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Hydrographic Equipment & Software All on GSA Schedule GS-07F-5715P EdgeTech Marine Side Scan Sonars, Subbottom Profilers & Integrated Systems Hemisphere GPS USGS conducted laboratory mea- surements from August to Septem- ber 2011 for different percentages of multiviscosity oil that was titrated and then stirred into seawater. The different peaks apparently correlate with differ- ent oil-droplet size populations in the mix. The results showed very strong IP phase shifts, nearly 500 milliradians for 3 percent oil in seawater. To put this in perspective, an economic porphyry copper deposit on land may give an IP anomaly up to 20 milliradians in field surveys and laboratory core measure- ments. The experimental surveys in the Gulf of Mexico suggest that it is pos- sible to maintain a noise envelope as low as approximately 1 milliradian, which suggests a detection threshold for oil below 0.1 percent in seawater. Marine IP Potential IP is a surface-sensitive physical property, which means it is ideal for mapping very low concentrations of finely disseminated metallic-luster sul- fide minerals (with the notable excep- tion of sphalerite), but IP also responds to ilmenite, oil in seawater and metal- lic debris. It has been reported that IP responds to magnetite but not to hema- tite. Placer heavy minerals such as gold, Chesapeake Technology SonarWiz Teledyne ODOM Echo Sounders & Multibeam Systems Teledyne TSS Motion & Navigation Systems zircon, rare earth elements and plati- num tend to accumulate in the same places as ilmenite, and ilmenite can be detected at concentrations as low as 0.01 percent. If ilmenite can thus be tracked with a streamer and if vibrac- oring shows that it is associated with gold (e.g., in coastal Alaska and South Africa), platinum (e.g., in Sierra Leone) or rare earth elements (e.g., in the At- lantic Continental Shelf), then marine IP can be an effective precious-metal prospecting tool for the subseafloor. Experiments are tentatively sched- uled for June to August 2013 offshore Nome, Alaska, to determine if marine IP responds directly to gold. The marine IP approach also works for ferrous and nonferrous metallic de- bris, such as shipwrecks and unexplod- ed ordnance, in dispersed concentra- tions on and beneath the seafloor. 50 st / SEPTEMBER 2012 Conclusions Laboratory and sea trials suggest that IP is a powerful tool for mapping the subseafloor for placer minerals, buried wrecks, metallic debris and www.sea-technology.com unexploded ordnance. IP responds to sulfide minerals, titanium-bearing placer sands and metallic debris even when covered by younger inert sedi- ments. Perhaps more significant, in light of a number of recent environmental disasters, is that the technology could also be used to detect and map dis- persed oil in seawater down to very low concentrations. The apparent fre- quency dependence of the IP response to oil-droplet size suggests that it is possible to monitor the biodegrada- tion of dispersed oil. Acknowledgments This work has been supported by the U.S. Geological Survey's Marine Geology, Mineral Resources and Vol- cano Science Centers, as well as by NOAA, Coastal Carolina University and the Mississippi Mineral Resourc- es Institute. Williamson & Associates (Seattle, Washington) and Zonge In- ternational (Tucson, Arizona) are co- operative research and development agreement partners. Rick Hoblitt, Seth Moran and John Ewert provided helpful technical re- views. References For a list of references, contact Jeff Wynn at jwynn@usgs.gov. n Jeff Wynn is a research geophysicist and science manager with the U.S. Geological Survey. He has earned pat- ents for the USGS on the marine tech- nologies discussed in this article and published more than 200 papers in ge- ology, geophysics, volcanology, miner- al resources and hypervelocity-impact physics. He is also a jujitsu sensei. Mike Williamson is a geologist in Wash- ington State with a 40-year interest in mining, developed as a teenager work- ing in a coal mine. Subsequent educa- tion 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 Tuc- son, Arizona, at Zonge International. He was previously a project manager at hydroGEOPHYSICS and a hydrolo- gist at the U.S. Geological Survey.