Low-resistivity Body Research Articles

Research on Subsurface Electrical Structure Based on a Dense Geomagnetic Array in Southern Yunnan.

The electrical resistivity of subsurface rocks is one of the important sensitive parameters characterizing the internal physics of the Earth. Currently, research on subsurface electrical structures using geomagnetic sounding methods primarily focuses on two approaches: the first is based on observations from a few geomagnetic stations, which have low spatial resolution and cannot effectively describe the distribution of anomalies; the second is based on mobile geomagnetic observations, which have low temporal resolution and cannot promptly reflect anomalies. To address these issues, this study deployed a dense geomagnetic array for long-term observation in the southern segment of the Xiaojiang Fault Zone in the Yuxi area of southern Yunnan. This setup aims to promptly capture seismic magnetic anomalies, providing more data support and fundamental information for short-term earthquake prediction. Based on the long-term observation data from the dense array, the study of the subsurface electrical structure is carried out. The results indicate that during the observation period, which was seismically quiet, the regional subsurface electrical structure remained stable. A large-scale subsurface low-resistivity body was observed in the region, and the electrical structures at the two ends of the southern segment of the Xiaojiang Fault Zone were found to be completely different.

Lithospheric resistivity structure of the Xuefeng Orogenic Belt and its implications for intracontinental deformation in South China

The Xuefeng Orogenic Belt (XFOB), located in the central part of the South China Block, is a typical Mesozoic intracontinental orogen in the central Jiangnan Orogenic Belt. By collecting magnetotelluric (MT) data across the XFOB, we obtained the resistivity structure of the lithosphere, which sheds light on the Mesozoic intracontinental orogenic processes in the XFOB. The resistivity structure reveals a low-resistivity body (<10 Ω∙m), beneath the XFOB, dipping southeast wards from a depth of 10 km to the bottom of the crust. This conductor is interpreted as a relic of the lower detachment zone, which coincides with low-density areas obtained from joint inversion of seismic models. It is believed to result from mineral fluids migrating along the thrust fault and squeezing sulfides into folds. Four low-resistivity bodies were identified at three extensional locations along the Jiangshan-Shaoxing Fault and at the Cili-Baojing Fault. The low-resistivity body (<10 Ω∙m) at the junction of the Shaoyang and the Hengyang Basin is located at the point where the Moho depth thins. The variation trend of the terrestrial heat flow values, with this low-resistivity body as the plate boundary, is consistent with the average variation of the terrestrial heat flow values within the block. We propose that the low-resistivity body under the Qidong-Yongzhou-Guilin fault conforms to the characteristics of the suture zone in the resistivity structure. Its existence indicates that the missing location of the Jiangshan-Shaoxing suture zone of the Yangtze and Cathaysia Block in the middle-southwest section of the South China Block is the Qidong-Yongzhou-Guilin fault. The Yangtze Block and the Hengyang Basin show high resistivity, the depth of which reaches 100 km and 40 km, respectively. Based on the resistivity model and geological data, the XFOB experienced Triassic compression, leading to basement decollement, thrusting, and nappe structures due to low-angle Paleo-Pacific Plate subduction. This compression also led to the uplift of the orogenic belt. Moreover, under the tension caused by the high-angle retreat of the Paleo-Pacific Plate, the Cretaceous extensional tectonics led to detachment along the thrust faults, forming half-graben and basin structures along the margins.

