C. David Chadwell

Measuring the onset of locking in the Peru–Chile trench with GPS and acoustic measurements

The subduction zone off the west coast of South America marks the convergence of the oceanic Nazca plate and the continental South America plate. Nazca–South America convergence over the past 23 million years has created the 6-km-deep Peru–Chile trench, 150 km offshore. High pressure between the plates creates a locked zone, leading to deformation of the overriding plate. The surface area of this locked zone is thought to control the magnitude of co-seismic release and is limited by pressure, temperature, sediment type and fluid content. Here we present seafloor deformation data from the submerged South America plate obtained from a combination of Global Positioning System (GPS) receivers and acoustic transponders. We estimate that the measured horizontal surface motion perpendicular to the trench is consistent with a model having no slip along the thrust fault between 2 and 40 km depth. A tsunami in 1996, 200 km north of our site, was interpreted as being the result of an anomalously shallow interplate earthquake. Seismic coupling at shallow depths, such as we observe, may explain why co-seismic events in the Peruvian subduction zone create large tsunamis.

Estimation and correction for the effect of sound velocity variation on GPS/Acoustic seafloor positioning : An experiment off Hawaii Island

A GPS/Acoustic experiment on the southeastern slope of Hawaii Island presented precise seafloor positioning in the condition of large water depth (2.5—4.5 km) and large velocity variations. We estimated sound velocity variations from acoustic ranging, and found that temperature variation can well explain the velocity variation. The effect of daily variation in the sound velocity amounted to +/- 0.7 m on acoustic ranging of 4—7 km with a fixed velocity structure. CTD data observed about every 3 hours could decrease the range residuals to +/- 0.4 m. These large residuals were fairly well canceled in the positioning of the array center of three acoustic transponders. The estimated precision of the array center positioning was about 3 cm in latitude and longitude.

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 (48ˆ11′ N, 127ˆ12′ 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.

Direct estimation of absolute precipitable water in oceanic regions by GPS tracking of a coastal buoy

A buoy-based GPS receiver and meteorological sensor are used to estimate directly the absolute precipitable water (PW) overlying a coastal ocean site 8 km from shore. During an 11-day experiment, one-second GPS data collected at the buoy and at a shore station are combined with 30-second data from four distant GPS stations to estimate the buoy position, zenith wet delay, phase biases, and receiver and satellite clocks using double-differenced phase processing with ambiguity resolution. GPS-derived PW at the buoy compared to radiosonde measurements (20) and to half-hourly GPS-PW values (384) from the nearby shore station show an rms agreement of ±1.5 mm and ±1.8 mm, respectively. Hourly means (170) of the GPS-measured vertical motion of the buoy show a ±24 mm rms agreement with a NOAA tide gauge, equivalent to about 4 mm of PW. GPS-derived PW from buoys may have the potential to improve weather forecasting, calibration of satellite-based sensors, and climate studies.

Active control of passive acoustic fields: Passive synthetic aperture/Doppler beamforming with data from an autonomous vehicle

The maneuverability of autonomous underwater vehicles (AUVs) equipped with hull-mounted arrays provides the opportunity to actively modify received acoustic fields to optimize extraction of information. This paper uses ocean acoustic data collected by an AUV-mounted two-dimensional hydrophone array, with overall dimension one-tenth wavelength at 200-500 Hz, to demonstrate aspects of this control through vehicle motion. Source localization is performed using Doppler shifts measured at a set of receiver velocities by both single elements and a physical array. Results show that a source in the presence of a 10-dB higher-level interferer having exactly the same frequency content (as measured by a stationary receiver) is properly localized and that white-noise-constrained adaptive beamforming applied to the physical aperture data in combination with Doppler beamforming provides greater spatial resolution than physical-aperture-alone beamforming and significantly lower sidelobes than single element Doppler beamforming. A new broadband beamformer that adjusts for variations in vehicle velocity on a sample by sample basis is demonstrated with data collected during a high-acceleration maneuver. The importance of including the cost of energy expenditure in determining optimal vehicle motion is demonstrated through simulation, further illustrating how the vehicle characteristics are an integral part of the signal/array processing structure.

Shipboard towers for Global Positioning System antennas

Abstract 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 mm 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.

Acoustic Ray-Trace Equations for Seafloor Geodesy

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 Snells Law with ellipsoidal geometry.

High‐resolution repeat bathymetric mapping using a mid‐size autonomous underwater vehicle

Shipborne bathymetric surveys south of Santa Barbara, CA indicate that massive slope failures have occurred along the northern flank of the Santa Barbara Basin. This region is seismically active and has the highest sedimentation rates along the California coast due to rapid erosion of the Transverse Ranges. A set of geodetic monitoring systems is being developed to study the character and deformation of the ocean bottom in this region, particularly that of a prominent east‐west trending crack aligned with the head scarp of one of the underwater landslides. One component is a high‐resolution repeat bathymetric mapping system composed of a multibeam sonar mounted inside a Bluefin 21 AUV. Navigation to the decimeter level is provided by an inertial navigation system and accurate depth sensor, and by a long baseline (LBL) acoustic transponder system previously developed to monitor tectonic plate motions. The quality of the data, particularly that from the high‐precision LBL system, is enhanced significantly b...

Passive acoustic localization with an AUV‐mounted hydrophone array

A mid‐size Odyssey IIb autonomous underwater vehicle (AUV) was retrofitted with the advanced vectored‐thrust system presently installed on AUVs manufactured by Bluefin Robotics, Inc. Subsequent modifications to this thrust system decreased the radiated acoustic and vibration noise levels recorded by an eight‐element hydrophone array mounted on the AUV’s inner shroud by 20 to 50 dB across the 20 Hz to 10 kHz band. This reduction in self‐noise levels to near, or at, background ocean noise levels permits the use of the vehicle‐mounted hydrophone array in passive ocean acoustic studies. One example is the application of passive synthetic aperture processing techniques to provide greater spatial resolution estimates of the direction of low frequency sources. Doppler spreading caused by medium motion is a limiting factor in array gain. At mid frequencies (1–10 kHz), the complexity of the received acoustic field created by scattering off the AUV body is partly captured in the array processing by the use of repli...

Precision acoustic geodetic measurement of seafloor motion over 10 km

The technique deploys three or more precision transponders (PXP) on the seafloor, collects simultaneous round‐trip travel times (±5 μs) from a hydrophone towed uniformly around the network above the seafloor (300 m), and measures the sound velocity (±10 ppm) and the relative depth of the hydrophone (±10 cm). These observations, modeled as ray traces through a spherical ocean model and ellipsoidal geodesic ray distances, are inverted to determine simultaneously, the centimeter level position of the transponders and towed hydrophone locations. Three PXPs were deployed on the Pacific and three on the Juan de Fuca plate to determine the pattern of deformation around the Juan de Fuca Ridge. Measurements were collected in 1994, 1995, and planned for 1996. Since 1991, at least four PXPs have been maintained seaward of the Cascadia Subduction Zone to measure the subduction rate using the Global Positioning System. No measurable relative horizontal motion among these PXPs is expected providing a test of system rep...