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

Abstract

"A geophysical survey was performed for Palace Development Corporation (PDC) by Blackhawk Geosciences, Inc. (BGI) from March 27 through March 31, 1990. The purpose of this survey was to assist the evaluation of fresh water resources near Kahuku Ranch on the southern end of the Island of Hawaii, at the Riviera Resort Development. 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 allowing rainfall to rapidly infiltrate into the ground and migrate downward to the water table, and eventually discharge 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 specific 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."