstract Vector controlled-source audio-frequency magnetotellurics (CSAMT) emits electromagnetic waves with a low directivity coefficient using a grounded horizontal electric dipole (HED). During observations in the far-field region (FfR), only a fraction of radiation energy is used. To improve the signal-to-noise ratio (SNR) in the FfR, it is necessary to increase the power of the transmitter or shorten the transceiver distance, but it will restrict the development of vector CSAMT. Comparatively, the tensor CSAMT uses HEDs in two directions to measure geological bodies in every direction. If the geological conditions of two HEDs are vastly different, it is easy to cause issues, such as an imbalance of the SNR in two directions of the receiving point. This paper presents a comparative study of a novel arithmetic amplitude-phase (AAP) weighted beamforming method (BFM) in vector and tensor CSAMT, which uses multiple HEDs to transmit signals with controlled amplitude ratio and phase difference to modulate the wavefront, thereby achieving beam steering and other additional functions (such as amplitude compensation). By concentrating the radiation energy in the area of interest (AoI) using BFM, the SNR can be largely regulated. The comparative simulation and analysis demonstrate that the BFM has advantages in improving energy utilization and beam steering in vector and tensor CSAMT. On the premise of same power and transceiver distance, using the AAP-weighted BFM, the SNR received in the AoI can meet the requirements better than that received via the traditional method.

Deep exploration of Jiaodong type gold deposit, taking Shanhou gold deposit, southern part of Zhaoping fault as an example

Crustal response induced by the subduction of Paleo-Pacific Plate, is crucial to study the metallogenic law and prediction of Jiaodong type gold deposit. Gold deposits are preferentially hosted in the lithospheric-scale second-order faults, which provide migration channels, mineralization corridors, and the physicochemical conditions. Super-large gold deposits in northwestern Jiaobei Terrane are generally controlled by regional linear faults, such as Sansandao, Jiaojia, Zhaoping and Qixia faults. However, Zhaoping fault is exposed sporadically (in Xiadian and Jiangjiayao gold deposits) beneath a thick cover of Quaternary basalts, which indicating that limited information can be obtained by traditional geological survey. In this study, high-precision geophysical surveys such as gravity and controlled source audio frequency magnetotellurics (CSAMT) are conducted in the Shanhou gold deposit, which hosted in southern part of Zhaoping fault. Three integrated geophysical profiles results further identified the spatial distribution of second-order oblique-slip faults of Zhaoping fault and established the relationships between brittle deformation and mineralization. The spatial variability of stratum (high density 2.82 g/cm3 and intermediate to high specific resistivity 4000 Ω m), brittle-slip faults (intermediate density 2.73 g/cm3 and low specific resistivity 200 Ω m) and magmatite (low density 2.57 g/cm3 high specific resistivity 8000 Ω m) are identified by two inversion techniques, including the man-machine interactive inversion technology and nonlinear conjugate gradient technology. In addition, the geophysical inversion pseudo sections delicately portray a smooth-out waveform low specific resistivity anomaly, which strongly couples with the spatial ore-hosted locations and enrichment places. The gold mineralized alteration zones generally dips to southeast and the angle changing from 45° to 30°, which are strong coupling with the large-scale acid magmatic intrusion and extensional tectonics occurred in the late Mesozoic. Finally, according to the deep prospecting prediction, a gold ore body with 10 m in thickness was revealed by drilling below the depth of 1,015 m, which is hosting in the brittle deformation of Zhaoping fracture zone.

A novel phased-array transmitting source in controlled-source audio-frequency magnetotellurics

Abstract In traditional controlled-source audio-frequency magnetotellurics (CSAMT), the orthogonal magnetic and electric field components in the far-field region are recorded to calculate the apparent resistivity. However, as the distance between the receiver and the transmitter increases, the electromagnetic signal weakens considerably. Moreover, as the location of the transmitter is fixed, the location of the far-field region also is fixed, which limits the exploration range. In this study, a novel phased-array transmitting source was proposed to overcome this limitation. In traditional CSAMT, a grounded single-dipole long-line source is used to transmit the electromagnetic wave. This source has a low gain and no directionality, resulting in a low signal-to-noise ratio (SNR) in the area of interest (AoI). Conversely, by using a phased-array source, beam control and stable directional radiation will be achieved so that the electromagnetic energy should be capable of effectively focusing within the AoI. Without increasing the transmitting power, the SNR should be conveniently enhanced and the size of the AoI can be greatly increased. In this paper, the characteristics of the radiation field of a single-dipole long-line source and its application in traditional CSAMT are analyzed. The design principle and the method of implementation of a novel phased-array source are provided. The feasibility and effectiveness of the proposed source are verified by simulations and experiments, and a prototype is realized.

The Effects of Magnetic Susceptibility on Controlled-Source Audio-Frequency Magnetotellurics

The Effects of Magnetic Susceptibility on Controlled-Source Audio-Frequency Magnetotellurics

Application of controlled-source audio-frequency magnetotellurics (CSAMT) for goaf detection: a case study in the Fangzi coal mine China

Application of controlled-source audio-frequency magnetotellurics (CSAMT) for goaf detection: a case study in the Fangzi coal mine China

Inversion of the Temperature and Depth of Geothermal Reservoirs Using Controlled Source Audio Frequency Magnetotellurics and Hydrogeochemical Method

The detection of the depth and temperature of deep geothermal reservoirs suffers great uncertainty if it is completed using the simple combination of traditional geophysical and geochemical methods. Given this, this study investigated the combined utilization of multiple methods including the multi-information joint detection using the controlled source audio frequency magnetotellurics (CSAMT) method and geothermometers in the Qutan geothermal field. First, the reservoir temperature and the depth of geothermal water circulation in the geothermal field were estimated using the geochemical method. Afterward, the relationship between the CSAMT resistivity and temperature of strata was explored through borehole temperature measurement. Based on this, the depth and temperature of geothermal reservoirs in the geothermal field were estimated again. According to the comprehensive analysis and comparison of the thermal reservoir temperatures and depths obtained using the two methods, the shallow reservoirs in the Qutan geothermal field have a top burial depth of approximately 423 m, a thickness of about 300 m, a temperature of about 79°C, and a fluid circulation depth of about 1959 m.

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.