A preliminary geothermal evaluation of the Mokapu Peninsula on the island of Oahu, Hawaii

Abstract

Preliminary geological, geochemical, and geophysical field surveys have been conducted on Mokapu Peninsula on the island of Oahu in an effort to determine whether sufficient indications of geothermal potential exist within or adjacent to the peninsula to justify further, more detailed, exploratory efforts. An evaluation of existing geologic data as well as recently completed mapping on Mokapu indicate that the peninsula is located on the edge of or immediately adjacent to the inferred caldera of Koolau volcano. There are at least three post-erosional volcanic vents located on the peninsula and several more form small islands adjacent to it. The age of this post-erosional activity has been estimated to be at least 400,000 years before present. The post-erosional events, on the basis of mineralogical and geochemical evidence, are not considered to have been a renewal of the older (1.8 million years before present) Koolau activity, but rather were a series of independent, short-lived eruptive episodes. Geochemical investigations conducted within and around the Mokapu Peninsula included mercury-soil surveys and radon ground-gas surveys as well as a limited evaluation of groundwater chemistry. Numerous difficulties were encountered in the interpretation of the soil-gas geochemical data because of the high degree of cultural activity associated with the U.S. Marine Corps Air Station at Kaneohe; however, one area in the southwest quadrant of the peninsula was tentatively identified as a low-order geochemical anomaly in which observed levels of mercury and radon were both significantly higher than background values. These anomalous values were tentatively attributed to increased soil permeability or possibly to slightly elevated subsurface temperatures. Groundwater sampling on Mokapu Peninsula was severely restricted because of the absence of wells within the study area and thus water chemistry analyses were limited to the Nuupia fish ponds. Samples obtained in the fish ponds were found to be seawater diluted with varying amounts of fresh groundwater. Although no thermal alteration of the water chemistry was evident for this area, an evaluation of existing groundwater chemical data for adjacent areas to the south and east of Mokapu suggests that some low-level thermal alteration may be present within shallow aquifers overlying the inferred Koolau caldera. Schlumberger resistivity soundings were completed in three locations on the peninsula: KVSI, in the northeast quadrant within the Ulupau crater, KVS2 in the northwest quadrant along the main jet runway, and KVS3 in the southeast along Mokapu Road. At KVSl a relatively high resistivity was encountered to a depth of approximately 20 meters below sea level, which was underlain by a basement resistivity of about 2 to 3 ohm-meters. At KVS2 and KVS3 similar resistivities of 2 to 3 ohm-meters were detected at much shallower depths (approximately equivalent to local sea level) below a thin, moderately resistive layer having an impedance ranging from 15 to 118 ohm-meters. Although the basement resistivity values are somewhat lower than would be expected for seawater-saturated basalt, and therefore could be interpreted as arising from a thermal anomaly, it is considered far more probable that the resistivities observed correspond to a low-resistivity seawater-saturated clay layer underlying the peninsula. In the context of the geothermal potential of the Mokapu Peninsula, the results of the present survey can be summarized as follows: 1. The geological data suggest that the post-erosional volcanism associated with the Mokapu Peninsula was of such a short duration and is of such great age that it is considered unlikely that significant remnant heat would be found beneath these structures. Although remnant heat may still be present within the magma chamber of Koolau volcano, there is presently no geologic evidence to substantiate its occurrence. 2. The geochemical data available indicate that one area within the peninsula may be slightly anomalous; however, no firm conclusions can be drawn concerning its relationship to a potential heat source. Limited groundwater geochemical data for the peninsula do not suggest the presence of thermally altered groundwater although some indication of groundwater anomalies have been identified several kilometers to the south of Mokapu Peninsula. 3. The results of geophysical surveys suggest that the peninsula is underlain by seawatersaturated clays at local ambient temperatures. The probability of there being an exploitable high-temperature resource beneath the Mokapu Peninsula is extremely low, and the probability for a low-temperature resource, at economically viable depths, is also very low.