IMAGING THE AFEN SLIDE FROM COMMERCIAL 3D SEISMIC METHODOLOGY AND COMPARISONS WITH HIGH-RESOLUTION DATA. J. BULAT British Geological Survey, Murchison House, West Mains Road, Edinburgh EH9 3LA, UK Abstract The Afen Slide lies in deep waters within the Faroe Shetland Channel (FSC) and was first recognised on TOBI data in 1996. Subsequently, it was observed on a 3D seismic survey that had been acquired in 1995. This paper presents the latest image of the slide generated from 3D seismic and the methodology used in attenuation of geophysical artefacts. It is demonstrated that 3D seismic has the potential to produce highly detailed images of seafloor features in deep-water areas comparable with swath or TOBI. Keywords: Submarine slide, 3D seismic, imaging, Faroe Shetland Channel Figure 1. Location of the Afen Slide superimposed on bathymetry map (in metres) of the Faroe- Shetland Channel. 1. Introduction The Afen Slide is located 95 km northwest of the Shetland Islands in water depths of 830-1120m (Wilson et al. 2002). It was first recognised on Southampton Oceanographic Centre s dual 32kHz sidescan sonar Towed Ocean Bottom Instrument (TOBI) data acquired for the Atlantic Frontiers Environmental Network (AFEN) in 1996 (Masson, 2001). Figure 1 is a sketch map indicating the location of the slide and Figure 2 is part of the TOBI data over the slide. The TOBI survey was conducted with a notional swath 205
206 Bulat width of 3km each side of the towed vehicle and a total swath width with overlay of 5 to 5.5km in water depths from approximately 800-1200m. The slide scour attains a maximum width of 3km. The length of the scour and debris lobe combined is 12 km. The TOBI image shows the debris fan and part of the scour well, but does not identify the head wall in its entirety. This is due to insufficient data coverage as a result of the wide track spacing. As the only major slide in the FSC it has been the focus of much study (Bulat and Long 1997, Holmes et al. 1997, Holmes et al. 1999, Masson 2001, Wilson et al. 2002). Figure 2. Part of a TOBI image over the Afen Slide. Data supplied by Dr. D.G. Masson, SOC. This paper presents the latest version of the Afen Slide image and the methodology used for the isolation and attenuation of survey footprint effects on the seabed event. Comparisons with high-resolution data are also presented that illustrate the limits in vertical and spatial resolution of such images. NW SE Headwall of slide Toe of debris slope Figure 3. Arbitrary line along the axis of the Afen Slide from the 3D seismic volume used to generate the images presented in this paper. The location of the line is shown in figure 4. Note the subtle nature of the slide s main features on the vertical profile.
Imaging the Afen slide 207 2. Imaging using 3D seismic surveys In 1995, Shell U.K. commissioned a 3D survey for hydrocarbon exploration that was subsequently found to cover the area of the Afen Slide. As the intended exploration target was deep, temporal resolution and sampling is low (4ms sample rate and a dominant frequency at the seabed of 30Hz). However, as is typical with 3D seismic, the data has a high areal sampling (25m grid). As part of a regional imaging study of the seabed using 3D seismic datasets over the FSC (Bulat and Long 1997), undertaken on behalf of the Western Frontiers Association (WFA), a detailed study over the slide was performed based on 3D horizon picks provided by consortium members. The results from this study were very encouraging in that many seabed features were clearly imaged, albeit with data artefacts. To investigate the slide further, Shell U.K. provided a SEGY file of the 3D volume which was loaded at 16-bit resolution. Figure 3 shows an 11 10 3 Figure 4. Depth converted raw horizon pick over the Afen Slide from 16-bit seismic data visualised with ER Mapper. Water velocity of 1.5 km/s used for depth conversion. The image was generated using a standard shading algorithm with illumination from the northeast. Although the image shows much detail, the presence of northeast-southwest corrugations, presumed to be survey footprint, reduces the overall image quality. White lines indicate the position of the seismic profiles presented in this paper. White numerals indicate the associated figures numbers.
