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Feature Article

Induced Polarization for Subseafloor, Deep-Ocean Mapping

By Jeff Wynn
Research Geophysicist
U.S. Geological Survey
Vancouver, Washington

Mike Williamson
Williamson & Associates
Seattle, Washington

John Fleming
Zonge International
Tucson, Arizona

A marine IP array towed in the deep ocean as part of a USGS technology patent application.

The U.S. Geological Survey (USGS) has developed and patented an electrical geophysical technology called marine 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 southeastern 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' deposits 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 seawater, 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.

Induced Polarization
IP is a subtle surface-effect, electrophysical phenomenon that was first observed by Conrad Schlumberger in 1912. Under an induced voltage, charge adsorbs onto certain mineral grains. When the induced voltage is removed, a time-delayed charge release can be measured. In modern IP systems, this reaction is measured as a phase shift between transmitted and received signals. The frequency at which the maximum phase shift occurs has been shown in other studies to be diagnostic of the specific minerals being polarized. For instance, the pyrite response peaks below 0.1 hertz, while ilmenite peaks around 4 hertz. Both these minerals are particularly strong responders to IP.

Subseafloor Mineral Surveys Using IP
For a subseafloor minerals application, the USGS uses a single streamer with a single current transmitter dipole, followed by multiple receiver dipoles at increasing distances. This allows mapping of IP-reactive minerals at different depths using various receiver channels.

Southeast Africa Survey. In a two-month survey carried out off the coast of southeast Africa in 2007, paleochannels known to have transported ilmenite from land during a previous low-seawater stand can be seen in bathymetry and also in coarser results from gridded vibracoring carried out earlier. The IP anomaly coincides closely with the bathymetric signature of the northern paleochannel, seen approximately in contoured ilmenite assays. A much larger IP anomaly lies south and east of this paleochannel, where one would expect the Agulhas Current along with longshore currents to have dispersed the ilmenite over time. A large debris field from a wreck was also observed with resistivity and IP data during this survey, in an area well-known for shipwrecks caused by rogue waves.

Gulf of Mexico Survey. A subseafloor survey was carried out in the Gulf of Mexico in which broad IP anomalies correlated with ilmenite-bearing black sands east of Cat Island, south of Gulfport, Mississippi. This site was chosen because it is a known locus of heavy placers draining the entire Mississippi Basin.

A narrow IP increase, coincident with a small drop in resistivity, was observed in the profile near where a bronze cannon was earlier dredged. This area is shoal rich, with a number of known wrecks dating back to the 17th century, and more metallic debris remains below the seafloor.

The USGS has also observed this kind of anomaly in the Atlantic, near the entrance to Murrell's Inlet, South Carolina. Additionally, IP anomalies are routinely observed on land over pipelines. These observations led to the belief that unexploded ordnance buried beneath younger sediments would be a logical target for marine IP. Two resistivity lows in the Cat Island profile, without coincident IP anomalies, lie over dredged channels leading to the east side of the island. These channels have been refilled by storm surges with less-consolidated sediments, which are expected to have higher seawater content and thus give the lower resistivity anomalies observed. To continue this article please click here.

Jeff Wynn is a research geophysicist and science manager with the U.S. Geological Survey. He has earned patents for the USGS on the marine technologies discussed in this article and published more than 200 papers in geology, geophysics, volcanology, mineral resources and hypervelocity-impact physics. He is also a jujitsu sensei.

Mike Williamson is a geologist in Washington State with a 40-year interest in mining, developed as a teenager working in a coal mine. Subsequent education and U.S. Navy experience led him to the marine environment, but he still likes to dig holes and develops remote-sensing methodologies to find missing ships.

John Fleming is a senior geophysicist and field operations manager in Tucson, Arizona, at Zonge International. He was previously a project manager at hydroGEOPHYSICS and a hydrologist at the U.S. Geological Survey.

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