Fast quantitative interpretation of surface-borehole TEM using transient impulse peak time

Due to the complex response law of the surface-borehole Transient Electromagnetic Method (TEM), qualitative analysis and semi-quantitative interpretation are the most widely used interpretation methods. However, the methods cannot obtain the resistivity parameter of the earth directly. To solve this problem, a fast quantitative interpretation method based on transient impulse peak time is proposed. Firstly, the expression of uniform half-space surface-borehole transient electromagnetic response is given, and the impulse time characteristics of surface-borehole transient electromagnetic response are analyzed. The results show that the deeper the receiving point is buried and the higher the earth conductivity is, the later the transient impulse time is. Based on the previous research results, the function between impulse time and earth conductivity is deduced, and the apparent resistivity is defined according to inverse function theory. In order to obtain the true resistivity of the earth, an improved resistivity recovery algorithm based on the diffusion velocity of the underground electromagnetic field is derived. The algorithm is applied to design the uniform half-space model, two-layer model and three-layer model respectively according to the common working mode in practice. Synthetic and measured examples show that the method of defining apparent resistivity based on impulse time can reflect the variation trend of earth resistivity well, but it has strong volume effect. The improved algorithm based on the diffusion velocity of the electromagnetic field can effectively weaken the volume effect, recover the resistivity value and reflect the electrical interface more accurately. This algorithm does not need iterative forward modeling calculation of complex models, so it has high efficiency, and can quantitatively recover the value of earth resistivity, which make it suitable for quick preliminary quantitative interpretation of the surface-borehole TEM. However, the “overshoot” and “undershoot” phenomena in the apparent resistivity definition should also be carefully considered when the algorithm is applied to data processing and interpretation to avoid incorrect interpretation.