208 Bulat example of the seismic data along the axis of the slide. A 3D horizon of the seabed was generated using a Landmark Graphics workstation and then imaged using ER Mapper. This raw 3D horizon is presented in Figure 4. Of particular note is that the subtle features observed on the vertical profile form a consistent surface containing significant detail. The form of the slide is clearly visible with headwall and debris lobe clearly defined. However, finer detail is partially obscured by the presence of northeastsouthwest trending lineations or corrugations in the surface. The northeasterly illumination direction was chosen to minimise the impact of these features on the image. The linear noise correlates with the acquisition direction of the 3D survey and is commonly referred to as survey footprint. 2.1 SURVEY FOOTPRINT Marfurt et al. (1998) define survey footprint as any pattern of noise that correlates with the survey acquisition geometry and describe some of the causes of this phenomenon. These include poor survey design aliasing backscattered noise, imprecision in survey parameters (such as feathering angle in marine 3D surveys) and inaccuracies in certain seismic processing steps such as migration. Survey footprint distorts both amplitude and phase of a reflector and exhibits itself in 3D horizons as minor time shifts between adjacent lines giving rise to a corrugated effect. This type of coherent noise is harmful to derived products such as coherency volumes and dip magnitude and azimuth maps derived from 3D horizons. The complex nature of survey footprint generally makes it difficult to remove from a whole seismic data volume. However, on a strong reflector such as the seabed, where other noise is minor in comparison to survey footprint, an empirical method for estimating and removing Figure 5. Published Afen Slide image generated using BLS correction on 8-bit horizon data. Illuminated from SW. 2.2 SURVEY FOOTPRINT ATTENUATION this coherent energy over a limited area where the seabed is regionally simple does present itself. If it is assumed that the observed linear features are wholly artefacts and not genuine features then the problem reduces to isolating the features and then subtracting them
Imaging the Afen slide 209 from the image. Because the seabed is regionally smooth, the short wavelength of the linear anomalies can be used to isolate them. Figure 6. ProMAX display of 3D horizon transformed into a pseudo-seismic profile after the application of a 20 Hz high-pass filter, equivalent to a spatial filter of 1250m in the cross-line direction on the original horizon. Our initial attempt at removing the artefacts calculated a mean shift for each line relative to a high order polynomial surface fitted to the input 3D horizon provided by Shell U.K. based on an 8-bit data volume. The method was described as Bulk Line Shift (BLS) and the resulting image has been published (Bulat and Long 1998, Holmes et al. 1997 and 1999). The image is reproduced here as Figure 5. The main drawback to the BLS technique is that it assumes that there is a single shift value along the whole line. Closer examination of the raw 3D horizon suggests that this is not always the case. Figure 7. ProMAX display of the result of applying a weighted median trace mix over 101 traces to the high-pass data in figure 6.(equivalent to an averaging over 2500m in the in-line direction). Seismic processing packages, such as ProMAX, have many types of data analysis and enhancement tools. In particular, spatial filtering can be simulated by treating spatial data as time series and using standard time series filters. Thus, the raw 3D horizon was converted into a pseudo-2d seismic profile where the cross-line became the CDP and the in-line direction treated as two-way times, while the original twoway time became the amplitude in the pseudo-2d profile. As the main variation in slope is in the cross-line direction (i.e. two-way time) a simple high-pass frequency filter suffices to remove the regional slope from the horizon. Figure 6 shows the result of removing spatial frequencies longer than 1250m. The slide boundaries show up clearly as do the linear footprint anomalies. Fortunately, the slide boundaries are normal to the footprint anomalies. Thus, we can discriminate against these by applying a weighted median trace mix. Figure 7 shows the result of applying a trace mix over 101 traces as an additional processing step. The Afen Slide outline has been removed and what remains is assumed to be footprint anomalies. It is clear from Figure 7 that the assumption made in the standard BLS technique of a single time shift for each line is an oversimplification and will generate errors in parts of the image. The new refinement to the BLS approach, outlined above, is here termed the weighted BLS correction. The estimated survey footprint is transformed into a new 3D
210 Bulat Figure8. Weighted BLS corrected horizon illuminated from the northeast. The image was generated using ER Mapper s shiny algorithm. horizon which is subtracted from the original horizon. Figure 8 is the weighted BLScorrected horizon imaged using ER Mappers s 'shiny' algorithm which treats the surface as having reflection highlights as well as areas of shade. It is immediately apparent that the image is far sharper than that presented in Figure 5 and that features previously
