6.0 Survey Methodology

 To investigate the possibility that shipwreck remains might still lie within the boundaries of the project area, Sea Surveyor, Inc. conducted magnetometer investigations of the five survey areas.  WSA and Sea Surveyor, Inc. also conducted a second side scan sonar survey, using a high-resolution, 600 kHz system over Blossom, Shag, Arch and Harding rocks.    The Golden Gate Mound was not reexamined during the maritime archaeology side scan sonar survey because the magnetic data and the side scan images obtained during the geophysical survey were judged a sufficient assessment of the potential for cultural resources in that area.

 6.1 Marine Magnetometer

Sea Surveyor, Inc. conducted a marine magnetometer survey of the five submerged rocks and surrounding areas in Central San Francisco Bay as part of the maritime archaeological investigation in support of the Rocks Removal Project.  The presence of ferrous-metal on or below the seafloor was determined using a Geometrics Model G-881 cesium-vapor marine magnetometer.  The magnetometer recorded local variations in the earth's magnetic field as measured by the marine sensor that was towed 61-meters (200 feet) behind the 26 foot survey vessel Betty Jo, owned and operated by Sea Surveyor, Inc.  The magnetometer data were displayed on the navigation monitor screen aboard the vessel, and the digital data were recorded in the navigation computer at one-second intervals along with the navigation data.  The magnetometer has a sensitivity of one gamma, with instrument noise-level no greater than three gammas, peak to peak.

 The magnetometer survey was conducted on 2-4 November 2000.  Navigation during the survey was based on an Omnistar differentially corrected Global Positioning System (GPS) that incorporated positioning information into its on-board navigational computer to maintain horizontal control of the survey.  Survey lines were run at nominal 45-meter (150 foot) intervals, and the survey tracklines are shown in (Figure 14) and (Figure 15).

 6.2 Side Scan Sonar

Before recommending the maritime archaeology side scan sonar survey, WSA’s maritime archaeologist and the staff of Sea Surveyor, Inc. reviewed the magnetometer data and the side scan sonar data obtained during the geophysical survey to determine if, in combination with the magnetic data, the acoustic data would be adequate to identify and assess the potential of any submerged cultural resources.  Based on this analysis, WSA recommended additional side scan sonar surveys in the survey areas surrounding Blossom, Harding, Arch, and Shag rocks.

 The side-scan sonar surveys were conducted on 15 November 2000, and 19 January 2001 using a Marine Sonics Sea Scan PC 600 kHz system.  The sensor for the side-scan sonar was towed 33 meters behind the survey vessel, the Betty Jo, which employed the same differential GPS navigation system used during the magnetometer survey.  The appropriate "layback" was programmed into the navigation software to adjust for the distance the sonar’s sensor was towed behind the boat's GPS antennae.

 The Marine Sonics Sea Scan PC 600 kHz side scan sonar uses a 600-kHz transducer to produce a plan view image of the seafloor to the left and right of the survey trackline.  The acoustic side-scan images were recorded digitally, processed, and displayed in real-time through Marine Sonics’ proprietary software, which was mounted in an on-board Fieldworks computer.

 The side-scan sonar surveys were run along previously-programmed, parallel survey lines, spaced at 50 meter intervals and oriented in the same direction as the tidal currents over the 5 sites.  The side-scan sonar data were collected at a range of 50 meters (164 feet) on each side of the trackline, allowing all parts of the seafloor to be viewed from two different directions with 100 percent overlap.  Tracklines from the side-scan sonar survey are shown in (Figure 16) and (Figure 17).

 WSA analyzed the side scan sonar records using Marine Sonics’ proprietary software, Sea Scan PC Review.  Acoustic targets were correlated with the magnetic anomalies identified in the magnetometer surveys of the five rock features.  Each target was analyzed to determine its nature, and an assessment was made of its potential for National Register eligibility.