Guoqiang Xue

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.

Control of the waveform dispersion effect and applications in a TEM imaging technique for identifying underground objects

The pseudo-seismic waves obtained by the transient electromagnetic method (TEM) can be used to explain geological conformations of underground objects. However, the directly transformed pseudo-wave always has a dispersive and broader waveform so that it cannot clearly indicate the geological–electrical boundary of the explored underground objects. This study shows that the reason for the waveform dispersion is because the variance of the kernel function with the Gauss distribution is the real time; the wave distribution range will increase with increasing time. Therefore, this study presents a technique for enhancing the resolving power of pseudo-seismic data processed by a de-convolution method, and detailed theory and algorithms are reported. The pseudo-seismic signals of two geological–electrical models are taken as the examples to verify the present algorithm. Results show that the pseudo-seismic waves after de-convolution processes have sharper waveforms and narrower distribution zones, and this can effectively control the waveform broadening phenomenon and is able to enhance the resolving power using the TEM pseudo-seismic waves to survey the boundary of underground objects.

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.

Extracting the Virtual Reflected Wavelet from TEM Data Based on Regularizing Method

A pseudo-seismic interpretation method is an alternative way to process and explain transient electromagnetic (TEM) data, and has become a popular research field in recent years. TEM signals which satisfy the diffusion equation can be converted by means of a mathematical transformation into ones which obey the wave equation. For an ill-posed problem of this kind of transformation, a sub-regularization algorithm is developed in this paper to extract a virtual wavelet of the TEM field. According to the conventional designation of TEM recordings, the entire integration period is divided into seven time intervals. In order to avoid low accuracy in the calculations, high-density wavefield data has been calculated based on the former sub-division. Therefore, the virtual wavelet can be extracted successfully by using an optimized algorithm to obtain high-density integral coefficients for all time windows, and a satisfactory condition number of the coefficient matrix while taking a different channel number in each time period. The Tikhonov regularization inversion scheme is used to determine the optimal parameters based on minimizing a least squares misfit, and the Newton iterative formula is used to obtain optimal regularization parameters. Both synthetic model simulations and a real data interpretation example indicate that the proposed pseudo-seismic wavefield method is a suitable alternative way to interpret TEM data.

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.

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.

Correlation analysis and imaging technique of TEM data

Although the transient electromagnetic method (TEM) has been used in geo-exploration for many years, the inversion precision of TEM data is still very limited and new techniques are needed to improve TEM data interpretation. Mathematically, TEM data can be converted into a series of virtual waves. Multi-aperture electromagnetic modelling shows that the coherence of multi-aperture TEM and echo waves measured at adjacent positions from the same geological body is high. Therefore, it is feasible to apply multi-aperture synthesis to TEM data. Based on the idea of synthetic aperture radar, a new data-processing method has been developed that uses superposition to realise multi-aperture data integration as well as Kirchhoff migration and imaging. After the pseudo-wavelet extraction from TEM data, the traditional approach of profile-based multi-aperture synthesis has been developed for each survey station. Furthermore, the traditional single point approach was applied for multiple point coverage. The technology of synthetic aperture improves TEM resolution, rendering it possible to extract information from TEM data that cannot be obtained by conventional methods. Experiments with both synthetic and survey data show that synthetic aperture imaging is effective, paving the way for developing a new TEM imaging technology. Mathematically, transient electromagnetic method (TEM) data can be converted into virtual waves. It is possible to process TEM data using the synthesised imaging method, which originates from radar data processing. The technology can improve TEM resolution. Both synthetic and survey data show that synthetic aperture imaging is an effective way of developing TEM imaging technology.