A geophysical site investigation of a water supply scheme ? case study

Abstract

Microgravity and EM resistivity surveys were carried out on three areas for the purpose of site investigation, to detect any possible cavities in a karstic area where a water supply project is to be erected. The survey also included measurements near the project site, along two profiles crossing two previously drilled boreholes in which cavities were found. Gravity lows and resistivity highs coincided with the positions of known cavities. The survey on the three areas indicates possible near-surface cavities ranging in depth between 2.5?7 m and extended 3?4 m. These depths have been estimated by using inverse problem formulae and two-dimensional interpretation of the gravity anomalies. Meanwhile, comparison of the gravity and EM-resistivity maps help in delineating near surface cavities. Future seismic activity forecasting, using the algorithm of McGuire based on Cornell's probabilistic technique, has been used to evaluate ground acceleration and return period at the site. It is found that for 100 years return period, the value of peak ground acceleration is 0.05 g. From these results it is recommended that detailed geophysical investigation prior to any drilling be carried out. This would locate the weak zones and, with drilling information, allow a better geophysical interpretation.