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When encountering sedimentary rocks with obvious laminations or fracture development zones, the conductivity of the conductive medium in different directions will change significantly, and the subsurface medium will exhibit macroscopic conductivity anisotropy. To analyze the impact of electrical anisotropy on the surface–borehole transient electromagnetic exploration method, we used the finite element method to investigate the electrical anisotropy surface–borehole transient electromagnetic three-dimensional (3D) forward algorithm, in which we used a tetrahedral mesh to spatially discretize the time–domain Maxwell equation. Then, we discretized it using the second-order backward Eulerian difference method, and we obtained the fields through the PARDISO solver. The validity and correctness of the algorithm were verified through comparison of a one-dimensional (1D) anisotropic model, a complex three-dimensional (3D) isotropic model, and a three-dimensional (3D) anisotropic half-space model. A typical anisotropic geological model was constructed to analyze the effects of anisotropic strata and anomalies in the different principal axis directions on the surface–borehole transient electromagnetic response. The results show that the response of the anisotropic medium is related to the direction of the transmitting source, and the response pattern is complex and volatile. The electrical anisotropic anomaly does affect the amplitude, which should be given special attention when performing surface–borehole transient electromagnetic inversion interpretation.

The permeability of the fluid-bearing rock is an important parameter for reservoir prediction. The Kozeny-Carman (K-C) formulation based on electrical measurements effectively characterizes the permeability-resistivity relationship of rocks with a single mineral composition or high porosity. The complex pore structure and mineral composition of compacted reservoirs affect induced polarization (IP) characteristics, indirectly limiting the applicability of conventional electrical K-C models. The permeability of fluid-bearing rocks is an important parameter for reservoir prediction. The theoretical chargeability of the modified generalized effective medium theory of induced polarization (MGEMTIP) model includes the effects of various conductive minerals. Due to the disconnection assumption of the disturbed medium in the MGEMTIP, there is a significant difference between the theoretical chargeability and the measured chargeability, and the difference is a sensitive parameter of rock permeability. A semi-empirical reservoir permeability prediction model is proposed based on the MGEMTIP. Theoretically and experimentally, the prediction model based on MGEMTIP is compared with the two electrical K-C models. Under the condition that the rock does not contain low-resistivity minerals, the prediction model based on MGEMTIP is theoretically equivalent to the K-C model. The experimental results show that this prediction model is more suitable for low-porosity and low-permeability rocks containing low-resistivity minerals, and the prediction results can be effectively restricted to the same order of magnitude. From the perspective of differences between model assumptions and natural rocks, the prediction model provides a semi-empirical relationship between complex mineral IP characteristics and permeability. Combined with the geological information of the survey area, the permeability prediction model can provide a theoretical basis for reservoir permeability prediction based on electromagnetic exploration.

The spectral-element (SE) method, which is based on the Galerkin technique, has been gradually implemented in geophysical electromagnetic (EM) 3D simulation. The accuracy and efficiency of this approach, implemented for deformed hexahedral and regular meshes, has been verified for airborne EM forward modeling. One advantage of the SE method over the conventional finite-element method is that it provides accurate results for earth models that can be adequately parameterized using a coarse mesh. However, realistic models can contain important small-scale conductivity variations or larger features with complicated boundaries. To overcome the limitations imposed by using the same mesh to parameterize the model and for implementing the forward-modeling approach, we have developed an adaptation of the conventional SE method. This is inspired by the ideas behind the element-free Galerkin (EFG) method, in which the conductivity is no longer assumed to be constant within a cell; instead, it is handled via the same kind of numerical integration as in the EFG method. This allows a coarse, regular hexahedral mesh to be used for the forward modeling for complex earth models. After presenting the theory for this new SE approach, we test it for airborne EM modeling of 1D and 3D models to verify its flexibility and accuracy. Finally, we model the Ovoid Zone massive sulfide ore body located at Voisey’s Bay, Labrador, Canada, to illustrate the flexibility and practicality of our approach.

