Geophysical survey for ground water evaluation, West Beach Estates area, Oahu, Hawaii

Abstract

TDEM soundings were conducted in the West Beach Area along four lines during March and July 1990, to attempt to map a postulated hydrologic barrier inferred from prior drilling results. Heads calculated from the TDEM data, using the Ghyben Herzberg relation along line lN, show good agreement with measured heads in nearby boreholes. The likely location of the hydrologic barrier has been delineated on three of the survey lines (lS, lN, 2N) and the position of the barrier. On line .2S the barrier was not detected, mainly because measurements could not be made far enough to the east to intercept the expected position of the barrier. The location of the hydrologic barrier is inferred from (i) an increase in the resistivity of the lower layer (possibly due to dolomitization or other lithologic change), and (ii) a rapid change in depth to the salt water saturated zone east of the barrier. This rapid change in depth to salt water was detected on three lines. Accuracy in determining depth to the salt water saturated zone is expected to be about ± 5% of the total depth. If it is important to determine the course of the hydrologic barrier further to the south then additional soundings on the east side of line 2S would be necessary, and an additional line parallel and south of line 2S would further assist in locating its position.