Huasen Zhong

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.

The Extraction of TEM Response from Pseudo Random Binary Sequence Source EM Data

This paper presents a method for converting the full waveform electromagnetic response excited by the pseudo-random binary sequences (PRBS) into subsurface resistivity cross section, based on the recovery of the transient electromagnetic diffusion field. The impulse response of the earth is extracted from the full waveform data efficiently using the Wiener-Hopf equation. Then, the backward accumulation is used to obtain the step off response from the extracted earth impulse response. An OCCAM inversion routine, which incorporates the band limitation of the derived transient, is adopted to produce the geoelectric models. Finally, the method was applied to the experimental field data set, and the consistency between the recovered models and the geological condition verifies the validity of this method.

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.