As one of four major black soil areas in the world, the black soil area in northeast China boasts fertile and rare black soil resources, and its protection and utilization have always attracted much attention. The changes in land uses/cover affect the structure, function, and health of the ecosystem and thus are greatly significant for the sustainable utilization of land resources. Based on the data on the land uses/cover in 1970, 1995, 2010, and 2020, this study analyzed the spatio-temporal characteristics of the change in the land uses of the black soil area in northeast China over the past half century using the methods, such as transfer matrix, comprehensive dynamic degree, and spatial change model. The results are as follows. ①The areas of construction land, cultivated land, and grassland in the black soil area of northeast China generally increased over the past half century, whereas the areas of wetland, woodland, and unused land generally decreased. The changes in land types mainly occurred in the Sanjiang Plain, the northwestern Songnen Plain, and the Xiliaohe Plain. ②The change in each land type was dominated by the alternating quantity, with a large amount of spatial position shift occurring. ③The degree of land utilization slightly increased yearly but is still lower than the national level, suggesting considerable potential for land development and utilization. Therefore, further studies should focus on the spatio-temporal characteristics and mechanisms of the change in the land uses of the black soil area in northeast China, as well as their impacts on the habitats, ecological service functions, and balance of carbon budget, to fully utilize and protect the black soil area and optimize the national spatial layout.

The soil organic carbon (SOC) and total nitrogen (TN) contents serve as the basis for sustainable agriculture and the key to soil fertility and play an important role in maintaining the soil nutrient content. Based on the soil nutrient element data obtained from the 1:250,000 land quality geochemical survey and the second national soil census, this study investigated the spatio-temporal variations in the SOC and TN contents in the surface soil of the Muling River-Xingkai Lake Plain from 1979 to 2019 using the geostatistical method and the geographic information system (GIS) technique. The results show that the SOC and TN contents in the surface soil of the study area are mainly at grade Ⅲ or above. They are rich or highly rich overall but are deficient locally. During 40 years of evolution, the SOC and TN contents decreased by 25.65% and 29.87%, respectively, and the C/N ratio increased by 6.00%. Compared with those in 1979, the SOC and TN contents in the surface soil decreased in different soil types except for paddy soil. The transition from the natural ecosystem into farmland also decreased the SOC and TN contents. These results indicate that factors such as changes in soil types and land uses have significant effects on the degree of the spatial variations in soil nutrients in the study area.

This study investigated the clay mineral composition of the soil profiles of cropland for soybean/maize rotation, forest land, grassland, and farmland returned to forest in the black soil regions of the northern Songnen Plain through field investigation, the analysis of soil chemical composition, the X-ray diffraction (XRD) spectrum analysis of soil clay minerals, and the analysis of soil weathering degree. The purpose of this study is to explore the chemical composition, clay mineral composition, and weathering change patterns of the soil profiles of the land for different uses and their environmental implications. The results show that in the soil profiles, the land for different uses is in the order of grassland > cropland > forest land > wasteland regarding the potassium, sodium, and calcium contents, in the order of cropland > forest land > grassland > wasteland regarding the organic carbon content in the soil at a depth less than 60 cm, in the order of wasteland > grassland > forest land > cropland regarding the organic carbon content in the soil at a depth greater than 60~80 cm, in the order of farmland returned to forest and wasteland > forest land > cropland > grassland regarding the soil weathering degree, and in the order of wasteland > grassland and forest land > cropland in terms of the soil leaching degree. The clay minerals in the soil of land for different uses area mainly consist of montmorillonites, kaolinites, vermiculites, hydromicas, and chlorites. The cropland, forest land, grassland, and wasteland are dominated by the 2:1 type of non-expansive clay minerals, the 2:1 type of non-expansive clay minerals, the 2:1 type of expansive clay minerals, and the 1:1 type of clay minerals, respectively. As shown by the comparison of the clay mineral assemblages and their contents in the soil of land for different uses in the study area, as well as the calculation of the weathering degree of the soil profiles, minerals such as chlorites in the soil transition to kaolinites and montmorillonites, hydromicas transition to vermiculites, and the environment has changed from a dry and cold climate to a wet and hot climate as the weathering degree and pedogenesis deepen.

