Seismic reflection profiles are acquired by means of an acoustic source (usually generated electromagnetically or with compressed air), and a hydrophone or hydrophone array. Both elements are typically towed in the water behind a survey vessel. The sound source emits a short acoustic pulse, which propogates through the water and sediment columns. The acoustic energy is reflected at density boundaries (such as the seafloor or sediment layers beneath the seafloor), and detected at the hydro- phone. This process is repeated at intervals ranging between 250 milliseconds and 10 seconds depending on the source type. In this way a two-dimensional image of the geologic structure beneath the ship track is constructed. Seismic data are stored in SEG-Y format, which is a standard digital format that can be read and manipulated by most seismic-processing software packages. The SEG-Y file format includes a 3200-byte descriptive header that should contain detailed information regarding the data acquisition and processing parameters. The navigation data are stored in flat ASCII files as latitude and longitude values that can be used to plot track maps of the seismic profiles at any scale or map projection desired. The data and format of the navigation files are described in the file prologue.
Marine seismic reflection data are used to image and map sedimentary and structural features of the seafloor and subsurface. These data are useful in mapping faults (such as the San Andreas and Hayward Faults) where they pass under the waters of the San Francisco Bay, and in assessing other submarine geologic characteristics and features.