Seafloor Positioning Research Articles

Detecting horizontal gradient of sound speed in ocean

Abstract We propose a new approach to monitor the horizontal gradient of sound speed in ocean for its correction on seafloor positioning using the GPS/acoustic technique. The new method requires five seafloor transponders to solve five parameters: δx, horizontal position of a transponder array; δt, a common delay in traveltimes due to the stratified component of sound speed; ∇ t, the gradient of traveltime delays among the transponders associated with the sound speed gradient. We also numerically evaluate the geometrical strength of the five transponders’ layout and observation point to avoid possible trade-off among the parameters.

Observation of coseismic seafloor crustal deformation due to M7 class offshore earthquakes

We have developed an observation system for seafloor crustal deformation measurement combining the two following techniques: the precise acoustic ranging and kinematic GPS (Global Positioning System) positioning. On July 14, 2003, we installed the units for this observation system at the seafloor of the Kumano Basin, Japan (33.58°N, 136.60°E). We performed repeated observation six times on 1) July 15–16 and 21, 2004, 2) November 9–10, 2004, 3) May 18 and 20, 2005, 4) July 19–20, 2005, 5) August 18–19, 2005, and 6) August 29–30, 2005. The present observation system can stably determine the seafloor position with an RMS repeatability of ±3 cm in both horizontal components. Two M7 class offshore intraplate earthquakes occurred only 60 km from the Kumano Basin site on September 5, 2004. We observed the coseismic horizontal displacement of 18 cm at the Kumano Basin site using the presently developed seafloor observation system. Our observation system can provide seafloor crustal deformation associated with crustal activities in offshore areas.

Initial Results of GPS/Acoustic Seaf loor Positioning using a Small Towed Buoy off Miyagi Prefecture, Northeastern Japan

Initial Results of GPS/Acoustic Seaf loor Positioning using a Small Towed Buoy off Miyagi Prefecture, Northeastern Japan

Development of a GPS/Acoustic Seafloor Positioning System for 6, 000 m Water Depth and its Trial Experiments at Sea

Development of a GPS/Acoustic Seafloor Positioning System for 6, 000 m Water Depth and its Trial Experiments at Sea

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.

Seafloor positioning system with GPS-acoustic link for crustal dynamics observation—a preliminary result from experiments in the sea—

Many active plate boundaries, such as mid-ocean ridges and trenches, are under the sea. Seafloor crustal deformation data will contribute significantly to understanding the nature of the tectonic processes at these plate boundaries. We have developed a seafloor positioning system with a GPS-acoustic link. This system consists of two main components; (1) the surface positioning by differential GPS to on-land reference and (2) the precise acoustic ranging using the M-sequence between the surface and seafloor references. The position and attitude of the surface GPS-acoustic link unit are determined from the GPS observations. Simultaneously, the acoustic ranging between the surface unit and seafloor references are carried out. The positions of the seafloor references are determined from these observations and a sound-speed structure model of the seawater. We performed preliminary seafloor positioning experiments. In these experiments, simple 1-D structure models are assumed for the sound-speed in the sea. The results show that the positions of the seafloor references are estimated with an accuracy on order of 10 cm. The residuals for acoustic ranging imply that there are systematic differences between the assumed and real sound-speed structure. It is necessary to reduce the uncertainties of sound-speed structures for more accurate positioning.

Sea‐floor positioning with global positioning system–acoustic link system

The present study describes a new system designed and developed for observing crustal deformation on the sea floor. The system consists of two parts, the kinematic positioning by global positioning system (GPS) and acoustic ranging techniques. Since the location of a site at the ocean bottom relative to a reference site on land cannot be determined directly, the procedure was divided into two steps. First, the position of a vessel was determined using differential and kinematic GPS techniques, and then the position of a reference point at the ocean bottom was located relative to the vessel using the acoustic ranging technique. Thus, the location of the ocean‐bottom station is determined relative to the reference sites on land in the global reference system. The accuracy was tested in several ways. In one experiment, a buoy was used as the surface station which linked the positions of a GPS receiver to the acoustic transponder. Assuming a simple sound velocity profile of the seawater, the position of an ocean‐bottom reference point was estimated with an accuracy of several meters. Thus, with the present system, it is difficult to observe ocean‐bottom crustal deformations generated by typical plate motions. Methods are being investigated to improve the observation system for more accurate sea floor positioning.

Performance of a sonar altimeter in the nearshore

A 1 MHz sonar altimeter with automatic gain control is shown to provide accurate estimates of the distance between the instrument and the seafloor. Laboratory experiments indicate that distance estimates degrade slightly when the bottom is rough or sloped and when sediment is suspended in the water column. Results from field tests, both within and seaward of the surf zone, show some degradation owing to a combination of suspended sediment and bubbles, bed undulations, and perhaps the dynamic nature of the sand bottom under waves. Seaward of the surf zone the bottom can be located within ±2 cm nearly continuously, whereas inside the surf zone the bottom can be located only intermittently and the accuracy decreases to ±3 cm. A 300 m long cross-shore transect of 16 altimeters was deployed from the shoreline to about 4 m depth for 3 months in summer-fall 1994 near Duck, NC. Results show that the altimeters are robust and can usually provide estimates of the seafloor position every few minutes even in the surf zone during large storms.