Hidden magma reservoirs: Insights from helium and carbon isotopic compositions in shallow groundwater beneath the Wudalianchi volcanic field, Northeastern China

The lack of strong geothermal manifestations and limited fumarolic activity has sparked widespread debate about the presence of active magma reservoirs beneath the Wudalianchi volcanic field (WVF). Helium (He) in hydrological systems is sensitive and responsive to the mixture of mantle-derived He, making it an excellent tool for deciphering mantle-derived magmatic processes. Through investigating dissolved He and inorganic carbon (DIC) in shallow groundwater of the WVF, we found that He captures mantle characteristics with R/Ra ratios (R equals to 3He/4He ratios, Ra is atmospheric 3He/4He ratio of 1.39 × 10−6) ranging from 0.07 to 2.27 and the highest proportion of mantle-derived He is 30.38 %. In contrast, DIC shows minimal mantle influence, with low concentrations (1.91–7.39 mmol/L) and depleted δ13C-DIC values ranging from −17.9 ‰ to −13.6 ‰. The calculated mantle 4He fluxes in groundwater are 2–3 orders of magnitude higher than that in stable continents and show the signature of volcanic degassing. The high mantle He fluxes and high upward flow rates suggest the efficient release and transfer of mantle-derived He into shallow groundwater through magma degassing. Magma-aging model effectively constraints the period of magmatic activity to within the last 47 ka. These findings, consistent with seismic noise imaging results, support the presence of magma reservoirs in the upper–middle crust. The absence of magmatic carbon signature in groundwater suggests the decoupling of magmatic He and magmatic CO2. Magmatic CO2 may be removed during ascent, possibly being trapped through dissolution or precipitation processes within the low-resistivity bodies identified at ∼2.5 km depth by magnetotelluric imaging. These findings not only contribute to resolving the controversy surrounding the existence of magma reservoirs beneath the WVF but also have broader implications in guiding the exploration and validation of hidden magmatic systems.

Relationship between electrical conductivity, metallogeny, and lithospheric structure in south China

The South China Block (SCB) is located at the junction of the Pacific, Eurasian, and Indo-Australian plates. Their interaction led to large-scale multi-stage mineralization in the SCB during the Mesozoic. Several regional ore-concentration areas, such as the Middle-Lower Yangtze Metallogenic Belt (MLYMB), the Qinzhou-Hangzhou Metallogenic Belt (QHMB), the Wuyishan Metallogenic Belt (WYMB), and the Nanling Metallogenic Belt (NLMB) were formed during this process. However, the dominant mineral types of these metallogenic belts are different. To study the deep mechanisms of the different metallogenic types developed in the same tectonic background at almost the same period, the magnetotelluric sounding (MT) data from 691 sites located mainly within the eastern SCB are employed to obtain a regional lithospheric 3-D resistivity model.It could be concluded from the resistivity model that deep low-resistivity anomalies and mantle upwelling channels mainly controlled almost all the Mesozoic deposits. Large-scale low-resistivity bodies extend from the crust to the upper mantle beneath the MLYMB and the QHMB, interpreted as channels of mantle-derived magma and melts/fluids containing metallogenic elements upwelling. However, the MLYMB and the QHMB have different upper crustal ore-conducting and ore-controlling structures, thus forming porphyry copper and copper-bearing tungsten deposits. Small-scale low-resistivity anomalies are invading the high-resistivity crust in the NLMB. And remelting the upper crust, mixing, and intrusion of crust-source magma, enriching tungsten-tin elements near the NLMB. Larger-scale low-resistivity anomalies exist deep in the WYMB, remelting the lower crust and resulting in the formation of porphyry copper in it. Significantly, the upper-mantle low-resistivity anomalies beneath the eastern SCB show a spatial distribution that is gradually shallowing from south to north, probably indicating that the asthenospheric materials are upwelling from south to north, corresponding with the changing progressively of magma and metallogenic activities. We propose that the lithospheric delamination and asthenospheric upwelling caused by the far-field effects of the Paleo-Pacific Plate subduction are the sources of solid magmatic activities and related metallogeny.

