Use of Soil-Gas Helium Concentrations for Earthquake Prediction: Limitations Imposed by Diurnal Variation

The diurnal variations in soil-gas helium concentration have been monitored at depths of 0.5-2.0 m in three localities in Colorado, Wyoming, and C•lifornia.'Bar•metric p e•su TM, air temperature, wind speed, soil temperature and moisture, relative humidity, and precipitation were also measured. The helium variation below a 1-m sampling depth usually did not exceed the analytical sensitivity of 10 ppm He. The meteorologic parameters that have the greatest effect on helium variation are wind speed and precipitation; another factor is the atmospheric pumping created by air temperature changes and its associated effect on near-surface soil moisture content. The absolute helium variation rarely exceeds 1% of the background helium concentration in air and can easily be compensated for because it follows a regular daily pattern. Similar diurnal changes in soil-gas helium concentration would not impose any severe limitations on the use of these data for earthquake prediction purposes.