The use of soil-gas helium concentrations for earthquake prediction: Studies of factors causing diurnal variation

Abstract

The diurnal variation in the soil-gas helium concentration was monitored at depths of 0.5-2 m. Barometric pressure, air temperature, wind speed, soil temperature, soil moisture, relative humidity, and precipitation were also monitored. The helium variation below a 1-m sampling depth usually did not exceed the analytical sensitivity limit of +10 ppb helium. The meteorological parameters that had the greatest effect on the helium variation is wind speed and precipitation; another factor causing variations was the atmospheric pumping created by air-temperature changes and its associated effect on the near-surface soil moisture content. The absolute helium variation rarely exceeded 1 percent of the background helium concentration in air. This minor variation could be corrected because it followed a regular daily pattern. Diurnal changes in the soil-gas helium concentration did not impose any severe limitations on the use of helium soil-gas data collected for earthquake prediction purposes.