Xingong Tang

The application of 2D AMT and MT method in basement exploration in Brazil

A 2-D Audio Frequency Magnetotelluric (AMT) and Magnetotelluric (MT) survey were successfully completed in area of São Francisco, Brazil last year. Totally 40 MT sites were designed to cover part of one seismic line in order to compare the data of two geophysical methods. The main objective of this project is to supply the resistivity profile and data comparison with seismic exploration to outline the depth of basement ranging from the surface to 4,000 meters. The optimal inversion method and interpretation strategy for this 2-D AMT/MT data was first selected through the comparison of different modeling and inversion tryout results using a variety of inversion methods and actual field 2-D AMT/MT data. Upon applying the optimal inversion method and interpretation strategy, the depth and distribution of the basement was estimated based on the 2-D AMT/MT inversion results. From the 2D inversion of different modes, one can see that the depth of basement of this area is roughly from surface to about 1.2 km depth and it was relatively flat with a little fluctuation, which is correlate well with the result of seismic data. The interpretation result of the 2-D AMT/MT surveys indicates that AMT/MT methods were proven to be the effective methods for prospecting deep structures which in many cases appear to correlate well with geological structures.

Study of Influential Parameters for Transient Electromagnetic Method

Series of models are designed to test the influences of some parameters such as resistivity of overburden layer, basement and transmitter-receiver layouts, to a given 3D conductive anomaly in the layered earth model by transient electromagnetic (TEM) method, which is often used in engineering exploration. The forward modeling procedure used is formulated with volume integral equation based on tensor Greens function. The response of electromagnetic components are first calculated in frequency domain and then transformed to time domain by digital filtering. Through a series of modeling, it shows that the transient response of both e x and V z components are larger if the overburden layer and basement are resistive. The models also illustrate that S-s layouts are not sensitive to the variation of basement resistivity and it is more suitable for surveying shallow target.

Analysis of Improvement Effects of the Remote Reference Processing Technique to MT Data

The principle of remote reference processing technique for magneto telluric (MT) data is firstly introduced. Then take a MT exploration case history in Shanghai as an example, the improvement effect before and after remote reference processing of MT data using the magnetic data from the remote site located in Hang zhou, which is more than 200 kilometers away from Shanghai, is presented and discussed. The results show that remote reference technique is an effective way to improve data quality in MT data acquisition and processing.

Forward Simulation Results of CSAMT Data on Considering IP Effect

The CSAMT responses of geoelectric media are actually the accumulation of electromagnetic response and induced polarization under the stimulation of controlled source. The resistivity is dealt as a parameter which is scarcely related to the frequency when we proceed the forward and inversion with field data. Due to the induced polarization of underground layers, however, the resistivity of polarized layers is complex number relating to the frequencies and using only resistivity will lead an inaccurate result. This paper provides a primary trial in solving this problem that not only the current research situation of basic theory of IP in recent years, but also a few of preliminary modeling results are presented