Magnetotelluric exploration of Newberry Volcano, Oregon, for volcano properties and geothermal resources

Newberry Volcano in Oregon, USA, is a back-arc shield volcano with a summit caldera at the intersection of the Basin and Range geologic province and the Cascade arc. Newberry is unusual in that it is exceptionally hot, voluminous, and has a history of bimodal volcanism with four Holocene obsidian flows within the caldera. Newberry has been the subject of several geothermal studies since the 1980s, providing a large dataset for understanding the volcano and its potential for energy production. Our experiment has two components: 1) produce a 3D electrical resistivity model of the volcano by inverting approximately 200 magnetotelluric (MT) stations; 2) monitor water injected during a geothermal stimulation in hot, dry rock on the volcano's western flank using continuous MT recording.

When monitoring changes in resistivity from a fluid injection, it is helpful to use methods that are sensitive to changes in dimensionality as the subsurface becomes more three-dimensional. However, a combination of high noise levels and a small injection volume made interpretations based on MT impedance inconclusive. We therefore developed time-domain analysis methods to look for responses in the EM vectors to changes in well pressure and flow rate. Because the magnitude of the cross product of two vectors depends on how orthogonal or parallel they are, and the three-dimensionality of a resistivity structure is an expression of how parallel the electric and magnetic field vectors are, the magnitude of their cross product can be used to find changes in dimensionality without the impedance tensor's need for an assumed coordinate system. We present results of this analysis.

Our study of the volcano shows an unusually high resistivity for the magma chamber below the caldera. Where previous seismic velocity models show an anomaly that is slow enough that magma is required, resistivity is approximately 70 Ω m. The high resistivity of the magma provides constraints on its composition. Its resistivity is consistent with a highly silicic and very dry magma. This is in agreement with its history of obsidian eruptions during the Holocene, suggesting low volatile content and high silica. Its dryness suggests that its source is the same anhydrous tholeiitic basalt identified from isotope characteristics as a source of mafic lavas on the volcano's flanks.