HIGH-RESOLUTION 3D AND PSEUDO-3D SEISMIC INVESTIGATIONS IN SHALLOW WATER ENVIRONMENTS

W. VERSTEEG*, M. VERSCHURE~, J.-P. HENRIET** and M. DE BATIST* *~enard Centre of Marine Geology, Universiteit Gent, Krijgslaan 281-S8, B-9000 Gent, Belgium ** Wpartement GCosciences Marines, IFREMER - Centre de Brest, BP 70, F-29280 PlouzanC, France

SUMMARY

Standard 3D seismic acquisition methods developed in the petroleum exploration industry can be downscaled to any application in the world of university research or civil engineering. Depending on the areal extent, positioning accuracy and scale of the Wbgical target structures, we argue that the most cost efficient solution entails the substitution of a , h e 3D approach with a pseudo-3D one, with an appropriately dense network of 2D profiles, in combination with computer aided 3D modelling. This method has been used to map e.g. clay tectonic faults in the Ypresian Clay on a hectometric scale. However, we also demonstrate that water-born high resolution 3D seismic acquisition is possible with the novel SEISCAT method, developed at RCMG for mapping a small singular clay diapir under the river Scheldt near Antwerp on a metric scale.

1. INTRODUCTION

2D high-resolution seismic methods are most commonly used for site investigations in a shallow water environment. However, a detailed analysis of complex structures for e.g. geotechnical purposes requires data coherence in three dimensions. In other words, 3D seismic acquisition methods developed in the petroleum exploration industry should be downscaled to the world of university research or civil engineering. Corsmit et a2.l have demonstrated that this is possible in land acquisition, however at the expense of numerous man-hours for moving source and g'eophones. Similarly, in marine practice, ship time and precise dynamic positioning of both source and receivers may be factors of prohibitive cost, that can prevent such a simple strategy in water-born seismics.

In this paper, water-born 3D and pseudo-3D seismic surveys at two different scales are presented, in which the type of approach is controlled by the scale of the target structure and by considerations of economy.

2. 3D SEISMICS AND THE SEISCAT APPROACH ON A CLAY DIAPIR

The SEISCAT 3D acquisition system On the river Scheldt, RCMG had already identified a clay diapir with a dense network of

boomer sections in 1982. The deformed reflectors of interest lie in a depth range of 20-40 m and bulge uvwards by 1 m over an area of 50x50 mz. This isolated structure became the ideal subiect for a test of t h e 3D acquisition. With the given sources (signal frequency of around 1 kHz) and Aepth range, Fresnel zone is about 4 m across. An adeauate samdine of such a small Fresnel zone imdies a bin si72 L- - - . . - - - - - - - -

of 1x1 m2. Consequently, the positioning iystem mist h v e a dynamic precision the decimetre range. Only a laser auto-tracking system currently satisfies this requirement. In such a system, a reflector vrism is automatically followed by an on-shore laser ranging theodolite. In order to minimize relative position errors, this phsm needs to be as close as possiblE to-the source and receivers. This has lead to the develo~ment of SEISCATZ, a 3D acquisition system that consists of an array of 12 dual-channel streamers towed from a source-bearing citamaran (fig. 1). The reflector prism w k mounted on top of the frame. The streamer spacing of 1 m ensured satisfactory coverage of the 1x1 mZ bins.

Two types of sources have been used during the experiment: an EG&G Uniboom and a S15- watergun, with modified mouthpiece and lashed horizontally to a fender at 0.2 m depth, which improved the high frequency content of the watergun signal. Position coordinates of the prism were written in the external header of each shot file.

3D processing The first processing step consisted of reading the shot positions from the external header and

correcting them for measurement and telemetric transmission delay. The boomer and watergun seismic traces have then been sorted with SSL's Phoenix Vector processing software into two separate grids of lxlm2 bins. Most of the bins were covered five times or more. A crucial step during the processing of the very high-resolution data was to correct for tidal influences.

Fig. 1: Lay-out of the SEISCAT 3D acquisition system

On the river Scheldt the tidal range amounts to 6 m (8 ms TWT),,which is high compared with the dominant wavelength of about 1 m. All traces were corrected to a common level by adding to each trace a time shift based on the vertical coordinate of the prism crown. Tidal action, however, had one advantage : multiple reflections arrived at different reflection times and were therefore largely suppressed in the process of stacking.

Since the areal coverage was greater for the boomer data, time slices separated by only 0.25 ms from this 3D data set are shown in fig. 2. As one looks deeper and deeper into the data set, concentric reflector pattems grow from the centre outwards towards the base of the diapir. A slight northeastward dip effect, corresponding to the general dip of the Tertiary strata in this area can also be observed.

Fig. 2: Time slices with 0.25 ms interval, showing concentric reflector pattems over clay diapir, and slight dip to NE. Sector measures 50x180 m2; bin size is 1x1 m2.

3. PSEUDO-3D SEISMICS ON A DENSE FAULT PATTERN

The other test site was situated in the northem-most part of the Belgian continental shelf, north of the North Hinder Bank. High resolution reconnaissance reflection surveys with a multi-electrode sparker had revealed a dense fracturation pattern confined to the Ieper/London clay. The faults have a throw of up to 10 m, their spacing ranges between 50-500 m, and they can be traced to a depth of about 70 m.

The Fresnel zone for the above-mentioned sources at these larger depths is about 8 m in diameter. Bins need to be as small as 2x2 m2 in order to adequately sample this zone. The dynamic positioning accuracy should therefore not exceed 1 m, but no land based positioning system offered this accuracy in the considered offshore area. In addition, the lateral extension of the target structure would entail a huge number of bins, and consequently a 3D data set too large to process with limited means. In order to elucidate the 3D nature of the fault system, it was therefore decided to use a network of 2D profiles.

A network of perpendicularly intersecting profiles with a constant maze width cab only unambiguously reveal linear structures with a length and spacing of at least twice the maze width. Such structures can then be said to be covered in pseudo-3D mode. Information on smaller or denser features may be 'spatially aliased', that is, small faults may appear as part of larger ones (fig. 3).

For the given fault system, a network of perpendicular sections spaced 50 m apart was considered to be sufficient for a pseudo-3D survey. The SYLEDIS system is the best available off- shore positioning system in this area of the North Sea, and has a precision of 3-6 m. In order not to diminish precisions much more, the survey was done with a single channel streamer, and with a zero- offset source-receiver configuration.

- profile

true fault trace on a given horizon

fault cut with small throw

fault cut with large throw

1 interpreted fault

alternative , -= interpretations

Fig. 3: Spatial aliasing in a pseudo 3D seismic network. The network consists of two perpendicular sets of parallel seismic profiles that intersect faults at fault cuts. The interpretation of faults comes down to the correlation of fault cuts guided by geological rules of thumb. A large fault (1) crosses a number of profiles and is therefore easy to correlate. Fault 2 is also quite obvious, apart from its termination to the right. Either fault cut a or b may be the true continuation of fault 2 : they have equal throw in the same direction, and both cuts line up with the rest of fault 2. The smaller fault through b may therefore incorrectly appear as, or alias part of a big fault. This network still discloses fault 3, but not faults 4 and 5. