Audio-frequency Magnetotellurics Research Articles

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.

Audio Magnetotellurics Study of the Geoelectric Structure across the Zhugongtang Giant Lead–Zinc Deposit, NW Guizhou Province, China

Non-invasive geophysical exploration methods a play key role in the exploration of ore deposits. In the present study, the audio-frequency magnetotelluric (AMT) method was applied to metallic mineral exploration. The metallic mineral deposit targeted was the recently discovered super large lead–zinc deposit of the Zhugongtang mining area of Hezhang County in the northwestern Guizhou province in China. The main objectives of this study were to estimate the geoelectric strike and generate geoelectric models that estimate both the depth and distribution of resistivity structures across the deposit. To achieve the objectives, we deployed sixty-one (61) AMT survey sites with an interstation separation of 20 m on a 1280 m survey track perpendicular to the geological strike across the Zhugongtang deposit. We operated in fifty-three (53) frequencies in the range 1–10,400 Hz to record the resistivity distribution of subsurface to a depth of more than 1200 m. The results from the AMT data computations estimated the geoelectric strike that varies between NE285° and NE315°. This range of strikes suggested that structures across the deposit are oriented in the NW–SE direction. Obtained two-dimensional (2D) models elucidated a remarkably low resistivity body (<15 Ωm) at an elevation of less than 1600 m above sea level (>0.50 km depth), thus extending to great depth and were interpreted as lead–zinc mineralization. Furthermore, low resistivity (<63 Ωm) features were imaged both in superficial and deeper depths and interpreted as shale, sandstone, claystone, and silty mudstone units. Dolomite and limestone lithologies were found widely distributed with high resistivity (>1000 Ωm). Bioclastic limestone and dolomite limestone were inferred and characterized by moderate-high resistivity (>250 Ωm) and were not widely distributed. A unit of basalts was found with moderate resistivity (>63 Ωm). In addition, it was also found that regions with high number of faults tend to have low resistivity values compared to regions with a low fault number. In summary, this case study presents the results of applying an AMT approach to explore the conductivity characteristics of structures across the Zhugongtang deposit. The findings may contribute to the literature about this deposit.

Application of Audio Frequency Magnetotelluric Method in the Exploration of Water Conveying Tunnel in Dagouhe Area

Application of Audio Frequency Magnetotelluric Method in the Exploration of Water Conveying Tunnel in Dagouhe 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.

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.

Geothermal exploration using numerical simulation and a comprehensive electromagnetic method

Geothermal resources are a type of important renewable clean energy with high development and usage potential. However, at the same time, geothermal resources are buried deep below the Earth’s surface, they involve significant exploiting hazards. Geophysical methods are frequently required to determine the depth and location of geothermal resources. Among geophysical methods, transient electromagnetic method (TEM) and controlled source audio-frequency magnetotellurics (CSAMT) have comparably high effectiveness and accuracy. This article first introduces the algorithm theory of the finite volume method, then establishes a simplified geothermal system resistivity model. Next, based on this model, simulation analyses of the abilities of CSAMT and TEM were performed to distinguish target faults with different resistivities, dip angles and widths. Finally, combined with theoretical analysis, taking a comprehensive electromagnetic exploration of CSAMT and TEM in Huairen County, Shanxi Province, China as an example, the effectiveness of these two methods in exploring typical geothermal resources is verified. It is shown that both methods can reflect the geoelectric structure from different perspectives, and the efficacy in geothermal exploration using comprehensive electromagnetic method is demonstrated.

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.

Modified symbiotic organisms search (SOS) algorithm for controlled-source audio-frequency magnetotellurics (CSAMT) one-dimensional (1D) modelling

Modified symbiotic organisms search (SOS) algorithm for controlled-source audio-frequency magnetotellurics (CSAMT) one-dimensional (1D) modelling

Application and Significance of Geological, Geochemical, and Geophysical Methods in the Nanpo Gold Field in Laos

