Anomalies and artifacts

The previous chapters have shown how sidescan sonar data were acquired, how they were (or should be) processed and interpreted, and examples have been shown at all depths and in all underwater environments so far studied with sidescan sonar. Sidescan sonar imagery, like any data, is rarely devoid of anomalies and artifacts. They may be easy to spot or mistake for real features, and they may be difficult to interpret or remedy. The present chapter aims at showing most sources of errors and artifacts, how they can be avoided during processing, and how to recognize and interpret them when they do occur. This will be demonstrated by drawing both on the most recent theoretical studies on the subject, and on real-world examples from a variety of applications. The reader may also find it profitable to look at a short (but now somewhat dated) publication from EG&G Marine Instruments (

Fish and Carr, 1990

)

1

which presents sidescan sonar operations in very shallow water, mainly for the detection of man-made structures. The different sections of this chapter follow the acoustic wave from transmission to reception and processing. This includes propagation through the water column, backscattering toward the sonar platform, processing, and final interpretation. All these stages are prone to errors and artifacts; some of them are unavoidable, but all of them should be recognizable.