Controlled-Source EM Experiment at Mt. Hood, Oregon

Abstract

As apart of a joint federal and state geothermal resource assessment of the Mt. Hood stratovolcano in Oregon, Lawrence Berkeley Laboratory performed a series of deep electromagnetic soundings over the frequency range of 0.1 to 200 Hz. The soundings, performed with a large moment horizontal loop systems, permit an analysis of earth resistivity in the region. Horizontal loop sources were placed a three locations around the volcano and magnetic fields were recorded at nine receiver sites located between 1 and 2 km from the individual sources. Square wave currents of up to 150 A were impressed into the loops and at each receiver location amplitude and phase spectra or elliptically of normal and radial magnetic fields were analyzed to obtain one dimensional resistivity models. Layered earth inversions yield similar two layer models of a resistive surface layer, 0.5 to 0.7 km thick, over lying a conductive layer of indeterminate thickness. A sounding at the north side of the mountain shows a 3 ohm meter layer at a depth of 0.7 km. This result agrees well with magnetotelluric results in the same area. The cause of the high conductivity zone may be high temperature, water saturated conditions beneath the cold meteoric water zone.