Fast Deterministic Designature/Demultiple Of 2D And 3D 4C-OBS Data

Abstract

Abstract Four-component ocean bottom seismic (4C-OBS) surveying, in contrast to conventional towed streamer acquisition, records the complete seismic wavefield. When developing seismic processing tools for 4C-OBS data one therefore can liberate one's mind from the line of thought used in the development of seismic processing methodology for streamer seismic. In particular, using the complete wavefield source signature deconvolution and the troublesome problem of attenuation of sea-surface related multiples can be specially designed for 4C-OBS data. The attenuation of these events is an essential prerequisite for an accurate seismic imaging. In 2001, Amundsen et al. published a fully data-driven method that without any information transforms, during processing in a computer, the recorded 4C-OBS data with the sea surface present into those data that would be recorded in a hypothetical 4C-OBS experiment with the sea surface absent. Removing the sea surface is equivalent to removing all seasurface related multiples. Further, this method automatically designatures the 4C-OBS data. Under the model assumption of a layered earth, the designature/demultiple method reduces to deterministic deconvolution where the deconvolution operator is designed from the inverse of the downgoing acoustic field. This assumption, however, does not severly limit the method's practical use in attenuating free-surface related multiples even in geologically quite complex areas. Further, this designature/demultiple scheme is the only method in the class of free-surface demultiple methods that straightforwardly is implemented in 3D for removing free-surface multiples in today's geometries of 4C-OBS surveys. The method, which is numerically fast, is implemented by several seismic contractors. During the presentation we show results of designature/demultiple processing on half a dozen 4C-OBS deep-water and medium-water depth exploration lines. Introduction Soon after the introduction of four-component ocean bottom seismic (4C-OBS) technology (Berg et al., 1994a,b; Ikelle and Amundsen, 2004) efforts to develop 2D and 3D wave-equation prestack depth imaging intensified (Amundsen et al., 2000; Arntsen and Røsten, 2002). To obtain accurate and detailed imaging, however, attenuation of multiple energy while preserving the character of primaries is required. Through the 1990's till today one has seen many excellent developments of wave-equation based free-surface demultiple algorithms for 2D and 3D streamer seismic and land seismic data (see, e.g., Fokkema and van den Berg, 1990; Verschuur et al., 1992; Matson and Weglein, 1996; Matson, 1997; Weglein et al., 1997; Ziolkowski et al., 1999; Lokshtanov, 1999; Ikelle, 1999a,b; Ikelle et al., 2003; Kleemeyer et al., 2003; Hokstad and Sollie, 2003). The attractiveness of these methods is that they do not require any information about the subsurface. A possible disadvantage is that these methods require information of the source signature. Further, the streamerseismic demultiple methods can not straightforwardly be adapted to 4C-OBS data, in particular not for 3D ocean bottom seismic surveys. Amundsen (2001), Amundsen et al. (2001), and Holvik (2003) published an alternative way of removing free-surface multiples in the case that the complete wavefield is recorded in the seismic experiment. For streamer seismic, the complete wavefield on the streamer is the pressure field and the vertical component of the particle velocity (or vertical pressure gradient).