Geothermal resources and deep tectonic in Leh Ladakh (NW Himalaya), India: Inference from magnetotelluric studies

The geothermal resources and their connectivity with the deep tectonic are meaningful to understand through their structural geometry in the NW Himalayas. Therefore, over the NW-SE profiles, 23 audio Magnetotelluric (AMT) sites and over the SW-NE profile, 15 broadband Magnetotelluric (BBMT) sites were carefully chosen to provide the detailed structure. The electrical models were produced from a 2-D inversion algorithm based on the non-linear conjugate gradient method. The Chumathang-Mahe and Puga-Sumdo regions offer excellent geothermal potential as heat/fluid passes through the Mahe Fault (MF), Zildat Fault (ZF) and Kiagor Tso Fault (KTF). These faults are well revealed in the electrical models beneath the subsurface. The Puga Valley has an excellent geothermal resource at shallow depth. A ∼1000 m thick sulfide mineral body is defined with high conductivity (∼1 Ω-m). The crustal structure shows a highly conductive mid-crust beneath the Ladakh batholiths with no manifestation roots. The signs of the Indus-Tsangpo Suture Zone (ITSZ) and Shyok Suture Zone (SSZ) are well represented in the crustal model and are associated with the steep-dipping faults. The Chumathang-Mahe and Puga-Sumdo geothermal regions are connected with low resistivity body (C1) at shallow depths in the crustal model along the SW-NE profile and offer a path for deep fluid flow. The frictional heat generated in the process of the Indian plate subducting beneath the Tibetan plateau and the collision zone between India and Asia carried to melt. The high heat surface flow indicates the thermal origin of low resistivity. Thus, a ∼10 sq. km low resistivity reservoir is identified beneath the Tso Morari dome in the crustal model that overlies a resistivity feature. The high conductivity at mid-crust determined at active tectonic fabric may be potential geothermal resources yielding high well-productivity.

Electrical Structure of Southwestern Longmenshan Fault Zone: Insights into Seismogenic Structure of 2013 and 2022 Lushan Earthquakes

On 1 June 2022, a magnitude 6.1 earthquake struck the southern segment of the Longmenshan fault zone on the eastern edge of the Tibetan Plateau, once again causing casualties and economic losses. Understanding the deep-seated dynamic mechanisms that lead to seismic events in the Lushan earthquake area and assessing the potential hazards in seismic gap areas are of significant importance. In this study, we utilized 118 magnetotelluric datasets collected from the Lushan earthquake area and employed three-dimensional electromagnetic inversion with topographic considerations to characterize the deep-seated three-dimensional resistivity structure of the Lushan earthquake area. The results reveal that the Shuangshi–Dachuan fault in the Lushan earthquake area can be divided into two relatively low-resistivity zones: a western zone dipping southeastward and an eastern zone with a steeper slightly northwestern dip. These two zones intersect at a depth of approximately 20 km, forming an extensional pattern resembling a “Y” shape. The epicenters of both the 2013 and 2022 Lushan earthquakes are primarily located in the upper constricted portion of the pocket-like low-resistivity body at depth. The distribution of seismic aftershocks is confined within the region enclosed by the high-resistivity body, following the pattern of the Y-shaped low-resistivity zone.

Three-Dimensional Forward Modeling of Transient Electromagnetic Method Considering Induced Polarization Effect Based on Spectral Element Method