Currently, the use of Cagniard apparent resistivity and phase is still the main method and means to process and interpret controlled-source audio-frequency magnetotelluric (CSAMT) sounding data. The CSAMT data must meet the conditions in the far zone to use the magnetotelluric (MT) sounding inversion interpretation method. The conditions in the far zone are directly related to transceiver distance; Hence, the distance measured by the CSAMT method is directly related to the reliability and credibility of the data. In this study, based on the formation wave and ground wave of the analytical expression for the electric field in a uniform half-space condition, the response laws of the formation wave and the ground wave are analyzed. After rigorous mathematical derivation, the formula for determining the optimal transceiver distance of the CSAMT is obtained, and the steps for using this formula in practical application are given. The actual field application verifies the rationality of the derived formula. This study provides a reference for the design of a CSAMT field source, and effectively guides the determination of CSAMT transceiver distance in the field.

The conventional 3D magnetotelluric (MT) forward modeling and inversions generally assume an isotropic earth model. However, wrong results can be obtained when using an isotropic model to interpret the data influenced by the anisotropy. To effectively model and recover the earth structures including anisotropy, we develop a 3D MT inversion framework for a triaxial anisotropic model. We use the unstructured finite-element method for our forward modeling. This offers more possibility to simulate more complex underground geology and topography. To solve the inverse modeling problem, we use a limited-memory quasi-Newton algorithm (L-BFGS) with a parallel direct solver for optimization that avoids the explicit calculation of the Hessian matrix and saves the memory and computational time. We validate our algorithm via numerical experiments on both synthetic data and MT survey data from the US Array project.

In this study, we analyzed the geological, gravity, magnetic, and electrical characteristics of depressions in the Erlian Basin. Based on the results of these analyses, we could identify four combined feature parameters showing strong correlations and sensibilities to the reservoir oil-bearing conditions: the average residual gravity anomaly, the average magnetic anomaly, the average depth of the conductive key layer, and the average elevation of the depressions. The feature parameters of the 65 depressions distributed in the whole basin were statistically analyzed: each of them showed a Gaussian distribution and had the basis of Bayesian theory. Our Bayesian predictions allowed the definition of a formula to calculate the posterior probability of oil occurrence in the depressions based on the combined characteristic parameters. The feasibility of this prediction method was verified by considering the results obtained for the 22 drilled depressions. Subsequently, we were able to determine the oil-bearing threshold of hydrocarbon potential for the depressions in the Erlian Basin, which can be used as a standard for quantitative optimizations. Finally, the proposed prediction method was used to calculate the probability of hydrocarbons in the other 43 depressions. Based on this probability and on the oil-bearing threshold, the five depressions with the highest potential were selected as targets for future seismic explorations and drilling. We conclude that the proposed method, which makes full use of massive gravity, magnetic, electric, and geological data, is fast, effective, and allows quantitative optimizations; hence, it will be of great value for the comprehensive geophysical evaluation of oil and gas in basins with depression group characteristics.

The Permian igneous rocks in the Sichuan Basin represent a major breakthrough, opening up a new prospect for oil and gas exploration, and igneous reservoirs have become a new field of oil and gas exploration. Gravity-magnetic-electric exploration is an effective means of identifying igneous rocks and helps in reducing the multiplicity of the prediction results. However, the lithology of igneous rocks is quite different, and the exploration theory and evaluation techniques need urgently to be improved. In order to deeply study the response characteristics of the gravity-magnetic-electric and physical properties of the Permian igneous rocks in the Sichuan Basin and their relationships with the reservoir parameters, physical property testing was carried out on outcrop samples of the Permian igneous rocks in southwestern Sichuan. The comprehensive physical properties of the samples with different lithologies, including basalt, tuff, and volcanic breccia, were analyzed and studied. Based on the geological characteristics of the igneous rocks, such as the mineral composition, microstructure, and reservoir properties, a multi-parameter intersection relationship model for the resistivity, polarizability, density, magnetic susceptibility, and their relationships with the reservoir parameters was established, and effective parameters favorable for igneous rock identification and reservoir evaluation were identified. The results of this study provide a physical basis and technical support for non-seismic exploration of igneous oil and gas reservoirs in the Sichuan Basin.

