Palos Verdes

Acoustic Survey of Palos Verdes Shelf and Vicinity

Example of an acoustic record showing a canyon.

An acoustic survey was conducted on the shelf, slope, and basins off the Palos Verdes Peninsula in order to obtain information about the distribution and fate of the deposit from the White's Point diffusers, here called "the effluent-affected sediment deposit". The survey used instruments that profile the strata beneath the seafloor (high-resolution acoustic-reflection profilers) and that image the seafloor in plan view (side scanning sonar). The objectives were to:

• define the distribution of the effluent-affected sediment deposit in three dimensions
• identify pathways and conduits of sediment transport
• identify areas of erosion and deposition
• describe the seafloor morphology

The extent of the effluent-affected sediment deposit was mapped from its appearance in acoustic-reflection profiles. The conclusion that the mapped deposit contains effluent from the White's Point diffusers is based on the deposit's geometry and location (extending from the diffuser pipes and mostly in the direction of regional currents) and on the presence of a low-density surface layer of sediment that contains chemical compounds identified with the effluent, in cores collected from within the mapped extent of the deposit.

The Palos Verdes shelf is a short (12 km), narrow (<3.5 km) section of shallow seafloor off the Palos Verdes Peninsula, situated between the larger San Pedro and Santa Monica Shelves. The shelf seafloor typically is about 75- to 100-m water depth. Steeply sloping seafloor, here termed the "basin slope", lies seaward of the shelf. It descends to the San Pedro Basin where the maximum water depth exceeds 900 m.

The general approach to the survey was to collect image and profile records along regularly spaced, straight tracklines approximately parallel to isobaths on the Palos Verdes and Redondo Shelves, the basin slope, and within San Pedro Basin. Tracklines were spaced so that adjacent side-scan images have substantial overlap, enabling construction of an image mosaic portraying the entire area of the seafloor. In shallow water (less than about 200 m), the total swath width of the images typically was 300 m, and tracklines were spaced at 240 m. In deep water, the swath width was increased to 1,000 m along tracklines spaced at 750 m. Subbottom profiles were collected concurrently with the side-scan images along tracklines, using a combination of profilers. A limited number of profiler tracklines were run normal and oblique to the isobath-parallel tracklines, and zig-zag profiler tracklines were run across some canyons and on the shelf.

The acoustic instruments used were:

• very high-resolution chirp seismic-reflection profiler
• high-resolution 3.5-kHz seismic-reflection profiler
• 10-kHz bathymetric profiler
• digital high-resolution side-scanning sonar

The ship's position along tracklines was monitored by diffrential GPS, GPS and a shore-based transponder (Del Norte system ) navigation system.

For additional information, contact Homa Lee