2. Imaging of the Near-Surface

Acoustic sounding of the seabed and its immediate sub-surface produces different types of returns, based on aspect, roughness and physical characteristics of the different targets. This is presented within both seismic (low frequencies) and sonar (high frequencies) contexts. Both authors describe their own studies in controlled laboratory environments and at sea, contrasting larger-scale benthic habitats with lower-scale buried targets. The key results are then compared with existing research in acoustic Computed Tomography. Each example builds up to a set of recommendations, about the source(s) and signals; about the receivers and their spacing; about the hardware (constrained by the harsh realities of underwater environments); and about the requirements of bespoke signal processing drawing on a large range of independent techniques. The relative merits of standard projectors and parametric arrays are discussed and the important roles of acoustic attenuation within the seabed and non-specular returns are introduced.