Electrical exploration has become an important means of shale gas reservoir exploration and evaluation, and is expected to play a key role in the later stages of reservoir fracturing and development. At present, the research on the electrical response characteristics of shale gas reservoirs and their relationship with reservoir parameters is extensive and in-depth, but there is little research on their complex resistivity anisotropy characteristics and influencing factors, which restricts petrophysical modeling and reservoir parameter prediction, and reduces the reliability of shale gas exploration and reservoir evaluation by electromagnetic methods. In this paper, shale samples from the Longmaxi Formation and the Wufeng Formation of shale gas wells in southern Sichuan were collected, the complex resistivity of 34 shales in bedding direction and vertical bedding direction were measured, and the induced polarization (IP) parameters of shales were extracted by inversion. The electrical anisotropy response characteristics under different temperature and pressure conditions were analyzed, and the influencing factors and laws of complex resistivity anisotropy of shales were revealed. Combined with the test results of shale porosity and permeability, the evaluation model of resistivity, polarizability and porosity and permeability parameters was established. The research results have formed a set of testing methods and analysis techniques for electrical anisotropy of shale reservoirs, which are mainly based on complex resistivity parameter testing. It is helpful to understand the electrical anisotropy characteristics of shale gas reservoirs in southern Sichuan; this will provide the theoretical and physical basis for shale gas reservoir evaluation and fracturing monitoring by electrical exploration.

With the rapid development of the economy, the demand for mineral resources has increased dramatically. Because it is difficult to explore new mines in deep and peripheral areas, new theories, technologies, methods, and equipment are needed. How to accurately and effectively carry out new mine exploration has become an urgent research topic. In this study, based on the requirements of mine exploration within a depth of 3000 m, an array receiver for borehole-induced polarization was developed. Each channel of the receiver array independently acquires data, and it is able to carry out full waveform acquisition in the dual voltage and voltage difference mode. The structure of the receiver array can be flexibly adjusted to obtain array observations of induced polarization in different ways. Experiments demonstrated that the device works well at a high temperature of 155 °C, its resolution reached 0.1 μV, and the error between the different channels was less than 3%. Its dynamic range is 180 db. Through experiments on induced polarization in a demonstration mining area, the apparent resistivity curve and the apparent polarization curve were determined to be consistent with the known well-logging information, and the interpretation results were in agreement with the stratum information.

This paper reports a kind of OFDM underwater acoustic communication technology based on Sagnac fiber optic hydrophone. It adopts the fiber optic hydrophone technology based on Sagnac fiber ring composed of 3×3 fiber coupler. The sampling rate of the system is 48kHz, the fiber optic hydrophone can achieve up to 12.3kHz underwater acoustic detection bandwidth and -140dB (0dB re 1rad/ μPa) sound pressure sensitivity. According to the test results of the underwater acoustic communication experiment, the underwater acoustic communication rate is up to 24kbps, the bit error rate is at the magnitude of 10−2 , and the simulation communication rate is up to 100kbps. Sagnac fiber optic hydrophone shows excellent low-frequency suppression ability and simple structure, which are very suitable for underwater acoustic communication.

Magnetometric Resistivity(MMR) Method, which can use the power supply method of the traditional apparent resistivity method to measure the magnetic field. At present, the application research abroad is relatively extensive, but the domestic(China) research on the application of the MMR method is very few, and it is not even well-known. This paper is based on the MMR theoretical method under the point source DC condition, combined with the abnormal potential method and the modified Biot-Savart law, and using the three-dimensional numerical calculation method of the finite difference method to calculate the abnormal potential field, electric field, and magnetic field on the matlab2018a platform. Calculate, realize the multi-physics simulation in the electromagnetic field through Matlab platform programming, and verify the correctness of the algorithm by the spherical anomaly model with an analytical solution. Through the visual simulation of the three-dimensional data volume of the multi-physics field in the electromagnetic field, we can better understand the response mechanism of the electromagnetic field under DC conditions and grasp their three-dimensional spatial distribution rules. It is hoped that the research in this article can help the research of MMR Personnel better use this method for exploration.

