The artificial source electromagnetic method is an important technique for deep resource exploration. The key challenge is to improve the depth and precision of detection through innovative technology. The detection mode of electromagnetic methods is evolving from far-field to near-field, and the study of near-source electromagnetic methods has become an international research frontier in recent years. Building on the recent advancements in wide-field electromagnetic methods and multi-channel transient electromagnetic methods, the short-offset transient electromagnetic method (SOTEM) has been further developed and proposed. The distinguishing features of SOTEM are its stronger signals and wider bandwidth, which are advantageous for achieving the detection requirements of greater depth and higher precision. This special issue presents eight articles covering the methods, techniques, software, and applications of SOTEM, providing strong support for the growing need for high-resolution subsurface detection.

The extended spatial autocorrelation (ESPAC)-based microtremor exploration(natural-source surface wave exploration) technology has been extensively used in shallow formation exploration owing to its simplicity,efficiency,and accuracy.However,the imaging effect of dispersion energy extracted based on the ESPAC method is unsatisfactory in practical applications.In particular,different observation array arrangements influence the extraction of dispersion curves from collected data.By investigating the imaging principle of the ESPAC method,this study conducted the simulation experiment of natural-source microtremor recording through ambient noise simulation.It compared the differences in dispersion energy under various dominant frequency distributions of wavelets.Moreover,it quantitatively analyzed the influence of different station arrangements and acquisition durations on the imaging quality of dispersion energy.The comparative study reveals the imaging patterns of the ESPAC method in shallow surface exploration.The ESPAC method can maximize the imaging quality of dispersion energy in the fundamental mode while considering both efficiency and exploration costs.The results of this study were applied to engineering application cases to further verify the simulation results.

The grounded-source short-offset transient electromagnetic (SOTEM) method involves many parameters in field data acquisition. The selection of these parameters is closely associated with the signal quality and detection sensitivity of measured data. Based on the relevant provisions in the organization standard, Technical specification for grounded-source short-offset transient electromagnetic method (T/CGS 002—2021), issued by the Chinese Geophysical Society, numerical emulations, and practical cases, this study analyzed and expounded the selection criteria of critical parameters like transmitting source length, transmitting fundamental frequency, offset, device type, and observation component. The insights obtained in this study are significant for guiding the field construction of the SOTEM device and leveraging its detection performance.

The gold ore zone in the Daduhe area of Shimian County, Sichuan Province, is located in the rare metal-precious metal-nonferrous polymetal metallogenic belt in the southern segment of the Songpan-Ganzi orogenic belt. It exhibits complex structures and steep terrains, severely restricting prospecting and exploration. Based on the geological settings, conditions, and geophysical characteristics for regional mineralization, this study investigated the typical Luoluoping gold deposit in Shimian by conducting deep geophysical exploration centered on audio-frequency magnetotellurics (AMT). Three low-resistivity anomalies (M1, M2, and M3) were identified, corresponding to the mineralized alteration zone I, the gold mineralization zone III, and the gold-copper ore body, respectively, demonstrating the effectiveness and applicability of geophysical methods for deep prospecting. Combined with geological understanding and engineering verification, this study summarized geological and geophysical prospecting markers for the typical gold deposit, establishing the geological-geophysical prospecting model for the gold ore zone in the study area. Therefore, this study provides technical support for deep prospecting in the study area.

Unfavorable geobodies such as Karsts, weak soil, and water-rich areas are extensively distributed in China. Under heavy rainfall, they are prone to geologic hazards like collapse. A severe geological collapse occurred in Xiacun Town, Yushui District, Xinyu City, near the Shanghai-Kunming high-speed railway. The space around the collapsed foundation pit was limited, with many interference sources like underground pipelines. With early signals subjected to the mutual inductance effects of receiver and transmitter coils, the conventional transient electromagnetic method exhibited low detection accuracy and anti-interference ability, encountering significant shallow blind zones. To locate unfavorable geobodies in the study area and provide suggestions for the prevention and control of geologic hazards, this study innovatively applied the opposing-coils transient electromagnetic method (OCTEM), supplemented by borehole-based verification. The results show that: (1) The OCTEM exhibited high accuracy, as demonstrated by the high consistency between the geophysical exploration results and the drilling results of the study area; (2) The low-resistivity zone spread across the study area, and the low-resistivity anomalies revealed by geophysical exploration were caused by groundwater according to borehole-based verification; (3) The strata from top to bottom were composed of soft plastic silty clay, hard plastic silty clay, soft plastic silty clay, and moderately weathered limestones; (4) The subsurface micro-confined water in the collapse area surged upward, gradually eroding the soft plastic silty clay layer around the area. The static water level in the collapsed foundation pit manifested an elevation of 55.60 m, located approximately 1.4 m below the surface; (5) A groundwater channel existed under the collapse area, with soil caves formed in the limestone layer under the prolonged erosion effect of water flow; (6) Long-term groundwater extraction may expand the underground seepage zone; (7) The administrative department in charge must promptly contain groundwater in the collapse area to prevent it from further eroding the surrounding unconsolidated soil layer.

