A SYSTEMATIC APPROACH TO THE INTERPRETATION OF MAGNETOTELLURIC DATA IN VOLCANIC ENVIRONMENTS WITH APPLICATIONS TO THE QUEST FOR MAGMA IN LONG VALLEY, CALIFORNIA

Abstract

Magnetotelluric (MT) data in Long Valley, California, are principally sensitive to the complex structure in the rocks filling the caldera. While the overall shapes of the sounding curves are set by the deep crustal structure and mantle, the lateral variations of that response can be entirely explained using structure of the upper 1600 m. A conductive body at depths from 1350 to 1600 m is present in the northern, southern, and western moats and beneath the resurgent dome. This conductor effectively masks structures at intermediate depths. We believe that this conductor represents fractured Bishop Tuff within the caldera and possibly graphitic metasediments beneath it. Electrical logs from wells penetrating these formations are used to corroborate our interpretation. A large, conductive (5 ohm m) body of magma is precluded by the MT data. The response to a spherical chamber of 4‐km radius at a depth of 10 km is several times larger than the mismatch between our modeled and observed sounding curves. Small (2‐km radius), isolated pockets of magma may be present, however. We have developed a systematic approach to interpreting MT data in a complex volcanic environment. We clustered sites on the basis of similar responses to perceive patterns in spite of severe distortions by local heterogeneity. Our model was built from the surface downward, incorporating other published geophysical surveys and a time domain electromagnetic survey released by Unocal. Three‐dimensional modeling was employed from the start, and our interpretation was based primarily upon the phases. This systematic approach of stripping off the known, shallow structure in order to study the intermediate and deeper structure was quite successful. An important finding of our study is that the phase is sensitive to shallow (1350 m), three‐dimensional structure even at frequencies of 0.01 Hz. Three‐dimensional modeling simply cannot be avoided in complex geological environments.