An examination of a dredged channel using sector scanning sonar in side-scan mode

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Abstract
In October 1974, a sector scanning sonar adapted for operation in side-scan mode was used to examine a channel dredged in 1971 within the sandwave field at Longsand Head (Outer Thames Estuary). Trials of the system were successful. The range and resolution of the records obtained were comparable with those of EG & G side-scan sonar but the provision of automatic gain control permitted the detection of seabed features which had hitherto been unresolved by side-scan sonar. Resurvey of a channel dredged through the sandwave field off Longsand Head showed that sections of the major sandwaves removed by dredging had shown only a limited tendency to reform in the 3-year period since the close of dredging operations. A system of dunes (5 m wavelength) had developed in the fine infill material which covers the floor of the channel. Intermittent noise signals due to sediment movement were recorded by the modified scanner system during each survey leg and provide some qualitative impression of the patchiness of sediment movement on sandwave crests.

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