STATICS & 3D SEISMIC VIBRATOR ATTRIBUTES – SIRTE BASIN: LIBYA Authors: Chris Webb* Chris Izatt, Bashir Rabti (BG Libya)

* Speaker

Abstract Integration of geological concepts, satellite data, vibrator attribute and static shifts derived from refraction analysis of 3D seismic proved an effective static method that corrects significant time anomalies under buried wadi/channels that have virtually no present day topographic expression. Examples are shown of: - Shallow buried wadis and their effect on seismic before and after static corrections - Key Vibrator attributes and buried wadi identification Introduction BG International acquired the Sirte123-1 and Sirte123-2 licences in Libya, EPSA IV-Bid Round 2. The licence agreement was signed in December 2005. Sirte licences123-1 and 2 cover an area of 5,000 km2. 3D seismic was acquired during 2007 (WEBB et al 2007), ahead of a drilling campaign in 2008, to complete the commitments on the licences. The seismic programme took 6 months to acquire and covered 3,700 km2. A brief expansion on what vibrator attributes are and how we consider they relate to seismic static issues is covered in the following two sections. Vibrator Attributes Vibrator attributes are derived from monitoring the system electronics during land acquisition and in particular those associated with controlling the vibrator plate movements. Data such as the force and the distortion of the vibrator sweep are recorded e.g. on soft terrain, when the vibrator plate loses energy and sinks into the ground, the electronic system compensates by applying increased force. This data can be plotted to generate maps which are then related to the physical characteristics of the very near surface. The WesternGeco Crew 1914 used the Sercel VE432 Vibrator Digital Control System and generated 6 attributes as listed below:

• Ground Viscosity • Ground Stiffness • Peak distortion • Average distortion • Peak force • Average force

There are major benefits of these data:

- Automatically recorded for every vibrator (8 vibrators used in this case) and at every vibrator point. So a very high data density ensues that helps define edges of seismic static zones.

- These data are available as the survey is being shot and thus can be used to more efficiently review static issues and relate the surface to the shallow geology.

Shallow Geology and Seismic Statics

It is considered that seismic statics relate to vertical zones from the surface topography down to the shallow subsurface (ref LEY et al 2005, BRIDLE et al 2006). There are 2 levels of concern for statics in this project. 1. The topography of the surface i.e. in effect elevation changes. There is a well known

static technique for correcting the seismic time picture for changes in elevation, the so-called elevation statics method.

2. Near surface effects and shallow geology (eg buried wadi channels), which can be corrected using refraction statics. These zones have an effect on vibrator attributes, probably resulting from changes in geology/soil conditions down to 30-50m below the surface (ref LEY et al 2005, BRIDLE et al 2006).

Results and Discussion Early discussions before the survey with other operators in the area and within BG indicated that upholes would not be critical in this area due to the largely low relief flat plain cut by one active Wadi/Channel system in the West (Fig. 1). It was considered that surface consistent refraction statics would be sufficient to resolve the static corrections required. This was tried with good effect on the BG 2D test line acquired in early 2006 some 10 months before the start of the 3D. However, it was not until end 2006 that reprocessing of the legacy 2D seismic by PGS first indicated the benefit of the refraction technique to correct some areas with anomalous time depressions. A coincidence was noted with a sinuous pale coloured band on the satellite pictures that coincided with some of the identified static anomalies (Fig. 2). The fact that the feature had no topographic expression, unlike the other, readily identified, active western Wadi, led to the view that it represented a buried (or filled-in) extinct Wadi. During the period of seismic acquisition the 3D vibrator attributes produced from early field processing showed clearly the buried Wadi channel outline. In August 2007 after a significant portion of the 3D had been completed, more complete vibrator attribute maps revealed the buried Wadi fairly precisely and it was realised this could aid definition of where attention was needed for static correction in the final processing (and later depth conversion). The final maps for two of the most revealing attributes are shown (Fig.3 & 4). The high viscosity attribute coinciding with the low velocity Wadi channel infill is similar to that described by BRIDLE et al 2007 and LEY et al 2006. The geometry and sinuous nature of the attributes seem to match nearby present day more active Wadi systems and additional channels/buried Wadis are shown to the East that have no obvious colour change on the satellite images. The ground viscosity attribute (Fig. 3) we consider was one of the better attributes for seeing these buried channels/wadis. The average distortion attribute (Fig. 4) was also one of the better attributes for seeing the buried channels/wadis. Once identified, two methods for statics correction were attempted. The elevation static method corrects the active Wadi effectively, but the refraction static was needed to correct the buried Wadi channels, as shown in work on a 3D sub-cube area (Fig. 5). This is best shown by looking at the seismic data at the early stages of seismic processing. The main attribute map (Fig. 3) shows 3 lines across these buried Wadis - the legacy reprocessed 2D line in the north and 2 parallel raw stack 3D lines for comparison. Of note are the thin wadi in north widening to south. The 2D line is shown

