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Aside all geophysical, geotechnical and offshore applications dealing with deep penetration in sea bottom (up to several kilometres), sub-bottom imaging for searching for objects deals with shallow sediment imaging (a few meters). Shallow penetration will lead to compromise frequency of a few kilohertz. Such an imaging is of interest in both military and civilian application domains: buried mine hunting, boulder detection, buried cables and pipelines as well as offshore industry components and underwater archaeology. The sedimentation of the sediment is highly variable between a recently buried object and the old buried ones. This will lead to very different properties of covering sediments. All these applications require a penetration of a few meters in the sediment and of a few centimetres in resolution. Synthetic aperture sonar, SAS, is a good candidate for such a situation as it breaks the link between the resolution and the array size. Nevertheless, the presence on the sediment may lead to a defocusing effect due to velocity changes (depending on the sedimentation history). First this defocusing effect will be investigated and several successful scenarios will be given for various situations. The second limitation of sub-bottom imaging is that one has to deal with volume reverberation (in opposition to surface reverberation in conventional sidescan sonar). Echoes from surface and volume reverberation may, of course, overlap in both time and angle. Thus, one will need to discriminate interface echoes from sub-bottom ones. Dual-band systems can be the solution for that problem. Results of dual-band imaging (cantered around about 10 and 100 kHz) will be shown with two geometries: conventional sidescan geometry and planar SAS).
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