Two Different Electrical Properties Can Improve Transoceanic Cable-Route Mapping Available to Purchase

Recent experience has shown that burial assessment surveys, consisting of continuous resistivity profiling, in coordination with periodic cone-penetrometer tests, provides critical information to engineers planning transoceanic cable routes. The expense of the new generation of high-bandwidth optical-fiber cables and their vulnerability to trawling, dredging, and various geological hazards such as landslides makes cable burial several meters below the watersediment interface increasingly important. Burial, however, is highly dependent upon the ripability (plowability) of the top several meters of ocean-floor sediments. Consequently, ocean-floor surveys beginning with side-scan sonar have become an integral part of modern transoceanic cable-laying operations, such as FLAG (Fiberoptic Link Around the Globe). In most cases, conepenetrometers can characterize these sediments for purposes of assessing the potential for plowing or hydraulic trenching, but these measurements are not continuous. Electrical resistivity, on the other hand, can be measured continuously. It can also distinguish lowporosity from high-porosity (i.e., probably ripable) materials, as well as layering providing depth to any shallow consolidated unit below the sediment-water interface. However, resistivity alone cannot distinguish lithologies- for instance, it cannot separate low-porosity basalt from low-porosity clay. For nearly half a century, the induced polarization method (IP) has been used on land to map certain polarizeable mineral species, in particular sulfides such as pyrite (FeS) and ihnenite (FeTi03). The latter is a titanium-bearing mineral found in metamorphic rocks and in some coastal beach sands. The IP method works by forming a cumulative double-layer capacitive effect, called the Warburg Impedance, at the mineral-electrolyte interface in the presence of an induced potential voltage (hence the name "induced polarization"). The method is thus surface-sensitive, not volume-sensitive, making it highly effective when working with very small percentages of these minerals. Typically, IP can detect ilmenite below 0.1% by volume*.