A new fast imaging method for inline electrical source transient electromagnetic data

Abstract

The transient electromagnetic (TEM) method is a widely used geophysical technique for obtaining electrical information about subsurface. In recent years, the electric source TEM method has gained popularity, particularly the inline configuration, which is highly sensitive to resistive targets. One of the key challenges in TEM research is the transformation of observed responses into resistivity-depth profiles. This paper introduces a new fast imaging method that addresses this challenge by directly converting the responses to resistivity-depth information. As the relationship between the TEM response and the resistivity is an implicit nonlinear function, we cannot get an explicit formula for the resistivity. Our method started from constructing a time constant using the peak-time of the impulse response and derived an explicit formula for the apparent resistivity. As a result, we can efficiently estimate the resistivity from the impulse response. Then, an apparent depth is defined based on the maximum depth of the electric field excited by a grounded wire source. Hence, the time-variant impulse response can now be imaged into the variance of resistivity with depth. To improve the accuracy of extracting the peak-time of the impulse response, an interpolation and re-extraction scheme is designed and applied. Numerical and field data examples are used to verify the effectiveness of the proposed fast imaging method. The results show that, compared with conventional apparent-resistivity imaging method, the proposed method is based on analytical derivation and can be used for at the full time range of the responses, thus with higher stability and calculation efficiency. Finally,