Core hole drilling and the “rain curtain” phenomenon at Newberry Volcano, Oregon

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Abstract
Two core holes have been completed on the flanks of Newberry Volcano, Oregon. Core hole GEO N‐1 has a heat flow of 180 mW m−2, reflecting subsurface temperatures, sufficient for commercial exploitation of geothermally generated electricity. GEO N‐3, which has a heat flow of 86 mW m−2, is less encouraging. We emphasize the “rain curtain” effect with the hope that a detailed discussion of this phenomenon at two distinct localities will lead to a better understanding of the physical processes in operation. Core hole GEO N‐1 was cored to a depth of 1387 m at a site located 9.3 km south of the center of the volcano. Core hole GEO N‐3 was cored to a depth of 1220 m at a site located 12.6 km north of the center of the volcano. Both core holes penetrated interbedded pyroclastic lava flows and lithic tuffs ranging in composition from basalt to rhyolite, basaltic andésite being the most common rock type. Potassium‐argon age dates range up to 2 Ma. Caving and sloughing were encountered in both core holes at depths near the regional water table. Both core holes penetrate three distinct thermal regimes. The uppermost regime is isothermal at mean air temperature down to about 900–1000 m (the rain curtain). A thermally conductive regime exists near the bottom of each core hole. Separating these two thermal regimes is a transition zone exhibiting considerable hydrologic disturbance. The uppermost isothermal regime is characterized by (1) a low and fairly uniform response on the gamma ray log (N‐1, N‐3) and the electrical conductivity log (N‐1), (2) temperatures below surface ambient measured downhole with a maximum recording thermometer (MRT) during periodic pauses in drilling operations (N‐1, N‐3), and (3) drilling fluids whose chemistry does not reflect an influx of geothermal fluids (N‐3). In contrast, the thermally conductive regime is characterized by (1) a high and variable response on the gamma ray log (N‐1, N‐3) and on the electrical conductivity log (N‐1), (2) temperatures (MRT) measured downhole during pauses in drilling which are above ambient and which track in situ conditions (N‐1, N‐3) and (3) drilling fluids whose chemistry clearly reveals a geothermal component (N‐3). The transition zone is characterized by major washouts in the caliper log (N‐1, N‐3), a major anomaly in the mercury content of the rocks (N‐1), an extremely strong response on the gamma ray log (N‐1, N‐3) and electrical conductivity log (N‐1), and a major self‐potential anomaly (N‐1). Smectite alteration, which seems to control the results of surface geoelectrical studies, begins in the isothermal regime close to and perhaps associated with the regional water table.