Dongyang Hou

A Comparison of Different-Mode Fields Generated from Grounded-Wire Source Based on the 1D Model

AbstractsTraditional TEM study mainly focuses on the generation and application of the TE field using a loop or grounded-wire source; but in recent decades, lots of efforts have been made for implementation of the TM field and even the integration of the TE field with the TM one into anomaly detection in the subsurface. However, no applicable principles have been proposed for selecting the optimal electromagnetic field for various subsurface targets. The transient electromagnetic (TEM) fields generated from grounded-wire source consist of the TE-mode response (current-carrying wire), the TE–TM mode response (grounding ends) and the combined TEM-mode response (current-carrying wire and grounding ends). This study performs a comparison of TE/TE–TM/TEM fields by generating them from grounded-wire source and testing their distribution characteristics, detection depth, and sensitivity to anomalies, using both synthetic 1D model and two field surveys in China. The comparisons demonstrate that, the detection depth of the TE–TM field is smaller than those of both the TE and combined TEM fields. Meanwhile, for electric field, the TE–TM response provides a better detection than the TEM one, but with an uneven distribution. Therefore, the TE–TM electric field requires well-designed arrangements of receiving positions when applied to real projects. For the magnetic field, the TEM response has the best detection capability compared to the TE and TE–TM ones, but is least sensitive to layer thickness and resistivity, especially for an embedded layer with low resistivity.

Short-offset grounded-wire TEM method for efficient detection of mined-out areas in vegetation-covered mountainous coalfields

The in-loop transient electromagnetic (TEM) method has been widely used for reliable investigation of mined-out areas of the subsurface. However, the method has limited applications in mountainous coalfields that are often covered with tall vegetation: first, it requires extra work to clear the surface and set up the TEM equipment; more importantly, dense vegetation restricts the required layout of a standard rectangular loop, and such geometry irregularity can decrease the detection accuracy. This study proposes using short-offset grounded-wire TEM (SOTEM) for enhanced detection of the mined-out areas in vegetation-covered mountainous fields. The distance between the SOTEM source and receivers is flexible and thereby the receiving location can be freely adjusted according to the environmental conditions of the surveying area, without reducing the detection resolution. Properties of the proposed SOTEM method are well tested before real applications, and our test indicates that SOTEM has an axis-symmetric field distribution, higher sensitivity to anomaly, and larger detection depth, compared to the conventional large-loop TEM method. Application to one coalfield in Changzhi (Shanxi Province, China) demonstrates the added value of implementing SOTEM for detecting the mined-out areas in this field and the results are well verified by the drilling result. The objective of this study was to overcome the limitations in using the loop-source TEM method and realise the efficient detection of mined-out areas using the short-offset grounded-wire TEM (SOTEM) method in vegetation-covered mountainous coalfields. The feasibility of SOTEM is verified by the drilling result in the field test.

Investigation of Axial Electric Field Measurements with Grounded-Wire TEM Surveys

The grounded-wire transient electromagnetic (TEM) surveying is often performed along the equatorial direction with its observation lines paralleling to the transmitting wire with a certain transmitter–receiver distance. However, such method takes into account only the equatorial component of the electromagnetic field, and a little effort has been made on incorporating the other major component along the transmitting wire, here denoted as axial field. To obtain a comprehensive understanding of its fundamental characteristics and guide the designing of the corresponding observation system for reliable anomaly detection, this study for the first time investigates the axial electric field from three crucial aspects, including its decay curve, plane distribution, and anomaly sensitivity, through both synthetic modeling and real application to one major coal field in China. The results demonstrate a higher sensitivity to both high- and low-resistivity anomalies by the electric field in axial direction and confirm its great potentials for robust anomaly detection in the subsurface.

