Electromagnetic Induction Studies of the Meager Creek Geothermal Area, British Columbia

Included in Research as Applied Geophysics No. 37, Geophysics Laboratory, University of Toronto.The magnetotelluric (MT) method has been among the most useful techniques for the mapping of geothermal zones. Its quantitative interpretation in terms of three-dimensional structures, however, has been hindered by the complexity of these environments and by the vast amounts of computer resources used to model such structures with the present numerical techniques. This thesis presents, for the first time, an interpretation of actual MT observations in terms of multiple three-dimensional confined conductors by means of a rapid low-frequency technique. Meager Creek is one of the most promising geothermal reservoirs in Canada. It is located at the base of a Quaternary volcano in southern British Columbia. This zone is characterized by impressive topographic reliefs, by an irregular and heterogeneous overburden, and by several conductive zones with different geometries. The electrical structure of the area of study is estimated through careful one- and two-dimensional interpretations of the MT and controlled source electromagnetic (CSEM) sounding data and with the use of previous studies in the region. The MT survey consists of seven sites crossing the thermal anomaly, measuring the three magnetic and two electric field natural variations. Responses at forty frequencies from 380 to .00055 Hz are obtained. The CSEM study consists of 22 sounding sites measuring the vertical magnetic field at nine frequencies from 3 to 1000 Hz where a finite loop is used as energizing source. A rapid integral equation algorithm is developed to approximate the MT galvanic response by ignoring the low-frequency inductive coupling present in the complete solution. The model consists of a set of arbitrary dipping plates embedded in a stratified earth. The method is based on the representation of current channeling by a distribution of surface current dipoles on the plane of the plate. Several different tests with existing published solid body results obtained from the complete solution are performed, showing a good agreement and a saving in computer execution time of at least one order of magnitude.Ph.D