The telecontrol electrode array is a new type of electrode array for power supply and observations that utilizes the carrier waves of the power-supply and measurement wires used in conventional electrical prospecting to transmit signals. By remotely controlling a series of coded electrode switches between various wires and their connections, this array allows for orderly power supply and measurements. This array enables flexible spacings between electrodes and the gradual expansion of the distances between power supply electrodes via remote control, thus achieving a gradient-based sounding array. The borrowing-line remote controller can be used combined with single-channel, multi-channel, and high-density resistivity instruments, and the combined arrays can perform 2D/3D electrical prospecting more effectively. Followed by the brief introduction of the principle of the borrowed-line telecontrol electrode array, this study discusses the applications and examples of this array and several combined arrays in the resistivity method and induced polarization (IP).

Magnetotelluric (MT) data collected from offshore areas are generally distorted due to coastal effects, posing challenges in obtaining true subterranean electrical structures. Based on the model of half-space with seawater, 2D electrical isotropy and anisotropy models, and 3D electrical isotropy model, this study analyzed the distortion patterns of MT responses under coastal effects through forward modeling. Moreover, it conducted 2D and 3D inversions of MT data with or without seawater constraints. Key findings are as follows: (1) The initial frequency points of distortion in MT apparent resistivity curves are closely correlated to the distances from survey points to seawater; (2) Compared to MT responses free from coastal effects, the high-frequency sections exhibit increased amplitudes in real induction vectors only at survey points in offshore areas and higher phase tensor ellipticity, whereas the low-frequency sections display expanded influence areas subjected to coastal effects; (3) Excluding seawater constraints, the inversion results show false anomalies in offshore areas and poor reconstruction effects on subsurface anomalies. Considering seawater constraints, the constrained inversion can effectively suppress the distortion caused by coastal effects. Overall, this study will provide a significant reference for the collection, processing, and interpretation of MT data from offshore areas.

Within a coal mine in Peixian County, Xuzhou City, brick-red Paleogene and Neogene strata were deposited in the faulted basin during the Cenozoic, with extensive Quaternary strata overlying various strata. The Quaternary, Jurassic, and Cretaceous strata exhibit thick overburden, up to over 500 m. This study explored the coal mine using the short-offset transient electromagnetic (SOTEM) method. Based on geoelectric conditions, reasonable observation parameters were designed to obtain the subterranean electric structure within a burial depth of 1500 m. Goafs were detected at a burial depth of 900 m, with their delineated boundaries aligning with the mining situation of the coal mine. The results of this study serve as a reference for detecting goafs with thick overburden in North China-type coalfields.

Marine controlled-source electromagnetics (MCSEM) is used to explore resources such as oil and gas hydrates, as well as deep geological structures, by revealing resistivity differences below the seabed. Different excitation frequencies correspond to varying detection depths. To enhance the resistivity imaging of targets beneath the seabed, this study investigated the MCSEM-based technology for generating arbitrary-frequency waveforms to flexibly alter the excitation frequency and improve the exploration effectiveness and efficiency. Using the direct digital frequency synthesis (DDS) chip AD9833 and the joint control of a microcontroller and a complex programmable logic device (CPLD), this study achieved the generation of arbitrary-frequency waveforms with limited precision. The test results indicate that this technology can effectively enhance the spectral adaptability and flexibility of MCSEM.

Soil fertility is a significant indicator that measures the ability of soil to provide various nutrients required by vegetation and reflects the crop producing power of soil. To ascertain the soil fertility of different forest stand types in the Nanshan forest farm of Jiyuan City, this study investigated the soil of four forest stand types: Platycladus orientalis, Robinia pseudoacacia, Quercus variabilis, and walnut trees. It analyzed the soil nutrients of the forest stand types by determining the pH, organic matter, total nutrients (TN,TP,TK), and available nutrients (AN,AP,AK) of soil at different depths (0~40 cm). Moreover, it assessed the comprehensive soil fertility using the improved Nemerow comprehensive index method. The results show that: (1) Except pH, the physicochemical factors of the soil of the forest stand types in the study area exhibited moderate variations, and the soil TP, TK, AP, AK, and pH manifested significant differences; (2) With an increase in soil depth, the soil in the study area showed increased pH and bulk density, decreased organic matter, TN, and available nutrients, and almost unchanged TP and TK; (3) The assessment of soil nutrient abundance indicated that the TN, TP, and available nutrients were relatively poor in the soil of the study area; (4) Based on a comprehensive assessment, the soil fertility of four forest stand types at different depths decreased in the order of Robinia pseudoacacia, Quercus variabilis, Platycladus orientalis, and walnut trees. Their soil fertility was generally at an average level, with low values observed at depths ranging from 20 to 40 cm. The comprehensive analysis indicates that TN, TP, and available nutrients are deficient in the study area. Therefore, applying appropriate organic fertilizers, nitrogen, and phosphorus is recommended for the study area.

The Mangling ore concentration area with intense magmatic activity has become a focal area for deep prospecting in the North Qinling tectonic belt in recent years. The formation of molybdenum deposits in this area is closely related to small Late Jurassic acidic intrusions. To achieve breakthroughs in deep ore prospection within this area, this study conducted the wide-field electromagnetic sounding over the concealed Yaozhuang intrusion delineated based on gravity anomalies. The results indicate the presence of pronounced high-resistivity anomalies at depth, and it is inferred that the protruding part of the anomalies corresponds to the concealed Yaozhuang intrusion. The resistivity inversion results roughly delineated the variations in the top surface of the intrusion, with the elevations and N-S width of the top surface estimated at -300~620 m and 1300~1600 m, respectively. Drilling in the most favorable deep mineralized part confirmed the presence of the concealed intrusion and concealed molybdenum ore bodies. The results of this study demonstrate that the wide-field electromagnetic sounding method exhibits great sounding depths and high resolutions, serving as an effective method for deep ore prospecting in the Mangling ore concentration area.

