Abstract:

Geodetic data, particularly using electronic distance measuring and space-based techniques, are central to terrestrial studies of crustal deformation. A decade ago, no comparable capabilities existed for use in the Earth's extensive ocean-covered areas. It had been clear for some time, however, that underwater acoustics could play a key role, replacing or extending the electromagnetic methods used in nearly all terrestrial systems. Three classes of centimeter-capable systems have emerged. For short ranges, direct path measurements using high frequencies are preferred. At intermediate ranges (1--10 km) marker transponders can be interrogated from an intermediate vehicle and their relative positions calculated. Beyond about 10 km, inadequate knowledge of sound speed limits purely acoustic systems, but a combined GPS/acoustic method has been devised to cope with the need for data over important longer base lines. Since 1990, several systems have been implemented in the context of geologically relevant problems, particularly in the northeast Pacific. As operating experience has grown and real data are being analyzed, useful initial results are emerging in conjunction with insights into the problems that must be solved when moving toward future developments. [Work reported has been supported by NSF, NASA, NOAA, PGC, and USGS.]