The crust preserves key geological records of the Earth’s long-term tectonic evolution, hosts a variety of mineral resources, and is the source region of many natural hazards such as earthquakes and volcanic activity. Deep seismic sounding (DSS) is an important geophysical approach for investigating the structure and material composition of the crust and lithosphere. Owing to its large source energy, long observation profiles, and great investigation depth, DSS provides unique advantages and plays an irreplaceable role in probing deep Earth structures. This paper systematically reviews fundamental principles, technology, history, current status, and development trends of DSS methods. Looking forward, DSS is expected to play an increasingly important role in deep Earth exploration. Addressing key scientific questions in typical tectonic regions, as well as national strategic demands such as energy security, mineral resource supply, and environmentally sustainable development, requires further advances in related technologies. These include source technologies (e.g., air-gun sources and controlled seismic sources), observation technologies (e.g., joint acquisition of DSS and deep seismic reflection data, integrated active-passive observations based on dense short-period arrays, and distributed fiber-optic sensing techniques), and data processing and imaging methods (e.g., joint imaging of active and passive seismic data, integrated DSS and deep seismic reflection imaging, and full-waveform inversion), DSS methods will play an increasingly significant role in the field of deep structural investigation and resource exploration.

The Mandula-Tongchuan-Fuling ultra-long seismic profile spans a total length of 1700 km, traversing from north to south through the Yin Mountains Orogenic Belt, the Ordos Basin, the Qinling Orogenic Belt, and extending to the southern edge of the Sichuan Basin near Fuling, with a diverse interlacing of basin and mountain systems. Based on the upper crustal velocity structure image using the refraction seismic data of this profile, our study has confirmed that: (1) The Qinling-Daba Orogenic Belt belongs to the same crystalline basement. The basin-mountain zone basement is highly undulating and orderly, forming a coupled chain of diverse basin-mountain sequences; (2) The two major basins have thick depressions cut by faults, with the crystalline basement layers whose depth reaching 4.0 similar to 6.0 km (Ordos Basin) and 6.0 similar to 10.0 km (Sichuan Basin); (3) Along the profile, the southern two orogenic belts are metallogenic belts, while the two sedimentary basins are areas for the accumulation and storage of oil and gas (or enrichment areas).

In the geophysical study of the Tibetan Plateau,the velocity distribution of crust and mantle media is the basis for exploring the behavior,trajectory and dynamic response of material movement in the hinterland and surrounding areas of the plateau.Through the high-precision artificial source seismic exploration and inversion implemented in plateau areas for more than 50 years,the characterization of the fine structure of crust and mantle medium and the velocity distribution of each sequence have been obtained.Based on artificial source seismic deep exploration,reference and natural seismic and MT sounding results,the average velocity structure model of the crust and mantle layer medium in the Tibetan Plateau is accordingly established.Paying attention to both physical structure and velocity structure,developing high-precision observation equipment and multi-dimensional information fusion technology,and establishing the fine velocity structure model of the crust and mantle layer medium of the Tibetan Plateau,are the future research direction.

It is widely accepted that the horizontal tectonic forces and lateral rheological contrast control the strain localization and, consequently, the evolution of mountain ranges. However, in active orogenesis involving massive surface materials redistribution, to what extent and in which way do surface processes and strong contrast in vertical rheological properties manage the stress transmission and strain partitioning, have been the subject of critical debate and discussion. The LongMenShan (LMS) area exhibits the steepest topographic gradient around the Tibetan Plateau. Understanding the interplay among the abnormal steep relief, slow convergence rate, and rapid erosion rate is crucial for comprehending how surface processes and vertical rheological configurations contribute to the evolution mechanisms of the Eastern Tibetan Plateau (ETP). In this study, 2-D finite element models were employed to retrieve robust relationships between surface topography and deep material transport in the ETP and LMS. These models considered fundamental mechanical parameters, particularly the erosion rate and mid-crustal rheological configuration. The results indicate that regional tectonization governs the tectonodeformation mechanism and surface geomorphic characteristics of the study area. Differential erosion is shown to significantly influence the shaping of peripheral topography and alter the substantial seismic potential across different faults. Between the LongRiBa Faults (LRBF) and the LMS, the weak mid-crust material decouples the upper and lower crusts via vertical strain partitioning. This decoupling affects the formation of deep detachment structures in the LMS and promotes the brittle shortening of the upper crust, contributing to the region's steep relief. Lastly, our results suggest that channel flow and brittle crustal shortening modes are not mutually competitive but rather coexist in different regions of the ETP and LMS. Deep processes, influenced by inherited lithospheric heterogeneities, play a decisive role during the formation of the ETP and LMS. Our results underscore the importance for understanding the underpinning geodynamic mechanisms that shape the eastern margin of the Tibetan Plateau.

