Geophysical Studies of a Deep Borehole in the Canadian Shield in NE Alberta
Geophysical Studies of a Deep Borehole in the Canadian Shield in NE Alberta
Previously Published Material: Parts of this talk will be given at the DGG conference - Hannover, Germany in March. The VSP component has been published in the Int. J. of Geosciences.
Abstract ID#: 35014
English Abstract:
With the support of the German-Canadian Helmholtz-Alberta Initiative we were able to obtain support to carry out an extensive suite of geophysical investiations of a nearly 2.4 km deep borehole drilled through the sedimentary cover into 1.9 km of metamorphic rocks of the Taltson Magmatic Zone in NE Alberta. This geological unit is primarily a biotite gneiss. This work was motivated as part of geothermal reconnaissance studies in the search for low-enthalphy geothermal resources in Alberta. Regrettably, the temperature at the bottom of the well is only about 50 °C. Despite this, however, there are a number of pertinent findings of more general interest. Geophysical logging and borehole seismic investigations were jointly conducted by the GFZ-Potsdam and the University of Alberta, withadditional data provided by the owner of the borehole and in later commerical logging. Logs acquired are resistivity, magnetic susceptibility, full waveform sonic, dipole sonic, density, neutron, and ultrasonic televiewer. High resolution zero offset VSP data were obtained with a receiver spacing of 2.5 m from 1800 m to surface. A number of walk-a-way VSP measurements were also obtained simultaneous to surface seismic acquisition. A number of fracture zones are encountered along the borehole and correlate well with normal electrical logs, full waveform sonic logs, and image logs. These fractures, too, appear to be filled with highly saline brines as they correspond to sharp peaks in the electrical conductivity. These fractures may provide pathways for fluid flow in an engineered geothermal system. The fracture zones may be related to dipping seismic reflectors seen in deep profiles adjacent to the borehole site. An alternative explanation could be the existence, on the basis of high magnetic susceptibility, of mafic sills with differing impedance contrasts. Analysis of this exceptional geophysical data set is still underway. This includes detailed analysis of the walka-way VSP data to measure seismic anisotropy of the crust and of the image logs to provide measures of in situ stress directions and possibly magnitudes. We hope in future to develop this site into a long term deep observatory.