The transient electromagnetic method (TEM) is widely used in the exploration of mineral, petroleum, and geothermal resources due to its sensitivity to low-resistivity bodies, limited site constraints, and strong resistance to interference. In practical applications, the TEM often uses a long wire source instead of an idealized horizontal electric dipole (HED) source as the excitation source. This is due to the complex external conditions and the relatively large distance between the receiving zone and the transmitter source. Compared to the HED, the long wire source can provide a larger excitation current, generating stronger signals to meet the requirements of a higher signal-to-noise ratio or deeper exploration. It also produces longer-duration signals, thereby providing better resolution. Additionally, for the interpretation of TEM data, three-dimensional forward modeling plays a crucial role. However, the mature traditional TEM forward method is based on a simple, sometimes inappropriate model, as it is well established that the induced polarization (IP) effect is widely present in the deep earth, especially in oil and gas reservoirs. The presence of the IP effect results in negative responses in field data that do not conform to the traditional theoretical decay law of TEM, which can significantly impact data processing and inversion results. To address this issue, a TEM forward modeling method considering the IP effect based on the spectral element method (SEM) has been developed in this study. Firstly, starting from the Helmholtz equation satisfied by the time domain electric field, we introduce the Debye model with polarization information into the forward modeling by utilizing the differential form of Ohm’s law. As a result, we derive the boundary value problem for the time domain electric field that considers the induced polarization effect. Using Gauss–Lobatto–Legendre (GLL) polynomials as the basis functions, the SEM is employed to discretize the governing equations at each time step and obtain spectral element discretization equations. Then, temporal discretization equations are derived using the second-order backward Euler formula, and the linear system of equations is solved using the Pardiso direct solver. Finally, the electromagnetic responses at any time channel are obtained via SEM interpolation and numerical integration, thereby achieving three-dimensional TEM forward modeling considering the IP effect. The results indicate that this method can effectively reflect the spatial distribution of polarizable subsurface media. It provides valuable references for studying the polarization parameters of subsurface media and performing a three-dimensional inversion of TEM data considering the induced polarization effect.

3D forward modeling and response characteristics of low-resistivity overburden of the CFS-PML absorbing boundary for ground-well transient electromagnetic method

This study used the stable and convergent Dufort-Frankel method to differentially discretize the diffusion equation of the ground-well transient electromagnetic secondary field. The absorption boundary condition of complex frequency-shifted perfectly matched layer (CFS-PML) was used for truncation so that the low-frequency electromagnetic wave can be better absorbed at the model boundary. A typical three-dimensional (3D) homogeneous half-space model was established and a low-resistivity cube model was analyzed under the half-space condition. The response patterns and drivers of the low-resistivity cube model were discussed under the influence of a low-resistivity overburden. The absorption boundary conditions of CFS-PML significantly affected the low-frequency electromagnetic waves. For a low-resistivity cube around the borehole, its response curve exhibited a single-peak, and the extreme point of the curve corresponded to the center of the low-resistivity body. When the low-resistivity cube was directly below the borehole, the response curve showed three extreme values (two high and one low), with the low corresponding to the center of the low-resistivity body. The total field response of the low-resistivity overburden was stronger than that of the uniform half-space model due to the low-resistivity shielding effect of electromagnetic waves. When the receiving-transmitting distance gradually increased, the effect of the low-resistivity overburden was gradually weakened, and the response of the low-resistivity cube was strengthened. It was affected by the ratio of the overburden resistivity to the resistivity of the low-resistivity body.

Study on the shielding effect of low resistance body when using transient electromagnetic to detect urban underground space.

Transient electromagnetic Method (TEM) is an efficient geophysical detection technology suitable for detection of urban near-surface space. However, its detection results are well affected by the low resistance anomaly, which interferes with the interpretation of the inversion results. This article used finite element method to simulate the entire process of urban underground pipeline under TEM detection. The causes of interference and the degree of interference under different working conditions were analyzed. The results demonstrate that low resistance anomaly in magnetic field will caused electromagnetic energy absorption and resulting eddy current losses, which lead to a distortion of the primary magnetic field in the vicinity of the pipeline, and formation of a weak field zone beneath the pipeline. The size and shape of the shielding zone are affected by burial depth, transmitter coil diameter, and anomaly size. When the burial depth exceeds 10 times the diameter of the coil or pipeline, the shielding range stabilizes at 1.5-2 times the pipeline's transverse diameter. Moreover, when the pipeline's transverse diameter exceeds twice the transmitter coil diameter, the weak field zone beneath the pipeline will transform into a strong field zone, this is due to the refractive and reflective effects of the electromagnetic field. Finally, experiments were conducted and the inverted results was found to be larger than the actual pipeline diameter, with an error margin similar to that explained by the simulation. These results have implications for high accuracy detecting underground pipelines in urban areas.