The Galinge iron deposit in Qinghai is overlain by deposits measuring greater than 150 m in thickness. The great burial depths of ore bodies lead to gentle magnetic anomaly morphology, making it difficult to characterize the spatial distribution of ore bodies. Therefore, this study employed three-dimensional magnetic anomaly inversion to determine the three-dimensional distribution characteristics of subsurface magnetic intensity in the study area. Given the prior information of non-magnetic surrounding rocks, the three-dimensional magnetic intensity model clearly presented the spatial distribution of the ore bodies and reflected the presence of intense magnetic bodies at depths of less than 500 m in existing boreholes. Accordingly, it can be inferred that there exist concealed ore bodies at depths exceeding 500 m in the study area. The results of this study suggest that three-dimensional magnetic anomaly inversion can effectively improve target identification, providing clear information on the horizontal positions, depths, and scales of magnetic ore bodies. The proposed inversion method can offer strong support for drilling design and reserve estimation, warranting promotion in detailed exploration of solid minerals.

The transform from transient electromagnetic (TEM) field to pseudo wavefield is the basis for using the imaging technique. The TEM field follows the diffusion equation, while the pseudo wavefield follows the wave equation. These two can be transformed into each other via the Q-transform. As the integral transformation from TEM field to pseudo wavefield is ill-posed, numerical method is used to get the desired pseudo wavefield. As a result, it is often the case that the methods and parameters are need to be optimized to get a optimal pseudo wavefield. Therefore, here in this paper, we derived the analytical solution of the pseudo wavefield from the TEM field. We start from the analytical formula of TEM field, and utilize the Laplace transform to establish the analytical solution pairs. The pairs can be used to test the accuracy and stability of the numerical method, as well as examining the features of the pseudo wavefield to get an effective imaging method.

Tunnel detection in complex environments requires fine-scale detection of small unfavorable geobodies like karst caves and fissures. Hence, this study designed a tunnel construction model with a small karst cave in front of the tunnel face. A borehole was drilled at the center point of the tunnel face towards the construction direction, and then an electrical source was put into the borehole for excitation. Array data acquisition was conducted on the tunnel face. The 3D forward modeling based on transient electromagnetic data was performed using the time-domain finite element method. As indicated by the results, the electromagnetic sounding of the target was achieved through the movement of the electrical source, and the planar position of the unfavorable geobody was determined based on the distribution patterns of the electromagnetic field on the tunnel face. Therefore, electrical source excitation in a borehole can enhance the detection ability of the transient electromagnetic method on small karst caves, serving as a feasible method for improving the accuracy of tunnel seismic prediction.

Black soil layer thicknesses, anessential attribute of black soil,serves as a significant indicator for measuring the fertility and erosion degree of black soil. Their spatial prediction holds critical significance for supporting China's black land conservation and ensuring food security.Considering the diagnostic characteristics of black soil layers in soil system classification, this study regarded soil layers with organic carbon content higher than 6×10^-3 of soil parent materials as black soil layers.Moreover, it derived the calculation formula for the thicknesses of black soil layers relying on the exponential distribution pattern of organic carbon in the vertical soil profile. Based on the 62 896 topsoil and 15 687 deepsoil organic carbon data obtained from the multi-purpose regional geochemical survey, this study conducted detailed spatial prediction of the thicknesses of black soil layers in the Songliao Plain and analyzed their relationship with soil types and climate factors. Key findings are as follows:(1) The thicknesses of black soil layers in the Songliao Plain range from 0 to 165 cm, with a median of 23.33 cm;(2) The spatial distribution of black soil layers exhibits significant heterogeneity, characterized by thin southwestern and thick northeastern portions;(3) The black soil layers of swampy soil and peat soil manifest the largest average thicknesses between 60 and 80 cm, followed by those of typical black soil (average thickness: 56 cm) and those of albic soil and meadow soil (average thickness: 40~50 cm);(4) The spatial distributions of the thicknesses of black soil layers are closely associated with climatic conditions, primarily showing a significant negative correlation with temperature and a positive correlation with rainfall;(5) The mean annual temperature of 0 ℃ is a significant temperature threshold for the development of thick black soil layers.Above this temperature, the average thickness of black soil layers exceeds 80 cm and no longer changes with temperature. With global warming, the southward shift of this 0 ℃ is otherm may significantly influence the thicknesses of black soil layers.

In response to the need for large-scale exploration of radioactive minerals in high-relief areas, this study developed an airborne gamma-ray spectrometer based on small-sized NaI (Tl) crystals, which was integrated with the F-120 unmanned aerial vehicle (UAV) to form an airborne gamma-ray spectrometry (AGRS) system. The UAV-AGRS system was applied to the follow-up geochemical survey of uranium deposits in a certain area of South China. The survey results show roughly consistent locations and morphologies for high-anomaly zones with the ground gamma-ray spectrometry results. Moreover, the results of this study exhibit more detailed anomaly morphologies, and more significant responses of uranium content-related uranium channel data to known uranium occurrences in the study area, suggesting superior data. Therefore, the UAV-AGRS system shows promising potential as an effective alternative to ground gamma-ray spectrometry for the follow-up geochemical survey of uranium deposits in high-relief areas.