This paper presents a comprehensive review on recent development and research conducted in domestic and international underground laboratories. We first introduce the differences in three environments—surface, mountain tunnel cavities and underground coal mine tunnels—by examining cosmic ray background, ambient noises related to gravity and seismic measurement, and electromagnetic noises in magnetic and magnetotelluric measurements. We highlight potential misuse of the term Underground Lab or Deep Underground Lab when describing observations in different physical fields. We introduce unique features of underground coal mine tunnels in China, such as large spaces, ultra-quiet conditions, and ultra-clean environments. When comparing with mountain tunnel cavities and borehole observations, coal mine tunnel observations have superior long-term stability and high precision. Through observations and comparisons of multi-physic fields at surface and the deep underground, we find that the higher SNR seismic observations conducted in deep underground tunnels in coal mines are beneficial to improve velocity tomography of the solid earth. The gravity observation with a Superconducting Quantum Interference Device (SQUID) makes it possibly to capture slow earthquake, which has not been observed previously in the Chinese mainland. SQUID magnetic observations can detect fluctuations as weak as femto-Tesla (fT), enabling us to explore the attenuation of Schumann Resonance down to the solid Earth. This opens opportunities to investigate the connections between the Earth’s magnetic field and the interactions within the human brain and heart. To improve the precision of quantum measurement, we should consider the possible effects of weak magnetic disturbances in deep underground environments. Finally, we discuss the importance of deep underground laboratories, observing facilities and techniques deployed in these laboratories, and their possible connection with respect to “deep space” and “deep ocean” exploration, emphasizing the need for focused research on various scientific challenges. We hope to encourage greater attention to deep underground laboratory and high-precision scientific observation.

We develop a Fortran package with high programming optimization and parallel computing for simulating high-frequency (>1 Hz) teleseismic wavefields using a hybrid numerical method that couples the finite-difference (FD) and frequency-wavenumber (FK) methods. This method can simulate the interactions of incoming teleseismic wavefields with local heterogeneities but reduce computational region to a much smaller localized domain, which can significantly reduce the computing cost of the high-frequency teleseismic wavefields. The local heterogeneities are allowed to vary arbitrarily in a localized heterogeneous domain. In this package, the geographical locations of earthquakes are permitted, which can consider the real azimuthal effect of the source. Numerical benchmark tests first demonstrate the effectiveness of the developed method for P- and S-wave receiver functions (RFs). The consistent travel times of synthetic and theoretical RFs phases demonstrate its high accuracy. Application on a dense array generally obtains consistent RFs profiles with observed ones and successfully reproduces the observed common-converted-point (CCP) stacking image, which further verifies the effectiveness of the presented method. In addition, statistics of the timeconsuming of typical models illustrate the high efficiency of this package, which needs very little computing resources even to be feasible on a laptop.

Determining the water content in the lithospheric mantle is crucial for understanding its dynamic evolution. Because the electrical conductivity of mantle minerals is particularly sensitive to water, the magnetotelluric (MT) method becomes a vital tool to determine the water content in the lithospheric mantle. Here we used broadband and long-period MT data collected along a 600-km-long, NS-trending profile to obtain the electrical resistivity structure of the lithosphere across the southwestern South China Block. By combining the results of laboratory electrical conductivity measurements of mantle minerals, xenolith-derived composition, and geotherm information, we further estimated the water content of the lithospheric mantle. The results show that the Youjiang Basin has a relatively thin lithosphere segmented by zones of low-resistivity that spatially coincide with major faults. The relatively conductive mantle lithosphere could be explained by the combined effects of water in nominally anhydrous minerals, sulfide and phlogopite. Combined with regional tectonic context, we proposed that H2O-rich fluids derived from the previously subducted slabs and related metasomatic processes lead to lithospheric hydration and thinning within the Youjiang Basin. Additionally, such processes, together with magmatic-hydrothermal activities, likely contribute to the formation of gold deposits within the basin. By contrast, the lithosphere beneath the Yangtze Craton is characterized by high resistivity extending to a depth of similar to 200 km, representing a typical cratonic lithosphere that has not undergone significant tectonic modification and contains no or very little water.

