This special issue of the CSEG RECORDER features three contributed articles describing recent developments in electromagnetic (EM) methods. While EM methods were some of the first geophysical methods to be developed for mineral exploration, they were rapidly surpassed by the number of seismic surveys undertaken in hydrocarbon exploration. Thus today the majority of geophysicists are seismologists who have a range of opinions about EM methods! Over the last 30 years, I have heard many concerns about the value of EM geophysics from my seismic colleagues! While many of these are valid, a number of criticisms are based on very early versions of EM methods that were around 40-50 years ago and were indeed very limited in what they could tell us about the Earth beneath our feet. Electromagnetic geophysics has developed significantly in the three decades I have worked with these methods, and I hope that the articles in this issue give a sense of what is now possible.
The articles are ordered by the depth of investigation, and each describes some significant new findings. Two common themes are (1) how they can be effectively used in combination with other geophysical methods, such as seismic exploration and (2) the extra value that is obtained when a 3-D approach is used in EM data collection and analysis.
Sarah Devriese and Doug Oldenburg (Imaging SAGD steam chambers: traditional ERT vs broadband electromagnetic methods) describe an application of electrical and EM methods to the problem of imaging steam chambers formed during SAGD extraction of heavy oil reservoirs. They show that a fully 3-D approach using data collection with bore hole instruments, combined with 3-D inversion, is capable of imaging the steam chamber. It is also possible to detect time variations in reservoir structure through the associated changes in resistivity. Seismic constraints are shown to sharpen the resistivity image, illustrating the value of a multi-technique approach.
David Goldak and Ryan Olson (New developments in audio-magnetotelluric exploration: Case study from Darnley Bay area, N.W.T) present an update on the audio-magnetotelluric method which uses natural EM signals in the 10,000 – 1 Hz range to image the subsurface. New instruments and inversion algorithms have greatly improved the efficiency of this method which is finding new applications in several exploration areas. The time-domain approach developed by EM Pulse geophysics is novel and has some clear advantages over the more widely used frequency domain approach.
The final article by Martyn Unsworth (Magnetotelluric studies of lithospheric structure beneath Western Canada: insights into plate tectonics both past and present) illustrates what can be achieved with deeper-sounding magnetotelluric (MT) surveys in studies of the structure and history of the continents. A fully 3-D approach has removed many of the ambiguities in MT data analysis. While the examples in this article are of tectonic studies, it is clear that similar benefits are being obtained in commercial MT exploration with a 3-D approach.
I would like to thank the authors for their contributions, and hope that they are useful to CSEG RECORDER readers.
About the Author(s)
Martyn Unsworth is a professor in both the Department of Physics and the Department of Earth and Atmospheric Sciences at the University of Alberta. He received his BA in Natural Sciences from Cambridge University in 1986 and his PhD in Earth Sciences from Cambridge University in 1991. Before moving to the University of Alberta in 2000, he worked as a post-doc at the University of British Columbia and as a Research Professor at the University of Washington. His primary research interest is in the application of magnetotellurics to tectonic studies, volcanology, mineral exploration and geothermal investigations. In recent years he has worked in several areas of Canada, China, Turkey, Bolivia, Argentina, and Antarctica.