Further Analysis of 3D Magnetotelluric Measurements Over the Coso Geothermal Field

Abstract

At last year’s GRC annual meeting we presented initial results of a 3D investigation of the Coso Geothermal field utilizing a dense grid of magnetotelluric (MT) stations plus a single line of contiguous bipole array profiling over the east flank of the field (Newman et al., 2005). Motivation for this study is that electrical resistivity/ conductivity mapping can contribute to better improved understanding of enhanced geothermal systems (EGS) by imaging the geometry, bounds and controlling structures in existing production, and by monitoring changes in the underground resistivity properties in the vicinity of injection due to fracture porosity enhancement. Initial analysis of the Coso MT data was carried out using 2D MT imaging technology to construct an initial 3D resistivity model from a series of 2D resistivity images obtained using the inline electric field measurements (Zxy impedance elements) along different measurement transects. This model was then refined through a 3D inversion process. The 3D resisitivity model clearly showed the controlling geological structures influencing well production at Coso and shows correlations with mapped surface features such as faults and regional geoelectric strike. Further analysis of the data, including lower frequency data to 0.3 Hz, clearly shows that 3D analysis allows for refinement in positioning of conductivity contacts when compared to purely 2D inversion, critical for well placement. Also the conductivity model has now been correlated with an acoustic and shear velocity model derived by Wu and Lees (1999) and Lees and Wu (2000) to show that the near-vertical high conductivity structure on the eastern flank of the producing field is also a zone of increase acoustic velocity and increased Vp/Vs ratio.