Although the early-Precambrian crystalline basement is now only sporadically exposed in the northern and south-western parts of the Yangtze Block, it is supposed to have a widespread distribution beneath its Neoproterozoic and Phanerozoic covers. Here we present results of regional aeromagnetic data processing in consideration of remanent magnetization to investigate the spatial distribution of the early-Precambrian basement buried deep under the Upper Yangtze Block and surrounding areas. The direct analytic signal amplitude of the aeromagnetic anomalies, which is less affected by the magnetization direction, reveals a broader basement below the Sichuan Basin, extending far north to the Micang Mountain. A comparison between the direct analytic signal amplitude with the reduction to the pole aeromagnetic anomalies indicates that possible remanent magnetization exists beneath the Micang Mountain near the boundary between the Qinling Orogen and Sichuan Basin. The automatic depth from extreme points transform is then performed on the direct analytic signal amplitude to estimate the depth to the early-Precambrian crystalline basement. A synthetic model of a magnetic interface with remanent magnetization and random noise shows that the depth from extreme points method is able to resolve variable basement depths. Application of the depth from extreme points method to the direct analytic signal amplitude of the Upper Yangtze Block presents meaningful results about the early-Precambrian crystalline basement undulations. It is shallow and uplifted beneath the Sichuan Basin, extending north to the Qinling-Dabie Orogen, probably corresponding to the ancient Chuanzhong palaeo-uplift. Although it gradually deepens to the east, the deepest basement is buried under the Jiangnan Orogen, which is likely associated with the collision-induced crustal thickening between the Yangtze and Cathaysia blocks during the assembly of the Columbia supercontinent. Large gas fields around the Sichuan Basin are found at the slopes or depressions between basement uplifts, indicating that the deep marine carbonate rocks in the south and east of Sichuan Basin, particularly those located at the slopes or depressions between ancient basement uplifts, are favourable targets for further petroleum exploration.

Most of the subcontinental lithospheric mantle (SCLM) beneath Proterozoic cratons consists of refertilized Archaean SCLM. Variations in SCLM composition and its physical properties significantly affect the stabilization and preservation of the ancient continents. In this paper, aeromagnetic data are analyzed to reveal the magnetic structure of the lithospheric mantle beneath two major Precambrian blocks in central-eastern China, i.e., the Upper Yangtze Block (UYB) and Ordos Block (OB). After being reduced to the pole, the Fourier power spectrum of the aeromagnetic anomalies is calculated to determine the depth to magnetic sources. Considering the lower spatial resolution of the power spectral analysis in dealing with the long-wavelength aeromagnetic anomalies, we applied the scale-normalized continuous wavelet transform (CWT) on the magnetic data to trace the magnetic sources, with special focus on deeper ones. Synthetical model of a magnetic layer and application to the profile data validate the effectiveness of this scale normalization scheme in improving the wavenumber/spatial resolution in the CWT scalogram. In order to present a detailed magnetic structure, we carried out 2.5D forward modeling work on the magnetic data of a 2280 km-long nearly N-S profile across the UYB and OB. Due to the inherent ambiguity in the modeling results, the CWT-based spectral analysis is successfully adopted to provide source depth constraints for the initial model. The constrained forward modeling results indicate strong inhomogeneities among main tectonic blocks of studied area, like humans have different fingerprints. The magnetization of OB is larger than that of UYB since its Archean to Paleoproterozoic metamorphic basement are widely exposed at the surface, while the Precambrian basement of UYB is mostly overlain by unmetamorphosed Sinian cover and weakly metamorphosed Neoproterozoic strata. The most interesting aspect is that deep-seated magnetic sources might reside in the uppermost mantle of UYB and OB, suggesting vertical layering in the SCLM in cold cratonic regions.

