St. Petersburg Coastal and Marine Science Center
Seismic Profiles or Seismic Reflection Surveys can be used to complement streaming resistivity, feature identification, and geologic framework.
Three types of seismic sub-bottom profilers are used by the USGS.
Chirp towfish; Shallow multi-frequency seismic (CHIRP) system to identify shallow stratigraphic layers. From: http://pubs.usgs.gov/ds/2007/254/#Anchor-29169
A seismic survey can be used to map details of sediment deposits and geologic structure below the seafloor. Similar to an acoustic depth profiler, the seismic profiling technique passes sound waves from a towfish through the water column and into the seafloor. The return signal, or pulse, is detected by a hydrophone and relayed to a computing system where it is measured and plotted as a two-dimensional diagram parallel to the ship's track. The computer displays the two-way travel time (TWTT) of the acoustical pulse in milliseconds from which depths to stratigraphic horizons can be determined. Differences in the underlying sediments appear as deviations in the amplitude and velocity of the signal. Sand, clay and carbonate sediment layers each have different velocity ranges. Consistent amplitude changes appear as continuous horizons on the seismic profiles. Logs from wells or sediment cores can be helpful for interpretation by ground-truthing the seismic reflections. Note the lateral (*) and vertical (*) discontinuities in the example.
Seismic profiles are important to marine mapping efforts because they allow scientists to "see" below the seabed surface. This allows them to detect potential mineral resources, identify potential hazards and aquifer links, and to determine the influence of geology on seafloor habitats and resources. Seismic surveys are particularly useful in Florida to identify sand deposits and buried sinkholes (see example profile above). Marine seismic reflection data are used to map sedimentary and structural features of the seafloor and subsurface. The two-dimensional seismic reflection data are useful in identifying underlying layers of sediment and rock. Scientists can assess submarine geologic features for special applications such as mapping sand deposits or identifying fractures and collapse features.
USGS equipment: Digital CHIRP EdgeTech sub-bottom profiler SB-424 and SB 512i with Triton Elics SBLogger (FSSB) acquisition software; Applied Acoustic CSP-1000, LVB Boomer System, ITI hydrophone ST-5, Benthos Mesh hydrophone with Triton Elics SBLogger seismic software and SBPWhiz seismic software; Knudsen 320BP 28/200 KHz bathymetric system.