Interpretation of shallow electrical features from electromagnetic and magnetotelluric surveys at Mount Hood, Oregon

Abstract

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.