The ancient eastward-dipping subduction zone reactivated during the Mesozoic between the Alxa and Ordos Blocks: Electrical evidence from an extended MT profile

This paper investigates the characteristics of the electrical structure and key geological issues of the Alxa and Ordos blocks using a lithospheric resistivity model obtained from a west-east trending magnetotelluric sounding profile. The electrical model shows that the lithosphere of the Alxa Block is high resistivity, except for two crustal low-resistivity bodies. The crust of the Helanshan Orogenic Belt is characterized by medium to high resistivity surrounded by two low-resistivity bodies on both sides. The upper crust of the Ordos Block exhibits discontinuous high resistivity. Still, the lower crust and upper mantle show distinctly low resistivity, which connects with the shallow low-resistivity body above it. The lithosphere of the Lüliang Uplift and the Shanxi Graben is generally characterized by high resistivity, with only localized low-resistivity anomaly found near Moho and in the shallow part of the Shanxi Graben. We propose that the Alxa Block has a stable cratonic lithosphere and that the crustal low-resistivity body within it is a remnant of the southward subducted oceanic crust of the Paleo-Asian Ocean. The low-resistivity body in the lower crust of the Trans-North China Orogen is also interpreted as a remnant of oceanic crust; the upper mantle low-resistivity body is proposed to be a partial melt resulting from the upwelling of asthenosphere caused by the combination of westward subduction of the Paleo-Pacific Plate and northeastward growth of the Tibetan Plateau. This low-resistivity anomaly spreads to the shallow crust, with its lateral distribution to the east and west being blocked by the Lüliang Orogenic Belt and the Alxa Block, respectively, then consequently upwelling along the Yinchuan Graben during the Mesozoic and Cenozoic. We propose that the Alxa Block has been subducted to the Ordos Block along the Helan Mountain in the Early-Middle Paleozoic, which may have formed an ancient suture zone. This ancient suture zone was reactivated in the Mesozoic and Cenozoic by the Paleo-Pacific Ocean subduction and northeastward growth of the Tibetan Plateau, becoming a material upwelling channel and ablating the roots of the Helan Mountain.

Spatial heterogeneity of the lithospheric destruction of the North China Craton: Evidence from an extended magnetotelluric sounding profile

To study the spatial heterogeneity of the North China Craton (NCC) destruction, this paper used a magnetotelluric sounding (MT) profile that passes through almost the entire NCC from west to east. Three-dimensional inversion is used to obtain a lithospheric resistivity model of the NCC. The results show that the upper crust of the Ordos Block is characterized by high resistivity. The lower crust to the upper mantle is characterized by low resistivity. The resistivity structure below the Trans-North China Orogen (TNCO) has stratification features; The Shanxi Graben shows high-low-high-low resistivity features from the upper crust to the asthenosphere; The lithosphere of the Lüliang and Taihang uplifts show high-resistivity features, and only some local relatively low-resistivity areas appear at the crust-mantle boundary. The upper crust on both sides of the Tan-Lu Fault Zone is characterized by high resistivity, but the resistivity structures of the lower crust and the lithospheric mantle are significantly different; The lower crust and the lithospheric mantle of the Sulu Orogenic Belt on the east are characterized by high resistivity; The Luxi Uplift on the west is represented by low resistivity. We propose that the mantle low-resistivity bodies (C1 and C4) of the Western and Eastern blocks may be related to the upwelling of partial melting materials along the ancient structurally weak zones in the lithosphere. The TNCO still has a typical Archean cratonic lithosphere, and the low-resistivity body C2 may be the remnant of the subducted oceanic crust. The Tan-Lu Fault Zone is structurally weak in the Eastern Block, while its western branch is a channel for the asthenospheric upwelling. We propose that the lithosphere of the northwestern Ordos Block and the Yinchuan-Hetao area is being destructed, and the TNCO is in the initial stage of being destructed. In contrast, the lithosphere of the Eastern Block has been severely destructed. In conclusion, affected by the subduction of the paleo-Pacific plate and the collision of the Indian and Eurasian plates, the ancient structures in the NCC were reactivated in the Mesozoic and Cenozoic, resulting in the spatial heterogeneity of the NCC destruction.

