Publication Abstract

An acoustic backscatter instrument to measure near-bed sediment processes

A.M. Smeron, J.M. Rees and C.E. Vincent

Acoustic backscatter (ABS) measurement is a non-intrusive technique for the monitoring of water-suspended sediment particles and changing seabed characteristics. An acoustic backscatter instrumentation package comprising acoustic sensors, data acquisition, storage and control electronics, and data extraction and reduction software, is presented.

The sensor comprises acoustic transducers that emit pulses of sound, which are incident on the seabed. They receive sound reflected by the sea bed and suspended sediment in the intervening water mass. The instrument records the amplitude of reflected sound at gated intervals, thus building are flected sound profile. With low angles of incidence, the technique may be used to monitor the formation and progress of seabed ripples. Perpendicular incidence angles will yield information on sediment suspension between the sensor head and the sea bed, and on the erosion or accretion of the bed level. The paper includes a discussion on the theoretical principles behind instrument calibration. Calibration is generally carried out under laboratory conditions in a turbidity tank using either samples of sea bed material from the deployment site or samples of material collected from suspended sediment sample tubes called Booner tubes. Calibration takes account of the different particle resolutions ofthe three acoustic frequencies and the non-linear acoustic response over the profile length. The recorded data may then be analysed to determine actual particle concentrations and size for the given sand or mud mixture.

Recent ABS data from their minipod lander deployments by the Directorate of Fisheries Research show storm resuspension at Tees Bay, north east England. On 19 February 1996, at the height of a large storm (significant wave height 4.8 m), optical sensors were fully saturated leaving only acoustic methods of estimating siuspended sediment concentrations. Individual groups ofwaves were recorded, with a crest to trough height of 5.5 m, resulting in wave orbital velocities of 1.68 m/s at the seabed (mean water depth 31 m). Suspended sediment concentrations from the Acoustic Backscatter system reflect the wave groups.

Reference:

A.M. Smeron, J.M. Rees and C.E. Vincent, 1998. An acoustic backscatter instrument to measure near-bed sediment processes.nternational Ocean Systems Design, 2: 5