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A magnetotelluric survey, with a reference magnetometer for noise cancellation, was conducted at accessible locations around Mount Hood, Oregon. Thirty‐eight tensor magnetotelluric (MT) and remote telluric stations were set up in clusters around the volcano except for the northwest quadrant, a wilderness area. Because of limited access, station locations were restricted to elevations below 1829 m, or no closer than 5 km from the 3424‐m summit. On the basis of the MT results, three areas were later investigated in more detail using a large‐moment, controlled‐source electromagnetic (EM) system developed at Lawrence Berkeley Laboratory and the University of California at Berkeley. One‐dimensional interpretations of EM and MT data on the northeast flank of the mountain near the Cloud Cap eruptive center and on the south flank near Timberline Lodge show a similar subsurface resistivity pattern: a resistive surface layer 400–700 m thick, underlain by a conductive layer with variable thickness and resistivity of <20 ohm m. It is speculated that the surface layer consists of volcanics partially saturated with cold meteoric water. The underlying conductive zone is presumed to be volcanics saturated with water heated within the region of the central conduit and, possibly, at the Cloud Cap side vent. This hypothesis is supported by the existence of warm springs at the base of the mountain, most notably Swim Warm Springs on the south flank, and by several geothermal test wells, one of which penetrates the conductor south of Timberline Lodge. The MT data typically gave a shallower depth to the conductive zone than did the EM data. This is attributed, in part, to the error inherent in one‐dimensional MT interpretations of geologically or topographically complex areas. On the other hand, MT was better for resolving the thickness of the conductive layer and deeper structure. The MT data show evidence for a moderately conductive north‐south structure on the south flank below the Timberline Lodge and for a broad zone of late Tertiary intrusives concealed on the southeast flank.

Magnetotelluric data, with both electric and magnetic field references for noise cancellation, were collected at accessible locations around and as close as possible to the Mount Hood andesite‐dacite volcano. The purpose of the study was to identify and map conductive features and to relate them to the thermal regime of the region. Several conductors could be discerned. The shallowest, at a depth of around 500 m below the surface, was identified as a flow of heated water moving away from the summit; the deepest (∼50 km) might be a melt zone in the upper mantle. Of particular interest is an elongate conductor that strikes Nl0° W and extends from a depth of 12 km down to 22 km. Because the conductor strike is close to the trend of the chain of Cascade volcanoes and because of the high conductive thermal gradients reported for the area, this feature was initially believed to be a zone of partial melt following the volcanic axis. However, because no teleseismic P wave velocity anomaly has been found, the cause of the conductor is more problematic. While the existence of small zones of melt cannot be ruled out, it is possible that the conductor is caused by a large volume of intensely deformed rocks with brine‐filled microfractures.

and Mineral Industries, was responsible for a magnetotelluric survey at Mount Hood, Oregon. The survey was conducted as part of a geothermal resource assessment study that had the overall objective of stimulating geothermal exploration near stratovolcanoes in the High Cascade Range. A telluric-magnetotelluric (T-MT) survey was chosen as the electrical resistivity technique. Geonomics, Inc., a Berkeley geophysical company, was contracted to conduct the survey because it had a data acquisition system that would allow us to test the applicability of the T-MT method in a geologically complex area and t o use a second magnetometer as a remote reference for noise rejection. Geonomics also performed the conventional MT data processing, but the reference magnetic processing is being done by the Engineering Geoscience Group, University of California. Data were collected in overlapping bands from 0.002 to 40 Hz, although high levels of cultural noise, coupled with instrumental problems, decreased the usable high frequency response during the first phase of the two-phase field program. In the first phase a total of 2 9 stations in clusters, including five duplicate and two reference magnetometer stations, were occupied on the south side of the mountain below Timberline Lodge, at an elevation of 5900 feet. During the second phase 2 1 stations in clusters were occupied on the northeast, east, southeast and west sides of the mountain. The apparent resistivity soundings generally show very different viii characteristics from one cluster of stations to the next, and in some cases even between adjacent stations within a cluster. This is indicative of lateral discontinuities in conductivity, which are illustrated by plotting the principal resistivity directions at each station as a function of frequency. At low frequencies, less than 0.04 Hz, we find a uniform east-west structural trend at stations north, east and west of the mountain. South of the mountain the conductive strike direction changes to nearly north-south and this may show that the volcano is localized at a major structural change. Two s p e c i f i c areas of i n t e r e s t have been i d e n t i f i e d : (a) Two anomalous near-surface low-resistivity zones occur close to the Cloud Cap eruptive center on the northeast side of the volcano. Anomalous conditions diminish away from the recent (12,000y.B.P.) vent. zone (0.5 to 1.0 km in depth) occurs within the Mount Hood volcanic pile and its 2 ohm-m resistivity is difficult to explain. A deeper zone (2& km), of approximately 10 ohm-m, occurs within what may be the pre-Mount Hood Yakima basalts (Miocene), and may have geothermal potential. (b) The strongly linear north-south electric field polarizations observed on the south side could be significant. Warm water emanations in the area suggest faulting, but there is neither direct evidence for faults, nor do the MT results clearly indicate anomalous resistivity conditions at depth. The MT station closest to the warm springs yielded low apparent + The shallower resistivit...

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