Geostatistical and multivariate analyses are effective methods to determine the source of heavy metal elements in the soil. The comparison of the contents and distribution of heavy metal elements in the topsoil and subsoil in different geological setting areas can provide a basis for studying the sources of heavy metal elements in the soil. As one of the areas with concentrated marsh wetlands in China, the Sanjiang Plain has a temperate humid and subhumid continental monsoon climate, is an important grain production base, and has a low degree of industrial activities. This study investigated the main factors affecting the distribution of heavy metal elements in the soil of cultivated land of the Sanjiang Plain on a regional scale. Samples were collected from the topsoil and subsoil of the Sanjiang Plain. The minimum, maximum, median, and average contents, coefficient of variation, and concentration coefficient of eight heavy metal elements (i.e., As, Cr, Cu, Hg, Ni, Cd, Pb, and Zn) in the samples were calculated. The results are as follows. The Sanjiang Plain has superior soil environmental quality. The topsoil samples had slightly lower contents of heavy metal elements except for Cd (e.g., As, Cr, Cu, Hg, Ni, Pb, and Zn) than the subsoil samples. All these elements in the topsoil samples had concentration coefficients less than 2, except for those in the topsoil samples collected at certain points, which had high concentration coefficients of Cd, Hg, and As elements. The main factors influencing heavy metal elements in the topsoil and subsoil in five soil parent material regions were determined through geostatistical and Pearson correlation analyses. The comprehensive analysis results show that elements Cr, Cu, Ni, and Zn mainly originate from the soil parent materials, and Cd, Hg, and As may also be affected by human activities in some areas.

Obtaining accurate soil organic carbon (SOC) content in the Xingkai Lake Plain and the main factor controlling its spatial variation is greatly significant for controlling and restoring the SOC content and achieving sustainable agricultural development. This study investigated the spatial distribution characteristics of the SOC content in the Xingkai Lake Plain and their control factor based on 4,151 topsoil samples collected at a depth of 0~20 cm in the field. Moreover, it compared the effects of soil parent materials, soil texture, soil type, land use type, and land reclamation duration (year) on the spatial distribution of the SOC content in the plain through geostatistical and regression analyses. The results are as follows. The SOC content in the topsoil of the study area is 0.35%~14.49% (average: 2.80%). It has a coefficient of variation of 0.44, indicating moderate spatial variations. It has a nugget-to-sill ratio of 47.06%, indicating that its spatial distributions are affected by both structural and random factors. It is low in the middle and west and is high in the east and north overall. All these five factors have significant effects on the SOC content (P<0.01). Among them, soil parent materials, soil type, land use type, and land reclamation duration can independently account for 6.8%, 3.8%, 9.2%, and 3.3% of the spatial variations in the SOC content, respectively. By contrast, soil texture can independently account for 30.1% of the spatial variations of the SOC content, which is far greater than that of the other four factors. Therefore, soil texture is the main control factor in the spatial distribution of the SOC content in the study area.

As per the Specification of Land Quality Geochemical Assessment and the Soil Environment Quality-Risk Control Standard for Soil Contamination of Agricultural Land (Trial), this study evaluated the soil quality of the Xunke Plain and the soil environmental quality of the plain as green food-producing areas using the 1:250 000 geochemical survey data of land quality. The results show that the Xunke Plain has fertile land and an excellent environment and is suitable for the development of green agriculture. ①The Xunke Plain is rich or highly rich in 16 soil nutrients (i.e., N, P, K, organic matter, CaO, MgO, Fe₂O₃, S, B, Zn, Mn, Cu, Mo, Co, Ge, and V) except for CaO, MgO, Cu, and Zn, which are deficient, and Ge and B, which are highly deficient. This plain mainly has soil nutrients of highly rich and rich comprehensive grades, which cover an area of 6,088 km² and 4,984 km², respectively, accounting for 44.58% and 36.5% of the total area of the plain, respectively. ②This plain mainly has the soil environmental quality of risk-free comprehensive grade, which covers an area of 13,600 km², accounting for 99.59% of the total area of the plain. The portion with controllable risk covers an area of 56 km², accounting for 0.41%. ③This plain mainly has soil quality of excellent comprehensive grade, which covers an area of 11,028 km², accounting for 80.76% of the total area of the plain. The land portion with high-quality soil quality has an area of 2,072 km², accounting for 15.17%, and that with moderate soil quality covers an area of 556 km², accounting for 4.07%. There is no poor (fourth grade) or inferior (fifth grade) soil in the plain. ④The soil of the Xunke Plain meeting the requirements of Class-I (AA) and Class-II (A) green food-producing areas has an area of 13,396.45 km² and 148 km², respectively, accounting for 98.1% and 1.1% of the total area of the plain, respectively. The soil that is unsuitable to be used as green food-producing areas has an area of 112 km², accounting for 0.8% of the total area of the plain. Overall, the Xunke Plain enjoys sufficient soil nutrients and a clean environment and has great potential for developing as green food-producing areas.