The South China Block is formed by the assemblage of the Yangtze Craton and the Cathaysia Block in Neoproterozoic, with a disputed southwest boundary location. Based on a north-south orienting broadband seismic array crossing the Youjiang Basin and the East Sichuan fold belt, we obtained the variation of seismic upper mantle anisotropy beneath Southwest South China Block by SKS wave splitting measurements. The results show from 26 N to south, the fast wave polarization direction of Youjiang Basin is mainly E-W and NEE-SWW, with delay time varying between 0. 5 similar to 2. 5 s. The upper mantle anisotropy of the eastern Sichuan fold belt includes many null values and its fast wave polarization direction is dominantly E-W, with delay time varying between 0.5 similar to 1.5 s. In the transition zone of the Youjiang Basin and the East Sichuan fold belt, the upper mantle anisotropy is obviously weak. Combined with previous study of upper mantle velocity structure, we suggest the transition zone between East Sichuan fold belt and Youjiang Basin is the south boundary of Yangtze Craton where the extremely thick lithosphere of the East Sichuan fold belt thinned down to the south. The sharp lithospheric boundary change induced the upwelling of asthenosphere whose vertically motion resulted in the weak horizontal azimuth anisotropy above the area.

In the regions from the northern Ordos Basin to the Central Asian Orogenic Belt (CAOB), many major geologic events have successively happened, such as the craton formation in the early Pre-Cambrian, the closure of the Paleo-Asian Ocean in the Paleozoic, the intra-continental orogeny in the Mesozoic, and the rifting and sagging since the Cenozoic. There are large scales of energy and mineral resources, and the Hetao Seismic Belt, and thus the characteristics of the crustal and upper mantle structure would be helpful to reveal the geological evolution and mechanisms of resource enrichment and seismogenesis. In this paper, constrained by the P-wave velocity model constructed by a Deep Seismic Sounding (DSS) investigation along the same south-northward profile from Mandula Town in the Inner Mongolia Autonomous Region to Yanchuan County in the northern Shaanxi Province, China, we carried out the joint inversion of multi-frequency P-wave receiver functions (RFs) and surface wave dispersions to obtain the S-wave velocity model beneath the profile. And then, we conducted the Common Conversion Point (CCP) stacking of RFs to construct the geometric structure of the crust and upper mantle constrained by both models derived from the DSS and joint inversion mentioned above. Some characteristics of the crustal and upper mantle structure along this profile are revealed and listed as follows: (1) the Moho topography fluctuates within the range of 35～48 km, and shows a nearly symmetric pattern centered by the Hetao Graben System and featured by step-wise converging and deepening Moho discontinuity; (2) the Moho discontinuity beneath the Hetao Graben System is uplifted with a dome shape, and the domal area coupled with upper mantle low-velocity zone is just beneath the Khondalite Belt; (3) the seismicity in the transition zone of the Daqingshan, Wulashan, and Sertengshan mountains, show two spatially isolated patterns of the flower structure with distinct velocity architectures. These features indicate (1) the crustal pattern beneath the north part of the profile mainly reflects the strong north-south compression and shortening deformations induced by B-episode events of the Yanshanian Movement within a tectonic framework of the amalgamation of the Ordos and Yinshan blocks in the Paleo-Proterozoic and the orogeny of the CAOB in the Paleozoic, and also impose the subsequently extensional deformation since the Cenozoic, especially from the strong rifting and sagging since the Neogene; (2) At the collision zone between the Ordos and Yinshan blocks, was the rifting of the Hetao Graben System initiated in the Oligocene, which implying the junction zone between the continental nuclei of the craton is still tectonically weak up to today; and (3) the Daqingshan and Sertengshan piedmont faults are the major faults with normal and striking senses, while the Wulashan piedmont fault and the northern marginal fault both play the adjusting roles between them. The seismogenic fault of the 1996 Baotou MS6.4 earthquake is likely the deeply extending part of the eastern part of the south-dipping Sertengshan piedmont fault.