When transient electromagnetic investigation methods are carried out in the field, the measured data often contain both the induced polarization (IP) effect and the electromagnetic effect. In order to study the IP effect in the transient electromagnetic response, many researchers first calculate the electromagnetic field which considers the IP effect by replacing traditional resistivity with complex resistivity of the Cole-Cole model in the frequency domain. After the forward modeling calculation of the electromagnetic field in the frequency domain that considers the IP effect, the transient electromagnetic field in time-domain is obtained by a time-frequency transform algorithm. In this paper, the resistivity is directly replaced by the time-variant resistivity expression of the Cole-Cole model by using digital filter algorithms when we simulate the transient electromagnetic fields in time- domain. The calculated result of the Cole-Cole model in time-domain and in frequency-domain are consistent with each other, as observed in the horizontal electric field and the vertical magnetic field comparisons, which indicates the correctness of the numerical computation method adopted in this paper. The research presented herein allows us to observe the influence of the IP effect on transient electromagnetic field as well as study the mechanisms of IP directly.

The deterministic geophysical inversion methods are dominant when inverting magnetotelluric data whereby its results largely depends on the assumed initial model and only a single representative solution is obtained. A common problem to this approach is that all inversion techniques suffer from non-uniqueness since all model solutions are subjected to errors, under-determination and uncertainty. A statistical approach in nature is a possible solution to this problem as it can provide extensive information about unknown parameters. In this paper, we developed a 1D Bayesian inversion code based Metropolis-Hastings algorithm whereby the uncertainty of the earth model parameters were quantified by examining the posterior model distribution. As a test, we applied the inversion algorithm to synthetic model data obtained from available literature based on a three layer model (K, H, A and Q). The frequency for the magnetotelluric impedance data was generated from 0.01 to 100 Hz. A 5% Gaussian noise was added at each frequency in order to simulate errors to the synthetic results. The developed algorithm has been successfully applied to all types of models and results obtained have demonstrated a good compatibility with the initial synthetic model data.

Monitoring and delineating the spatial distribution of shale fracturing is fundamentally important to shale gas production. Standard monitoring methods, such as time-lapse seismic, cross-well seismic and micro-seismic methods, are expensive, timeconsuming, and do not show the changes in the formation with time. The resistivities of hydraulic fracturing fluid and reservoir rocks were measured. The results suggest that the injection fluid and consequently the injected reservoir are characterized by very low resistivity and high chargeability. This allows using of the controlled-source electromagnetic method (CSEM) to monitor shale gas hydraulic fracturing. Based on the geoelectrical model which was proposed according to the well-log and seismic data in the test area the change rule of the reacted electrical field was studied to account for the change of shale resistivity, and then the normalized residual resistivity method for time lapse processing was given. The time-domain electromagnetic method (TDEM) was used to continuously monitor the shale gas fracturing at the Fulin shale gas field in southern China. A high-power transmitter and multi-channel transient electromagnetic receiver array were adopted. 9 h time series of Ex component of 224 sites which were laid out on the surface and over three fracturing stages of a horizontal well at 2800 m depth was recorded. After data processing and calculation of the normalized resistivity residuals, the changes in the Ex signal were determined and a dynamic 3D image of the change in resistivity was constructed. This allows modeling the spatial distribution of the fracturing fluid. The model results suggest that TDEM is promising for monitoring hydraulic fracturing of shale.

In this paper two different geophysical electro-magnetic techniques, long-offset time domain EM and the magnetotelluric method , are jointly inverted using a variation of Bayesian inversion called Hybrid Monte-Carlo. Long-offset time domain EM is a method that records Earth response to a controlled source square wave signal, while the magnetotelluric method records Earth response to naturally occurring electro-magnetic signals. We obtain better understanding of subsurface resistivity structures because the information they provide is complementary and joint inversion reduces the set of permissible inversion models. In most situations where long-offset time domain EM is recorded magnetotelluric data is also collected, since receiver instrumentation is shared. Hybrid Monte Carlo uses the gradient of the forward model to enable larger jumps through the solution space than are permissible using the standard Metropolis-Hastings algorithm, avoiding the pitfall of random walk behavior and giving faster convergence. The method is successfully applied to a survey area near the edge of Wuhan China. We obtain inversion results in a highly urbanized area, where the magnetotelluric method is typically compromised by background electro-magnetic noise.

