Geophysical surveys for ground water evaluation near Waikoloa, South Kohala, Hawaii

Abstract

The objectives for the geophysical survey can be understood from the hydrogeologic cross-section, typical of a volcanic island. The volcanic rocks are generally highly permeable and rainfall rapidly infiltrates into the ground and migrates downward to the water table, and eventually discharges into the ocean. Fresh water in these settings is found in two environments. 1. Dike confined waters. Above the rift zone intrusive dikes originating from a magma source below can form ground water dams, and behind these natural dams significant quantities of ground water can be stored. 2.Basal fresh water. The high permeability of the volcanic rocks allows sea water to enter freely under the island, and a delicate balance is reached where a lens of fresh water floats on sea water. The Ghyben-Herzberg relation states that for every foot of fresh water head above sea level there will be 40 ft of fresh water below sea level. The basal water resource was the focus in the investigations for MLR. The drilling depth to the basal fresh water lens rapidly increases with elevation, and the objective of geophysical surveys is to determine the drilling depth to fresh water-and the thickness of the fresh water lens. The impetus for using geophysics is that the cost of a geophysical station is about one-five-hundredth of the cost of drilling a well at elevations above 1,000 ft. Geophysical surveys, combined with other hydrogeologic information, are used to provide optimum locations for well placement and well completion depths. The geophysical method employed was time domain electromagnetic (TDEM) soundings. This method was selected because it has proven effective in prior surveys in similar settings in Hawaii.