The Wuyuer River basin in Heilongjiang Province is an endemic area with a high incidence of Keshan and Kashin-Beck diseases. This study investigated the distribution characteristics of the total content and available contents of the trace elements necessary for plants (i.e., B, Cu, Fe, Mn, Mo, and Zn) in the Wuyuer River basin using the 1:250 000 land quality geochemical survey data of the black land in northeast China. The results are as follows. ①The soil in the basin is highly rich in Mn, rich in Fe, moderately rich in Cu and Mo, and deficient in B and Zn. ②All the trace elements have natural contents and are not rich or deficient in the surface and deep soils of the basin, except for Mo, which is slightly depleted in the soils. ③The soil in the basin is highly rich in available Cu and Fe, rich in available Mn, moderately rich in available B and Zn, and deficient in available Mo. ④It is recommended that Mo fertilizer should be applied in blocks deficient in Mo to increase the content of available Mo in the soil and to improve the yield and quality of agricultural products.

The geochemical characteristics of the major elements in the black soil profiles and their ratios are closely related to the formation and development of the soil. They reflect the degree of the formation and development of soil as well as the environmental changes in the provenance areas. Moreover, they are important geological records of environmental evolution and climate change in the Songnen Plain. To explore the origin, provenance, and chemical weathering intensity of the typical black soil in northeast China, this study analyzed the major elements of nine representative black soil profiles in the Hailun area and then compared these elements with the element geochemical characteristics of typical aeolian deposits. The results are as follows. ①The sum of the content of the major elements (SiO₂, Al₂O₃, and Fe₂O₃) and the UCC standardized curve of the Hailun black soil profiles were highly similar to those of typical aeolian deposits, indicating that the typical black soil in Hailun is aeolian; ②The average chemical index of alteration (CIA) was 63.97. The CIA-Na/K diagrams all indicated a low degree of chemical weathering. By comparison with the weathering intensity of typical aeolian deposits, the weathering intensity of the soil was in the order of loess in Huangshan, Harbin > loess in Xiashu, Zhenjiang > red clay in Xifeng > black soil in Hailun≈loess in Luochuan; ③The black soil at a depth of 0~30 cm has higher w(SiO₂)/w(TiO₂) and w(SiO₂)/w(Al₂O₃) ratios than the black soil below 30 cm, indicating that the late typical black soil in Hailun has a coarser grain size and higher quartz content. The geochemical characteristics of the major elements indicate that the typical black soil in the surface layer and the deep layer may not have the same climate, environment, and provenance conditions, which may change at a depth of approximately 30 cm.

Based on the measured contents of the main trace elements (B, Cu, Mn, Mo, and Zn) in 1175 surface soil samples (sampling depth: 0~20 cm), this study analyzed the contents and spatial distribution characteristics of the trace elements in the soil in Hailun City using both geostatistics and geographic information system (GIS). The results are as follows. The surface soil in the city is deficient in B and Mo. Elements Mn and Mo in the surface soil have nugget-to-sill ratios greater than 75%, with weak spatial autocorrelation. Elements B, Cu, and Zn in surface soil have nugget-to-sill ratios of 25%~75%, with moderate spatial correlation, and their spatial variations are mainly affected by natural factors. Elements B and Cu in the surface soil have similar spatial distribution, with the contents gradually decreasing from the center to the surroundings of the city. The Mn and Mo contents in the surface soil show an obvious zonal distribution, and the Zn content is higher in the north than in the south. There are both symbiotic and antagonistic relationships among trace elements. The soil organic matter content, soil parent materials, soil type, and land uses have different effects on the trace elements in the surface soil in the city.

