Dixie Valley: MT Survey at Geothermal Field, Nevada

Application

The survey goals were to:

• Resolve a fundamental structural uncertainty at the Dixie Valley thermal area
• Delineate fault zones which have experienced fluid flux as indicated by low resistivity
• Image the disposition of resistive, possible reservoir formations in the subsurface
• Investigate the generic capability of fully sampled electrical data for resolving subsurface structure.

Challenges

Historically, subsurface resistivity images are lacking in resolution due to limited data type, poor data sampling and inadequate inversion approaches.

Spartan MT Solution

The Spartan MT system uses proprietary inversion software developed by Phil Wannamaker, providing better tools, methods and data.  The new-generation array magnetotelluric (MT) system was applied in a continuous bipole deployment over three profiles at the Dixie Valley thermal area.  Using a remote reference, the Spartan MT survey was deployed on three, 4-6 mile long profiles totalling 14 line miles.  Each line crossed the Stillwater fault zone approximately at right angles, over the frequency range of 10 kHz to 0.03 Hz. These lines cross the Senator Fumeroles area, the Cottonwood Creek and main producing area, and the low permeability region.

This well-sampled data set was analyzed using a new inversion algorithm for MT image construction based on stabilization using a-priori constraints.

Spartan MT Results

The MT resistivity image corresponds well with geologic model.  Initial 2-D inversion suggests that shallow pediment basement rocks extend for a considerable distance (~1-1/2 km) southeastward from the topographic scarp of the Stillwater Range. A particularly low resistivity zone borders the interpreted main offsetting fault and may be due to alteration from geothermal fluid outflow and upflow.

Analysis has been carried out for the northerly line across the Senator fumeroles and Caithness geothermal wells.  For both TE (transverse electric) and TM (transverse magnetic) modes, low apparent resistivities are typical of the eastern half of the profile for all frequencies over the thickest section of Dixie Valley.  Low apparent resistivities over the western half occur at higher frequencies, but an increase in this quantity in both modes towards the lower frequencies is evident, indicating the shallowing of resistive basement in the westerly direction.