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DeSanto, JB, Sandwell DT, Chadwell CD.  2016.  Seafloor geodesy from repeated sidescan sonar surveys. Journal of Geophysical Research-Solid Earth. 121:4800-4813.   10.1002/2016jb013025   AbstractWebsite

Accurate seafloor geodetic methods are critical to the study of marine natural hazards such as megathrust earthquakes, landslides, and volcanoes. We propose digital image correlation of repeated shipboard sidescan sonar surveys as a measurement of seafloor deformation. We test this method using multibeam surveys collected in two locales: 2500m deep lightly sedimented seafloor on the flank of a spreading ridge and 4300m deep heavily sedimented seafloor far from any plate boundary. Correlation of these surveys are able to recover synthetic displacements in the across-track (range) direction accurate to within 1m and in the along-track (azimuth) direction accurate to within 1-10m. We attribute these accuracies to the inherent resolution of sidescan data being better in the range dimension than the azimuth dimension. These measurements are primarily limited by the accuracy of the ship navigation. Dual-frequency GPS units are accurate to approximate to 10cm, but single-frequency GPS units drift on the order of 1m/h and are insufficient for geodetic application.

Chadwell, CD, Sweeney AD.  2010.  Acoustic Ray-Trace Equations for Seafloor Geodesy. Marine Geodesy. 33:164-186.   10.1080/01490419.2010.492283   AbstractWebsite

One goal of seafloor geodesy is to measure horizontal deformation of the seafloor with millimeter resolution. A common technique precisely times an acoustic signal propagating between two points to estimate distance and then repeats the measurement over time. The accuracy of the distance estimate depends upon the travel time resolution, sound speed uncertainty, and the degree to which the path computed from propagation equations replicates the actual path traveled by the signal. In this paper, we address the error from ray propagation equations by comparing three approximations to Snell's Law with ellipsoidal geometry.

Chadwell, CD, Hildebrand JA, Spiess FN, Morton JL, Normark WR, Reiss CA.  1999.  No spreading across the southern Juan de Fuca Ridge axial cleft during 1994-1996. Geophysical Research Letters. 26:2525-2528.   10.1029/1999gl900570   AbstractWebsite

Direct-path acoustic measurements between seafloor transponders observed no significant extension (-10 +/- 14 mm/yr) from August 1994 to September 1996 at the southern Juan de Fuca Ridge (44 degrees 40' N and 130 degrees 20' W). The acoustic path for the measurement is a 691-m baseline straddling the axial cleft, which bounds the Pacific and Juan de Fuca plates. Given an expected full-spreading rate of 56 mm/yr, these data suggest that extension across this plate boundary occurs episodically within the narrow (similar to 1 km) region of the axial valley floor, and that active deformation is occurring between the axial cleft and the plate interior. A cleft-parallel 714-m baseline located 300 m to the west of the cleft on the Pacific plate monitored system performance and, as expected, observed no motion (+5 +/- 7 mm/yr) between the 1994 and 1996 surveys.

Chadwell, CD.  1999.  Reliability analysis for design of stake networks to measure glacier surface velocity. Journal of Glaciology. 45:154-164. AbstractWebsite

Measurement of glacier surface velocity provides some constraint on glacier flow models used to date ice cores recovered near the flow divide of remote high-altitude ice caps. The surface velocity is inferred from the change in position of a network of stakes estimated from the least-squares adjustment of geodetic observations-terrestrial and/or spaced-based-collected approximately year apart. The lack of outliers in and the random distribution of the post-fit observation residuals are regarded as evidence that the observations contain no blunders. However, if the network lacks sufficient geometric redundancy the estimated stake positions san shift to fit erroneous observations. To determine the maximum size of these potential undetected shifts, given the covariance of the observations and the approximate network geometry expressions are developed to analyze a network for redundancy number and marginally detectable blunders (internal reliability), and the position shifts from marginally detectable blunders (external reliability). Two stake networks, one on the col of Huascaran (9 degrees 07' S, 77 degrees 37' W; 6050 m a.s.l.) in the north-central Andes of Peru and one on the Guliya ice cap (35 degrees 17' N, 81 degrees 29' E; 6200 ma.s.l.) on the Qinghai-Tibetan Plateau in China, are examined for precision and internal and external reliability.