During seismic surveying, a seismic source is repeatedly actuated and seismic receivers, such as hydrophones in marine seismic surveying, receive energy direct from the sources and reflected from various boundaries or interfaces. In the case of marine seismic surveying, energy propagates into the earth and is reflected back to the hydrophones from subterranean boundaries or interfaces, for instance between strata of different types. The recorded seismic data can be processed to reveal information about the structure of the earth in the area being surveyed. However, such reflections are contaminated by other reflection paths. For instance, energy from the sources is reflected at the sea surface directly to the hydrophones. Also, energy can be reflected more than once between the sources and the receivers. Such multiple reflections can take place within the earth. Also, energy initially traveling downwards from the sources can be reflected upwardly and then downwardly again from the surface of the sea before arriving at the hydrophones. Reflections of this type are referred to as "free-surface multiple reflections". Free-surface multiple reflections can be classified according to their order, which is equal to the number of reflections from the free-surface. Thus, first order free-surface reflections comprise energy initially traveling downwardly from the sources (as opposed to "ghosting" where energy travels upwardly and is reflected from the free-surface), is reflected upwardly from the sea bed or a boundary below the sea bed, and is then reflected downwardly from the free-surface to the hydrophones. Second order free-surface multiple reflections undergo two downward reflections from the sea-surface before being detected by the hydrophones, and so on.
The invention makes use of a general concept of multiple attenuation using the inverse scattering or scattering Born series. For this general concept reference is made to the published International patent application WO 95/10787 and the British patent application No. 9426255.7 (published as GB-A-2296567).
From those documents it is known how to derive the scattering Born series for removing free surface multiples: EQU D.sub.P =D.sub.0 +AD.sub.1 +A.sup.2 D.sub.2 +A.sup.3 D.sub.3 + . . . ,[1]
wherein D.sub.P denotes the signals without free-surface multiples and A denotes the inverse of the source signature. The first term of the scattering series, D.sub.0, is the actual seismic data, the second term, D.sub.1, removes first order free-surface multiples, the following term, D.sub.2, removes second order free-surface multiples, and so forth.
It is an object of the present invention to improve and utilize the above method for sea bottom seismic acquisition operations.