Acoustic penetration of the seabed, with particular application to the detection of non-metallic buried cables

Damaged submarine fibre optic telecommunication cables are located by the use of remotely operated underwater vehicles. These are fitted with sensors which have the capability to detect the metal shielding in the cable cores. However, it is anticipated that the next generation of cables will have a much reduced metal content, their strength being derived from synthetic materials such as aramid fibres. This will make them practically invisible to present day detection systems.To this end, research has been conducted into the remote detection of a range of buried objects having a 'low acoustic contrast' using, primarily, acoustic techniques, with particular emphasis being given to the detection of buried cables. This involved the design of a system to reliably detect a small diameter (cm scale), cylindrical target buried to a depth of up to 1 metre below the seabed from a height of 1 metre above the seabed at a sea depth of 1 000m.A purpose-built, laboratory-scale, automated sensing system comprising a bistatic arrangement of adjustable, focused transducers was constructed. This was used successfully in the high resolution imaging of a range of buried objects, proving an acoustic detection system to be a feasible solution. The experimental investigation focused on the areas of waveform optimisation, optimal filtering and clutter reduction, with the signal processing system being optimised to achieve maximum separation of the target signal from the background.The lack of an experimentally verified model to describe the transmission of acoustic waves into the seabed was a complicating factor. Therefore, the propagation of compressional waves into and within a sandy sediment was investigated. This study was of particular academic interest in light of recent work in which anomalous acoustic penetration of seabed, where the grazing angle in the water column was sub-critical, has been observed. An experimental investigation showed that scattering from surface roughness is a viable explanation for this phenomenon.