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U.S. Geological Survey Open-File Report 2010-1030

Geophysical Surveys of the San Andreas and Crystal Springs Reservoir System including Seismic-Reflection Profiles and Swath Bathymetry, San Mateo County, California

Surveys

Chirp Seismic Reflection Survey

A high-resolution seismic-reflection survey in San Andreas Reservoir, San Mateo County (USGS cruise ID F-1-07-CA) was conducted June 12-14, 2007, (Figure 1) with the goal of imaging the uppermost few tens of meters of subbottom geology beneath the reservoir floor to show locations of offset or disrupted strata. The data were acquired using an Edgetech 512i Subbottom Profiling system towed along-side a 23-ft Boston whaler at an approximate survey speed of 3.5 km/hour. The towed instrument sled contained both the piezo-electric chirp seismic source and a short array of receiving hydrophones. During data acquisition, a high-frequency chirp pulse (30 ms length, 700 to 12,000 Hz bandwidth) was emitted 8 times per second, and the returning reflected signal was digitally recorded to create two-dimensional subbottom image profiles. Seventy-nine profiles were collected and are contained in sixteen files in standard geophysical SEG-Y format and are available for download from the Data Tables page. Differential GPS locations were recorded in the SEG-Y files in the standard trace header locations as milliseconds of arc.

Swath Bathymetry of San Andreas and Upper and Lower Crystal Springs Reservoirs

The bathymetric survey of San Andreas Reservoir (USGS cruise ID F-2-07-CA) began on November 27, 2007, and was completed on November 30, 2007. The bathymetric survey of Upper and Lower Crystal Springs Reservoir (USGS cruise ID F-1-08-CA) started on April 7, 2008, and was completed on April 15, 2008, excluding the weekend of April 12-13, 2008. For both surveys the weather was cooperative throughout, and conditions were good for marine surveying. No survey days were lost to equipment malfunctions.

About 5 km2 of lake bottom were surveyed over the course of 8 days. The San Andreas lake survey consisted of 30 track lines covering 1.63 x 106 m2; Lower Crystal Springs consisted of 66 track lines covering 1.95 x 106 m2; and Upper Crystal Springs consisted of 65 track lines covering 1.81 x 106 m2. See Figure 1 for the location of the reservoirs in San Mateo County, California.

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 a 25-ft Boston Whaler outfitted with a mobile electronics lab on the aft deck and powered by a portable generator. The small size of the boat lead to platform stability problems during the survey, however, a larger boat would have been impractical due to the size and condition of the available boat launches.

System specifications appear in Table 1.

Table 1. SWATHplus-m Sonar Specifications (SEA Ltd., 2004).
SpecificationValue
Sonar frequency234 kHz
Maximum water depth100 m
Maximum swath width300 m (typically 7 to 12 times water depth)
Resolution across track (best case)5 cm
Transmit pulse length34 µs to 500 µs
Ping repetition rate
150-m swath width10 pings per second
300-m swath width5 pings per second
Vertical accuracy (range dependant), in meters
  570.1
1140.2
1710.3
For more information, contact David Finlayson.

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