Geophysical survey for ground water exploration near Keauhou-Kona, Island of Hawaii
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1990-10-02
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Blackhawk Geosciences, Inc.
Tom Nance Water Resource Engineering
Tom Nance Water Resource Engineering
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Abstract
"This report contains the results of time domain electromagnetic (TDEM) geophysical surveys for ground water resource evaluation on the Honokohau Property ("the Property") north of Kailua-Kona on the Island of Hawaii. The survey was performed by Blackhawk Geosciences, Inc. (BGI) for Lanihau Partners L.P. ("Lanihau") during September 11 to September 13, 1990.
The main objective of the geophysical survey at the Property was to determine the elevation and thickness of the lens of fresh water floating on saline water. ... The volcanic rocks are generally highly permeable and this allows rainwater to percolate with little impedance directly downward through the island mass. 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.
Because the electrical resistivity of rock formations is highly dependent upon the salinity of ground water, electrical surface geophysical techniques can map the depth to salt water, and the thickness of the fresh water lens can then be estimated using the Ghyben-Herzberg principle.
The impetus for using geophysics is that the cost of a geophysical sounding is about one-thousandth the cost of completing 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 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."
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groundwater, TDEM, Honokohau, Big Island, Hawaii, Geology--Hawaii, Groundwater--Hawaii, Water-supply--Hawaii, Geology, Groundwater, Water-supply
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31 pages
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