Shallow soil mercury surveys are an inexpensive and effective exploration tool for geothermal resources. In a geothermal system at depth, mercury is leached from the country rock and transported to the surface by the geothermal fluids. The mercury is fixed by clays and organic material in the soil above the geothermal system and can be detected by analyzing the near-surface soil. Surface mercury surveys can be used in regional reconnaissance to discriminate prospective from nonprospective areas. Closely spaced mercury surveys over individual prospects typically enhance the structural understanding of the prospect and show which structures are important as migration pathways.

The use of probability graphs greatly enhances the interpretation of mercury geochemical data. Probability graphs have a logarithmic ordinate scale versus a cumulative percentage abscissa scale. This graph is arranged such that a typical lognormal population plots as a straight line on the graph. Deviations from a straight line show deviations in the data from a standard lognormal population distribution. The mercury data are easily plotted on the graph and can be evaluated visually. Anomalous and background populations can be separated consistently even when considerable overlap occurs between populations. In Nevada and California, soil mercury surveys have detected up to four distinct mercury populations in one area. These populations are related to background

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over different bed-rock types and to variations in leakage from the geothermal system at depth. An interpretation of the mercury populations in light of the available geologic and structural characteristics of a prospect can significantly increase the understanding of the geothermal resource.

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