As the main part of the Indosinian metallogenic province in the eastern part of the Tethys metallogenic domain, Southeast Asia has experienced multiple stages of tectonic magnetic activities accompanied by the formation of rich mineral resources. However, due to the undeveloped economy, low degree of geological work, dense vegetation cover, and lack of obvious prospecting marks, traditional geological prospecting work in the area is not optimal. Consequently, the combination of high-precision geophysics and geochemistry has become an important method of looking for ore bodies deep underground in this area. The Nanpo gold deposit is a hydrothermal gold deposit that occurs in the Indosinian felsic volcanic rock body, and its mineralization is closely related to felsic magmatism. This study carried out comprehensive geophysical and geochemical exploration methods of soil geochemical survey, induced polarization (IP) survey, and audio-frequency magnetotelluric (AMT) survey. Based on the characteristics of geophysical and geochemical anomalies, geological inference, and interpretation, the integrated geophysical and geochemical prospecting criteria of the ore area have been determined: The large-scale and overlapping Au-Ag-Cu anomaly area in the host felsic magmatic rocks (mainly diorite, monzodiorite and granodiorite) is a favorable metallogenic area. Two anomalies, P1–H1 and P3–H6, with the best metallogenetic conditions and the deepest extensions of the known ore bodies, were further selected as engineering verification targets. After the study of the drill core, gold (mineralized) bodies consistent with the anomalies were found, indicating that the combined method is suitable for the exploration of mineral resources in this area, and the prospecting effect is good. At the same time, the metallogenic prediction shows that the deep part of the mining area still has great metallogenic prospects and prospecting potential. The characteristics of geophysical and geochemical anomalies and prospecting experience in the study area can provide references for the prospecting of hydrothermal gold deposits in the Luang Prabang–Loei structural belt.

Static-shift suppression and anti-interference signal processing for CSAMT based on Guided Image Filtering

Shallow conductive heterogeneity can lead to static shifts ain the apparent resistivity sounding curve of controlled-source audio-frequency magnetotellurics (CSAMT). The static effect will shift the apparent resistivity curves along with axial log-log coordinates. Such an effect, if not properly processed, can distort the resistivity of rock formation and the depth of interfaces, and even make the geological structures unrecognizable. In this paper, we discuss the reasons and characteristics of the static shift and summarize the previous studies regarding static shift correction. Then, we propose the Guided Image Filtering algorithm to suppress static shifts in CSAMT. In detail, we use the multi-window superposition method to superimpose 1D signals into a 2D matrix image, which is subsequently processed with Guided Image Filtering. In the synthetic model study and field examples, the Guided Image Filtering algorithm has effectively corrected and suppressed static shifts, and finally improved the precision of data interpretation.

Sedimentary Setting and Ore-Forming Model in the Songtao Manganese Deposit, Southwestern China: Evidence from Audio-Frequency Magnetotelluric and Gravity Data

The break-up of the supercontinent Rodinia in the late Neoproterozoic led to the formation of the Nanhua rift basin within the South China Block. The Datangpo-type manganese deposit, which developed in the Nanhua rift basin, is one of the most important types of manganese deposits in South China. Although it is widely accepted that deep sedimentary structures significantly affect the manganese ore system, the relationship between the manganese deposits in South China and the Nanhua rifting process is still unclear. The origin of the manganese ore layer remains controversial. In this paper, we integrated the audio-frequency magnetotelluric (AMT) data, gravity data, and comprehensive geological and borehole data analysis to characterize the structure of the Datangpo-type manganese deposit in Songtao, Guizhou Province. The resistivity and density models produced an inclined layered structure, which correlated well with the coeval sediment strata of the Nanhua rift basin. A high-resistivity cap was observed from the surface to a depth of 800 m, corresponding to the Cambrian Loushanguan (ϵ3−4ls) and Palang dolomite formation (ϵ2p), which has helped the storage of the manganese ore. The most significant low-resistivity anomaly (25–40 Ω·m) resides at a depth of 1400 m in the Nantuo (Nh3n) gravel sandstone and Datangpo (Nh2d) silty and carbonaceous shale, corresponding to the ore-forming layer. This distinct low-resistivity layer was possibly produced by aqueous fluids and pyrite in the syn-sedimentary fault and alteration zone. The accumulations of sulfide minerals in the rock samples suggest a possible anoxic-euxinic deposition environment during the manganese mineralization and precipitation. The fault revealed in the resistivity models is perhaps a previous fault zone produced by extension in the Nanhua rifting process, which provided migration and upwelling channels for ore-forming minerals. Based on our resistivity models, density models, and geological survey, the manganese ore-forming model was derived, which can help to provide geophysical evidence for the origin of the Datangpo-type manganese deposit.

