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ter and receiver electrodes (a labor- and time-intensive project), but these measurements can be con- ducted in the sea as fast as an active streamer can be towed through the water or along the seafloor, as sea- water provides an immediate elec- trical connection to the substrate. If multiple cables are used, then Laboratory IP results for a range of oil-in-saltwater mixture. ity and IP data during this survey, in an area well-known for shipwrecks caused by rogue waves. Gulf of Mexico Survey. A subsea- floor survey was carried out in the Gulf of Mexico in which broad IP anoma- lies 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 Mis- sissippi 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 bur- ied 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 resis- tivity anomalies observed. Characterizing Oil in Seawater The IP principle has been used for decades to search for disseminated sulfide deposits (so-called "porphyry copper" ore bodies). The ocean analog www.sea-technology.com SEPTEMBER 2012 / st 49 appears to work in a similar fashion, with the peak frequency correlating in- versely with oil in the case of droplet size. Normally, a square wave voltage signal is transmitted into the seawater. A fast Fourier transform of transmit and receive signals, deconvolved against each other, allows characterization of the evolving oil-droplet size distribu- tion. IP measurements on land require the manual emplacement of transmit- towing the array in lawn-mower fashion will map the dispersed hy- drocarbons in 3D, or 4D with repeat surveys. As the peak polarization response frequency to oil droplets increases over time, it should thus be possible to monitor ongoing oil biodegradation. Existing methods used to moni- tor hydrocarbon plumes include Rosette sampling and use of an AUV- deployed membrane inlet mass spec- trometer. However, these are not as efficient as IP because the former requires hours of an immobilized ship to acquire data and captures only a few discrete verti- cal points, resulting in data that do not account for a plume's movement, and the latter gives only a one-dimensional profile of something that is inherently 3D and in motion. SEE US AT OCEANS '12—BOOTH #1902