Geophysical survey for ground water evaluation, TMK 8-7-14:02, Island of Hawaii

Abstract

This report contains the results ot time domain electromagnetic (TDEM) measurements acquired on April 27, 1990, on property owned by Cal-Pacific International, Inc. (TMK 8-7-14:02) on the Island of Hawaii. The data was acquired by Blackhawk Geosciences, Inc. at two locations. The specifications of the EM-37 and a brief technical note describing the basic theory of TDEM are given in Appendix A. In the schematic hydrogeologic section often encountered in volcanic islands, ground water is shown in two main occurrences: (1) In the basal mode the fresh water lies in static equilibrium on top of the basal saline water. In this case the Ghyben-Herzberg relationship states that for every foot that the fresh water is elevated above sea level, there will be 40 ft of fresh water below sea level. (2) Typically, further inland, ground water occurrences are often controlled by geologic structures such as dikes or other intrusives. In these cases ground water heads and production can be quite variable. On the property owned by Cal-Pacific International, Inc. the main ground water occurrence is expected to be in the basal mode. For basal mode water exploration, geophysical methods which measure the electrical resistivity of the subsurface are very effective. The reason for this is that electrical resistivity is directly influenced by changes in ground water salinity. By measuring the depth to saline water an estimate of the fresh water resource can be made (i.e., approximately the volume between sea level and the elevation of the saline water). The TDEM method was selected for this survey because it has the best sensitivity to lateral and vertical changes in resistivity compared to other geophysical methods.