Research and application of AMT in tunnel hidden goaf under complex conditions

In order to detect the distribution, development scale and shape of mining area in tunnel hidden coal measure strata, and provide intuitive and reliable geological basis for later design and construction, geological mapping combined with audio frequency magnetotelluric method was used for exploration. In this paper, firstly, the characteristics of tunnel hidden goaf and the basic principle of audio frequency magnetotelluric (AMT) method are briefly described; secondly, the key technologies of scientific acquisition and data processing flow of the method are analyzed; secondly, the geological characteristics of goaf under different conditions are analyzed, and the geological models are established for forward modeling; finally, the method is applied in yuchanshan tunnel of Ronglu expressway The results of electromagnetic imaging directly and accurately reflect the distribution position and scale of goaf, and provide reliable basic data for later design. Through theoretical analysis and practical application, the results show that audio frequency magnetotelluric method combined with geological mapping can play an important role in the investigation of tunnel hidden goaf, and can directly reflect the location, shape and other basic elements of goaf, providing more reliable basic data for subsequent investigation and design.

Application of Integrated Geophysical Methods for Site Suitability of Research Infrastructures (RIs) in China

Research Infrastructures (RIs) are essential to achieve excellence in innovative scientific research. However, because of limited land availability and specific geological requirements, evaluating the viability of a site for a new RI can be a challenging task. Stringent safety construction requirements include developing site-specific architectural and geoengineering solutions, minimizing construction disturbances, and reinforcing rock and soil in a timely fashion. For successful development of the RIs in China, such as the Daya Bay Neutrino Laboratory (DBNL) and the China Spallation Neutron Source (CSNS), an integrated approach of joint geophysical methods including the electrical resistivity tomography (ERT), controlled-source audio-frequency magneto telluric (CSAMT)), gravity and seismic refraction methods, and geological mapping and surveys were carried out. Geophysical parameters, such as electrical resistivity, density, and seismic velocity, show inverse proportion to the degree of rock fracturing or weathering. The results show that the low values of geophysical parameters suggest the weathered/fractured rock, while high values reveal the fresh bedrock. The Engineering Geological Suitability Index (EGSI) value can represent the individual EGSI values at a constant and summed over varying depths. EGSI methodology is an improvement on the existing siting process and has been applied to CSNS. Our integrated approach provides clearer insight into the subsurface for site suitability of RIs in challenging geological engineering conditions and removes any ambiguity caused by a single geophysical parameter. The obtained geological knowledge of the area not only provides engineers with much-needed information about the construction conditions of a potential site but also gives scientists the opportunity to explore the local geology. In this study, we demonstrate our innovative approach for siting RIs, as demonstrated by the synthetic evaluation of the site location and utilization for two established RIs (DBNL and CSNS).

Features of apparent resistivity anomaly of controlled-source audio-frequency magnetotellurics and prospects of oil shale in the Tongchuan area of the southern Ordos Basin, China

Four controlled-source audio-frequency magnetotelluric (CSAMT) profiles were conducted in early 2011 to investigate the distribution rules of oil shale in the Tongchuan area of the southern Ordos Basin. After terrain correction, we high-pass filtered the resulting images to prepare the 2D CSAMT apparent resistivity profiles for further analysis. Specifically, we have correlated the high-pass-filtered apparent resistivity anomalies to the distribution of oil shale seen in three available wells. The results reveal a close relationship between the oil shale distribution and the anomalously high-apparent-resistivity belt-like anomalies in the Tongchuan area. Our analysis indicates two prospective areas with different depths for oil shale within the study area.

Tipper data forward modeling and inversion of three-dimensional tensor CSAMT

The tensor controlled-source audio-frequency magnetotellurics (CSAMT) method has multiple sources, and its resolution is higher than scalar CSAMT. The two observed sets of orthogonal horizontal electric and magnetic fields within the tensor CSAMT are transformed into apparent resistivity, phase or impedance which are the same as magnetotellurics (MT). However, the common measurement of tensor CSAMT does not further imitate MT in acquiring the vertical magnetic field and calculating tipper data, which has been widely proven to improve resolution in MT. In some special areas (such as deserts), the electric field is not easy to collect, so the use of tensor CSAMT tipper data is expected to increase measuring points and improve inversion in these areas. Based on this, we used the staggered grid finite difference method to simulate and study the two types of tipper data from the tensor CSAMT. The forward simulation results showed that the tipper characteristics of tensor CSAMT were similar to those of MT to a certain extent. However, the tipper data showed serious asymmetric properties at low frequencies or near offset. The simulation results indicated that it was necessary to consider the influence of the transmitting source when using tensor CSAMT full-band tipper data. For this reason, the limited-memory Broyden-Fletcher-Goldfarb-Shanno (LBFGS) method was used to implement the tensor CSAMT three-dimensional (3D) tipper inversion. The inversion tests of the theoretical model proved that the tipper data could provide useful information and improve the 3D inversion resolution.