Detailed Structural Mapping of an Ocean Rift, Using Acoustic-Transponder Positioning: Problems of Net Maintenance (1981–86) and New Experiments Possible with Long-Life Transponders

Abstract

Since September 1981, the U.S. Geological Survey has continuously maintained an acoustic-transponder net on the axis of the Juan de Fuca Ridge off west-central Oregon. This transponder net has provided position control for deep-tow vehicles, samplers, and manned submersibles as part of a long-term program to study the structural, volcanic and hydrothermal processes along the 2,200m- deep ridge crest. Field operations in the area each fall utilize a net of four or five limited-lifespan transponders (both recoverable and expendable units); between operations, a skeletal net of one to three transponders is left. In addition to the difficulty of guaranteeing ship time each year for refurbishing the transponder net, servicing the net includes other problems: (1) time lost to deployment, net calibration, and recovery each year, (2) net rotation ambiguity resulting when only one transponder survives to the next field season; and (3) difficulties in reoccupation of sample localities using separate net calibration for each successive field season. We have anchored benchmark floats at several sampling sites to aid in future site reoccupation and to determine absoulute accuracy between successive net calibrations. Long-life (5 + years) transponders, capable of measuring round-trip travel times to 10 µs (Spiess and Boegeman, Eos., v. 65, p. 851, 1984), would allow or facilitate several new experiments, including: (1) geodetic determination of rifting rate, (2) monitoring of changes in mineral and faunal assemblages at hydrothermal vents, and (3) detection of sea-floor deformation.

Presentation given at symposium, but paper not available for publication.