The successful operation of geothermal wells on the eastern margin of the Yinchuan Basin suggests a new geothermal exploration orientation around the Huanghe fault and its secondary faults. To further investigate the controlling effect of the Huanghe fault on geothermal resources in the study area and pinpoint the exploration target, this study arranged four magnetotelluric (MT) lines, obtaining 71 survey points.The preprocessing and two-dimensional inversion of MT data yielded a two-dimensional electrical structure of the study area within a depth of 10 km.Based on the previous geological, gravity, microtremor, and controllable source audio magnetotellurics(CSAMT) data in the study area, this study further interpreted the strata and fault structures of the study area.It posited that the relatively-low-resistivity zone within the high-resistivity zone of the Ordovician basement reflected by the MT lines may be the secondary-fault development site at the rear edge of the Huanghe fault, which is caused by the fragmentation and water filling of Ordovician strata, showing certain exploration potential for geothermal resources.

Over recent years, large-scale geochemical surveys on a scale of 1:25,000 have been conducted in the eastern Kunlun region for geological prospecting. These surveys, characterized by the rapid and accurate location of anomaly sources, have been widely applied in mineral explorations, yielding encouraging mineral prospecting results. Using 1:25,000-scale stream sediment surveys, this study delineated 12 geochemical integrated anomalies of the element associations of Cu, Co, and Ni in the Hedong area, Mangya City, eastern Kunlun. These anomalies include one integrated anomaly dominated by Co, where various element anomalies exhibit high-degree overlap, relatively large scales, high intensity, and distinct concentration zoning. The Co anomaly is closely associated with the Ordovician Qimantag Group, demonstrating a high degree of spatial consistency. The subsequent anomaly verification reveals four cobalt mineralized zones within anomalies GA16, GA26, GA11, and GA39, with three ore bodies and multiple mineralized bodies of cobalt being identified within the mineralized zones. The comprehensive analysis indicates that the eastern Kunlun region enjoys excellent metallogenic geological conditions and favorable geochemical characteristics, with promising prospecting results having been achieved. Therefore, this region has the prospecting potential of medium to large-scale cobalt deposits.

This study performed forward modeling for the short-offset transient electromagnetic (SOTEM) responses of a three-dimensional geoelectric model using the three-dimensional finite-difference time-domain method. The results reveal that: (1) The attenuation curves of the E_x responses from the electric field above the goaf or collapse column were above the background response curve at early times but below it at later times; (2) A shorter offset corresponded to a higher relative anomaly of ∂B_z/∂t response signals. Increasing the length of the transmitting source could enhance the signal intensity while slightly decreasing the relative anomaly; (3) In the case of survey lines close to the transmitting source, the overall ∂B_z/∂t responses of the goaf or collapse column were weaker than the background responses. For survey lines away from the transmitting source, the overall ∂B_z/∂t responses of the goaf or collapse column were stronger than the background responses. In summary, the calculation results demonstrate significant implications for understanding the ability of the SOTEM method to detect typical water-rich bodies in coal mines, laying a theoretical foundation for the parameter selection of SOTEM devices and their generalized application in the coal sector.

Surface-related multiple elimination(SRME) is vital in processing seismic data.At present,the SRME method most commonly used for processing marine data can effectively predict surface-related multiple models,achieving multiple elimination through matching and subtraction.However,this method requires full wave field information,thus placing higher demands on observation systems.The irregular land data acquisition makes it difficult to apply this method for processing land data.Hence,this study presented an improved SRME method,which performs moveout correction on adjacent channels for multiple prediction by constructing the minimum error constraint function,thus enhancing the adaptability to complex observation systems.The improved SRME method was applied to process the actual seismic data of land.Its effectiveness was validated by comparing the gathers,velocity spectra,prestack time migration profiles,and synthetic seismograms before and after SRME.

The Mianning-Dechang rare earth element (REE) metallogenic belt exhibits considerable resource potential. Previous prospecting was oriented to hard-rock REE deposits associated with Himalayan alkaline complexes, with ion adsorption-type REE deposits under-studied. To explore the mineralization prospect of granite weathering crusts widespread in the metallogenic belt, this study investigated the Shizishan area through multi-channel gamma-ray spectrometry, soil profile survey, and shallow drilling. One ion adsorption-type REE ore occurrence was identified in the granite weathering crust at Mosuoying. This study analyzed the radioactive response of the geological, geophysical, and geochemical profiles and ore contents, finding that the elemental contents of thorium (Th) and potassium (K) were highly indicative of REE and rare metal mineralization. Highly mineralized, industrial-scale ion adsorption-type REE deposits will likely occur when 6.25<w(Th)/w(K)<10, w(eTh)>37×10^-6, and w(eK)>4.2%. As revealed by the analysis of the response of radioactive anomalies to supergene weathering, thorium anomaly halos can effectively indicate granitic plutons while potassium anomaly halos can well delineate the extent of granite weathering crusts.

