Geochemical Soil Surveys as an Exploration Technique for Low-Enthalpy Geothermal Resources

Abstract

A geochemical soil survey was conducted over a sedimenthosted low-enthalpy resource at Naike, New Zealand. Samples were taken on a 250m x 250m grid, covering about from both the A and B horizons. Soil ammonia (or, more smctly, ammonium)was selected as the pathfinder species since this attains high levels in geothermal fluids from sedimentary-hosted reservoirs. The ammonia contentwas determined by a gas-sensing ammonia-selective electrode following a potassium chloride leach. The A horizon showed a stronger anomaly contrast than B horizon. In the A horizon, the mean background concentration, as determined by probability plots, was 2 mg with anomalous values up to and a threshold of 4 mg The results define a pattern of anomalies which parallel known faults, thereby implying a fracture dominated hydrology - a conclusion supported by previous geophysical surveys. This study demonstrates that soil surveys can be useful in the exploration for low-enthalpy resources, and shows that ammonia is an effective pathfinder to this type of geothermal field.