Electromagnetic Response Characteristics and Applications of Numerical Simulation of Geoelectricity in Water-Rich Areas of Mines

An ideal metallogenic and prospecting model has important guiding significance for aluminum ore development and geophysical exploration. Previous research in this field only focused on ore body evaluations and metallogenic belts. Selection of reasonable geophysical methods for exploration of bauxite deposits is still challenging. To solve these problems, a new metallogenic model based on the new geological information is established firstly. The ore-bearing rock and ore bodies of sedimentary bauxite deposits are strictly controlled by the paleo-topographic conditions. The undulating shapes of the paleo-topography control the scales and shapes of ore-bearing rock series and bauxite ore bodies. Then, different geophysical methods applied for exploration of bauxite deposit are analyzed based on the petrophysical measurement. For the areas with thick overburden and large buried depths, it was found that fixed-loop transient electromagnetic (TEM), high-precision gravity, and seismic exploration methods were most effective. In regard to the areas with thin overburden and small buried depths, direct current (DC) sounding, high-precision gravity, seismic exploration, and TEM methods were considered to be effective. However, for the areas with very shallow buried depths, DC sounding and seismic exploration methods were deemed to be more suitable. This study selected the actual exploration processes of the Wanggudong bauxite mining area in western Henan as an example, and the application of a DC sounding method for sedimentary-type bauxite was successfully introduced.

A long wire with large current source transient electromagnetic (TEM) monitoring, with a large detection depth, low cost, safety, and environmental protection, has unique advantages in the testing and identification of unconventional reservoir fluid and the evaluation of stimulated reservoir volume. So, the TEM 3D forward modeling method has become a research hotspot. Although the finite-element method (FEM) is a type of numerical algorithm that has been widely applied in three-dimensional (3D) electromagnetic field forward modeling, the efficiency and accuracy of FEM require further improvement in order to meet the demand of fast 3D inversion. By increasing the order of the basis function and adjusting the principle of mesh discretization, the precision of the mixed-order spectral-element (SEM) result will be increased. The backward Euler scheme is an unconditionally stable technique which can ignore the impact of the scale of the time step. To achieve a better description of the nonlinear electromagnetic (EM) response of the grounded source TEM method and to optimize the efficiency and accuracy/precision of the 3D TEM forward modeling method significantly, we proposed the use of 3D TEM forward modeling based on the mixed-order SEM and the backward Euler scheme, which can obtain more accurate EM results with fewer degrees of freedom. To check its accuracy and efficiency, the 1D and 3D layered models are applied to compare the SEM results with the semi-analytical and FEM solutions. In addition, we analyzed the accuracy and efficiency of the SEM method for different types of order basis functions. Finally, we calculated the long-wire source TEM response for a practical 3D earth model of a shale gas reservoir for fracturing monitoring and tested the feasibility of the TEM method in a hydraulic fracturing monitoring area to further demonstrate the flexibility of the SEM method.

Unexploded ordnance (UXO) investigation is mainly carried out by the magnetic exploration. However, in this paper, the transient electromagnetic (TEM) method was used in order to detect UXO more accurately. Although the TEM method is usually used for deep sounding of mineral exploration, we used small loop TEM method to detect buried UXO. In order to understand the characteristics of the small loop TEM method, many experiments using small empty cans were carried out at Ito Campus in Kyushu University and a lot of features were discovered. For example, if the size of loop becomes smaller, the penetration depth becomes smaller. These results showed that the small loop TEM was applicable to UXO investigation. Based on these results, the test survey to detect the imitation UXOs using 180×180-cm-square loop was carried out. In this field experiment, there are various kinds of imitation UXOs, but all of the UXOs were detected. As a result, it was found that UXO detection by small loop TEM method is effective compared to magnetic method. Moreover, the small loop TEM method could narrow the range of UXO location to the size of 30×30cm-square.

An extremely high resolving capability of both direct current (DC) and transient electromagnetic (TEM) soundings using a vertical electric dipole (VED) source is shown. In contrast to conventional surface methods, electric, magnetic and EM fields generated by a VED are sensitive to the presence of very thin layers, even in the case of moderate resistivity contrasts. It is shown analytically that the field in asymptotic domains (long separations or late stage in DC and TEM methods, respectively) is exponentially related to geoelectric parameters of the section above an insulating basement. Numerical analysis, in turn, proves the high resolving capability of the methods under more realistic conditions, namely finite resistivity of the basement, moderate separations and measuring time.

Transient Electromagnetic (TEM) and Magnetic Resonance Sounding (MRS) methods were applied jointly to investigate lithological variations in the Dead Sea coast of Israel. The subsurface in this area is heterogeneous and composed of intercalated sand and clay layers over a salt rock, which is partly karstified. Groundwater is very saline, with a chloride concentration of 100-225 g/l. TEM is known as an efficient tool for investigating electrically conductive targets like saline water, but it is sensitive to the salinity of groundwater, the clay content and the porosity of rocks. MRS, however, is sensitive primary to groundwater volume but also to lithological variations in the subsurface. MRS is much less sensitive to variations in groundwater salinity in comparison with TEM. We show that MRS enables us to resolve the fundamental uncertainty in TEM interpretation caused by the equivalence between groundwater resistivity and lithology. Combining TEM and MRS in Nahal Hever area we have identified lateral extensions of different lithological formations.