After a disposal repository for high-level radioactive waste operates, the near-field surrounding rocks will be long in a thermal-hydrological-mechanical coupling environment. Therefore, their mechanical and permeability characteristics are crucial to the performance evaluation of the disposal repository. The surrounding rocks of the Beishan preselected area for the disposal of high-level radioactive waste in China are typical sparsely fractured granites, whose creep characteristics are directly related to the long-term safety of the disposal repository. Using water jet and wire cutting techniques, along with fracture surface blocking and combined sealing at rock sample ends, this study addressed the challenges of the sample preparation and sealing of fractured granite in thermal-hydrological-mechanical coupling triaxial tests. Based on this, multi-loading triaxial creep tests were conducted. The test results indicate that under the condition of multi-field coupling, the triaxial creep strength of the fractured granite was approximately 80% of its triaxial strength. Creep deformation increased with the axial load level, with lateral creeps more pronounced than axial creeps. For the compacted and crack propagation sections of the rock samples, both the axial strain rate and permeability decreased initially and then increased. In the case of consistent osmotic pressure difference, a higher osmotic water pressure within fractures would lead to decreased peak strength of the rock samples and result in greater lateral creep deformation.The results provide scientific support for the site selection, surrounding rock evaluation, engineering design and construction of the repository.

Non-destructive testing of the internal structural characteristics of ancient buildings is the key to preserving cultural relics.To determine the structures and internal defects of the ancient Great Wall in Linhai City,this study performed non-destructive testing of the wall structures in different orientations using a ground-penetrating radar (GPR) combining 100 MHz and 270 MHz antennas.The testing results show significant structural stratification in the ancient Great Wall.The GPR signal-reflected images reveal clear internal wall defects like pores,cracks,voids,and other hidden dangers.This study demonstrates the reliability of the GPR in detecting structures and defects of ancient walls,there by providing technical support for the structural protection of the ancient Great Wall.

In shallow engineering investigations, the small-loop transient electromagnetic (TEM) system is challenged by limitations such as overweight equipment, significant transmitter-receiver mutual inductance, and high manpower requirements. Hence, this study introduced an improved system. Based on the opposing-coils transient electromagnetic (OCTEM) theory, this study calculated the magnetic field distribution of the generalized opposing-coils antenna device. Furthermore, it designed and developed efficient portable antennas and the supporting system (collectively referred to as the portable OCTEM system). Subsequent field experiments demonstrate that while ensuring exploration accuracy, the portable OCTEM system can enhance the investigation efficiency by effectively mitigating the transmitter-receiver mutual inductance and significantly reducing manpower requirements. This study preliminarily verifies the feasibility of the portable OCTEM system, providing a novel technology route for developing downsized shallow exploration equipment.

In the past two decades, high-sensitivity ring laser gyroscopes have demonstrated the potential of rotational observation data in global seismology. Commercial fiber-optic three-component rotational seismometers have heralded a new development phase of rotational seismography. Field experiments for high-sensitivity portable fiber-optic rotational seismometers in China remain in the initial stage, whereas their relevant data analysis results have been obtained internationally. This study elucidated the co-located observation experiments on six components (6C, including three components of translational motions and three components of rotational motions) of an active source and a natural earthquake, involving experimental schemes, implementation steps, and subsequent data analysis. Moreover, this study revealed the primary factors influencing the experiment results by comparatively analyzing the similarities and differences of experiments. Fiber-optic rotational and conventional seismometers need to be fixed on the same rigid panel to ensure the consistency of received signals. Besides, proper ground coupling and burial processing contribute to high-quality experimental data. The experimental results indicate that water bodies will affect surface wave manifestation and P-wave clarity. These findings, enriching the practical experience in seismic rotational observation experiments, serve as a reference for the design of subsequent rotational observation experiments, thereby assisting in completing the experiments and obtaining higher-quality data. In terms of data application, this study optimized and substantiated the feasibility of the preprocessing scheme, with the backazimuth calculation accuracy improved by 58.8° and 50° at the two active-source measuring points, and by 24.1° and 29.4° at the two passive-source measuring points. The application of six-component seismic data from a single seismic station suggests that additional observation of rotational components can acquire more seismic wavefield information, thus the observation of rotational components can be employed to enhance the utilization of China's massive seismic observation data. Overall, fiber-optic rotational seismometers broaden the boundary of seismic monitoring technology, boost seismology research, and create new possibilities for future earthquake research.

Owing to the successful application of grounded-source in recent years, roadway detection using the grounded-source transient electromagnetic (TEM) method has attracted significant academic interest. Using the three-dimensional finite-difference time-domain (FDTD) method, this study simulated the grounded-source TEM responses of the anomalies in the roadway floors and between roadways. The simulation results are as follows: (1) For anomalies in the roadway floors, their surfaces show violent changes in the horizontal and vertical components of the electric field subjected to the surface charge of anomalies. The horizontal electric field exhibits a strong ability to discriminate low-resistivity bodies but a weak ability to discriminate high-resistivity bodies. The vertical electric field displays opposite response signs from the low- and high-resistivity bodies. The low- and high-resistivity bodies correspond to opposite ∂B_y/∂t responses in the early and late stages. Specifically, the response signs are negative in the early stage and positive in the late stage for low-resistivity bodies, whereas the results are contrary for high-resistivity bodies; (2) For anomalies between roadways, the horizontal electric field also exhibits a strong ability to discriminate low-resistivity bodies but a weak ability to discriminate high-resistivity bodies. The ∂B_z/∂t response curves are above the background curves in the model of high-resistivity bodies but below the background curves in the model of low-resistivity bodies, suggesting the discrimination ability of the ∂B_z/∂t response for both low- and high-resistivity bodies.