Using the gravity anomaly dataset of the 1320 km long profile from Mandula (Inner Mongolia) in the south, through Mongolia to the Kachujia (Russia) in the north of Baikal, the 2D crustal density structure model has been constructed in this study. We have analyzed the deep structural features along with the Moho interface of the profile including the gravity anomaly, the crustal density structure, the interface undulation and fault distribution for each tectonic unit. This result can provide the proof of gravity field for the further understanding of the crustal structure of the area across the profile, the boundaries and correlations of tectonic units, and the related continental dynamics.

The Youjiang Basin (YB) on the southwestern margin of the Yangtze Craton (YC) contains >200 Triassic or Cretaceous gold deposits, but deep dynamic processes contributing to large-scale gold accumulation remain unknown. Based on a south-north trending linear broadband seismic array, we obtained the crust and mantle structure beneath the YB and southwestern YC by P-wave and S-wave receiver functions and ambient noise tomography. Our results show a remarkable lithospheric boundary beneath the northern margin of the YB, which separates the YB with felsic thin crust and thin mantle lithosphere from the YC with both a thick crust and thick mantle lithosphere. A new structural image shows that the southern end of the YC Moho dips southward and overlaps with the YB Moho. A 5-km-thick high-velocity layer is also found at a depth of 10 km beneath the northern YB. Combining this information with previous tectonic and geological results, we reveal the process of Mesozoic lithospheric evolution in the southwestern South China Block. We infer that both the pre-Mesozoic weak crust and mantle lithosphere and the intense intracontinental orogenesis in the Early-Middle Triassic contributed to the formation of first-stage Au deposits in the YB. Lithospheric delamination in the Early Cretaceous thinned the lithosphere of YB rather than YC, forming a steep lithosphere-asthenosphere boundary beneath the transition zone between YB and YC, which favored upwelling of Au-rich asthenosphere below the thick cratonic root to the bottom of the crust in the northern YB and contributed to the second-stage Au deposits in the YB.

Characterized by lower background condition, underground laboratories provide important experimental space for many frontier researches and have become national key scientific research platforms in many countries. At present many countries in the world have established underground laboratories by using cavities in mountains or underground mining tunnel. Most of them are mainly for astrophysics and particle physics experiments, and few of them are aimed at geoscience, rock mechanics, dark ecology, etc. Based on the introduction of some important underground laboratories in the world, this paper focuses on observations and experiments in geophysics. And then the measurement indices of multi-physical fields that can be achieved by observing in "ultra-silent" and "ultra-clean" deep underground environment are emphasized. Aiming at ultra-high precision observations that can be realized in deep underground, this paper also discusses the challenges posed by deep geophysical observation to current instrument science, equipment and observation technology, and imply a series of key basic scientific problems on geophysics faced by deep underground observation.

Compared with the surface, the deep environment has the advantages of allowing “super-quiet and ultra-clean”-geophysical field observation with low vibration noise and little electromagnetic interference, which are conducive to therealization of long-term and high-precision observation of multi-physical fields, thus enabling the solution of a series of geoscience problems. In the Panyidong Coal Mine, where there are extensive underground tunnels at the depth of 848 m belowsea level, we carried out the first deep-underground geophysical observations, including radioactivity, gravity, magnetic, magne-totelluric, background vibration and six-component seismic observations. We concluded from these measurements that (1) the background of deep subsurface gravity noise in the long-period frequency band less than 2 Hz is nearly two orders ofmagnitude weaker than that in the surface observation environment; (2) the underground electric field is obviously weaker thanthe surface electric field, and the relatively high frequency of the underground field, greater than 1 Hz, is more than two orders of magnitude weaker than that of the surface electric field; the east-west magnetic field underground is approximately the same asthat at the surface; the relatively high-frequency north-south magnetic field underground, below 10 Hz, is at least one order ofmagnitude lower than that at the surface, showing that the underground has a clean electromagnetic environment; (3) in additionto the high-frequency and single-frequency noises introduced by underground human activities, the deep underground spacehas a sig-nificantly lower background vibration noise than the surface, which is very beneficial to the detection of weakearthquake and gravity signals; and (4) the underground roadway support system built with ferromagnetic material interferesthe geomagnetic field. We also found that for deep observation in the “ultra-quiet and ultra-clean” environment, the existinggeophysical equipment and observation technology have problems of poor adaptability and insufficient precision as well asdata cleaning problems, such as the effective separation of the signal and noise of deep observation data. It is also urgent tointerpret and comprehensively utilize these high-precision multi-physics observation data.

