Induced Polarization for Subseafloor, Deep-Ocean Mapping
Marine Induced Polarization Used for 3D Mapping Of Subseafloor Minerals and 4D Oil-in-Seawater Characterization
By Jeff Wynn Research Geophysicist U.S. Geological Survey Vancouver, Washington Mike Williamson President Williamson & Associates Seattle, Washington and John Fleming Geophysicist Zonge International Tucson, Arizona
he U.S. Geological Survey (USGS) has developed and patented an electrical geophysical technology called ma- rine induced polarization (IP) to map placer heavy minerals on and below the seafloor. A large-scale survey using marine IP, conducted with industry partners off the coast of south- eastern Africa, successfully identified a large offshore placer
titanium (ilmenite, or FeTiO3) resource beneath the modern seafloor. In other areas of the world, these "black sand" de- posits are associated with zircon and precious heavy metals, including gold, platinum and heavy rare earth elements. Recent laboratory experiments suggest that oil dispersed in the deep ocean contributes a large capacitance to sea- water, which has been measured in the laboratory with the same marine IP technology. Hydrocarbons in seawater can theoretically be detected down to a concentration below 0.1 percent, and the technique can track hydrocarbon plumes and monitor their degradation through time and space. A cooperative research and development agreement
with several private companies has been set up to exploit this placer-mineral and hydrocarbon-mapping technology, which can be useful for mapping wrecks and is promising for rapidly mapping buried unexploded ordnance.
IP is a subtle surface-effect, electrophysical phenomenon that was first observed by Conrad Schlumberger in 1912.
A marine IP array towed in the deep ocean as part of a USGS technology patent application.
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