Reducing Seismic Multiples in Very Shallow Water Archaeological Prospection

Summary The geophysical investigation of ancient harbour sites often includes the shallow water area close to the shore. In this environment, hydroacoustic methods are the methods of choice as they provide high resolution in the decimetre range with corresponding penetration depths of several meters. However, in shallow water areas multiple reflections are known to interfere with primary signals reflected by structures of interest. To overcome the problem of water layer multiples, we implemented and tested two model-driven approaches to remove water bottom multiple reflections, which are based on the forward modelling of seismic wave field including multiples. In the first approach, water column multiples are simply modeled and subtracted from the field data, the second approach takes advantage of reverse time migration and a deconvolution imaging condition. The methods are applied to synthetic and real multi-offset data in very shallow water. Both tested methods are able to reduce the signal amplitude of the water bottom multiple by up to ~85 %, if the reflection coefficient at the seafloor is determined with errors of less than 20 %.