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

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

Evaluation of a potential supercritical geothermal system in the Kuju region, central Kyushu, Japan

We derived the thermal structure of the Kuju region of volcanic activity in central Kyushu, Japan, to assess the potential existence of a supercritical geothermal system. Extrapolation of existing well-logging data suggests the presence of a high-temperature region beneath the area between the Hatchobaru geothermal power plant and Kuju-Iwoyama Volcano, which closely overlaps a low-resistivity body estimated from magnetotelluric data. Based on estimated temperatures exceeding 380 °C at the relatively shallow depth of -3.3 km above sea level, and considering evidence for magmatic fluid transport, this high-temperature region is a potential target for a supercritical geothermal reservoir.

Deep Electrical Structure of the Qilian Orogenic Belt with Dynamic Implications for the Northeastern Margin of the Tibetan Plateau: Revealed by 3D Magnetotelluric Inversion Using Unstructured Tetrahedral Elements

Abstract We present the results of a magnetotelluric (MT) array across the Qilian orogenic belt to elucidate the uplift mechanism of the northeastern Tibetan Plateau (Qinghai-Tibet Plateau). The array extends from the Qaidam basin in the south to the southern edge of the Alxa block in the north. Using the three-dimensional (3D) inversion of MT data based on unstructured tetrahedral elements, the electrical structure 100 km below the orogenic belt is obtained. The results show that there are high-resistivity bodies in the lithospheric mantle of the North Qilian and Hexi Corridor, which may represent the trace of southward subduction of the Asian lithosphere. Besides, there are partially molten bodies with low resistivity in the middle and lower crust below the Qilian orogenic belt, which may be caused by tectonic heat. The melt fraction of low-resistivity bodies is 2-5%, which indicates that the crustal flow from the Qiangtang and Songpan-Ganzi blocks is unable to penetrate beneath the Qilian orogenic belt. The low-resistivity bodies beneath the Qilian orogenic belt decouples the upper crust from the middle-lower crust. Owing to the continuous compression, the decoupled middle-lower crust has subsequently driven the northward movement of the upper crust, resulting in the uplift of the Qilian orogenic belt.

Exploring multiple electrical layers overlying coal seams using the transient electromagnetic method

The transient electromagnetic method (TEM) is widely applied in coal hydrogeological exploration owing to its sensitivity to low-resistivity bodies. However, when a coal seam is buried deep, particularly if there are multiple electrical layers in the vertical direction of the overlying stratum, the results of the calculation using the late-channel empirical formula of the TEM may no longer reflect the actual situation. In this study, we evaluated the principle of the one-dimensional (1D) Occam algorithm and the steps for performing an inversion. We proposed various two-, three-, and four-layer electrical models for inversion using the 1D Occam algorithm. Our results are consistent with the electrical distribution of the models, thus indicating the effectiveness of the algorithm. A test project of large-depth transient electromagnetic exploration in the Datong Coalfield in Shanxi, China, was selected for experimental verification. The 1D Occam inversion was used to successfully identify various electrical strata overlying the coal seam.

