Geohydrologic data from a 4,403-foot geothermal test hole, Mountain Home Air Force Base, Elmore County, Idaho

A 4,403-foot test hole was drilled on the Mountain Home Air Force Base in Elmore County, Idaho, for geothermal exploration. The hole was cored continuously below about 1,000 feet. Rock above 532 feet was principally basalt with interbedded sediment ranging in size from silt to coarse sand; from 532 to about 1,900 feet, rock was principally silty sand and silty clay. Below about 1,900 feet, rock was principally basalt. Selected geophysical and temperature logs were obtained; temperature at 3,960 feet was about 93 C. Logging below that depth was not possible because maximum depth capability of the logging unit was 4,000 feet. Thermal conductivity values (dry) of selected core samples ranged from 0.791 to 0.803 watts per meter degree kelvin in two fine-grained sedimentary rock samples and from 0.674 to 1.586 watts per meter degree kelvin in four basalt samples; values of saturated samples of the basalt ranged from 1.046 to 1.644 watts per meter degree kelvin. Dry density averaged 1.70 and 2.29 grams per cubic centimeter in two sedimentary and four basalt samples, respectively; wet density averaged 2.39 grams per cubic centimeter in the basalt. Two water samples were air lifted from below a mechanical packer set at 2,003 and at 3,462 feet. Chemical analyses were obtained for both samples; the shallower sample also was analyzed for deuterium and oxygen-18. INTRODUCTION Between November 1985 and July 1986, a 4,403-ft test hole on the Mountain Home Air Force Base in Elmore County was drilled, cored, and logged. The hole was drilled by the U.S. Air Force to determine the availability of water from geothermal aquifers to supply energy for space heating of military housing and other facilities on the base. The purpose of this report is to make data acquired from the test hole available to the public. Scope of this report includes generalized lithology, geophysical logs, temperature profile, thermal conductivity analyses, and chemical and isotopic analyses of water samples. U.S. Geological Survey participation in the work and publication of the data were funded by the U.S. Department of Energy, Geothermal Technology Division. LOCATION, DRILLING, AND CONSTRUCTION The drill site is located in the NW%NE%NE% section 27, township 4 south, range 5 east, on the Mountain Home Air Force Base about 8 mi southwest of Mountain Home (fig. 1). The site is in the western Snake River Plain, a broad, structural trough filled with as much as 10,000 ft of interbedded volcanic and sedimentary deposits (Lindholm, 1981). Target of the test hole was a series of silicic volcanic rocks, mostly rhyolite or quartz latite flows