The total nitrogen (TN), total phosphorus (TP), and nitrogen/phosphorus ratio (N/P) of soil are important indicators of soil fertility and quality. The study of their spatial heterogeneity is of great significance for the formulation of policies concerning soil nutrient management and ecological environment monitoring. Using methods such as geostatistical analysis and geographic information system (GIS), this study analyzed the spatial variation and influencing factors for the ecological stoichiometry of nitrogen and phosphorus in the surface soil (depth: 0~20 cm) of the Jiansanjiang area through soil sampling and laboratory tests. The results are as follows. The soil in the study area has average TN, TP, and N/P of 2.49×10^-3, 0.81×10^-3, and 3.20, respectively. The TN and N/P of the soil have high spatial autocorrelations, while the TP of the soil has a moderate spatial autocorrelation, all in line with the index model. Regarding the spatial distribution, zones with high TN content are mainly scattered in the form of patches in the east, south, and northwest of the Jiansanjiang area, while zones with low TN content are mainly scattered in the northwestern, central, and western portions of the area. Zones with high TP content are mainly distributed in the east and northwest, while zones with low TP content are in the central portion. Moreover, the N/P ratio is distributed in a mosaic-like pattern. Specifically, zones with high N/P ratios are distributed in the form of patches in the central, southern, and northeastern portions, while zones with low high N/P ratios are mainly distributed in the northwest. The types of the soil, the Quaternary, and land uses of the study area are important structural and random factors affecting the ecological stoichiometric characteristics of soil nitrogen and phosphorus, while the types of the soil parent materials and landforms have little effect on these characteristics.

The geochemical classification of soils is significant for agricultural and ecological regionalization. Based on the data on major elements in soil obtained from the multi-purpose regional geochemical survey, this study conducted the geochemical classification for a typical black soil area in northeast China using the principal component analysis combined with the K-means clustering algorithm (also referred to as the principal component clustering method). The results are as follows. The soil parent materials are the main factor controlling the characteristics of major elements in the soil. It is the most appropriate to divide the soil samples from the typical black soil area into five categories using the principal component clustering method. Various samples had significantly different major element contents (P<0.05). The geochemical classification results corresponded to the Quaternary geological units to a certain degree and can better reflect the actual distribution of soil parent materials. Moreover, the high SiO₂ content in the black soil area in the southern Songhua River indicates desertification, to which much attention should be paid in the protection of the black soil.

This study conducted the inversion of the organic matter content in the soil of the black soil area in Sunwu County, Heilongjiang Province using the Sentinel-2A multispectral remote sensing images and the surveyed soil data. After preprocessing the images, the characteristic bands were selected through correlation analysis and using the random forest (RF) method. Subsequently, a multispectral inversion model for the organic matter content of the soil was built using the partial least square method and the BP neural network, and the inversion of the organic matter content of the soil in the Hongqi Forest Farm was conducted. According to the obtained results, the bands selected based on the reciprocal of the logarithm of the first-order differential of reflectance through the correlation analysis and the combined bands selected using the RF method can effectively improve the inversion precision of the organic matter content in the soil, and the RF-BP neural network model for the combined bands yielded the optimal inversion performance (R²=0.7245 and RMSE=1.3127%). The results of this study will provide technical support and reference for the dynamic monitoring of the organic matter content in soils.

This study restored the balanced cross sections of the Tabei sag and calculated the extension amounts and extension rates of the sag using three typical seismic profiles selected from the western Tabei sag,Erlian Basin using the 2D Move software.Then,it analyzed the geological and tectonic evolution characteristics of the Tabei sag since the Early Cretaceous.The results indicate that:(1)Due to the effects of the main normal fault layer on the northwest side,the whole Tabei sag has evolved into a single-faulted dustpan-shaped faulted lake basin that is faulted in the west and is overlapped in the east.Moreover,it consists of a fault in the lower part and a depression in the upper part vertically.(2)The variation characteristics of the extension amounts and extension rates in each period of the Tabei sag match the tectonic evolution process of the sag,and all of them show a strong to weak trend,indicating that the extension in the Tabei sag is in a weak-strong-weak trend.(3)The Tabei sag mainly underwent four tectonic movements:①During the sedimentary period of the Aershan Formation,the faults in the Tabei sag began to develop,the sag subsided under the control of normal fault activities, and the Tabei sag began to take shape.②During the sedimentary period of the Tenggeer formation,the faults inside the sag continued to develop,and the whole sag was significantly controlled by the fault depression.The extension amount and extension rate gradually increased and reached their peaks during the upper Tenggeer formation.③During the sedimentary period of the Saihan formation,the Tabei sag was slightly affected by faults overall,and the sedimentary center was gradually far away from the main fault. During this period, the Tabei sag was characterized by depression,subsidence,and overlap unconformity.④Since the end of the Saihan formation,the fault depression activities have disappeared,the basin has gradually contracted, and the terrain has tended to be flat.Since then,the Tabei sag has been continuously uplifting.

