Power Imaging — A Passive Electromagnetic Hydrocarbon Detection Method: Examples from Railroad Valley, Nevada
Power Imaging is a passive electromagnetic method used for pre-drilling detection of hydrocarbons in onshore settings. Power Imaging is a geophysical prospecting method that utilizes the electric power grid as a continuous source of energy for investigating the earth’s subsurface geological structure, stratigraphy, and hydrocarbon potential.
The electric power grid induces electromagnetic waves in the earth. These electromagnetic waves are at specific frequencies, which are harmonics and sub-harmonics of 60 hertz (50 hertz in many areas of the world). These secondary harmonics include multiples of 60, 30, 15 and 7.5 hertz (or 50, 25, 12.5 and 6.25 hertz) and extend up to frequencies in the tens of kilohertz. The waves propagate into the subsurface as plane waves and encounter the various geologic boundaries. Those boundaries having dielectric and/or conductivity contrast reflect a portion of the waves back to the earth’s surface. With continuous sourcing from the electric power grid, the waves resonate between the subsurface boundaries and the surface of the earth. Because the power grid as a whole creates the resonance, the distance and attitude to any one or more power lines is not important. In fact an effort is made to null out the effects of nearby power lines. The power grid induced waves become organized such that there is a direct relationship between the many resonating frequencies and the depths to the various geologic boundaries.:
Because of the electrical contrast between hydrocarbon bearing rocks and their surrounding formations, an electromagnetic signature can be detected by measuring the resonant frequencies at the surface of the earth. Interpretation of this signature yields an electromagnetic hydrocarbon indicator or EHI thus allowing the direct detection of hydrocarbons, along with the depth and approximate thickness of the hydrocarbons. This presentation will be illustrated with examples from Railroad Valley, Nevada.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California