High-Frequency Sound Interaction in Ocean Sediments
2 July 1999

Synthetic Aperture Sonar Measurements

K.W. Commander and J.T. Christoff
Naval Surface Warfare Center
Coastal Systems Station
Panama City, FL 32407-7001
(850) 234-4851, -4886 (fax), CommanderKW@ncsc.navy.mil
(850) 234-4104, -4886 (fax), ChristoffJT@ncsc.navy.mil

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The objectives of the SAS experiments are to (1) obtain high quality SAS data in conjunction with highly characterized environmental data, (2) obtain bistatic surface, bottom, and target scattering data, and (3) determine the effect of time on artificially buried mines in a sandy bottom. The first objective will be met with the help of the environmental data from other DRI participants and from the 2-D CTD chain measurements from the second CSS support ship. The second objective will be met with the assistance of NSW Buried Target Sonar System project's underwater sonar tower transmitters and passive data collection by the SAS tow vehicle. The third objective will be achieved by burying targets in April and again in October (in the same proximity) and analyzing the differences (if any) in their scattering signatures.

SAS System

The Synthetic Aperture Sonar (SAS) experiments will be conducted with the HF/LFSAS system. It is housed in a variable depth, stabilized towed vehicle and consists of a double sided 55 cm line array operating a high and low frequency SAS simultaneously. The two center frequencies are 20 kHz and 180 kHz with a 10 and 30 kHz bandwidth, respectively. The system has a theoretical resolution of 2.5 cm for the high frequency and 7.5 cm for the low frequency operating in a monostatic mode. The stave data are delivered to the topside computer by a fiber optic cable where they are recorded on a high-density data recorder.

CTD Towed Chain System

The CTD Chain System consists of 11 high-resolution temperature/conductivity sensors configured as a vertical line array with a depressor attached to the bottom. The sensor array is towed from a boom attached to a support vessel. The array of sensors is towed at speeds up to 4 knots and has an update rate of 10 Hz. The CTD system will provide near real-time 2-D temperature/conductivity profiles between the SAS system and the target field or any other region of interest.

Bistatic Projectors

The NSW Buried Target Sonar System project will provide a 20 kHz narrow-beam sonar mounted on an underwater tower to produce the transmissions for the bistatic surface, bottom, and target measurements. The tower will be located sufficiently close to the line of targets so that it can easily insonify them for the bistatic measurements. Details of NSW equipment configuration are found under Dr. Joe Lopes' project description.

Target Field

The SAS target field will consist of four cylindrical targets of interest and two burial scars (without targets). Two cylindrical targets and one burial scar will be placed on or in the sea bottom in April 1999 and the remaining three placed just prior to testing in October. There will be two cylindrical targets that are fully buried with approximately 10 cm of sand over the target and two cylindrical targets that are partially buried (approximately 50% of the volume below the sand bottom). The third type target will be a burial scar (covered crater) approximately the same size as the cylindrical targets.

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