Sandstone type uranium deposits in Erlian Basin are concentrated in the Ba-Sai-Qi ancient channel. The northern part of the channel is mostly covered by basalt, which increases the difficulty of uranium exploration. According to the electrical characteristics of the basalt-covered area, the audio frequency magnetotelluric survey (AMT) was optimized and carried out to obtain uranium metallogenic information such as fracture, sand body, basalt distribution, basement, and tectonic framework, providing deep electrical data support for evaluating the prospecting prospect in the area. There are ternary electrical structures-basalt cap, sedimentary strata and basement rock in basalt-covered area. The sand bodies and fractures of Saihan Formation favorable for uranium mineralization are well developed in the area, and the deep thermal fluid can provide heat source for uranium superimposed mineralization. Uranium accumulation in sedimentary strata near basalt caprock should be considered in uranium exploration, and also uranium source conditions. Therefore, it is suggested to find out the NNW-trending fracture distribution in the west of basalt-covered area so as to provide a basis for uranium exploration of hydrothermal superimposed mineralization.

Physical property parameters are the link between geophysics and geology and are crucial to the rational inversion of geophysical interpretation and inference. To systematically grasp the physical properties of rocks (minerals) in Hanzhong and Ankang areas to the south of the Qinling Mountains, improve interpretation accuracy, and serve basic geological research and the surveys and evaluation of mineral resources, this study collected 458 physical property points of rocks, 13 740 pieces of susceptibility data, and 458 sets of energy spectrum data in the hinterland of the Hanzhong and Ankang areas at the southern foot of the Qinling Mountains. Then, this study classified and made statistics of the physical properties of rocks. The results of magnetic susceptibility data show that the Paleoproterozoic basic and partial intermediate rock masses are ferromagnetic geological bodies in the study area, with an average magnetic susceptibility of greater than 4 000×10^-5 SI, and gabbro has the highest magnetic susceptibility; the dominant magnetic strata in the area are the Paleoproterozoic Xixiang Group and the tuff in the Sinian Yaolinghe Formation, which have average magnetic susceptibility of mostly close to 1 000×10^-5 SI, and the high magnetic susceptibility can reach more than 2 000×10^-5 SI. According to laboratory testing, most of the magnetic geological bodies in the study area have induced magnetism, and a few tuffs, gabbros, and magnetite-bearing gabbros in the study area have strong remanence, which must be considered in the anomaly inversion. The physical property analysis based on energy spectrum parameters shows that the Paleozoic and Sinian metamorphic rocks in the study area have high radioactivity and highly variable content, among which black rocks (carbonaceous) generally have high radioactivity. As indicated by two examples of western Miaoba Village and the southeastern Shiquan County, magnetic, radioactive, and physical-property parameters are important for the interpretation of magnetic and radioactive data, and they establish a link between aerial magnetic and radioactive information and geological information, improving the reliability of interpretation.

This study explored the deep occurrence characteristics of the Malugou fault and its secondary faults in the study area based on the data obtained using audio-frequency magnetotellurics (AMT), aiming to provide a basis for the drilling layout and improve the effects and benefits of the geological exploration of uranium deposits. The feasibility of the AMT method was demonstrated by analyzing the response characteristics of the forward theoretical model of the Malugou fault and its secondary faults. Based on this, this study deduced and interpreted the inverted resistivity cross-sections by combining geological setting, the electrical parameters of rocks, and the drilling data collected. As a result, the deep development characteristics of faults in the study area were roughly identified, and the details are as follows. The Malugou fault (F₁₀₁) has an NW strike, a dip angle of approximately 80°, and dip directions of NE and SW along sections X501-X502 and sections X502-X506, respectively. Secondary faults F₁₀₂ and F₁₀₃ have an NW strike, a dip direction of NE, and a dip angle of 75°~80°. The above-mentioned faults are characterized by large cutting depths and high dip angles.

The quality of seismic data plays a critical role in geological interpretation.However,the real seismic data usually contain a lot of noise,leading to fuzzy strata and unclear fault structures.The non-local means (NLM) filtering algorithm can effectively suppress random noise,but its computational efficiency is low.Therefore,it has limitations when being applied to large-scale seismic data processing.This study proposed a fast adaptive NLM algorithm,for which the computational efficiency was improved using the centrosymmetric data integration algorithm and the filtering parameters were adaptively adjusted using the standard deviation of similarity to estimate the homogeneity,thus further improving the noise attenuation effect.Therefore,the modified NLM filtering algorithm can effectively improve computational efficiency and enhance the noise attenuation effect.Furthermore,the feasibility and effectiveness of the algorithm were verified using model data and actual data.

