Interpretation of self‐potential anomaly over idealized bodies and analysis of ambiguity using very fast simulated annealing global optimization technique

Abstract

ABSTRACTAn efficient and reliable approach is developed for the interpretation of self‐potential anomaly measured over idealized bodies (sphere, horizontal and vertical cylinder) using a very fast simulated annealing (VFSA) global optimization method. Since VFSA optimization lends itself to a number of good‐fitting models in a vast multidimensional model space, the nature of ambiguity in the interpretation has also been investigated simultaneously. The study reveals that, while optimizing all model parameters (electric dipole density, horizontal location, depth, polarization angle and shape factor) together, the VFSA approach yields a number of equivalent solutions. It has been observed that the shape factor plays an important role in finding a reliable estimate of other model parameters. The analysis of ambiguity shows that a small change in the shape factor produces a large change in the estimated electric dipole density. Accordingly, inaccurate estimates of other model parameters have also been obtained. It has been observed that the optimization method is able to determine all the model parameters accurately when shape factor is fixed. Therefore, interpretation of Self‐potential data is carried out by adapting a two‐step procedure. In the first step, all the model parameters are optimized. The inversion results obtained after the first step indicates the value of shape factor is around 1.5, 1.0 or 0.5. Subsequently in the second step, the shape factor is fixed to 1.5, 1.0 or 0.5 and other model parameters are optimized. In this way, the most reliable result has been obtained, and ambiguity in the interpretation has become insignificant. The efficacy of this approach is demonstrated using noise‐free and noisy synthetic data and three field examples from different areas. One field example is interpreted using multiple targets to show the efficacy of the developed approach in dealing with optimization of a large number of model parameters. The computation time of the two‐step procedure is very short (35 s for each step). It is highlighted that, even if the shape factor is known either from a priori geological information or anomaly contour map, interpretation should be performed in two steps to obtain the most reliable estimate of various model parameters as well as confirmation of geometrical shape of the subsurface structure.