3D seismic imaging of the Nesjavellir geothermal field, SW-Iceland

We present a detailed seismic imaging of the harnessed Nesjavellir geothermal area, SW-Iceland, which is one of several geothermal fields on the flanks of the Hengill volcano. We map thevP,vS, andvPvSratio using seismic data recorded in 2016–2020 and compare them with both a resistivity model of the same area and the rock temperature as measured in boreholes. The results show that the shallower crust (depth less than 1 km) is characterized by lowvPandvS, and highvPvSratio (around 1.9). Shallow low resistivity values at similar depths in the same area have been interpreted as the smectite clay cap of the system. At depths between 1 and 3 km the observed lowvPvSratio of 1.64–1.70 correlates with high resistivity values. In this area, characterized by temperatures larger than 240°C, the seismicity appears to be sparse and located close to the production wells. This seismicity has been interpreted as induced by the production in combination with naturally occurring earthquakes. At depths greater than 4 km, highvPvSratio of 1.9 correlates well with low resistivity values. In the valley of Nesjavellir, a deep-seated conductive body, domes up at about 4.500 m b.sl. and coincides spatially with a significant highvPvSratio anomaly (&amp;gt;1.9). Above these anomalies an elevated temperature is registered according to borehole temperature data. This is proposed here to be caused by hot 600°C–900°C cooling intrusives, close to the brittle ductile transition—probably the heat source(s) of the geothermal field above. These anomalies are at the same location as the last fissure eruption in Hengill almost 2,000 years ago. The NNE-SSW trending, deeper seismic cluster at 3–6 km depth is located at the edge of this highvPvSanomaly. The heat source of the Nesjavellir geothermal field is most likely connected to this most recent volcanism as reflected by the deep-seated low resistivity body and highvPvSratio, located beneath the deep fault that connects the flow path of the high temperature geothermal fluid, resulting in an actively producing reservoir.

The Whole-Space Modeling of the Hazardous Geological Body ahead of the Tunnel Face by the Transient Electromagnetic Method

Karst caves are widely distributed in southwest China, causing difficulties and disasters for tunnel construction. To better detect the krast caves in front of the tunnels under construction using the transient electromagnetic method, in this paper we propose a 3-D finite element method to simulate the multi-parameter transient electromagnetic response of unfavorable geological bodies in a whole-space. First, the models of vertical water-filled faults, water-filled caves and complex geological bodies in front of the tunnel face are established. The horizontal electric field component and the vertical magnetic field component at different time in the whole-space are researched. Secondly, the electromagnetic response features of the caves with different resistivity, buried depths and scales are studied. We found that the resistivity of the target body is 10 times larger than that of the surrounding rocks, and the anomaly amplitude increases obviously with the growing distance from the target body. The deeper the buried depth, the later the anomaly appears and the smaller the anomaly amplitude. The larger the target size, the longer the transient electromagnetic response delay and the larger the anomaly amplitude. We arranged a measuring line on the tunnel face. The full-time apparent resistivity section shows the position and characteristics of the low-resistivity anomalous body, indicating that the transient electromagnetic method (TEM) has obvious advantages in detecting the low-resistivity body in front of the tunnel face. Finally, the TEM is successfully applied to the advanced detection of a karst tunnel to get the electrical distribution of the surrounding rocks in front of the tunnel face. According to the geological conditions of the excavated tunnel, the validity of the TEM in the tunnel advanced prediction is verified.

Electrical structures of the Yinchuan Basin and adjacent area, western North China Craton, inferred from magnetotelluric imaging

• The Alxa and Ordos blocks are characterized by simple, laterally uniform, layered electrical structures. • The East Helanshan and Yellow River faults correspond to major electrical boundaries beneath the Yinchuan Basin. • Electrically conductive crustal weak zone is the main driver of earthquakes in the Yinchuan Basin. The Yinchuan Basin is located in the contact zone between the Alxa block and the Ordos block, North China Craton . The Yinchuan Basin also lies within the northern segment of the North-South seismic zone in China, which has experienced frequent earthquakes, including the M 8.0 Pingluo earthquake in 1739. In this study, two magnetotelluric profiles that cross the Yinchuan Basin and the adjacent Helanshan Uplift were used to obtain a 3-D image of the crustal electrical structure in this region. The results show that the electrical structure of the Yinchuan Basin and its surrounding areas differ not only in the east–west direction but also from north to south. The Yinchuan Basin and Helanshan Uplift comprise a typical basin-range tectonic feature , with a high-resistivity body in the mountain range and a low-resistivity body in the basin. Within this region, the East Helanshan and Yellow River faults appear as significant large-scale electrical boundaries along the east and west edges of a seismically imaged “flower” structure within the deep part of the Yinchuan Basin. The Yinchuan Fault is believed to extend northward as a buried fault, and here we speculate that it may be the master fault of the “flower” structure. The epicenter of the 1739 Pingluo earthquake falls along the boundary between high and low resistivity bodies. The low-resistivity body, within the Yinchuan Basin, extends to a depth of more than 50 km and is regarded as a weak zone; we speculate that the Pingluo earthquake was triggered by the interaction of multiple seismogenic faults within that weak zone. In addition, the alternating high and low-resistivity bodies in Yinchuan Basin and Helanshan Uplift, along which deformation is accommodated between the stable Alxa and Ordos blocks, likely defines the geodynamic framework of this earthquake. Comparing the electrical structure from the southwestern and northeastern margins of the Ordos block, we can conclude that the different tectonic frameworks can explain different kinematic properties within these active fault zones.

