Export 3 results:
Sort by: Author [ Title  (Asc)] Type Year
A B [C] D E F G H I J K L M N O P Q R S T U V W X Y Z   [Show ALL]
Sweeney, AD, Chadwell CD, Hildebrand JA.  2006.  Calibration of a seawater sound velocimeter. IEEE Journal of Oceanic Engineering. 31:454-461.   10.1109/joe.2004.836582   AbstractWebsite

We calibrated a sound velocimeter to a precision of +/- 0.034 m/s using Del Grosso's sound-speed equation for seawater at temperatures of 2, 7.2, 11.7, and 18 degrees C in a tank of seawater of salinity 33.95 at one atmosphere. The sound velocimeter measures the time-of-flight of a 4-MHz acoustic pulse over a 20-cm path by adjusting the carrier frequency within a 70-kHz band until the pulse and its echo are inphase. We used the adjustable carrier frequency to determine the internal timing characteristics of the sound velocimeter to nanosecond precision. Similarly, sound-speed measurements at four different temperatures determined the acoustic pathlength to micrometer precision. The velocimeter was deployed in the ocean from the surface to 4500 dbar alongside conductivity, temperature, and pressure sensors (CTD). We demonstrated agreement of +/- 0.05 m/s (three parts in 10(5))-with CTD-derived sound speed using Del Grosso's seawater equation from 500 to 4500 dbar after removing a bias and a trend.

Sweeney, AD, Chadwell DC, Hildebrand JA, Spiess FN.  2005.  Centimeter-Level Positioning of Seafloor Acoustic Transponders from a Deeply-Towed Interrogator. Marine Geodesy. 28:39-70.: Taylor & Francis   10.1080/01490410590884502   AbstractWebsite

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?11? N, 127?12? 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.

Maksymowicz, A, Chadwell CD, Ruiz J, Trehu AM, Contreras-Reyes E, Weinrebe W, Diaz-Naveas J, Gibson JC, Lonsdale P, Tryon MD.  2017.  Coseismic seafloor deformation in the trench region during the Mw8.8 Maule megathrust earthquake. Scientific Reports. 7   10.1038/srep45918   AbstractWebsite

The M-w 8.8 megathrust earthquake that occurred on 27 February 2010 offshore the Maule region of central Chile triggered a destructive tsunami. Whether the earthquake rupture extended to the shallow part of the plate boundary near the trench remains controversial. The up-dip limit of rupture during large subduction zone earthquakes has important implications for tsunami generation and for the rheological behavior of the sedimentary prism in accretionary margins. However, in general, the slip models derived from tsunami wave modeling and seismological data are poorly constrained by direct seafloor geodetic observations. We difference swath bathymetric data acquired across the trench in 2008, 2011 and 2012 and find similar to 3-5 m of uplift of the seafloor landward of the deformation front, at the eastern edge of the trench. Modeling suggests this is compatible with slip extending seaward, at least, to within similar to 6 km of the deformation front. After the M-w 9.0 Tohoku-oki earthquake, this result for the Maule earthquake represents only the second time that repeated bathymetric data has been used to detect the deformation following megathrust earthquakes, providing methodological guidelines for this relatively inexpensive way of obtaining seafloor geodetic data across subduction zone.