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