Electrical resistivity structure across the Tethyan tectonic belt in western Yunnan, SW China

The Tethyan tectonic belt in western Yunnan, SW China, has witnessed a complex, multistage Tethyan evolution since the Paleozoic, including a late stage of continental collision between the Indian and Eurasian plates in the Cenozoic. Consequently, this belt has become a collage of Gondwana-derived microcontinental blocks and arc terranes wedged between two continents, including the Tengchong block, Baoshan block, Lincang block, and Simao block from west to east. To better understand the structure of these ancient continental blocks and the dynamic mechanism of the Indian-Eurasian continental collision, we investigate the electrical resistivity structure of the crust and uppermost mantle in the region. Magnetotelluric data were collected along a NW-SE striking transect crossing multiple tectonic terranes and were then modeled with a 3-D approach. The final resistivity model shows conductors beneath the Tengchong volcanic area, which may represent magma chambers that feed the active volcanism in the area. The electrical structure between the Tengchong area and the Wanding fault consists of layer-like lower crustal conductors along the transect, which may indicate mechanically weak layers where the tectonic escape of the Tibetan Plateau may occur via lower crustal flow. The Lincang block shows higher resistivity characteristics than the surrounding crust and demonstrates a trend of eastwardly napping resistors onto the Simao block, making up a crustal boundary of high-resistivity and low-resistivity bodies. This observation suggests that the Changning-Menglian suture between the Baoshan block and the Lincang block may be the boundary between Gondwana and Cathaysia.

2-D Magnetotelluric Multiparameter Joint Inversion Considering the Induced Polarization Effect

Magnetotelluric (MT) is an important geophysical exploration method that uses natural sources to study the electrical structure of the earth. This method is advantageous owing to its low cost, large exploration depth range, and high resolution for low-resistivity bodies. However, traditional MT modelling approaches can only invert the resistivity parameters of geological bodies, while natural geological bodies also exhibit the induced polarization (IP) effect. Applying the IP effect of geological bodies could be effective for exploring mineral resources, such as polymetallic ores, oil (gas) fields, and coal fields. To incorporate the IP information into MT inversion, we proposed a multi-parameter MT joint inversion algorithm that considers the IP effect. We used the Cole-Cole model to integrate four IP parameters into the forward algorithm: the zero-frequency resistivity ρ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> , chargeability η, frequency exponent <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</i> , and time constant <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">t</i> . The influence of each IP parameter on the forward response was analyzed by forward simulations, and we concluded that the parameters ρ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">and</i> η should be considered in the inversion. A cross-gradient function was introduced into the objective function of the Occam inversion method to constrain the structural consistency of ρ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> and η. Model testing and practical application results illustrated that the algorithm can not only obtain the subsurface resistivity structure that can be achieved by the traditional MT method but also reveal the distribution of the chargeability parameter. The additional information obtained using this algorithm is conducive to interpreting specific geological structures that cannot be distinguished by the traditional MT method.