Precise acoustic measurements of seafloor transponder and towed-vehicle positions

• Published in: The Journal of the Acoustical Society of America 2000-05, Vol.107 (5_Supplement), p.2807-2807
• Main Authors: Sweeney, Aaron D., Chadwell, Carl D., Hildebrand, John A., Spiess, Fred N.
• Format: Article
• Language: English
• ISSN: 0001-4966; 1520-8524
• DOI: 10.1121/1.429043

We describe a project using acoustic techniques to determine the pattern of seafloor deformation between annual measurements. Using cross-correlation between transmitted and received signals, round-trip travel times precise to a few microseconds were achieved between a towed vehicle and seafloor transponders. The technique is sensitive to sound-speed changes as small as 0.01 m/s. The positions of the transponders and towed vehicle are calculated in a least-squares sense from a ray-trace model, providing relative positions for seafloor transponders with an accuracy of a few centimeters. Two experimental sites were surveyed. The first is in the interior of the Juan de Fuca plate, where the temporal variability of the near-bottom sound speed was 0.05 m/s, with little range dependence. Deformation at the few-centimenter level was neither observed nor expected due to remoteness from plate boundaries. The second site straddled the Juan de Fuca spreading center. Measurements at this site suggest temporal sound-speed variations of about 0.15 m/s, and range dependence due to the presence of localized hydrothermal vents. These measurements are also consistent with little or no deformation over a 2-year interval, in agreement with other measurements, although the spatio–temporal sound speed changes create larger overall error in position determination.