Chirp seismic-reflection data from field activity L-1-06-SF collected offshore Bolinas to San Francisco, California on 2006-09-30

Chirp seismic-reflection data were collected using an Edgetech 512 Chirp subbottom profiling system consisting of a source transducer and an array of receiving hydrophones housed in a 500-lb “fish” towed at a depth of several meters below the sea surface. The swept-frequency “chirp” source signal is between 500 and 4,500 Hz, and the data are recorded by hydrophones located on the bottom of the fish and recorded in SEG-Y 32-bit floating point using a Triton SB-Logger. 15 lines were collected and line “01” was used as a test line and was not recorded. GPS position information was inserted into the SEG-Y trace headers in header bytes 73 and 77, with a scalar of -1000 (noted in header byte 71).

Chirp seismic-reflection SEG-Y files consist of the internally transformed amplitude envelope channel corrected for the depth of the fish below the sea surface using Paradigm Geophysical Echos seismic processing software. 1) SEG-Y channel with sequence number “1” was written to a SEG-Y file; 2) The fish depth header word (byte location 49 in SEG-Y file) was read and converted from millimeters to two-way travel time to create a fish-depth static value; and 3) The fish depth static was applied to the data to correct for the vertical up and down movement of the Chirp fish. UTM 10N x and y position data were inserted into SEG-Y header bytes 81 and 85. Additionally, ship-heave statics were calculated using SIOSEIS seismic processing. The statics were created with a seafloor auto-tracker to detect the seafloor two-way travel time for each shot. This tracker defaults to the time value of the previous shot on low amplitude traces. Corrections were applied to the data to filter out the effect of the sea swell. A seafloor mute was applied before the final processed lines were output in SEG-Y format.

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