SEISRES - a Visual C++ program for the sequential inversion of seismic re-fraction and geoelectric data

Refraction seismic and geoelectric methods are usually applied for the deli neation of near surface structures in environmental, engineering and hydrog eological investigations. When applied independently, these techniques yiel d sufficiently accurate subsurface models. But the inversion of these data may also lead to incorrect. parameter estimation specially in complicated g eological situations, namely, blind zone problems in seismics, suppression and equivalence problems in geoelectrics. Stability and non-uniqueness can be reduced to a great extent by integrating physically different sets of da ta into a joint or sequential inversion scheme. In the present paper, we ai m at introducing one such algorithm, wherein the seismic refraction and DC resistivity inversion routines are amalgamated. Even though the seismic and geoelectric methods may independently see different interfaces due to comp letely different physical responses, the joint or sequential inversion need s a common parameter, the layer thickness being the one in this situation. The proposed scheme is coded in Visual C++ on Microsoft Windows '95 environ ment using the concept of object-oriented programming. The program SEISRES is exclusively menu driven and customised for running on personal computers . It has several options, namely, seismic ray inversion for near-surface es timation, curve dissemination for generating a starting model using seismic depth section, 1D resistivity inversion for Schlumberger and Wenner single electrode arrays using evolutionary programming for global optimisation, 1 D forward calculations, creation of resistivity data sets from Wenner multi -electrode pseudo-section and construction of quasi-2D geoelectric section of the subsurface. The software is tested on a variety of synthetic example s with complex litho-stratigraphic relationships. The present paper deals w ith three such synthetic examples for the delineation of an aquifer in a th ree-layer setting in the first two examples and the detection of a thin con ductive clay lens embedded in an aquifer in a four layered earth model emul ating an equivalence problem. The seismic-guided 1D Schlumberger and Wenner inversions and the quasi-2D sections from pseudo-section interpretation le d to subsurface information with better precision compared to the direct 2D inversion. A detailed field investigation was undertaken in Midnapur Distr ict for ground water prospecting. The results of two test sites at Satkui a nd Tangasol are presented here for judging the performance of the software package. Available borehole lithologs and geophysical logs validated the fi ndings from SEISRES. The strength of the present scheme lies in its ability to model the subsurface seismically and geoelectrically, even in 2D enviro nment by 1D approximation on a laptop/personal computer at-site cost-effect ively. (C) 2000 Elsevier Science Ltd. All rights reserved.