Geophysical survey for ground water exploration near Keauhou-Kona, Island of Hawaii

Abstract

"This report contains the results of a geophysical survey to assist the evaluation of fresh water resources near Keauhou-Kona on the Island of Hawaii. The work was performed for Kamehameha Investment Corporation (KIC) between January 9 and January 24, 1990. The objectives for the geophysical survey can be understood from the generalized hydrogeologic cross-section, typical of a volcanic island, shown in Figure 1-1. 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 -- Typically, 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. In cases of hydrostatic equilibrium, the Ghyben-Herzberg relation states that for every foot of fresh water head above sea level there will be about 40 ft of fresh water below sea level. The basal water resource was the focus in the investigations for KIC. The drilling depth to the basal fresh water lens rapidly increases with elevation, and the objective of geophysical surveys was 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 typically less than about one-five hundredth of the cost of drilling a well at elevations above 1,000 ft. Geophysical surveys are used to provide optimum locations for well placement and information about optimum well completion depths."