Recent onshore work by several investigators indicates that ilmenite, an important titanium bearing constituent of placer heavy-mineral assembles, has a strong and, distinctive induced polarization (IP) spectral signature. Reconnaissance geologic studies of the Atlantic Continental Shelf also suggest that there are potentially large concentrations of ilmenite and other placer minerals in paleo beach-complex deposits, at least offshore of Virginia and Georgia. Our experiments with a seafloor-bottom-configured IP streamer off the Virginia' coast, if verified by further work, may mean that IP can be used offshore to map these resources. During an 8.2-km run lasting 50 minutes, several anomalies were observed that exceeded the typically 3 to 4 mrad noise envelope. One, anomaly, peaking smoothly at 8 mrad over 80 m, was found to coincide with a coast-parallel geomorphic feature that could reasonably be expected to host placer heavy minerals. Modeling and previous analyses of grab samples from the area suggest that this anomaly might indicate a significant deposit. Because it's not necessary to plant electrodes in the seafloor, data acquisition could be extremely efficient. Success depends on whether the technique proves reliable for continuous measurements, especially in conjunction with other marine operations. We believe this is only a problem of improving the engineering. If so, it is now now theoretically possible to conduct reconnaissance IP survey .of very large areas of the continental shelf, guiding shipborne vibracore sampling much more efficiently than heretofore done.
The induced polarization (IP) phenomenon has been known to detect disseminated sulfide minerals since the late teens of this century'. The method itself has seen extensive use in onshore exploration, mainly for disseminated sulfides, since the late 1940's. Until recently, however, it had not been used in a marine environment because it was felt that the high conductivity of sea water would lead to current channeling and insignificant penetration of the seafloor, and consequently unuseable IP measurements.
There are abundant examples in the literature of the use of the electrical resistivity method to map the sub seafloor, beginning with Schlumberger and others who mapped bedrock beneath a harbor in Algiers. Subsequent papers describe other Efforts4, 5, 6, 7, 8, 9, 10 Corwin and others11, 12, 13 demonstrated with modeling and coring that vertical sounding information of a quality comparable to that acquired on land can be acquired in a marine environment, even while the operating vessel was underway.
A pioneering effort to conduct IP measurements in a marine environment was reported by Scott and others14 using a surface-towed IP streamer. The initial IP results of this system were reported as discouraging, due to high noise levels, but Scott (written communication) indicates that improvements in signal-to-noise have permitted subsequent acquisition of useful IP data.
Against this background, Wynn and others15 have demonstrated that certain titanium minerals could be measured, even identified and quantified in situ, using IP and spectral IP measurements from a surface array onshore.
