Application of pre-stack wave equation datuming to remove interface scattering in sub-basalt imaging*

In recent years sub-basalt imaging has been a problem of general interest in the petroleum industry. Sub-basalt images are often known to be of very poor quality. Conventional reflection seismic methods are usually not very successful at imaging either the internal flow structure of the basalt or the sub-basalt structures. The ability to image at depth is critically dependent on transmitting enough energy to the target. The matter has often been related to high velocity in the basalt layers, which yields high impedance contrast and a subsequent limited penetration of the P waves. It has recently been seen that this is not the major controlling factor in the imaging problem. First of all, basalts very often do not have such a high velocity, and even when they do, sufficient seismic energy is usually transmitted and propagates through basalt (e.g. O’Doherty & Bean 1999; Planke & Eldholm 1999). Here we show that the imaging problem is due to a combination of factors, amongst which the internal and interface structures of flood basalt play an important role. Both the heterogeneous nature of basalt interfaces and its internal structure strongly affect sub-basalt seismic imagery. The role of wave scattering in seismic imagery is well documented in the literature (Gibson & Levander 1988; Pullammanappallil et al. 1997; Martini et al. 2001) as well as the effects of irregular interfaces on wave propagation (Paul & Campillo 1988; Purnell et al. 1990; Hestholm & Ruud 2000; Ruud & Hestholm 2000). Lava cake geometry, intra-lava velocity variations and inter-layering between lava and sediments create a very heterogeneous structure that attenuates, scatters and multiply reflects the energy transmitted into the layers. Some of these mechanisms have been evoked, either separately or together, to account for poor sub-basalt imagery. In heterogeneous media, scattering occurs as a result of the interactions of seismic waves with spatial variations in the material properties of the medium, variations that range in size from several seismic wavelengths to a small fraction of the wavelength. The scattering regime depends upon the relationship between the propagating wavelength and the scale of the heterogeneity. Scattering by heterogeneities affects a number of seismic observables, including amplitudes, traveltimes, spectra and waveforms. Another important consideration is the ratio between the thickness of the heterogeneous layer and the wavelength propagating through it. To facilitate the generation of a significant amount of body wave scattering, the layer has to be thick enough to enable the propagation of a few cycles through it, notwithstanding the fact that high frequencies are preferentially scattered (Leary 1995). This reasoning may go some way toward explaining the conclusions that many authors have reached that low frequencies are better than high frequencies in sub-basalt exploration. With low frequencies (long wavelengths) less cycles travel into the heterogeneous layers, and the long wavelengths miss the small scale variations in the media. Therefore, the wavefield is less scattered. We emphasize the importance of this factor as scattered waves can seriously contaminate lower structures. Scattering in highly heterogeneous layers ensures that waves continue to be recorded at time and depths corresponding to lower layers, which are then not properly imaged due to this contamination. The degree of this contamination is controlled by the ‘size’ of heterogeneity in the layer, and the wavelength propagating through it. For ‘thick’ layers, where internal body wave scattering is a major cause of image contamination, migration and velocity control have been shown to be of extreme importance (Martini et al. 2001). Basalt boundary surfaces are not expected to be smooth but to contain roughness at a range of scales, including the seismic wavelength. Again, the scale of corrugations with respect to incident wavelength is the important factor. Irregular interfaces have a disruptive effect on wave propagation. So, both internal and interface scattering contribute substantially to the problem, one or the other being the dominant factor according to a balance between different parameters, such as basalt layer thickness, nature of the interfaces and seismic wavelength. Numerical investigations indicate that interface scattering dominates over body scattering in the sub-basalt imaging problem (Martini & Bean, in press). In this work we concentrate on the interface aspect. As a possible solution to the detrimental effect that interface scattering can have on deeper reflector imaging and continuity, a wave equation based pre-stack datuming has been tested, both on synthetic and real field data.