Background

Foth was contracted by L3 Maripro, Inc. to acquire multibeam bathymetry and backscatter imagery of the planned Sylmar Ground Return System (SGRS) location, in the vicinity of Pacific Palisades, California. The purpose of the survey was to document any debris or other obstacles that may impede the pending SGRS installation, ensuring the project trunk cables and offshore vaults are installed onto soft sedimentation avoiding hard-bottom habitat.

The survey was conducted using a high frequency multibeam sonar with a small beam width to maximize detection capabilities for the water depths of the survey area. The multibeam sonar provided full coverage of the bathymetry in the survey area as well as backscatter imagery. The backscatter imagery from the sonar was used in conjunction with the bathymetry to identify any debris or other potential obstructions on the seafloor, including areas of hard bottom, such as rocks. In addition to the multibeam data, the survey crew also mobilized an underwater camera to use in the event that any features were detected by the multibeam requiring additional documentation.

The timing of the project provided a significant challenge, as L3 Maripro needed the project deliverables quickly to prevent further delays in their cable installation. Our team responded rapidly, completing the delivery of all survey products within a two and a half week timespan.

Results

The client initially scoped the project to require survey work performed by a Remotely Operated Vehicle. Foth discussed alternative options with the client to save time and money. Foth recommended the use of a high-frequency multibeam sonar with a small beam footprint to maximize feature detection capability. The imagery mosaic from the multibeam backscatter was overlaid with the multibeam bathymetry to provide a faster, more cost-effective method to meet the site clearance requirements. This methodology saved significant money and time.

Foth’s expertise in multibeam surveying, specifically with integrating the backscatter imagery with the bathymetry, played a key role in this project. The survey determined that the proposed cable installation did not conflict with any debris or hard-bottom features on the seafloor, with the exception of some areas of scattered rocks nearshore. The GIS products delivered to the client illustrated the sediment type and seafloor features in the path of the proposed cable, clearing the path for the client’s final permitting to begin construction on their pending project.