Precise acoustic measurements of seafloor transponder and towed‐vehicle positions

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.