Geophysical surveys for ground water investigations, Central Oahu, Hawaii

Abstract

Time domain electromagnetic (TDEM) soundings were used to assist in the ground water evaluation of the Central Oahu area on the island of Oahu, Hawaii. The results of the TDEM survey show: (1) The unconformity between the Waianae and Koolau lava flows was detected and mapped on four TDEM survey lines from line 3N toward the south and not detected north of line 3N. Good agreement was observed about the presence, depth of occurrence, and thickness of the unconformity mapped in a borehole and interpreted from TDEM soundings. From TDEM measurements along lines on both sides of the unconformity no consistent influence of the unconformity on fresh water head could be observed. (2) The elevation of the fresh-salt water interface was mapped throughout the survey area. Over most of the survey area the elevation of the salt water interface ranges from -600 ft to -900 ft below msl. This range is consistent with observations of fresh water head in wells, although locally some differences between TDEM interpretations and heads observed in wells occur. An important conclusion from the TDEM survey is that large fresh water resources exist uniformly throughout the survey area with heads above 15 ft.