The current hydrocarbon exploration targets are concealed,scattered,thin,and small.These characteristics put forward higher requirements for the migration imaging technique.Owing to the special acquisition method,the data derived from borehole seismic have the advantages of high resolution,rich wave field information,and less interference.In theory,borehole seismic can be used to realize high-precision imaging of complex reservoirs,such as concealed,scattered,thin and small ones around the well.Well types greatly limit the layout of the seismic sources.In practice,besides vertical wells,there are also many types of wells,such as inclined wells,curved inclined wells,and horizontal wells.For different well types,the seismic sources at the same depth have different positions and the same number of seismic sources have different spatial distributions,leading to significantly different seismic wave propagation paths and further affecting the imaging quality.However,there is no qualitative or quantitative understanding of the effects of well types on migration imaging currently.Using the visco-acoustic inverse time migration imaging method,this study analyzed the effects of well types on migration quality by comparing the seismic migration imaging results of theoretical models under various well types.The numerical results provide the qualitative relationships between well types and borehole seismic migration imaging quality and effective imaging range.The results also provide corresponding theoretical support for the design of a borehole seismic acquisition system.

The numerical simulation of visco-acoustic media using the high-order Fourier finite-difference method can reflect the seismic response of high-dip strata with geo-absorption effects more accurately.It can adapt to any lateral speed changes and suppress the dispersion and background noise at large dip angles caused by the finite-difference method.The migration precision of the high-dip strata depends on the determination of the constant coefficient of the difference operator and the calculation of the order.In this study,the gradient descent method was used to optimize the high-order finite-difference correction item in the Fourier finite-difference operator.According to the optimization results of the relative errors and constraint coefficients,the approximation effects of higher-order equations were achieved without increasing the order of the equation and then were expanded to viscoelastic media.Using the designed model,it can be concluded that the proposed method is applicable to the forward simulation of the strong spatial variable speed media with absorption and attenuation effects and has high calculation accuracy and efficiency.Accurate seismic numerical simulation of complex geological structures further confirmed the effectiveness of this method.

Data reconstruction is a critical preliminary work in the processing of seismic data.Compressed sensing (CS) has been well applied in data reconstruction.The key to CS is random sampling,which converts the mutual coherent alias caused by regular under-sampling into lower-amplitude incoherent noise. But traditional sampling methods lack constraints on sampling points, resulting in excessive noise interference. The segmented random sampling (SRS) method can effectively control the distance between sampling points. Furthermore, a single mathematical transformation will lead to incomplete sparse representation and impact data reconstruction. The morphological component analysis (MCA) can decompose a signal into several components with outstanding morphological features to approximate the complex internal structure of data. A new dictionary combination (Shearlet+DCT) has been found under the MCA framework, and the block coordinate relaxation (BCR) algorithm has been used to get the optimal solution to obtain desired reconstruction results. Tests of real data have proven that the proposed method can produce good effects when used to reconstruct the SRS data.

The iterated closest contour point (ICCP) algorithm results in large matching errors and even false matching in areas with small geomagnetic variations. Given this, this study established an error model for the ICCP algorithm in order to determine the causes of the large matching errors in the areas. Based on this model, this study proposed an iterated search closest contour point (ISCCP) algorithm suitable for geomagnetic gradient matching for navigation. In the proposed algorithm, the closest point search method was improved based on the ICCP, and an iterative search was performed using the geomagnetic gradient information of three orthogonal directions. The simulation results show that the average point error of the ISCCP algorithm can be controlled within half a grid length, with a precision about 15 times higher than that of the ICCP algorithm. Therefore, the ISCCP algorithm proposed in this study can effectively eliminate the large matching errors in areas with small geomagnetic variations caused by the ICCP algorithm.

Industrial frequency noise comes from the electromagnetic noise produced in social activities, and it causes apparent resistivity curves to become pathological or divergent. To improve the accuracy of data processing and interpretation, this study used the Hilbert-Huang transform (HHT) to remove the common power frequency noise in EH-4 data. According to the time series processing and analysis results of measured data, this method can self-adaptively decompose signals according to the time-scale characteristics of the data and successfully remove the industrial frequency noise in the data, thus providing an effective way to remove the noise in magnetotelluric signals. In addition, this study also analyzed the serious modal aliasing and "end effect" occurring in the process of the empirical mode decomposition and decomposed simulation signals and the time series of measured data using the ensemble empirical mode decomposition (EEMD), effectively solving problems such as modal aliasing.

