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(Top) An example of a seafloor marine IP profile ac- quired in the Gulf of Mexico, with several different features identified. The baseline system IP response is about 13 to 14 milliradians. Typical seawater has a resistivity of about 0.3 ohm-meters. (Bottom) Part of a five-week-long marine IP survey conducted off the coast of southeast Africa. For instance, the pyrite response peaks below 0.1 hertz, while ilmenite peaks around 4 hertz. Both these minerals are particularly strong re- sponders 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 di- poles at increasing distances. This allows mapping of IP-reactive min- erals at different depths using vari- ous 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-seawa- ter stand can be seen in bathym- etry and also in coarser results from gridded vibracoring car- 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 stud- ies to be diagnostic of the specific minerals being polarized. ried 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 resistiv- 48 st / SEPTEMBER 2012 www.sea-technology.com