A TEST OF GEOPHYSICAL METHODS FOR GEOTHERMAL EXPLORATION IN THE PACIFIC NORTHWEST: MT. HOOD, OREGON

Three reconnaissance geophysical methods -- shallow magnetic surveys, electrical resistivity surveys and shallow refraction microtremor (SeisOpt® ReMi™) surveys -- were tested for fault recognition and geothermal exploration in a small area on the north flank of Mt. Hood, Oregon. ReMi refraction microtremor surveys were the most effective of these geophysical techniques for identifying faults. They also documented the vertical component of slip preserved near the ground surface. Magnetic surveys detected most of the known faults, but with varying degrees of confidence. Electrical resistivity surveys were not effective for reconnaissance studies in this environment. None of these methods was as good as LiDAR imagery for identifying active faults that offset the ground surface. Analyses of spring geochemistry, intended to be an independent evaluation of geothermal potential near the faults, were ineffective because of dilution by meteoric water.

Although the Cascade volcanoes clearly represent a potential source of geothermal energy, the high rainfall, meltwater from glaciers and permanent snowfields, and resulting thick soil cover and dense vegetation in the region rule out many of the exploration methods that are effective in other environments. This study entailed a series of near-surface, small-scale geophysical surveys across Q fault traces that had been previously identified on LiDAR imagery. The study area spanned an accommodation zone between the west-dipping normal faults of the Q Blue Ridge fault zone, and the east-dipping normal faults of the Q Gate Creek fault zone. The pervasive fracturing associated with accommodation zones makes them favorable structural settings for geothermal activity. Surface water samples for geochemical analysis were collected at Blowdown Ridge, in the accommodation zone.

Although none of the geophysical tools used in this study provide direct evidence of geothermal resources, some of them could be useful in reconnaissance exploration in the Pacific Northwest. In this study, shallow refraction microtremor (SeisOpt® ReMi™) surveys were second only to LiDAR imagery in their ability to locate faults. Magnetic surveys are promising for initial reconnaissance, to identify areas of interest for further studies. Both are portable, quick and relatively inexpensive.