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Chadwell, CD, Spiess FN, Hildebrand JA, Young LE, Purcell, George J, Dragert H, Segawa J, Fujimoto H, Okubo S.  1997.  Sea floor strain measurement using GPS and acoustics. International Association of Geodesy Symposia. 117:682-689., New York, NY, International (III): Springer-Verlag, New York, NY AbstractWebsite
Burgmann, R, Chadwell D.  2014.  Seafloor geodesy. Annual Review of Earth and Planetary Sciences, Vol 42. 42:509-534.   10.1146/annurev-earth-060313-054953   AbstractWebsite

Seafloor geodetic techniques allow for measurements of crustal deformation over the similar to 70% of Earth's surface that is inaccessible to the standard tools of tectonic geodesy. Precise underwater measurement of position, displacement, strain, and gravity poses technical, logistical, and cost challenges. Nonetheless, acoustic ranging; pressure sensors; underwater strain-, tilt- and gravimeters; and repeat multibeam sonar and seismic measurements are able to capture small-scale or regional deformation with approximately centimeter-level precision. Pioneering seafloor geodetic measurements offshore Japan, Cascadia, and Hawaii have substantially contributed to advances in our understanding of the motion and deformation of oceanic tectonic plates, earthquake cycle deformation in subduction zones, and the deformation of submarine volcanoes. Nontectonic deformation related to down-slope mass movement and underwater extraction of hydrocarbons or other resources represent other important targets. Recent technological advances promise further improvements in precision as well as the development of smaller, more autonomous, and less costly seafloor geodetic systems.

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.  2003.  Shipboard towers for Global Positioning System antennas. Ocean Engineering. 30:1467-1487.   10.1016/s0029-8018(02)00141-5   AbstractWebsite

Two 12.2 m-high towers for mounting Global Positioning System (GPS) receiver antennas were designed and constructed to provide millimeter-level stability while maintaining portability and accessibility to satellites and deck spaces. A combination of guys and a 3-m horizontal strut provide roll and pitch stability of 2-3 rum observed from 0.1 seconds to 12 days using a combination of GPS and optical/laser devices. The shipboard antenna mounts connect sub-aerial GPS positioning to underwater acoustic ranging that determine the centimeter-level location of seafloor transponders. Observed annually, these seafloor geodetic positions measure seafloor crustal motion for geophysical studies. (C) 2003 Elsevier Science Ltd. All rights reserved.