With the urbanization in recent years, the power line interference noise in electromagnetic signal is increasing day by day, and has gradually become an unavoidable component of noises in magnetote...

Petrophysical study is playing an important role in oil and gas exploration. Shale gas and shale oil is blooming in recent years in many countries. Less rock physics knowledge is known about shale relatively to other rock type such as sandstone and limestone. In this paper, we carried out a rock physical study of shale core sample which is drilled from north China. The plan distribution of permeability, P wave velocity, S wave velocity and complex resistivity were acquired based on AutoScan-IIplatform. The results show that the permeability of the shale sample is basically low with values of 0.1 to several micro Darcy (mD) except some fracture areas in the surface, which has values of about several tens mD. The permeability can basically describe the distribution of the fracture. The complex resistivity has the similar characteristics with permeability, which is also roughly corresponding to the position of the facture. As for the Vp and Vs, although not very good correspondence with the surface, they are still approximately present the high and low velocity feature of the core sample as well. This result is significantly helpful for shale gas exploration and production.

Using one or two electric dipole transmitters in the Controlled source electromgnetic method, this paper calculate the transient electromagnetic responses of one and two 3D conductive anomalous bodies in the layered earth by varying the number and intensity of the dipole sources. The results of the models show that satisfactory resolution can be achieved when only one dipole source is used. The stronger the intensity of the electric dipole source, the better resolution for the anomalous body for the same offset. When two electric dipole transmitters laid each side of one 3D body, it means that the anomalous body locates in the middle of the sources and the resolution to the target becomes poor. The numerical results of the transient electromagnetic responses of two 3D bodies also support the idea that resolution is not good when two transmitters are used in each side of the targets simultaneously. As the conclusion, in order to get good resolution to the target, multi transmitters laid different side of the geological target should not be used the same time in the field survey.

The forward modeling procedure used in this article is formulated with the volume integral equation based on the tensor Green’s function. The electromagnetic components responses are first calculated in the frequency domain and then transformed to the time domain by digital filtering. The valley and hill topography with a layered earth is stimulated by a horizontal electric dipole (HED) transmitter, which is common in field surveys, and the TEM responses are calculated at the transmitter and receivers. The topography effects on the long offset electromagnetic transient (LOTEM) responses are discussed in detail. The results show that both valley and hill topography has significant effect on the LOTEM measurement. If the HED is located in the bottom of a valley, the distortion of the observed anomalous field at distance is severe. A valley at the receiver locations show a strong effect but are localized in space and time. In general, hill-shaped topography shows smaller effects no matter where its located. When the topography is located between source and receivers, the influence is negligible. We conclude that the location of the source is much more important than the receivers and it is critical to put the transmitter in an open flat area in the field survey.

Complex resistivity (CR) is one of an electromagnetic method which plays an important role in the exploration of oil and gas, underground water as well as solid mineral resources in recent years. Nowadays China is under fast developing and there is still a big gap between the supply and demand of mineral resources. As an effective controlled source electromagnetic method, CR method can be easily used to judge the content of resources, determine the target reservoir and select a favorable drilling area. In this paper, an introduction to CR method and its application in copper mine exploration in west China is present. The result shows that CR is an effective electromagnetic method in the exploration of deep mineral resources.

The shale gas is becoming a new strategic energy source in the world. To improving the efficiency of exploration and production of such a non-conventional resource has been a global choice of oil and gas powers. Many previous researches have revealed that China has equivalent amount of shale gas reserves as USA. China-based research has shown that the southern China is one of the most potential areas for shale gas exploration. However, the complex topography with carbonate covers and sophisticated geological structures in southern China is a main difficulty for seismic exploration to get reliable and high-resolution data. This paper alternatively present electromagnetic exploration methods that are applicable to shale gas exploration in such rugged area.