Sea Surveyor, Inc
SINCE 1985



Precision marine surveys for the engineering community



SF Hazardous Rocks



Client: U.S. Army Corps of Engineers, San Francisco District

Contract Value: $190K

A/EContract Value: $1 Million


3-D Multibeam Image of Blossom Rock

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


Geologic X-Section over Blossom Rock

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




Rock Location

Location of Hazardous Rocks in SF Bay


Side-Scan Image of Sunken Barge