Weiying Chen

Appraisal of an Array TEM Method in Detecting a Mined-Out Area Beneath a Conductive Layer

Abstract The transient electromagnetic method has been extensively used for the detection of mined-out area in China for the past few years. In the cases that the mined-out area is overlain by a conductive layer, the detection of the target layer is difficult with a traditional loop source TEM method. In order to detect the target layer in this condition, this paper presents a newly developed array TEM method, which uses a grounded wire source. The underground current density distribution and the responses of the grounded wire source TEM configuration are modeled to demonstrate that the target layer is detectable in this condition. The 1D OCCAM inversion routine is applied to the synthetic single station data and common middle point gather. The result reveals that the electric source TEM method is capable of recovering the resistive target layer beneath the conductive overburden. By contrast, the conductive target layer cannot be recovered unless the distance between the target layer and the conductive overburden is large. Compared with inversion result of the single station data, the inversion of common middle point gather can better recover the resistivity of the target layer. Finally, a case study illustrates that the array TEM method is successfully applied in recovering a water-filled mined-out area beneath a conductive overburden.

Quasi MT Inversion of Short-Offset Transient Electromagnetic Data

The short-offset transient electromagnetic method (SOTEM) has been extensively used for mineral and hydrocarbon exploration and hydrogeological investigations due to its ease of use and capability to generate diagnostic subsurface information. At present, the data processing methods of SOTEM are mainly focused on one dimensional inversion. To apply the proven inversion methods of frequency domain electromagnetic methods to SOTEM data, this paper presents a new transformation relation from time to frequency based on the similarity between SOTEM all-time apparent resistivity and magnetotelluric (MT) apparent resistivity. Results show that the transformation coefficients depend on the variation trend of SOTEM all-time apparent resistivity curves. Bostick inversion and conjugate gradient inversion techniques were applied to transformed SOTEM data and the results were validated by some simulated calculations and field measured data. This study provides a novel method to SOTEM data processing and a useful aid to join inversion with MT data.

A New Apparent Resistivity Formula for In-loop Fast Sounding TEM Theory and Application

The central-loop time-domain electromagnetic (TEM) method has been widely used in hydrogeological prospecting. Because this type of TEM configuration is relatively insensitive to lateral resistivity contrasts, the mathematical derivation of an apparent resistivity formula, valid for later recording times, is greatly simplified. However, no simple formula exists for an in-loop off-center TEM sounding. In this type of acquisition, the field response is recorded at several points surrounding the central location. In-loop TEM surveys are frequently employed to obtain high spatial resolution within engineering geophysical exploration. Although the apparent resistivity formula of a fixed-loop TEM system is also applicable for the non-central receiver points used in an in-loop TEM system, the apparent resistivity values need to be extracted using an iterative method that requires complicated integrations. Presently, no fast method exists that can give access to the apparent resistivity values in a direct manner. The main objective of this paper is to present a possible solution to this computational problem. A new solution is presented that takes as a starting point a circular transmitter loop where an analytical solution exists for non-central receiver points. This analytical solution is further approximated through the combined use of least-mean square (LSQR) determined polynomial coefficients and an equivalent circular loop to represent the rectangular loop. The new apparent resistivity formula, valid for in-loop TEM, has been tested on field data. A successful case study from central China is presented where an increased sensitivity to locate water-filled zones has been obtained.

Discovery of a Major Coal Deposit in China with the Use of a Modified CSAMT Method

Conventional use of the controlled-source audio-frequency magneto-telluric (CSAMT) method is based on calculating the apparent or Cagniard resistivity from the amplitude ratio of the horizontal electric and magnetic field components. However, direct comparison between these two components shows that the electric field is more sensitive to the underground medium resistivity than its magnetic counterpart. Thus, use of the electric component only should provide adequate information about the electric properties of the subsurface. The measurements typically belong to the far-field zone, but show a non-dipolar characteristic because of the source. In this paper, we therefore propose a simplified CSAMT technique based on measuring the electric field component only. As part of this new formulation, a related theoretical model for the electric field component accounting for the non-dipolar nature of the transmitter antenna is introduced. This is accompanied with a new apparent resistivity definition, including a procedure to transform it into pseudo-phase data, thus removing the static shifts. The potential of this modified CSAMT method is demonstrated using a field case from the Shanxi province in China. Until recently, it has been thought that no coal deposits exist in this region. However, application of the single-component CSAMT technique as advocated for here, revealed a major coal deposit, which was verified later by drilling.

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.

Research on the Application of a 3-m Transmitter Loop for TEM Surveys in Mountainous Areas

Large loop transient electromagnetic (TEM) is a widely used surface exploration method for mapping electrically conductive bodies. However, when working in the mountainous areas of China, it is difficult to lay out large enough transmitter loops for conventional surveys. An option for this type of terrain is a newly designed TEM survey configuration that uses a small 3-m square loop in combination with a high current (1,000 A to 2,000 A) TEM transmitter to excite a powerful EM field for detecting targets at greater depths. The detection depth of the system is analyzed. After comparing the responses as well as the detection depths between this special system and standard large loop configurations, it is shown that: 1) a small loop system can result in a detection depth similar to that of a large loop system; and 2) a small loop system is superior to a large loop during shallow depth target detection as well as in mountainous areas. Sufficient signal intensity can be obtained by a small central loop configuration equipped with a high current transmitter and a large equivalent area receiver coil. A large trial survey was conducted on the surface of mountainous areas in the Shanxi Province of China. This TEM survey showed that the 3-m by 3-m transmitter loop system was an effective method for deep TEM sounding in mountainous areas.

A Fast Topographic Correction Method for TEM Data

Topography affects electromagnetic data and leads to wrong interpretation of shallow resistivity layers. In this paper, we propose a new method for correcting topographic error in electromagnetic data. In order to achieve this, we used a small DC configuration to obtain the surface resistivity, and then used the surface resistivity to construct a new topographic correction formula. Both synthetic and field TEM and CSAMT data were processed using this method. The results show that our method is a fast, effective and practical tool for correcting topographic effects in electromagnetic data.