The mineral resources buried within the first deep space (0 similar to 500 m) in China cannot meet the national strategical requirement. As there is great potential of deep-seated mineral resources in China, the new scientific concept to explore large or ultra-large mineral resources in the second deep space (500 similar to 2000 m). This paper validates this scientific concept through two field examples implemented with electromagnetic method. First, high-resolution geophysical techniques, such as the near-source electromagnetic method, flight platform-based detection, noise removal, inversion or imaging method, and comprehensive interpretation, are analyzed. The authors conducted the copper polymetallic mine exploration by using the newly developed SOTEM method in Weichang deposit, Hebei province. 6 deep-seated mineralization abnormalities were revealed and were verified by subsequent drillings. Furthermore, based on a CSAMT survey, the deep structure and potential mining site are delineated in Jiaodong Peninsula. It is found that the independently distributed gold deposits such as Jiaojia, Matang, and Sizhuang, are connected in the second deep space, belong to the same gold deposit and thus become a rare supergiant gold deposit. Both two examples strongly support theory on mineralization and exploring mineral resources in the second deep space.

Geophysics plays an important role in the research and exploration of Earth sciences system, it is inevitable track for geophysics to break through the framework that is mainly based on desperation and deduction, then gradually heads towards to semi-quantization and quantization. Therefore, it is the demand of the times for geophysics to improve its precision to deepen the understanding of relevant scientific problem. Based on physical concept and its definition, the core issues of whether geophysics could control the commanding point in Earth sciences are continuous innovation and facilitating interdisciplinary. Hence, a truthfully high-precision observation and data collection, the acquisition of fine structure are not only the footstone of high-resolution geophysics but also the deep-seated gist of further understanding the formation and evolution problems of the crust and mantle in Earth sciences. According to systematically study and analysis, we suggest that (1) the qualitative description and the dependence of shallow layers process and phenomena should be overfulfilled. (2) The key of capturing the truth with high-resolution geophysics is to solidly grasp basic science theories. (3) High-precision geophysics is the footstone of our destination to further understanding of the Earth.

当前我国正处在快速工业化、经济腾飞,并步入一个世界科技大国向世界科技强国迈进的前夜,需要大量的资源和能源乃必然的发展轨迹.然而,我国地大但物并不博.为此,在我国快速发展中、在多元共享世界能源的同时,必须立足本土,建立起可靠的、稳定的且能保障长期供给的能源战略后备基地已刻不容缓.研究与分析当今世界和我国油、气、煤等化石能源发展的历程,结果表明:①第二深度空间(5000～10000 m)化石能源勘探和开发乃21世纪中叶前后的重要领域,此类能源是一次性能源消费的主体;②非常规油气、特别是页岩油气藏在最大限度地减少污染、强化高新科技应用于降低成本前提下,大力勘探、开发、利用乃未来化石能源发展的必然轨迹;③煤炭、煤层气与煤能转化,制油、煤制气,地下燃烧发电和煤化工产业的研发在能源结构中的主导地位不会改变.基于上述认识,在21世纪中叶前后乃至更长时期内,我国的能源匹配模式应为"第二深度常规油气"+"煤层气与煤能转换"+"非常规页岩气".

化石能源的勘探与开发对中国的快速工业化和经济腾飞做出了重大的贡献.同时面临着对外依存度不断增加和对生态环境的影响.为此不同类型新能源广为提出与试验非常重要.中国能源发展与研究现状表明:①中国能源结构为少油、缺气、富煤.而煤乃是未来清洁化石能源开发的源地;②煤层气在天然气能源开发与供给中的潜力巨大;③煤炭多元转化型能(煤转型能)和煤炭高效利用的轨迹研究说明,21世纪煤层气和煤炭多元转型能必是中国能源匹配中开发供给的主力.