Centimeter-Level Positioning of Seafloor Acoustic Transponders from a Deeply-Towed Interrogator

An array of three seafloor transponders was acoustically surveyed to centimeter precision with a deeply-towed interrogator. Measurements of two-way acoustic travel time and hydrostatic pressure made as the interrogator was towed above the array were combined in a least-squares adjustment to estimate the interrogator and transponder positions in two surveys spanning two years. No transponder displacements were expected at this site in the interior of the Juan de Fuca Plate (48^ˆ11′ N, 127^ˆ12′ W) due to the lack of active faults. This was confirmed to a precision of ±2 cm by least-squares adjustment. Marginally detectable blunders in the observations were shown to affect the transponder position estimates by no more than 3 mm, demonstrating the geometric strength of the data set. The accumulation of many hundreds of observations resulted in a significant computational burden on the least-squares inversion procedure. The sparseness of the normal matrix was exploited to reduce by a factor of 1000 the number of calculations. The acoustic survey results suggested that the near-bottom sound speed fields during the two surveys were in better agreement than inferred from yearly single-profile conductivity, temperature, and pressure (CTD) measurements.

Keywords:

• Seafloor positioning
• acoustic ranging

Acknowledgments

The authors thank the Marine Physical Laboratory Deep Tow group for engineering support, the Ocean Data Facility at SIO for calibration equipment, and Dr. Chris Yorath and his colleagues of the Canadian Pacific Geoscience Centre for assistance in installing transponder A. We also are indebted to the captains and crews of the Canadian R/V Tully, and Scripps Institution of Oceanography ships R/V Melville and R/V New Horizon for seagoing support, and to Jo Griffith for illustrations. This material is based on work supported by NSF grants OCE 87-16392, OCE 93-14611, and OCE 99-07247, and NASA grant NAG5-1914.

Figures 1, 4, and 11 were prepared with the Generic Mapping Tools (Wessel and Smith 1998).