Ground radon surveys for geothermal exploration in Hawaii

Abstract

Exploration for geothermal resources in Hawaii has required adaptation of conventional exploration techniques as well as the implementation of relatively new techniques because of the complexities introduced by the oceanic island environment and the basaltic lavas which form the islands. One exploration technique previously untried in Hawaii and used to only a very limited extent elsewhere, is the measurement of the concentration of the radioactive gas, radon, within ground gas at shallow depth. The measurement procedure applied utilizes alpha-particle sensitive film housed within an inverted 250 ml container. The device is exposed by burial in the ground for periods of 4 to 5 weeks. The films are easily developed in the laboratory using a constant temperature bath of NaOH which enhances perforations caused by alpha particle tracks enabling them to be counted under magnification. The establishment of numerous measurement stations within an area enables patterns of the concentration of radon to be mapped. To confirm whether technique could be successfully applied in Hawaii, surveys were conducted in four areas of different characteristics: the summit of Kilauea volcano and the lower part of its active east rift zone, on the is1and of Hawaii; the lower part of the extinct north rift of Haleakala volcano, Maui and within the extinct eroded Waianae caldera in western Oahu. The technique was demonstrated to have been successful, and enabled the delineation of anomalous zones associated with the volcanic structures and resulting from increased ground permeability and anomalous subsurface temperatures. The data further indicate the common development of ground gas convection systems within and around these structures. To further assist in determining the controls over subsurface radon, gamma-ray spectrometry analysis of rocks, soils and volcanic sublimates was also made and other properties of soils were examined. These data indicate that elevated concentrations of uranium, the main parent of radon, occur within secondary mineralization within Hawaii hydrothermal systems, and which appear to enhance measured concentrations of radon. Results of the study overall indicate that the radon measured is commonly derived from depths of several 10's of meters, but that in areas of increased permeability and heat measurable concentrations of radon can be transported from depths of several 100’s meters. As an exploration technique in Hawaii, such surveys were found to be cost-effective, being both rapid and inexpensive.