Effectively identifying the stratigraphic and landslide structures in landslide-prone areas is significant for disaster prevention and mitigation. By investigating the landslides in the Xuelang Mountain scenic spot in Wuxi, this study analyzed the differences between two-and three-dimensional inversion using the high-density resistivity method. Accordingly, this study explored methods for eliminating the banded effect in the three-dimensional inversion, performed three-dimensional resistivity inversion under the constraints of high-precision surface elevation data and borehole-derived prior information, and constructed a three-dimensional geological model for the study area. The results indicate that three-dimensional resistivity inversion enjoys distinct advantages in complex landslide surveys. The banded effect can be effectively suppressed by optimizing the grid spacing, damping coefficient, and filter parameters for inversion. Furthermore, the terrain-induced impacts and the multiplicity of solutions of the inversion can be significantly reduced using constraints of refined terrain data and prior information, thus improving the resolutions of stratigraphic boundaries and landslide structures. Through three-dimensional resistivity inversion and geological modeling, this study determined the three-dimensional stratigraphic structure, along with the spatial distributions of the landslide bodies and sliding surfaces, and investigated landslide mechanisms, providing important data for the survey and control of landslides in the study area.

The spectra of high-order pseudo-random electromagnetic signals encompass all the frequencies required for exploration engineering, it has the characteristics of enhancing work efficiency and strong anti-interference capability, and has been applied in electromagnetic exploration in urban environments. This study extracted effective information from high-order pseudo-random signals in the electromagnetic survey conducted in areas with strong powerline interference within the special exploration area of the Phase I engineering of the Jinan Urban Rail Transit Line 8. To efficiently extract high-quality effective information, an envelope assessment algorithm was combined with high-order pseudo-random signals. Specifically, the actual signal interference was accurately estimated by analyzing the spectral envelope values. This allows for screening received signals, thus further mitigating the impacts of powerline interference and its harmonics. As a result, more effective frequency and geoelectric information were obtained, providing abundant effective electromagnetic data for subsequent inversion and interpretation. The novel method serves as a technique for effective information extraction for future electromagnetic sounding in a complex urban environment.

Several thermal springs associated with tectonic activity occur along the Chuhe fault zone. Except for the Bantang thermal spring at the southern end, other springs along the fault zone exhibit unideal utilization of thermal energy. This affects the development of local industries. Therefore, it is necessary to delve into the geochemical characteristics and formation mechanism of typical thermal springs along the Chuhe fault zone. The purpose is to provide scientific evidence for the rational utilization and exploration methods of geothermal resources in the thermal spring groups in the future. Therefore, this study investigated thermal springs in the Bantang, Zhaoguan, and Xiangquan areas along the Chuhe fault zone, where nine samples of thermal spring water, cold well water, and surface water were collected individually. For these samples, the composition tests of 25 indices, including anions, cations, and major and trace elements, were tested, and the hydrogen and oxygen isotope values were determined. The analyses of the 25 measured indices, along with the investigation of the geothermal water source and controlling factors through hydrogen and oxygen isotope tracing, indicate that the thermal spring water in the three areas tends to be of the CaSO₄ type and is all closely related to the interactions between groundwater and surrounding rocks. The thermal spring water, cold well water, and surface water in Bantang and Zhaoguan show consanguinity, with geothermal water being directly recharged with local precipitation and surface water. In contrast, the thermal spring water, surface water, and cold well water in the Xiangquan area show weaker consanguinity, indicating different sources for the underground cold water runoff and geothermal water. This should be noted when determining the primary factors controlling the thermal spring in this area. The temperatures calculated using a chalcedony geothermometer were close to the temperatures of the hot water recovered on the surface. In contrast, the temperatures calculated using a quartz geothermometer approached the temperatures of deep geothermal reservoirs. The results of this study preliminarily reveal the geochemical characteristics, recharge relationships, and water-rock interactions of thermal spring water in the Chuhe fault zone and propose effective geochemical geothermometers for the study area. These contribute to deeper insights into the mechanisms and controlling factors of the thermal springs along the fault zone, as well as providing practical value and a methodological model for enhancing thermal spring functionality and geothermal resource exploitation and utilization in the future.

In response to the increasing demand for large-scale field seismic acquisition, this study developed a low-cost multifunctional nodal rotational seismometer (RBWL) with a micro-electro-mechanical system (MEMS) sensor, considering the functionality, economic feasibility, and the ease of arrangement. The RBWL employs a low-cost and low-power MEMS sensor to acquire seismic signals, involving three-component translational motions (T_x,T_y,T_z) and three-component rotational motions (R_x,R_y,R_z). To reduce the impacts of environmental factors on measurements, the system of the RBWL automatically records real-time information including temperature and attitude while performing compensation correction on the measurement results. For real-time monitoring and data transmission at acquisition nodes, the system establishes a data transmission link integrating 4G, cloud, and client, with the measured maximum data transmission rate up to 100 Mbps. The testing of H/V spectral ratios verifies the system functions and principal performance parameters of the RBWL and its effectiveness in engineering physical exploration.