Compared with Euler deconvolution, joint Euler deconvolution of multi-component gravity gradient data features higher calculation accuracy and inversion resolution. To eliminate the divergent solutions of calculation, different screening methods must be used in the application of joint Euler deconvolution of multi-component gravity gradient data, making the calculation process cumbersome. It is evident that effective screening methods and developing a piece of easy-to-use and visual software can improve the accuracy, convenience, and effects of the joint Euler deconvolution of multi-component gravity gradient data. Therefore, this study proposed the joint Euler deconvolution of gravity gradient data with the constraint of edge detection based on correlation coefficients. Moreover, on the design principles of the intuitive interfaces, practical functions, and concise codes, this study designed a software system with functions such as data/file management, two-dimensional/three-dimensional visualization, edge detection, and joint Euler deconvolution of multi-component gravity gradient data using Python language and its function library to meet the requirements of algorithm flow and functions. The accuracy of calculations and the practicability of the software have been verified through a theoretical model and tests of measured data, proving that the software designed in this study can improve the application effects.

Geographical base map is an important basic step and component of airborne geophysical thematic map; nevertheless, the previous treatment methods seem to be rather complicated. This paper analyzes the function and present situation of the thematic base map of aero geophysical exploration and introduces the spatial analysis method. With 1:250 000 public versions of terrain data, this paper discusses the idea and method of compiling the thematic base map of aerial geophysical exploration, and completes the vectorization of geographical elements. The Dazhou-Huaying survey area in Sichuan Province is chosen to perform test. It is shown that the basemap designed by this method is simple and practical and, compared with previous processing methods, it is more intelligent, and that the production results are fast and accurate and can meet the relevant precision requirements. This method provides a strong support for the accurate production of airborne geophysical thematic maps, and it is easy to use. At the same time, it also lays a foundation for the development of airborne geophysical exploration.

The processing and interpretation of the data derived using the transient electromagnetic (TEM) method are still mainly conducted through one-dimensional (1D) inversion presently. Therefore, developing an efficient and stable 1D inversion system with complete functions is greatly significant for further promoting the applications of the ground-based loop-source TEM method in China. This study developed such an inversion system, which involves four methods, namely minimum-structure inversion, Occam's inversion, laterally constrained inversion (LCI), and spatially constrained inversion (SCI). The first two methods are based on the Gauss-Newton method, and the others are based on the damped least-square method. This 1D inversion system was applied to detect the occurrence forms of the basalt rock masses in the Narenbaolige coalfield in Inner Mongolia. Then, the results derived from the minimum-structure inversion and Occam's inversion used in the 1D inversion system were compared with those obtained using the commercial software IX1D. As shown in the 2D pseudosection maps of resistivity obtained by these inversion methods, the basalt rock masses have similar occurrence forms and consistent resistivity range. Compared with drilling data, these 1D inversion results clearly reflected the distribution patterns of basalt rock masses in the Narenbaolige coalfield except for the upwelling channels of magmas. Afterward, the LCI and SCI were also applied to the coalfield. The results indicate a decrease in the resistivity differences of the basalt rock masses between adjacent survey points and an increase in the continuity of the interfaces between the basalts and sedimentary rocks.

Nonlinear optimization algorithms can be used to conduct a global search for the optimal solutions within a given parameter range, inherently making them highly competent in performing a global search and escaping from local extrema.In this study,an emerging nonlinear optimization algorithm-the sparrow search algorithm (SSA) was introduced for the inversion of Rayleigh wave dispersion curves.To address the problems of multiple parameters and local extrema, adaptive t-distribution was introduced.The data acquired from the inversion experiment of three theoretical models indicate that the improved SSA has high inversion accuracy,stability,and resistance to random noise compared with the conventional SSA.Furthermore,the improved SSA can yield better performance than particle swarm optimization and differential evolution algorithm due to its capability to achieve a more reasonable balance between the early global search and late local search in the process of iteration.

