Improving Exploration for Geothermal Resources with the Magnetotelluric Method

• Author / Creator

  Lee, Benjamin M

• This thesis investigates improvements in methods used for exploration for geothermal resources with the magnetotelluric (MT) method. Geothermal energy is a renewable resource that provides heat and electricity with low carbon emissions. Targets in conventional geothermal exploration are geothermal reservoirs, which are underground regions with hot fluids which can be extracted for direct use of heat or electricity production. Geothermal reservoirs are detected by geophysical methods such as MT, a technique utilizing naturally occurring electromagnetic signals to image the subsurface electrical resistivity. MT is useful for locating geothermal reservoirs because high-temperature hydrothermal alteration can make the reservoir relatively resistive. Improvements in the strategy used for MT exploration are needed to make this method more effective. In this thesis two case studies are described where conventional analysis and interpretation of MT data was inadequate for assessing the potential geothermal resource.
  The first study considered the Krafla geothermal field in Iceland. Supercritical fluids beneath the geothermal field could increase electric power output by an order of magnitude per well if used instead of steam. The IDDP-1 well was drilled in the year 2009 to reach the supercritical fluids at a depth of 4 to 5 km. However, drilling ended prematurely when magma was unexpectedly encountered at 2.1 km depth. This study investigates why the magma was not imaged with the existing MT data. First, improvements to the 3-D inversion of the Krafla MT data were implemented, including: (1) a 1-D resistivity model as a constraint on the final resistivity model; (2) full impedance tensor data instead of only the off-diagonal elements used by previous authors; and (3) model cells with horizontal dimensions of 100 by 100 m, which is a finer discretization than used by previous authors. The most prominent feature in the 3 D resistivity model is the low resistivity feature (C3) coincident with the bottom of the IDDP-1 well. The interpretation of C3 includes (1) partial melt, distributed in a dyke and sill complex, and (2) dehydrated chlorite and epidote minerals at temperatures above 500 to 600°C. Sensitivity tests revealed that the MT data were not sensitive to cubic-shaped magma bodies at the bottom of IDDP-1 that were 1 km3 or smaller with a resistivity between 0.1 and 30 Ωm. Therefore the MT data cannot preclude the existence of magma distributed in small pockets, such as the magma body intersected by IDDP-1. This study demonstrates that the limitations of the MT method should be considered before interpreting the deep parts of a resistivity model.
  The second study involves the Canoe Reach geothermal prospect located near Valemount, British Columbia. Although this area hosts the Canoe River thermal spring, there is no obvious heat source and it is unclear if the underground fluids represent a viable geothermal reservoir. MT surveys were performed at Canoe Reach to image the distribution of fluids in the Southern Rocky Mountain Trench Fault (SRMTF), which may be a permeable pathway for fluids to reach the Canoe River thermal spring. Careful interpretation of resistivity models is needed because low resistivity features may correspond to fluids or to other conductive materials within a rock such as graphite or sulfides. In addition, the Canoe Reach area contains deformed rocks with anisotropic textures that may be electrically anisotropic. Isotropic and anisotropic inversions of the Canoe Reach North MT data resulted in significantly different resistivity models. The anisotropic resistivity model contains simpler structure and is more consistent with the mapped geology, which shows that misinterpretation can occur if electrical anisotropy is not considered. An anisotropic feature in the SRMTF footwall has a low resistivity in the east-west direction and a high resistivity in the vertical direction, which is more easily explained by graphite or sulfides than by fluids in the rock. At Canoe Reach South, anisotropic features near the thermal spring with a low resistivity in the vertical direction may correspond to fluids within the SRMTF. The electrical anisotropy of fluids in faults could be explained by a preferred orientation of fractures and/or fault corrugations that increase permeability and decrease resistivity along the fault slip direction. Electrical anisotropy might be common in highly deformed geological settings, and should be routinely investigated with the increasing availability of 2-D and 3-D anisotropic modeling algorithms.
  

• Subjects / Keywords

  • geophysical imaging of magma
  • Iceland
  • Krafla
  • geothermal energy
  • electrical resistivity
  • magnetotellurics
  • Rocky Mountain Trench
  • electrical anisotropy

• Graduation date

  Fall 2020

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/r3-yt01-qj05

• License

  Permission is hereby granted to the University of Alberta Libraries to reproduce single copies of this thesis and to lend or sell such copies for private, scholarly or scientific research purposes only. Where the thesis is converted to, or otherwise made available in digital form, the University of Alberta will advise potential users of the thesis of these terms. The author reserves all other publication and other rights in association with the copyright in the thesis and, except as herein before provided, neither the thesis nor any substantial portion thereof may be printed or otherwise reproduced in any material form whatsoever without the author's prior written permission.