Accurately identifying faults is crucial for the exploration and exploitation of oil and gas fields,and further fault extraction based on this holds critical significance for later comprehensive research.At present,the commonly used fault extraction techniques primarily include automatic fault tracking,fault slice interpretation,and manual interpretation.However,these fault extraction techniques and their application in practical exploration and exploitation often face the following challenges.Automatic fault tracking based on an attribute volume often extracts faults with low accuracy and poor continuity,whereas fault slice interpretation and conventional manual interpretation require long work cycles.Hence,this study proposed a fault extraction technique based on structure-oriented filtering.First,the original poststack seismic data were processed through structure-oriented filtering to improve the quality of fundamental data and enhance the fault boundary features.Then,a relative isochronous model was established based on the filtered data volume,with sensitive attributes that can characterize faults extracted.Finally,based on the analysis of fault combination relationships,a comprehensive interpretation method combining plane and profile views was employed to extract faults.The technique proposed in this study has been successfully applied to a certain block of SB.As indicated by the application results,the proposed technique exhibits higher reliability,accuracy,and efficiency compared to the three commonly used fault extraction techniques, thus demonstrating high applicability.

The geomagnetic field is a vector field in space. However, traditional marine magnetic surveys focus on geomagnetic field intensity, failing to fully acquire and utilize rich information about the geomagnetic vector field. Given this, this study developed a towed marine geomagnetic vector measurement system. This system was designed to operate in dynamic marine conditions and ultimately acquire geomagnetic vector field data within the geographical coordinate system. Through sea trials of the system, grid line and repeat line measurements were obtained. After preprocessing the collected data, the measurement accuracy of repeat lines and crosspoints was better than 6.7 nT and 6 nT, respectively. The results of the sea test indicate that the measurement system has the preliminary ability to measure the geomagnetic vector field at sea, and it can be applied to geomagnetic field measurement tasks near or far sea, obtaining richer geomagnetic field information.

A strong shielding layer exists between Neogene and Meso-Paleozoic strata in the South Yellow Sea Basin,hindering the downward propagation of seismic wave energy.Consequently,reflected seismic waves in deep strata manifest low energy and poor wave group characteristics,severely constraining the oil and gas exploration in Meso-Paleozoic strata.To enhance the imaging quality of Meso-Paleozoic strata under the strong shielding layer,this study proposed a method for designing a high-energy airgun source rich in low-frequency components through extensive simulation analyses at the excitation end of the source.Finally,a 6050 in³ high-energy airgun source rich in low-frequency components was designed and applied to the streamer-based seismic data acquisition in the extremely shallow water area in the western Laoshan uplift.The seismic data acquired in the deep strata under the strong shielding layer demonstrate significantly improved quality,exhibiting a higher signal-to-noise ratio,more continuous seismic events,and clearer structural characteristics.

The Salt Lake geothermal field in Yuncheng City, Shanxi Province, China lies beneath a densely populated urban area, posing significant challenges to further geothermal exploration and extraction. Based on the distribution of geothermal gradients in the geothermal field, which are higher in the southwest and lower in the northeast, a NE-trending microtremor survey profile was arranged in the southern part of Yuncheng City, aimed at investigating the deep geothermal reservoir structure and NW-trending structures in the geothermal field. The 2D velocity structure profile reveals a pronounced low-velocity anomaly in the eastern part, which is supposed to be induced by the fault fracture zone formed by multiple NW-striking tensional faults. Spatially, this concealed fault zone roughly corresponds to the low-geothermal gradient anomaly in the northeastern Salt Lake geothermal field, suggesting that this fault fracture zone might facilitate the rapid infiltration of cold surface water, thereby lowering the temperature of deep rocks in the northeastern part, leading to the formation of a large-scale low-temperature anomaly zone. Additionally, the faults identified by the microtremor survey can be traceable and confirmed in a controlled source reflection seismic profile in the study area, demonstrating the complementary nature of the two methods. This study further reveals the deep geothermal structures of the Salt Lake geothermal field based on previous efforts. This study provides more valuable bases and guidance for future exploration and resource evaluation of geothermal fields in the region while also demonstrating the effectiveness and superiority of the microtremor survey method in research on urban geothermal resources.

Tight sandstones serve as significant oil and gas reservoirs.Their lithofacies identification can assist in further understanding the developmental characteristics of reservoirs.Combining core observations with log data processing,this study analyzed the lithofacies and sedimentary microfacies characteristics of tight sandstones in the Xinchang area and the internal relationships between lithofacies and sedimentary microfacies.Moreover,it constructed a random forest classification model with geological implications through data mining of sedimentary microfacies characteristics.The results show that:(1)Tight sandstones in the Xinchang area can be classified into seven typical lithofacies,including mudstone,siltstone with ripple lamination,massive fine sandstone,fine sandstone with parallel bedding,massive medium- to coarse-grained sandstone,and medium- to coarse-grained sandstone with parallel/cross bedding;(2)The sedimentary microfacies in the Xinchang area consist primarily of subaqueous distributary channel,subaqueous distributary bay,river-mouth bar,and prodeltaic mud,which are closely associated with the sedimentation of lithofacies;(3)In the classification model,the relative centroid(RM),root mean square deviation(GS),average median(AM),and average slope(M) of the gamma ray(GR) curve can be used as the characteristic parameters of sedimentary microfacies to increase the number of characteristics in the dataset;(4)Considering the characteristics of sedimentary microfacies,especially the energy and turbulence of water bodies,can significantly enhance the performance of the random forest classification model.Overall,the results of this study provide a novel approach for lithofacies identification using machine learning methods and a significant reference for oil and gas exploration in tight sandstones.

