MARINE GEOPHYSICAL SURVEY OF HAZARDOUS ROCKS

Client: U.S. Army Corps of Engineers Completion Date: 1999 Contract Value: $190K

Under contract to the San Francisco District, Corps of Engineers, Sea Surveyor conducted a highly-detailed

geophysical survey of five (5) geological features that pose a hazard-to-navigation to deep-draft vessels entering

San Francisco Bay. The 5 submerged hazardous rocks are located in Central San Francisco Bay between the

Golden Gate Bridge and Alcatraz Island, and include:

· Harding Rock

· Shag Rocks

· Arch Rock

· Blossom Rock, and

· Golden Gate Mound

Sea Surveyor mapped the submerged rocks and prepared detailed

bathymetric contour charts, geologic cross-sections, and isopach

maps showing the thickness of sediment layers. The submerged

3-D Multibeam Image of Blossom Rock Geologic X-Section over Blossom Rock

rocks were mapped using the following geophysical sensors:

· Multibeam Sonar and Survey-Grade Depthfinder,

·

Side-Scan Sonars (100 kHz and 600kHz)

· Two types of Subbottom Profilers

· Seismic Refraction Surveys

· Marine Magnetometer

The Seismic Refraction Survey, which measures the velocity of sound

passing through the rocks to estimate their density, determined

Golden Gate Mound to be unconsolidated sand rather than rock.

A comprehensive Marine Archaeological Survey found a sunken 200’

Location of Hazardous Rocks in SF Bay Side-Scan Image of Sunken Barge

960-C Grant Street, Benicia CA 94510

(707) 746-1853 info@seasurveyor.com

barge at Blossom Rock, and rock anchors drilled into Shag Rocks.