Shallow Water Sub-bottom Data
Acoustic sub-bottom profiling systems are used to determine physical properties of the seafloor and to image and characterise geological information a few metres below the seafloor. In recent years, acoustic methods have been used also to measure small scale sedimentary structures and processes in high temporal and spatial resolution. The systems have been widely adopted by marine researchers because of their ability to collect data rapidly and non-intrusively. Sub-bottom profiling systems can provide information on sedimentary environments which is valuable in developing models of benthic environments and habitats.
Sub-bottom profilers are usually comprised of single channel source that sends sound pulses into the shallow sub-seafloor sediments. The sound pulses bounce off different rocks according to differences in their acoustic impedance (hardness). The different times taken for this signal to be returned and recorded by the sub-bottom profiler indicates where the rocks are below the seafloor. The surface of the different rock strata beneaath the seabed are mapped over the study area.
There are a number of shallow sub-bottom profiler systems which operate at different frequencies. Different sub-bottom profiler systems are used depending on the objectives of the survey, water depths and prior knowledge of the rock types (if known). The 'Pinger' is a high frequency system which operates on a range of single frequencies between 3.5 kHz and 7 kHz. It can achieve seafloor penetration from just a few meters to more than 50 m, depending on the type of sediment. It is capable also of resolving stratigraphic boundaries down to a thickness of approximately 0.3 m. High frequency profilers are particularly useful for delineating shallow features such as faults, gas accumulations and palaeochannels. The 'Boomer' is a lower frequency system which usually operates from 500 Hz to 5 kHz and typically can penetrate to between 30 m and 100 m with a vertical resolution of 0.3 m to 1 m. The 'Sparker' system has an even lower frequency and can penetrate sediment and rocks down to around 1000 m in ideal conditions. The 'Chirp' sub-bottom profiler operates around a central frequency which is swept electronically across a range of frequencies, usually between 3 kHz to 40 kHz. It has similar sub-bottom penetration to the 'Pinger' system.