This study converted the frequency-domain electromagnetic diffusion equation into the wave equation in the fictitious domain based on the transformation relationship between the diffusion field and the fictitious wave field,achieving the numerical calculation of the electromagnetic field in the fictitious wave field.By introducing the complex frequency shifted perfectly matched layer(CFPML) boundary condition,the storage capacity of the computer memory decreased.Furthermore,by encompassing the air layer in the calculation domain,the complex processing of the ground-air interfaces was avoided.Compared to the uniform half-space analytical solution,the algorithm proposed in this study had relative errors of less than 3.5% and thus is effective and correct.Finally,the numerical simulation of a typical geoelectric model indicated that the 3D electromagnetic responses of multiple frequencies can be obtained through single forward modeling,suggesting an elevated calculation efficiency.The numerical simulation results also exhibit that the apparent resistivity calculated based on the fictitious wave field is insensitive to the field source effect and thus can effectively identify anomaly boundaries.

A variogram serves as a crucial tool for quantifying spatial correlations. However, existing variogram fitting methods often yield unstable results. This study proposed an automatic variogram fitting method based on deep learning, aiming to enhance the precision and stability of automatic fitting. The fitting of the experimental variogram is essentially a nonlinear optimization problem, which involves optimizing the matching between the experimental and theoretical variograms. The proposed method generated substantial training datasets using several sets of theoretical variograms with varying parameter values for training and learning in deep neural networks. The trained model was then used for the automatic fitting of the experimental variogram. Multiple sets of experimental results demonstrate that based on the robust fitting capability of deep neural networks, the proposed method manifested superior fitting stability and computational efficiency compared to the least squares method, providing a novel approach for automatic variogram fitting in geostatistics.

The method of structural superimposed halo to find blind ore is a new method to find blind ore, which is based on the study of the theory of primary halo to find blind ore-the axial zoning of primary halo, and puts forward two new theories of ‘primary superimposed halo theory’ and ‘structural superimposed halo theory’. The accuracy of blind ore prediction by superimposed halo of structure is determined by the correctness of prediction marks and indexes. Based on the summary of seven common signs of structural superimposed halos in the prediction of deep blind ore deposits in more than 100 hydrothermal deposits, four important quantitative qualitative prediction signs are upgraded to quantitative prediction indicators. The structural superimposed halo prediction signs and indicators of 17 different combinations of accurate prediction of blind ore and discrimination of ore body denudation degree in the deep and periphery of the mining area are described in detail, and a practical ideal model of structural superimposed halo for predicting blind ore and discriminating ore body denudation degree is established. The common prediction signs and indicators of the model have important guiding significance for the prediction of deep blind ore in typical hydrothermal deposits, and have achieved remarkable prospecting results in more than 100 mine applications.

The short-offset transient electromagnetic (SOTEM) data are typically processed using conventional inversion methods based on physical modeling, manifesting relatively low efficiency and difficulty in integrating priori information. In contrast, the data-driven inversion methods can enhance the inversion accuracy and efficiency but fail to ensure the generalization capability. To achieve high inversion accuracy and efficiency for SOTEM data and a reliable generalization capability, this study proposed an inversion method that integrates physical modeling with the data-driven approach, introducing the supervised descent method in machine learning into SOTEM data inversion. The proposed inversion method involves the offline training and online prediction stages. In the offline training stage, the prior information is flexibly integrated into the model training through a reasonable training dataset to obtain the average descent directions with implicit model features. In the online prediction stage, the physical modeling functions and the descent directions are employed to reconstruct the model parameters under the conventional inversion framework. In this study, the layered geodetic model was applied to design the training and test datasets for the 1D inversion of SOTEM data based on the supervised descent method. The inversion results were compared with those obtained using Occam's inversion algorithm, demonstrating that the proposed inversion method shows significantly enhanced inversion efficiency, higher inversion accuracy, and higher generalization capability.

Hidden disaster-causing factors in coal minesdenote the geological structures and unfavorable geobodies that are concealed in coal seams and surrounding rocks and may cause mine disasters during mining. Methane and radon are common harmful gases in coal mines, and their abnormal release is often accompanied by hidden disasters like unfavorable structures of coal seams and gas accumulation. With the Xinyuan coal mine in Yangquan City as the study area, this study selected two geochemical indices based on the microseepage mechanism:Methane and radon, which are highly sensitive to hidden disaster-causing geological factors. Building on the dynamic monitoring data of methane and radon in free hydrocarbons at 408 points, and the area survey data of methane in free and acid-hydrolyzed hydrocarbons at 416 points and soil radon at 651 points, this study obtained the geochemical characteristics reflecting the distributions of disaster-causing factors like the underlying water-bearing fracture zones, coal bed methane see page zones, microstructural fracture zones, and collapse column surround zones. Moreover, this study conducted joint exploration and analysis combined with the wide-field electromagnetic method, completing the verification and identification of hidden disaster-causing factors related to structures and gas accumulation in coal mines. Furthermore, it delineated the distribution sections of potential disaster-causing factors in the study area. This study demonstrates the applicability and practicability of the hydrocarbon microseepage theory in the exploration of hidden disaster-causing factors in coal mines. It also lays a foundation for the extensive application of methane-radon geochemical indices in the survey and exploration of hidden disaster-causing factors in coal mines, thus holding critical significance for enhancing the safety of coal mine production.