Nannan Zhou

Discovery of a Large-scale Porphyry Molybdenum Deposit in Tibet through a Modified TEM Exploration Method

During the last decade, there have been many exploration achievements in the Tibetan Gangdese metallogenic belt. The Sharang area of the Tibetan region is covered by a lowtemperature mineralized alterable clay that is considered to be a low-grade ore. Although small intervals of rich molybdenum (Mo) mineralization have been discovered, the ore deposit scale is limited and the condition of deep ore is still unknown. To explore these deeper targets, a modified large-loop TEM system was used in the Sharang area. The TEM receiver configuration is redesigned and the late-time resistivity equation of large-loop TEM has also been defined. During data processing, two regions with low resistivity anomalies were discovered. The interpreted results indicate that the main ore deposit is buried 200 m beneath the surface, and extends 600 m vertically. The total anomalous area associated with the ore deposit is estimated at 3.77 km 2 . The interpretation results are consistent with drilling data acquired after the geophysical survey. The results show that it is the first ultra-large porphyry molybdenum deposit that has been found in Tibet.

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.

Detection and monitoring of water-filled voids using transient electromagnetic method: a case study in Shanxi, China

Shanxi Province is a major coal base for the rapid development in China, but heavy production of coal resources has left abundant water-filled voids, creating significant geologic hazards around coal mines. It is necessary to quickly investigate the position and distribution of voids with effective geophysical methods. The feasibility of transient electromagnetic (TEM) method in detecting and monitoring water-filled voids is analyzed using 3D finite-element numerical simulation results as well as a case study in this paper. The case study was mainly to analyze and monitor the conditions around the site of a power transfer station under construction in the west of Shanxi province where many water-filled, mined-out voids existed. Central-loop TEM sounding was carried out three times from January 2010 to April 2011 around the building footprint. The low resistivity abnormal area had been repeatedly located and the survey results showed that the area of voids increased with time, which indicated that the ground at the site was in an unstable and dangerous state. The surveys provided more detailed geological information to the government for further safety arrangement and building re-design.

Physical simulation and application of a new TEM configuration

In some applications, especially for tunnel surveys and ancient tomb investigations, the scale of survey location is so small that it is impossible to lay a large enough transmitter loop for detection. A small-scale and high-power Transient Electromagnetic (TEM) transmitter configuration must be adapted to detect longer distance or greater depth. Redesigning the TEM surveying configuration may facilitate improving signal penetration and precision of TEM soundings. Based on physical simulation, a newly designed special-loop-source TEM survey configuration has been introduced, which employs four square apertures within a single large transmitter loop to excite stronger fields. The primary and secondary fields have been measured using both the new special-loop system and standard normal single loop TEM configuration for different receiver-transmitter separations and for different target depths. The response curves were compared, revealing that the primary field intensity and the secondary field response were improved by the special-loop transmitter system as compared to the standard system. The new special-loop configuration can be used for tunnel TEM prediction and other TEM investigations. A case study was conducted on tunnel forecasting in Hubei Province, China. This terra TEM survey showed that it is an effective and successful method for exploring and predicting challenging geological structures ahead of a tunnel wall during excavation.

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.

Comparison of the time-domain electromagnetic field from an infinitesimal point charge and dipole source

An electromagnetic field is generated through the accelerating movement of two equal but opposite charges of a single dipole. An electromagnetic field can also be generated by a time-varying infinitesimal point charge. In this study, a comparison between the electromagnetic fields of an infinitesimal point charge and a dipole has been presented. First, the time-domain potential function of a point source in a 3D conductive medium is derived. Then the electric and magnetic fields in a 3D homogeneous lossless space are derived via the relation between the potential and field. The field differences between the infinitesimal point charge and the dipole in the step-off time, far-source, and near-source zones are analyzed, and the accuracy of the solutions from these sources is investigated. It is also shown that the field of the infinitesimal point charge in the near-source zone is different from that of the dipole, whereas the far-source zone fields of these two sources are identical. The comparison of real and simulated data shows that the infinitesimal point charge represents the real source better than the dipole source.

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.