Abstract

I have used both d.c. resistivity and time-varying (0.2-5 Hz) magnetic field soundings to study the Ulupalakua-Makena area on the island of Maui, the site of the last historic eruption in 1790. Using generalized inversions of the combined data, I have estimated the electrical resistivity structure beneath eight magnetometer sites. The resistivity values I obtained for electrical basement (saltwater-saturated basalt) are reasonably consistent with a single value of 6 + or - 1 W/m at all eight sites, although there is a suggestion of an increase in resistivity away from the coastline. The resolution matrices for the magnetic data indicated that, in addition to resolving the resistivity of electrical basement, the data were significantly affected by a combination of the resistivity of the soil layer and the depth to basement. Use of the Schlumberger data in conjunction with the magnetic data removed this correlation and resulted in good resolution for the resistivity/thickness combinations of the tour upper model layers as well as for the resistivity of electrical basement. Assuming an average porosity of 20 percent for basalt and using a formation factor obtained for drillhole samples on the island of Hawaii, I estimate the basement temperature required to explain the interpreted resistivities is 59 + or - 13 degrees C. Since the penetration depth at the lowest frequency used is about 3 km, it is unlikely that these low resistivities can be explained by single flows having anomalously high porosities. I therefore conclude that the area may be of value as a low- to medium-temperature geothermal resource.

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