Due to the recent turn down of submarine telephone cables by their commercial operators, there is an opportunity to make planetary scale measurements of the geodlectric field variations on the unpowered cables at a number of locations around the world. Recent work indicates that the long (thousands of km) cables average out the motionally-induced electric field from ocean flows out to periods of 10-20 days, in contrast to point electric field sensors which are dominated by the ocean at 4 days or longer. This means that the cable electric field may be used for deep sounding of the electrical conductivity of the mantle by the magetotelluric methods to depths extending well into the lower mantle. This project is a preliminary magetotelluric study using existing data from the HAW-1 cable collected at Bell Labs and the installation of special high stability voltage monitoring equipment on three segments of TPC-1 extending from Hawaii to Midway, Midway to Wake, and Wake to Guam. This will enable the potential difference across the entire Pacific basin to be measured.
