Imagery and bathymetry

The previous chapter (Chapter 2) presented basic acoustic theory, from the generation of a sound wave to its scattering on/in the seabed and its reception by the sonar platform. Acoustic pressures are used to quantify the response of the seabed at the particular frequency used. These backscatter measurements are combined across-track to create a swath, and successive swaths are combined to form an image of backscatter variations. If the sonar is calibrated, these variations can provide generic measurements, called backscatter strengths, comparable with results from other surveys (with the same sonar at the same settings, or not). These processes are detailed in Section 3.2. Sidescan sonar imagery can be usefully compared and merged with bathymetry measurements. The newest generation of sidescan sonars can now produce imagery and bathymetry at the same time, using interferometry (explained in Section 3.3). These measurements of the seafloor topography are at the same resolution and co-registered with the measurements of the acoustic response of the seafloor. The large majority of sidescan sonars currently in use do not have interferometric capabilities though, so bathymetry is derived from other sources, like multibeam sonars. These measurements usually do not have the same ground resolution (e.g., 100m multibeam bathymetry and 6 m sonar imagery), and rarely benefit from the same exact geographic registration requiring the use of techniques like rubbersheeting (presented in Chapter 4). The problem is compounded by the fact that new multibeam systems also provide some forms of imagery, calculated by analysing each beam or only portions centered on the irst seabed return.