Millimeter-level precision in a seafloor geodesy experiment at the Discovery transform fault, East Pacific Rise

Direct-path acoustic ranging is a promising seafloor geodetic technique for continuous high-resolution monitoring of geodynamical process such as fault slip and magma intrusion. Here we report on a yearlong acoustic ranging experiment conducted across the discovery transform fault at similar to 4 degrees S on the East Pacific Rise. The ranging instruments utilized a novel acoustic signal designed to enhance precision. We find that, after correcting for variations in sound speed at the path end-points, the ranging measurements have a precision of similar to 1 mm over baselines approaching 1 km in length. The primary difficulty in this particular experiment was with the physical stability of the benchmarks, which were deployed free fall from a ship. Despite the stability issues, it appears that the portion of the transform fault that the array covered was locked during the year of our survey. The primary obstacle to continuous, high sample rate, high-precision geodetic monitoring of oceanic ridges and transform faults is now limited to the construction of geodetic monuments that are well anchored into bedrock.