Seismic reflection profiles are acquired by means of anacoustic source (usually generated electromagneticallyor with compressed air), and a hydrophone or hydrophonearray. Both elements are typically towed in the waterbehind a survey vessel. The sound source emits a shortacoustic pulse, which propogates through the water andsediment columns. The acoustic energy is reflected atdensity boundaries (such as the seafloor or sedimentlayers beneath the seafloor), and detected at the hydro-phone. This process is repeated at intervals rangingbetween 250 milliseconds and 10 seconds dependingon the source type. In this way a two-dimensional imageof the geologic structure beneath the ship track isconstructed.Seismic data are stored in SEG-Y format, which is astandard digital format that can be read and manipulatedby most seismic-processing software packages. TheSEG-Y file format includes a 3200-byte descriptive headerthat should contain detailed information regarding thedata acquisition and processing parameters.The navigation data are stored in flat ASCII files aslatitude and longitude values that can be used to plottrack maps of the seismic profiles at any scale or mapprojection desired. The data and format of the navigationfiles are described in the file prologue.
Marine seismic reflection data are used to image andmap sedimentary and structural features of the seafloorand subsurface. These data are useful in mappingfaults (such as the San Andreas and Hayward Faults)where they pass under the waters of the San FranciscoBay, and in assessing other submarine geologiccharacteristics and features.
Neither the U.S. Government, the Department of the Interior, nor the USGS, nor any of their employees, contractors, or subcontractors, make any warranty, express or implied, nor assume any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, nor represent that its use would not infringe on privately owned rights. The act of distribution shall not constitute any such warranty, and no responsibility is assumed by the USGS in the use of these data or related materials. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.