The microtremor exploration technique has been widely used in the geological exploration of a complex urban environment due to its strong spatial adaptability and anti-mechanical interference ability.The forward modeling of microtremor characteristic curves is all based on a horizontally layered medium model,while actual strata are typical inhomogeneous geological models.The inconsistency leads to the unsatisfactory fitting degree of characteristic curves.Therefore,it is necessary to conduct forward modeling research based on the condition of inhomogeneous media.Since the geological model based on the assumption of horizontally layered media cannot meet the requirements for high-precision forward modeling of microtremor characteristic curves,forward modeling needs to be conducted using an inhomogeneous model.Based on the dynamic optimization method,this study proposed the forward modeling technique of microtremor characteristic curves to realize the forward modeling of high-precision characteristic curves of inhomogeneous media.In this technique,the aperture range and the optimization coefficients varying with frequency were calculated based on the theoretical characteristic curves of equivalent horizontally layered media.Taking spectral ratio curves as an example,the forward modeling technique of microtremor characteristic curves using the dynamic optimization method can obtain the shear wave distribution of inhomogeneous media through inversion,achieving higher lateral resolution and exploration precision.

To improve the detection level of foundation piles and reduce major potential safety hazards, this study established three geoelectric models corresponding to a single pile, long-short piles, and a pile group through the 2.5D forward and inverse numerical simulations using the cross-well ultra-high-density resistivity imaging technology and analyzed the response characteristics and regularity of these models. Given the complex detection environment of foundation piles, this study further expounded the application characteristics and effects of the technology by combining two cases for the detection of the burial depths of piles. The results are as follows. The cross-well ultra-high-density resistivity imaging technology enjoys the advantages of high precision and flexible and convenient construction when being applied to the detection of pile buried depth. It can detect the lengths of foundation piles on a large scale rather than detecting one foundation pile using one detection hole, thus greatly improving the detection level of the burial depths of foundation piles.

Quickly and efficiently identifying the hidden dangers inducing ground collapse in cities are greatly significant for urban construction, disaster protection, and protection of people's properties. In this study, the urban shallow ground collapse was classified into cavities caused by dissolution, disorderly drainage of groundwater, and old civil air defense works. Through forward calculations, this study analyzed the response laws of the geological models of the three types of cavities using the opposing-coils transient electromagnetic method (OCTEM), as well as the various characteristics of attenuation curves of the models under high resistance and low resistance overburden strata. Moreover, this study investigated the electrical characteristics of the geological models of the three types of cavities using the rate of change in the transient electromagnetic responses of rocks. The forward results are as follows. Compared with the surrounding rocks, both the models of cavities caused by dissolution and disorderly drainage of groundwater showed low resistance characteristics, while the model of cavities caused by civil air defense works showed high resistance characteristics. The rates of change in the transient electromagnetic responses of the three models show that the opposing-coils technology has a good ability to identify the hidden dangers inducing all kinds of cavities. The application results of the OCTEM to the detection of three types of cavities in areas such as Kunming and Zhengzhou show that this method is effective for the detection of urban shallow cavities.

Aiming at the low-detection precision of traditional geophysical prospecting inversion methods for unfavorable geological conditions such as karst caves in the early detection stage of infrastructure projects, this study proposed a cross-hole resistivity detection and inversion method of karst caves based on the squirrel search algorithm to improve the performance of the traditional Tikhonov regularization-based sensitivity iteration method, which is sensitive to initial values and noise and easy to fall into local optimization. The detection results obtained using different intelligent search algorithms and sensitivity iteration methods were compared and analyzed using three numerical examples of small, large, and beaded karst caves. Moreover, an indoor physical model was also built to validate the proposed method. The results show that the inversion method based on the squirrel search algorithm has a high convergence speed and precision and can significantly improve the detection precision of karst caves using the cross-hole resistivity method.

The large fixed-source loop is a commonly used device in the transient electromagnetic method (TEM). However, moving its transmitter requires a lot of manpower and time, greatly reducing the efficiency of the method. The electrical source TEM enjoys the advantages of large detection depth, less terrain restriction, and high efficiency. However, its signal intensity severely attenuated and it has a low signal-to-noise ratio in the case of a large offset, which limit its detection precision to a certain extent. To achieve high-precise geological exploration, this study built a forward model using the multi-excitation source TEM. With the detection of a water-bearing goaf of the Weijiadi coal mine in Gansu Province as a case study, this study compared the exploration performance of the multi-excitation source TEM with that of the conventional large fixed-source loop and the single-excitation source TEM. As verified by drilling, the multi-excitation source TEM can deliver better exploration performance in the study area. The results of this study can provide technical support and a reference for goaf detection in adjacent and similar areas.