The borehole transient electromagnetic (TEM) method can be useful in deep mineral exploration to detect blind ore bodies beside or below the borehole, and is especially adapted to finding small-scale, deep, rich ore bodies. In this method a transmitting loop is deployed on the top surface of the Earth, while a receiving coil is moved down the borehole. As the borehole TEM method is limited by the borehole’s location and depth, so its exploration scope is limited. The surface to airborne TEM method is a semi-airborne TEM configuration that transmits on the surface and receives TEM response in air. The two systems are combined into one system in this study, sharing the transmission loop deployed on the surface. With this combined system, the TEM response in the borehole and in the air can be observed at the same time. This paper employs a joint interpretation method based on the equivalent filament, which is introduced to obtain more reliable geometric information for the target with both borehole and aerial TEM data. The eddy currents induced in a thin confined conductor can be represented by equivalent current filaments, and the distribution of filaments can reflect the position and geometry of the conductor. Therefore, geometric parameters of targets can be obtained by filament inversion, and the joint inversion can be more accurate with both borehole and aerial response. Numerical modeling results show that the joint inversion based on the equivalent filament results can reliably obtain the geometric parameters of the thin conductive plate embedded in half space.

With the purpose of monitoring coal seam roof grouting, an innovative surface–borehole transient electromagnetic (TEM) method to compare the Earth resistivity distribution of the coal seam roof before grouting and after grouting is proposed. The TEM secondary field time–decay curve of the proposed surface–borehole TEM method is compared with that of the traditional surface TEM method. It is found that the time–decay curve of the former is very different from that of the latter. For the first time, forward modeling algorithm has been proposed to evaluate the TEM response characteristics of Earth, with an inversion algorithm developed to invert the measured data into resistivity distribution. A field work has been implemented with the customized equipment to evaluate the surface–borehole TEM method. In the field work, the proposed surface–borehole TEM measurement is performed twice, i.e., before and after the coal seam roof grouting. The analysis results demonstrated that the resistivity distribution had a distinct change near the depth of grouting with low resistivity distribution replaced by high resistivity distribution, which validates the effectiveness of the proposed surface–borehole TEM method.

This abstract outlines the use of the transient electromagnetic (TEM) method for hydrogeophysical investigations. The TEM method has proven to be a strong tool for the delineation of water bearing sand and gravel layers.We discuss newly developed TEM systems (PATEM and HiTEM) along with developments in data interpretation. Finally the TEM part of the Aarhus Survey is presented. This survey covers an area of more than 100 km2 and about 6000 TEM soundings have been measured.

The surface-to-borehole transient electromagnetic (TEM) method, which uses a rectangular loop as its source, has shown great potential in mineral exploration at greater depth. However, recent studies have demonstrated that a grounded-wire source can resolve the resistive targets and provide detection at greater depths than the loop source. Thus, we propose a surface-to-borehole TEM method using a grounded wire as a transmitter at short offsets. We conducted numerical experiments to explain the variation of different parameters and its comparable effects on the TEM transients. Similarly, a systematic comparison of the cross-sectional electric field distribution maps, borehole profiles of the time derivatives of the magnetic field, qualitatively reveal that the proposed method induces detectable responses for conductive and resistive targets at greater depths. In the present study, unlike the loop source configuration, the time derivative of an underground magnetic field in horizontal direction showed high sensitivity to resistive bodies. Furthermore, 1-D inversion was performed to provide quantitative interpretation of the borehole data. The 1-D inversion of the surface and borehole data was realized using an Occam-type inversion scheme. The results of both synthetic and field data indicate that inverting the surface or borehole data separately resulted in degraded resolution in both shallow and deep earth. Thus, we also compared the results from the inversions of surface and borehole data in order to explore the merits of joint inversion in resolving shallow and deep structures.

Effect of Application of Transient Electromagnetic Method in Detection of Water-Inrushing Structures in Coal Mines

The general aim of this study is, for the first time, to investigate the inner structure of mud volcanoes in Azerbaijan using several near-surface electromagnetic methods. The central-loop transient electromagnetic (TEM) method and the radiomagnetotelluric (RMT) method were applied on mud volcanoes near Perekishkul, Azerbaijan. These methods complement each other in terms of depth of investigation. In particular, the RMT method generally resolves the very shallow resistivity structures (<10 m) directly beneath the mud volcanoes, whereas the TEM method provides a resolution of up to 100- to 150-m depth. The obtained data sets were processed and interpreted individually. The TEM data were interpreted by conventional 1-D inversion algorithms, i.e., Occam's and Marquardt inversion, and by a quasi-2-D laterally constrained inversion. The RMT data were interpreted by 2-D inversion. Subsequently, a combined model regarding the sediments and corresponding resistivities for the area around the mud volcanoes was derived. Overall, a resistivity range of 1–20 Ω-m was observed. The resistivity models obtained from TEM support the assumption of a three-layer subsurface for the resolved depth range. Also, a lateral resistivity variation caused by the mud volcanoes was observed. Furthermore, the RMT results indicate the presence of shallow low resistivity structures, which can be interpreted as mud reservoirs.

