Capture of Volatile Mercury with a Gold Needle and its Application to Geothermal Prospecting

Abstract

Out of geothermal fields, mercury in a gaseous form uprises in proportion to the underground geothermal activity. Such volatile mercury is captured at a number of survey points scattered systematically on the ground surface and following quantitative mercury measurements provide its horizontal distribution map, which could be an effective method for geothermal prospecting. In order to capture volatile mercury selectively, a gold needle, 1 mm in diameter 10 cm in length, is hung in a hole 1 m deep in the ground and is left for several successive days, and then is collected for determination. Mercury adsorbed on the gold needle is reliesed by heating and determined using an atomic absorption spectrophotometer whose detection limit is satisfactorily low, 0.1 ng. The rate of mercury adsorption more than 1 ng/day indicates the existence of promising geothermal resources beneath. In the process of estimating the degree of underground geothermal acitvity, it becomes necessary to know not only the absolute amount of adsorbed mercury but its distribution patterns such as localization, extension and direction. Several survey attempts by this method have successfully been accomplished in some geothermal and non-geothermal fields in Japan. It is noteworthy that this prospecting method is applicable even in less active geothermal areas without any visual geothermal manifestations. According to some laboratory experiments, momentary slight mercury adsorption precedes regular adsorption process. Regular adsorption of volatile mercury to a gold needle occures in accordance with the first order reaction in which the adsorbed amount increases in proportion to the surrounding concentration and/or the lapse of standing time.