Sub-bottom profilers

A sub-bottom profiler is effectively an echosounder that transmits a relatively low-frequency acoustic pulse that can penetrate the seabed. This signal is reflected off sub-surface boundaries between sediment or rock layers that have different acoustic impedance, which is related to density and sound speed within each layer (Figure 1). The strength of the reflected signal depends on the degree of impedance contrast. The returning sound waves are recorded by an array of hydrophones (towed behind the vessel), or by a transducer/transceiver, depending on the type of system. The first useful signal received represents the seabed-water interface, and shows the morphology of the seabed in a manner similar to a single beam echosounder. The time of arrival and intensity of subsequent impulses provides information about layers that exist below the seabed.

Several physical parameters of the acoustic signal emitted, such as output power, signal frequency, and pulse length affect the performance of the instrument and influence its usefulness in various marine environments. Increased output power allows greater penetration into the substrate. However, in the case of harder seabeds (e.g. gravels or highly compacted sands) or very shallow water, higher power will result in multiple reflections and more noise in the data. Higher frequency systems (up to 20 kHz) produce high definition data of sediment layers immediately below the seabed, and are able to discriminate between layers that are close together (e.g. 10’s of cms). Lower frequency systems give greater substrate penetration, but at a lower vertical resolution. Longer pulse length transmissions (or ‘pings’) yield more energy and result in greater penetration of substrate. However, they decrease the system resolution. The depth of penetration also depends on the hardness of the upper layers and is significantly limited by the presence of gas deposits.

Figure 1. Deployment of various shallow-water sub-bottom profiling systems. After Stoker et al.
(1997). [1][2]

1. Penrose J D, Siwabessy P J W, Gavrilov A, Parnum I, Hamilton L J, Bickers A, Brooke B, Ryan D A and Kennedy P: Acoustic techniques for seabed classification, September 2005, Cooperative Research Centre for Coastal Zone Estuary and Waterway Management, Technical Report 32
2. Stoker, M.S., Pheasant, J.B., Josenhans, H. (1997). ‘Seismic methods and
   interpretation.’ In T.A. Davies, T. Bell, A.K. Cooper, H. Josenhans, L.
   Polyak, A. Solheim, M.S. Stoker, J.A. Stravers (Eds.), Glaciated Continental
   Margins: An Atlas of Acoustic Images. Chapman and Hall, London, 1997.
   315 pp.