Due to the existence of the metal casing in production wells, it is difficult for a conventional well-logging method to achieve satisfactory results. To study the possibility of measuring the formation conductivity through casing by using a transient electromagnetic (TEM) method, characteristics of the response signals received at different source-receiver spacing are investigated. Based on the advantages of the response signals at long spacing in the separation of the direct and indirect coupled signals, the corresponding differential response signals calculated with different formation conductivities are studied further. Results show that: (1) the differential signals are three orders of magnitude smaller than that of the original response signals; (2) there is a maximum and a minimum in the differential signal; (3) with the increase of the spacing, the profile of differential signal hardly changes, but moves backward on the timeline and the amplitude decreases slowly, which is in sharp contrast to the original response signals and (4) the differential signal is proportional to the difference of corresponding formation conductivities and the maximum and minimum values in the response difference are the most sensitive. The research reveals the characteristics of TEM responses in cased-hole and the distribution of formation conductivity information, which is predicted to provide a new perspective for the design of long-spacing through-casing in the TEM conductivity logging tool and the signal processing method.

The process of voltage drop and recovery will cause violent electromagnetic transient response of DFIG, which may lead to problems such as generator over-current or over-voltage. This paper gives a deep analysis to the electromagnetic transient response characteristics of DFIG under different grid voltage drop failures. On the basis of establishing mathematical model of DFIG under space vector coordinates, the electromagnetic response characteristics under grid voltage symmetric and asymmetric drop failures are theoretically analyzed, and the specific response characteristics equations are deduced, then the simulation validation of the above theoretical deduction is carried out by using Matlab software. Meanwhile, the differences of DFIG electromagnetic response characteristics under different drop failure types are compared. The relevant factors that influence response degree of different drop types are concluded, which has guiding significance for further study of DFIG-LVRT technology.

At present, the investigation of seafloor massive sulfides (SMS) is currently strongly limited by the available technology to the detection of actively forming sites. This is due to the fact that the detection of SMS deposits mainly relies on plume detection of active hydrothermal venting in the water column and seafloor morphological observations. While these methods have proven to be valuable tools for the detection of actively forming SMS, they do not allow the detection of deposits which are no longer connected to hydrothermal activity and which are possibly covered by sediments or lava.
\nTherefore, to broaden the scope of investigations on SMS it is necessary to develop new technologies allowing for the detection of SMS deposits which have finished their active phase. In land-based exploration it has been common practice for several decades to use electromagnetic methods to detect and characterize massive sulphide deposits. However, when turning to marine investigations only a few electromagnetic experiments have ever been conducted on SMS (e.g. by Cairns et al., 1997; Kowalczyk, 2008). In a recent publication our workgroup has proposed to use the transient electromagnetic (TEM) method (Swidinsky et al., 2012). There, we have shown that the TEM method would in principle not only be useful for the detection of a deposit under a sediment cover, but could also yield valuable information about it's depth and potentially also it's thickness.
\nHere, we will give a proof of principle by showing results of the – to our knowledge – first successful experiment, in which the TEM method was used to find a buried SMS deposit at the Palinuro Seamount in the Tyrrhenian Sea.

ABSTRACTMutually constrained inversion in combination with laterally constrained inversion (MCI‐LCI) between transient electromagnetic (TEM) and direct current (DC) resistivity methods was successfully used to characterize a buried valley structure. Although both methods measure, in some sense, the electrical resistivity, or conductivity, of the subsurface, they sample different volumes and have different sensitivities, which are exploited with mutually and laterally constrained inversion of combined, coincident profile data sets. The output models incorporate the information from both data sets to obtain optimum layered 1D models, fitting both data sets and constraints.The set‐up of constraints contains three parts. First, we constrain the individual data sets along their profile using lateral constraints producing a chain of TEM data and a chain of DC data. Next, we merge the information from these two chains by setting up mutual constraints between the TEM and the DC models. Finally, we adjust the mutual constraints to resemble the increasing sampling volumes with depth, i.e. wide constraints at large depths and short constraints at shallow depths. All data sets are inverted simultaneously; a common objective function is minimized, and the number of output models is equal to the number of 1D soundings. The lateral and mutual constraints are part of the inversion, and consequently the output models are balanced between the constraints and the data‐model fit. Information from one model will spread to the neighbouring models through the constraints, helping to resolve parameters that are poorly resolved by any of the individual data sets.A field example illustrates that MCI‐LCI allows the governing information from each method to dominate the inversion process. Thus, the model resolution in both the shallow and the deeper parts of the model is significantly enhanced. This could not be obtained by inverting the two data sets separately with a subsequent comparison of the output models. Our results are confirmed by drill‐hole data.