Magnetotelluric Phase Tensor Applications to Geothermal Assessment in New Zealand and New Mexico

Magnetotelluric (MT) phase tensor analysis preserves the background (regional) phase response irrespective of galvanic distortion even if distorting inhomogeneities change between multiple MT deployments. This characteristic is the basis for repeat MT monitoring of the South Karapiti, New Zealand region near the Wairakei Power Station where 1-2 km-deep reinjection of spent geothermal fluids will commence soon. Deep electrical conductivity changes caused by this injection may be detected by background phase tensor changes independent of possible surficial changes, e.g., from drilling operations, or from differing sensor alignments during the multi-MT occupations. In 2010-2012 twenty MT sites within 1.5 km of a newly-drilled injection well were reoccupied by New Zealand GNS scientists and US students from NSF's International Research Experiences for Students program. Maps of phase tensor ellipses at various frequencies have identified frequency bands exhibiting good repeatability, therefore, they are potentially useful for detection of future brine injection. Final reoccupation of the MT sites is scheduled in 2013 after a large brine injection. In New Mexico, the 2012 SAGE program (Summer of Applied Geophysical Experience) applied phase tensor analysis to 8 MT soundings aimed at understanding the occurrence of anomalously high vertical and horizontal temperature gradients located approximately 25 km NW of Santa Fe. Plots of phase tensor ellipses allowed unique, distortion-free visualization of the dimensionality and directions of background geoelectric variations. Analysis of the plots as functions of frequency and location revealed a nearly one-dimensional (1-D) upper conductive (sedimentary) section. Variations in the orientations of the principal axes of phase tensor ellipses exposed an overall, deeper three-dimensional (3-D) geoelectric structure in the region. However, two sequential frequency bands revealed dominantly two-dimensional (2-D) regional features. They were identified as trends in the approximately 3 km-deep resistive basement and a midcrustal conductor at about 15 km-depth, each with differing geoelectric strike directions. The upper 1-D/2-D (sedimentary/basement) section permitted non 3-D analysis with results agreeing with drill hole, seismic, and gravity data.