Imaging the Afen slide 211 unobserved in the adjacent seabed are now imaged. To date this is the best image of the Afen Slide available. It is beyond the scope of this paper to present a geological interpretation of the image; this is presented elsewhere in this volume (Wilson et al.). However, it is clear that the image contains a high level of real information. 3. Comparison with deep-tow boomer records An important consideration regarding the use of 3D seismic is that of its vertical resolution. Deep-seismic 3D surveys typically employ low frequency sources. SW NE Examination of the 3D seismic volume shows that the dominant frequency of the seabed event is 30Hz implying a dominant wavelength of 25ms 50m (33ms two-way time) assuming a velocity of 1500m/s. Thus, the observed reflection is a composite response from all Figure 9. BGS deep tow boomer line 0002-07. The line traverses the slide. See Figure 4 for location. The green horizon is the weighted BLS corrected seabed event projected onto the 2D profile. The occasional data spikes are a product of the projection process. Otherwise there is good agreement with the boomer profile. NW 25ms SE Figure 10. BGS deep-tow boomer profile 0002-02. The line runs parallel to the slide. See Figure 4 for location. Note that the green horizon agrees not with the seabed but with a package of events just below it. reflectors within the first 50m of the seabed. In 2000, BGS acquired a grid of 20khz deep-tow boomer lines over the Afen Slide. Although there are problems with the determination of the exact position of the boomer, inherent in a deep-tow device, the survey does provide an opportunity to compare the 3D seabed horizon with the boomer profiles. Overall there is good agreement between the boomer lines and the seabed image. Figure 9 shows an example traversing the Afen Slide with the seabed horizon projected onto the profile. There is some smoothing but otherwise
212 Bulat good agreement. An example of the seabed image not in full agreement with the deeptow boomer is presented in Figure 10. Here the deep seismic seabed horizon clearly mimics the topography of a buried package of reflections just below the seabed. The difference is explicable by consideration of the seismic frequencies used in the survey. The 3D seabed horizon is an average response over a 33ms window. Where the geometry of shallow buried features dominate the time window, as seen in figure 10, the seabed image reflects these geometries. This has implications for the interpretation of the Afen Slide and indeed any study involving images derived from 3D surveys. Faults with minor vertical displacements, but persistent over the time window will be imaged, whereas very thin sedimentary units or point sources such as shipwrecks will not be observed. 4. Conclusions Commercial 3D seismic data are increasingly being used as a primary exploration tool in many parts of the world. This study shows that despite the low frequencies inherent in such data, images of deep-water seafloor features such as slides can be generated that stand comparison with those produced using other high-resolution systems, such as TOBI or swath bathymetry. 3D seismic data can contain artefacts, in particular survey footprint. However, these can be overcome with the use of processing techniques such as weighted BLS where circumstances permit the ready isolation of such artefacts. Comparison with high-resolution seismic reflection data illustrates the limits in vertical resolution of such images. It is important to understand these limits before interpretation as the images are highly detailed and can seduce the interpreter into over interpretation. In particular, it must always be understood that the image is that of the composite response of the first 50m (i.e. the dominant wavelength) of the seabed and near-seabed sediments. 5. Acknowledgments Thanks are due to the members of the Western Frontiers Association for funding the work and to Shell U.K. for providing the 3D seismic data volume. This paper is published with the permission of the Executive Director of the British Geological Survey (NERC). 6. References Bulat, J. and Long, D., 1998. Creation of seabed feature maps from 3D seismic horizon data sets. British Geological Survey Technical Report WB/98/38C. Bulat, J., and Long, D., 2001. Images of the seabed in the Faroe-Shetland Channel from commercial 3D seismic data. Marine Geophysical Researches 22: 345 367, 2001. Holmes, R., Bulat, J., Gillespie, E., Hine, N., Hobbs, P., Jones, S., Riding, J., Sankey, M., Tulloch, G., and Wilkinson, I.P., 1997. Geometry, processes of formation and timing of the AFEN submarine landslide west of Shetland. British Geological Survey Technical Report WB/97/33C.
Imaging the Afen slide 213 Holmes, R., Masson, D.G. and Sankey, M. 1999. Geometry and timing of the AFEN submarine landslide west of Shetland. In: abstracts North-east Atlantic Slope processes: multi-disciplinary approaches. Southampton Oceanography Centre, Southampton, p42. 1999. Marfurt, K.J., Scheet, R.M., Sharp, J.A and Harper, M.G., 1998. Suppression of the acquisition footprint for seismic sequence attribute mapping. Geophysics, 62: 1774-1778. Masson, D.G., 2001. Sedimentary processes shaping the eastern slope of the Faeroe-Shetland Channel. Continental Shelf Research, 21: 825-857. Wilson, C.K., Bulat, J. and Long, D., 2002. The Afen Slide. British Geological Survey Technical Report CR/02/291. Wilson, C.K., Long, D. and Bulat, J., 2003. The Afen Slide - a multistage slope failure in the Faroe - Shetland Channel. In: Submarine mass movements and their consequences. J.Locat and J Mienert Ed., Advances in Natural and Technological Hazards Research Series, KLUWER, (this volume).