Geophysical survey : ground water evaluation at mac farms, island of Hawaii

"This report contains the results of a geophysical survey to assist the evaluation of fresh water resources for Kapua Ventures at Mac Farms. The work was performed by Blackhawk Geosciences, Inc. (BGI) for Kapua Ventures on April 20, 21, 22 and 26, 1990. The objectives for the geophysical survey can be understood from the hydrogeologic cross-section, typical of a volcanic island, shown in Figure 1-1. 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 2. Basal fresh water. The basal water resource was the focus in the investigations for Kapua Ventures. 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. The specific objective of the geophysical survey over the Mac Farm area was to derive the thickness of the basal fresh water zone in the study area."Kapua Venture