Exploration of Deep Magnetite Deposit Under Thick and Conductive Overburden with Ex Component of SOTEM: A Case Study in China

In northern China, thick loess strata with low resistivity are widely present and can shield the deep induced electromagnetic field signal and influence the accuracy of deep targets while electromagnetic prospecting is being performed. To achieve deep target exploration with high resolution, a new technique known as the short offset transient electromagnetic method (SOTEM) is applied in this research area. This method can be used to perform near-source configuration surveys, but shows no obvious difference between the deep magnetite deposit and low-resistivity layer in this survey area. Therefore, it cannot explore the magnetite deposit directly using the traditional magnetic component (Hz) of SOTEM. The Ex component, which can more effectively distinguish the high-resistivity surrounding rock and overlying low-resistivity layer than the Hz component, is used for exploration. First, 2D forward modeling and sensitivity analysis of the high-resistivity body are carried out to determine the different responses of the Ex and Hz components, respectively. Moreover, to better determine the interface between the surrounding rock and low-resistivity layer, 1D Occam inversion and the generalized inverse matrix inversion method are used. Furthermore, aiming at the static effect of the Ex component in field exploration, joint inversion of the Ex and Hz components is carried out for comprehensive interpretation. Finally, a field test is conducted, and the drilling results are used for verification. The research results can provide an effective geophysical model for the exploration of contact metasomatic magnetite deposits.

The Shielding Effect of Low Resistivity Layer in TEM

Transient electromagnetic method can penetrate high-resistivity shielding layer and delineate underlying strata or structure. However, the shielding effects of low-resistivity layer on transient electromagnetic exploration are often ignored, especially for low-resistive overburden. It is of great importance to ensure exploration depth and improve detection precision by researching the relationship between low-resistive layer and electromagnetic field. In this paper, the propagation process of the subsurface and surface TEM field responses were simulated using finite-difference time-domain method (FDTD). And the characteristic response of low-resistivity overburden was studied using numerical modeling. The observation time with low-resistivity layer was estimated using observation time formula. It was shown that a low-resistivity layer will not only reduce the propagation speed of the field, but also weaken the response of low-resistivity target.

Bias in Transient Electromagnetic Method Due to Non-rectangular Loop

Rectangular loop is one of the most popular transient electromagnetic (TEM) devices, which is widely used in engineering, hydro-geological and ore deposit exploration. However, in theory, circular loop is used to approximately simulate the rectangular loop. Traditionally, this gives rise to a series of problems. Meanwhile, in practice, the rectangular loop is not easy to layout on field especially in complex terrains or mountainous areas. Therefore, we have modified the rectangular loop. The geometry of the loop may now be arbitrarily named, modified-rectangular loop. However, some bias will be caused if standard rectangular-loop parameters are used to invert the field data from modified rectangular loop. And this bias lowers the detection precision of the modified rectangular loop. Therefore, the bias caused by the approximation of circular and modified rectangular loop is analyzed in this paper. Firstly, the response distribution and bias between circular loop and rectangular loop are compared using induced voltage and primary field. Then, the forward method of vertical magnetic field from modified rectangular loop based on coordinate transformation is given. The bias between rectangular loop and modified rectangular loop are then analyzed using vertical magnetic field. From theoretical modeling that there is an indication that changing the loop geometry will result in some error and lower the detection precision.

The Response of In-loop Transient Electromagnetic Configuration

For the aim to assess the large loop transient electromagnetic method (TEM), an important method in engineering exploration, we study the distribution of vertical and horizontal magnetic component by calculating the 1-D response corresponding to in-loop and out-loop field point. A new algorithm for Dual Bessel integral has been developed. We divided the integral range \( (0,\infty ) \) into \( (0,\lambda_{0} ] \) and \( [\lambda_{0} ,\infty ) \). In the integral range of \( (0,\lambda_{0} ] \), based on the derivative relationship of Bessel integral, transforming the integral into the form of easily calculation. In the range of integral of \( [\lambda_{0} ,\infty ) \), Linear sine and cosine transformation algorithm can be used for the computation. We design models for numerical simulation and give the result. Based on the results, we draw a conclusion that vertical magnetic field component can be surveyed in the range of 50% inside of the loop and horizontal magnetic field component is so smaller than vertical component that it can not be well surveyed inside large loop.