Geothermal prospecting by ground radon measurements

Abstract

Abstract Radon-222 was measured using Kodak LR-115 film in the soils of 2500 locations near the Ngawha hot springs region, New Zealand, which is being exploited for geothermal power; the object was to determine its usefulness for predicting good drill sites. Unlike other surveys, which have shown large areas with consistent high radon values, anomalies here were scattered, and corresponded mainly with fault lineaments. The results suggested a major previously unnoticed fault. The sampling distance was 50 m. There was a strong seasonal effect on ground radon levels, with summer levels about ten times higher than winter levels. Swamps usually had measured radon levels of near zero because of the slow diffusion of radon in stagnant water, and even thermal areas (mainly in the swamps) usually had low measured values. However, where a fault crossed swamp it was sometimes detected, and with high signal/noise ratio, so swamps should be surveyed. Arguments from the radon levels found on different geologies show that at Ngawha radon has a maximum half value diffusion thickness of 7 m for lacustrine sediment and 25 m for basalt unless a permeable area is present (e.g., a fault). There was a weak correlation of radon levels with the temperatures found on drilling deep bores. Comparisons with the ROAC system and Alphacard system for measuring radon showed no statistical inter-correlations, but some qualitative correspondence of radon contours. The main strength of the method in regions with impermeable soils (such as at Ngawha), seems to be in detecting or confirming the presence of faults, and possibly (through them) indicating geological structure as deep as 300 m.