1. Prospecting for high-temperature water and/or steam close to the surface, that can be used profitably for electric power generation and/or direct heat utilization. Geothermal methods also may be used to locate geologic features that affect heat flow (salt domes, dikes, faults, etc.) or groundwater variations. 2. Measurements of variations in earth temperature that are not attributable to variations in solar heating. Diurnal temperature variations penetrate to about 1 m (and annual temperature variations to 20 m). See Poley and van Steveninck (1970).