Faeroe Sub-Basalt Seismic Imaging - a New Iterative Time Processing Approach

Abstract

Converted wave terminologyConversion occurs each time a wave impinges at non-normal incidence on an impedance contrast, and is accompanied by a partition of energy between different wave types (P, S, reflected, transmitted). Some of the possible conversions have such a low energy content that they can be considered negligible for seismic imaging purposes. Moreover, marine acquisition with a towed streamer requires that a P wave must arrive at the receiver. Among the many different types of waves analysed, some conversions appear to be particularly attractive:• PSSP: means P wave down to top basalt, conversion to S wave at top basalt, S propagation and reflection in subbasalt layers up to top basalt, conversion to P wave at top basalt and propagation up to receivers. This is the symmetrical converted wave (for a 1D layered medium, the downgoing and upgoing travel paths have the same wave mode travelling in the same layer) with the minimum number of conversions. Symmetrical waves are very interesting, because they allow the use of traditional processing tools developed for PP waves. • PSP: means P wave down to an impedance contrast, S wave reflected at the contrast up to sea bottom, conversion to P wave at sea bottom and propagation to

A study of sub-basalt depth imaging using the local Radon attributes on the Erlend Tertiary volcanic complex - North of Shetland, UK

“…Once V stac^ has been determined during pre-processing, the pre-processed subsets are migrated several times, using conventional Kirchhoff PSTM (e.g. Robein & Hanitzsch, 2001) for major wave types identified during pre-processing, as outlined by Barzaghi et al (2002). We begin with a smooth 2D model for which we search the principal reflections after PSTM has been applied with the velocity V m i g = V s t a ck-This leads in general to non-flat image gathers and requires additional (e.g.…”

“…According to the elastic migration principle (Kuo & Dai, 1984), we migrated data using a combination of velocities and event enhancement amplitude weights suitable for each event. This makes it possible to avoid misprocessing and misinterpreting non-PP arrivals as primary PP reflections, without applying labourintensive pre-processing that involves the use of handpicked mutes or interactive interpretations based on calculations of the traveltime response (Barzaghi et al, 2002;Van der Baan et al, 2003). In event-based migration with accurate velocities, migrated energy from assumed wave-propagation modes coincides with the actual reflector, although different parts of this reflector are imaged with each event (Kuo & Dai, 1984).…”

“…The precise mechanism for such noise varies from case to case (Martini & Bean, 2002;Maresh & White, 2005). Preliminary multi-wave-type or eventbased imaging, illumination and forward modelling studies (Purnell, 1992;Barzaghi et al, 2002;Van der Baan et al, 2003) have been particularly encouraging, suggesting the possibility of imaging beneath HVLs using converted waves generated at the interface between formations with strong velocity contrast. Traditionally, wide-angle data have been used to derive a starting large-scale velocity model by traveltime inversion (Fruehn et al, 2001).…”

Multi‐mode pre‐processing of long‐offset marine streamer data ‐ Faeroes‐Shetland Basin

“…The large impedance contrast between basalt and the surrounding sediments also generates strong converted waves (Purnell, 1992). These waves may contain more energy than the standard PP reflections, especially at long offsets (Barzaghi et al (2002). Hence, the challenge of multi-mode data preprocessing is to separate various wave modes associated with high impedance contrast interfaces and to remove unwanted arrivals such as water peg-leg multiples and converted PS waves from TB that often interfere with target reflections.…”

Improvement of sub‐basalt imaging using parabolic tau‐p transformation

“…Recent investigations have shown that wide-angle acquisition (e.g., Fliedner and White, 2001;Fruehn et al, 2001) combined with a broad-band, low-frequency source (White et al, 2002) has the potential to overcome some of the aforementioned difficulties. Further, development of processing techniques based on modeling (e.g., Hansen et al, 2001) and converted wave energy (e.g., Barzaghi et al, 2002) enhances our ability to extract sub-basalt information from seismic reflection data.…”