In 1983 the United States declared sovereign rights and jurisdiction over living and nonliving resources in an area extending 200 nautical miles (370 km) seaward from its shores. In response to the establishment of this Exclusive Economic Zone (EEZ), the U.S. Geological Survey (USGS) has implemented a program, called EEZ-Scan, to systematically map the EEZ, using the Geological Long- Range Inclined ASDIC (GLORIA) II longrange side scan sonar system developed by the Institute of Oceanographic Sciences (IOS) of Great Britain [Somers et al, 1978]. The first part of the EEZ-Scan field program was completed in the summer of 1984, when USGS and IOS scientists surveyed the EEZ off the western conterminous United States aboard the British research vessel Farnella (Figure 1). The west coast survey, requiring 96 days of ship time and four separate legs, has resulted in virtually total sonograph coverage of the sea floor from the continental shelf break to the 200-nautical mile limit between the Mexican and Canadian borders, an area of about 850,000 km2 . Other data collected on the cruises included two-channel digital seismic reflection and 3.5-kHz highresolution and 10-kHz bathymetric profiles, as well as towed magnetometer data along approximately 20,000 km of trackline spaced nominally at 30-km intervals.