Geochemical characteristics of soil gas in the Yanhuai basin, northern China

The geochemical backgrounds and origins of soil gases in the Yanhuai basin are discussed based on the regional seismogeological data and concentrations of Rn, Hg, CO2, H-2, He and CH4 in soil gas measured at 422 investigating sites in field during September to October 2007. The geochemical background values of Rn, Hg, CO2, H-2, He and CH4 are (8105.8 +/- 5937.4) Bq/m(3), (9.7 +/- 5.8) ng/m(3), (395.9 +/- 35.3)x10(-6), (4.0 +/- 2.3)x10(-6), (15.9 +/- 10.4)x10(-6) and (12.7 +/- 8.1)x10(-6), respectively. The geochemical backgrounds of the soil gases are higher in the eastern part of the Yanhuai basin. The main factors affecting the gasgeochemical backgrounds are gaseous origins, structure of the crust, faults, stratum and microbe activity. The higher values of gasgeochemical backgrounds in the eastern part are attributed to the existence of low-velocity zones in the upper crust, stronger tectonic activity and more contributions of Hg and He derived from the deep-earth and Rn originated from granite, corresponding to stronger seismic activity. The results can be applied to identifying seismic precursor from monitoring data of gases in the studied area.