before static (Fig. 6) and after static correction (Fig. 7) and included in these figures for reference are raw 3D seismic stacks before statics. The static solutions from the refraction work provide better definition and estimate of width of the buried Wadis (Figs.5).and also gives a better seismic time picture; even on the early raw stack 3D data (Figs. 8). Summary Buried Wadi channels and their associated spurious seismic time static anomalies may have gone unrecognized in older seismic surveys especially 2D with its lack of spatial sampling. This lack of recognition would be aggravated if reliance for statics was based solely on uphole surveys, which would have been unlikely to have drilled in the right places to accurately map the anomalies. It would seem clear in the future that best practice would be to integrate satellite radar, upholes and vibrator attributes rather than rely on just one of these methods. In this respect integration of shallow ground penetrating radar i.e. as supplied by shuttle orbits abbreviated as SIR-C/X-SAR (Space borne Imaging Radar-C/X-band Synthetic Aperture) appears to be of great interest and relevance for helping to map out these buried Wadi channels. A good example of this, albeit used for archaeological purposes, is shown in reference OSBORNE et al 2007 and a clearer image as shown in JET et al 1995 from the Kufra basin clearly delineates buried wadi systems. The availability of such data would be very useful for static analysis in future seismic surveys. With the seismic to be optimally processed it is considered best practise to use the vibrator attributes matched with shallow surface geology and topography to get the best static solution. This was a key for BG to get the best time image from the processing and subsequent depth conversion to the deeper target levels. Subsequent drilling verified the model used for depth conversion.

Conclusions Vibrator attributes The best attributes to identify buried Wadi channels are: 1. Ground Viscosity - High viscosity coincides with low velocity fill (buried Wadi) 2. Distortion - Low distortion coincides with low velocity fill (buried Wadi) Statics Spurious time depressions under buried Wadi channels are best corrected using the refraction static correction method

Future Considerations Obtaining early satellite and vibrator attribute data for use in: 1 Identifying areas to concentrate processing on for statics 2 Selective upholes locations where required. The ability to identify where to place upholes

during seismic acquisition, to optimize resolution of longer wavelength statics, is a major benefit (Pers.Comm. H. Crook 2009)

Future Investigations possible: 1 Can the static correction be derived mathematically from the vibrator attributes? (Ref.

BRIDLE et al 2007 and LEY et al 2006)

2 Special Radar images that seem to show buried Wadi systems (Ref. OSBORNE et al 2007) 3 Integration of all these above should deliver the best static solution

