Swath Bathymetry Surveys of the Monterey Bay Area from Point Año Nuevo to Moss Landing, San Mateo, Santa Cruz, and Monterey Counties, California

Surveys

Survey Planning

This survey was planned as one survey area, divided into 12 blocks (figure 1). The survey was completed in two field activities due to time constraints and other scheduled work for the mapping vessel. Block size was determined by survey line length, which was limited to about 5 km to keep file sizes manageable throughout the various processing steps. Block boundaries can be seen in the amplitude data where overlapping survey lines with different geometry leave traces in the backscatter.

Geodetic Control

Geodetic control for the two surveys was established using shore-based Global Positioning System (GPS) base stations broadcasting Real Time Kinematic (RTK) corrections to the survey vessel by way of UHF radio link. For all survey operations, base stations were scheduled to collect satellite observations and broadcast RTK corrections daily between 0500 and 2000 local time. GPS base stations were located on temporary control points established on fixed structures near the coast. The locations of the control points were selected to provide maximum RTK radio-signal coverage within the boundaries of the survey area. Due to the changing orientation of the coastline in the survey area, it was necessary to establish two control points in addition to a radio-signal repeater to achieve complete coverage. Horizontal and vertical positions of the control points were established by conducting static GPS surveys using both Ashtech and Trimble survey-grade GPS receivers. The GPS observations were post-processed using the National Geodetic Survey’s (NGS) Online Positioning User Service (OPUS) to achieve centimeter accuracy for the control point locations (appendix A). For standardization, all control-point coordinates were transformed to the 2002.0000 epoch using the HTDP software package created by the NGS.

For Block A and Block B of the survey area, a control point (“BGCK”) was established on a fixed structure on property owned by the Big Creek Lumber Company (figure 1). The structure, located on the edge of a bluff above Waddell Creek State Beach, provided an obstruction-free view of the sky for satellite observations and an expansive view of the northern survey area for line-of-sight RTK radio broadcasts. A fixed antenna mount was constructed to ensure that no accidental movement of the GPS antenna could occur. The base station and radio transmitter were powered using AC wall power to allow for continuous operation with minimal site visits by project personnel. Geodetic control for this point was established with a static GPS survey using an Ashtech Z-extreme survey grade GPS receiver. GPS observations were post-processed using OPUS to achieve an overall RMS error of 0.018 m. (see appendix A, GPS base-station datasheet, OPUS solution for BGCK survey, and HTDP transformation output).

For the southern part of the survey area including Monterey Bay, a control point (“WRIG”) was established on the roof of the Wrigley building, a multistory industrial building in Santa Cruz, Calif. (figure 1). The location provided a secure area for a GPS base station to be mounted with a good view of the sky and an ample horizon for RTK radio broadcasts. The GPS antenna mount was fastened to the roof of the building to ensure that no movement could occur during the surveys. All other equipment was housed inside the building and was powered using AC wall power. The base station was scheduled to collect satellite observations and broadcast RTK corrections daily between 0500 and 2000 local time. The geodetic control for this point was established by averaging position data from four static GPS surveys using a Trimble R7 survey grade GPS receiver. All GPS observations were post-processed using OPUS to achieve a mean overall RMS of 0.019 m. (see appendix B, GPS base-station datasheet, OPUS solutions for WRIG surveys, a table showing averages of coordinates, and HTDP transformation output).

In order to ensure full RTK coverage for the entire survey area, the radio broadcast of station WRIG was augmented using a repeater. The repeater consisted of a receiver tuned to receive RTK corrections on the broadcast frequency from base station WRIG and a transmitter set to broadcast the same corrections on a different frequency. The repeater was located at a private antenna installation on Mount Toro, at an approximate elevation of 1,030 m, approximately 30 km southeast of the southern extent of the survey area (figure 1). The repeater provided improved coverage in radio-link dead zones created by variable topography and changing coastline orientation in Monterey Bay.

Field activities S-7-09-MB and S-10-09-MB

The first bathymetric survey (S-7-09-MB) of the Monterey Bay area began on August 13, 2009 and was completed on September 3, 2009. The second bathymetric survey (S-10-09-MB) started on October 12, 2009 and was completed on December 16, 2009. For the first survey, the weather was mostly cooperative and conditions generally were good for marine surveying. No survey days were lost due to equipment malfunctions. During the second survey, several winter storms interfered with surveying for short periods. One survey day was lost owing to equipment failure.

About 512 square km of seafloor was surveyed during the two surveys. A total of 980 lines of data were collected during 59 survey days, covering a linear distance of about 3,280 km. Survey S-7-09-MB consisted of 16 days and 218 lines, and Survey S-10-09-MB was 43 days, with 762 lines collected. See figure 1 for the location of the survey area in and north of the Monterey Bay area, Calif.

The bathymetric surveys were done using a 234.5 kHz SEA (AP) Ltd. SWATHplus-M phase-differencing sidescan sonar. The sonar was pole-mounted on the 34-ft. USGS mapping vessel R/V Parke Snavely and affixed to a hull brace. RTK GPS position data were passed through a CodaOctupus F180 intertial measurement unit (IMU) to the sonar hardware and data-collection software. Sonar heads, GPS antennae, and the IMU were surveyed in place to a common reference frame with a Geodimeter 640 Total Station. The R/V Snavely was outfitted with three networked workstations and a navigation computer for use by the captain and survey crew for data collection and initial processing.

System specifications appear in Table 1:

For more information, contact David Finlayson.