Geothermal prospecting at ground surface by means of combination of N-packer and portable gaseous mercury analyzer PM-1A

Abstract

Surface Sniff method, SS method, is conducted by settling the thermometer-attached N-packer (N is after the initial letter of inventor's name, Noda and that of cooperative corporation, NIC) on the ground and connecting it to the portable mercury analyzer PM-1A. Thus emitted mercury from the ground into the N-packer is measured, where the standard measurement is fulfilled by 2 times successive measurements for 5 min. at 0.41/min of suction rate to get stable intrinsic mercury concentration reflecting underground environmental condition. The change of air temperature in the N-packer is the undesired and uncontrolled factor giving a large difference in mercury emission, therefore it becomes necessary to correct data of mercury concentration against temperature change in order to compare them to each other. The mercury concentration changes according to air temperature in such a manner as bearing a linear relation between logarithmic mercury concentration and reciprocal air temperature in °K. The equation for the temperature-concentration correction is thus shown as follows, log D₀=log Dobs+a{(1/Tstd)-(1/Tobs)}, where 'D₀' is corrected essential mercury concentration, 'Dobs' observed mercury concentration, 'a' the temperature-concentration coefficient depending on field property, 'Tstd' standard temperature and 'Tobs' observed air temperature in the N-packer during measurement. The distribution map based on the corrected mercury concentration is found to harmonize well with underground geothermal condition in the area. Mechanism of mercury emission at geothermal area has been turned out as following through some experimental measurements in two instinctive geothermal fields, Hachimantai, Tohoku and Kirishima, Kyushu, Japan. There occures gaseous mercury atmosphere near ground surface whose concentration is in accordance basically with underground condition, especially geothermal activity and is recontrolled according to the surrounding air temperature. Such intrinsic concentration of mercury is immediately restored even though mercury above the ground diminishes by environmental effects such as wind-blow etc. The distribution patterns of mercury upward and laterally from a ground spot are diffusion-like, which suggests diffusion mechanism manages transportation of mercury from the ground.