References

BRIDLE, R., LEY, R., AND AL-MUSTAFA, A. (2007). Near Surface models in Saudi Arabia. EAGE 2007, Geophysical Prospecting 55, 779-792 LEY, R., ADOLFS, W., BRIDLE, R., AL-HOMALI, M., VESNAVER, A., AND RAS, P. (2006). Section I – Madrid Workshop on Near-Surface 2005 Ground viscosity and stiffness measurements for near surface seismic Velocity. Geophysical Prospecting, EAGE 2006, 54, 751–762 JET PROPULSION LABORATORY IMAGING RADAR (1995) Wadi Kufra radar image (ImageP-45719).Available at http://southport.jpl.nasa.gov/cdrom/sirced03/cdrom/DATA/ LOCATION/AFRICA/WADIKUFR/WADIKUFR.HTM OSBORNE, A. H., VANCE, D., ROHLING, E. J., BARTON, N., ROGERSON, M., AND FELLO, N. (2008). A humid corridor across the Sahara for the migration of early modern humans out of Africa 120,000 years ago. PNAS October 28, 2008 vol. 105 no. 43 www.pnas.orgcgidoi10.1073pnas.0804472105

WEBB, C., RABTI, B., MILLINGTON, M., CROOK, H., AND HEATH, R. (2007). “Sparse 3D” Acquisition – An Efficient Exploration Technique" EAGE 2007 February, Tripoli.

Acknowledgements: Thanks to Piotr Synowiec (Seismic Survey observer BG consultant) for the great idea of plotting out the Vibrator attributes and WesternGeco for supplying the static images. Thanks to BG Libya & BG Advance: Howard Crook, Tim Marples, Nick Benfield and WesternGeco for help on the work. Thanks to BG Group and NOC for permission to publish.

Figure Captions Figure 1. Satellite based radar altimetry and elevation perspective image of Sirte 123/1 & 2 BG area outlined in orange Figure 2. High Res Satellite Image – False Colour with 3D survey area outlined in yellow, 2 km circular features are crop circles formed by irrigation Figure 3. Map of Vibrator Attribute - Ground Viscosity and location of example seismic lines. Red colours indicate high viscosity and coincide with shallow static anomalies with time pushdowns / low velocity fill. Figure 4. Map of Vibrator Attribute - Average Distortion. The pale blue colour indicates low distortion and coincides with shallow static anomalies with time pushdowns / low velocity fill. Figure 5. Map of Elevation static versus Refraction static on 3D sub-cube. Refraction statics correct for time pushdown under buried Wadi. Figure 6. Original legacy 2D seismic line (before statics) with blue line interpreted as Near Top Augila and raw stack 3D seismic lines before applied statics showing affect of buried Wadi and time pushdown Figure 7. Original legacy 2D seismic line (time depression after corrected by reprocessing and refraction statics) with blue line from uncorrected interpretation and for reference from the previous figure the raw stack 3D seismic lines before applied statics. Figure 8. 3D seismic (NW-SE) raw stack cross-line with initial static correction tests. Refraction statics is better for time pushdown under buried Wadi. Note reduction in time depressions to a more acceptable level at Top Augila Limestone Fm – Near Base Oligocene in the refraction statics test

Fig 1

Active Wadi with topographic expression

190 m Elevation

North

100 m Elevation

East area appears to be a benign surface level static environment with regional dip less than 1 degree

Fig 2

Active Wadi with topographic expression – elevation static issue

Roads, Great Man-made river pipes Slight Colour changes – near surface statics?

Wadi on Geographic Maps

Fig 3

2D 3D

3D

Buried Palaeo-Wadi Channel NE-SW system - little topographic expression

Active Wadi with topographic expression

Fig 4 Good attribute to image buried Wadi/channels

Active Wadi with topographic expression

2D 3D

3D

Fig 5

Active Wadi with topographic expression - elevation statics

Refraction statics identify in more detail the buried wadi,

Fig 6

Well A1-LP6D

2D Uncorrected

Base Oligocene – Augila

Black is hard peak

Slow/Soft Clastics

Red trough is hard 5 km

3D xline Raw stack

3D xline Raw stack

Buried Wadi channels (giving time low)

Fig 7

Well A1-LP6D Buried Wadi channels (giving time low)

3D xline Raw stack

2D - Corrected

5 km

3D xline Raw stack

Fig 8

Near Base Oligocene

elevation statics

Refraction statics

Buried Wadi/Channel

Basement