2010 Honorary Lecturer, The Pacific South

Electromagnetic phenomena underlie many of the methods used for finding and characterizing oil, minerals, and gas in the earth and are of vital interest to geophysicists. Applications span an enormous range of frequencies, from millihertz magnetotelluric readings to radar investigations in the hundreds of megahertz range. For a petrophysicist evaluating subsurface formations, the resistivity log has long been the main tool to discrimate oil- and gas- bearing zones from water-bearing zones. Dielectric tools, operating up to 1 GHz in frequency have been used in the past and are now making a comeback for specialist applications. One problem with dielectric logging was the generally poor understanding of the physics involved and the major differences between dielectric responses and standard electrical responses made at low frequency (i.e., a frequency much less than 1 MHz).

The popularity of resistivity for oil detection in petrophysics has in the past decade or so been taken up with renewed vigor in the search for oil and gas, with controlled source electromagnetics (CSEM) being applied widely, especially in frontier exploration settings. The need to monitor and verify safe, long-term disposal of CO₂ through geosequestration has also led to increased interest on onshore CSEM surveys, both from the surface and crosswell. The resurgence in EM methods has led many geophysicists with a predominantly seismic background to seek a better understanding of rock electrical properties and how they are measured. Petrophysicists are being asked tricky questions like “does this downhole log from my offset well give a good representation of the resistivity structure in my basin that will be seen in a CSEM survey?

The aim of the lecture is to outline the fundamentals of rock electrical behavior. Topics to be covered are:

• Introduction: High and low frequency realms of electromagnetic behavior and the crossover area between them
• A primer on dielectrics
• Making valid laboratory measurements of electrical properties
• Galvanic, inductive, and capacitive coupling into the earth
• Downhole “resistivity” tools and what they measure: i.e., galvanic, induction, and propagation type tools
• Log versus survey: some effects of scale and anisotropy

The focus is on sediments and sedimentary rocks, though many of the basic principles apply also to “hard rock” situations.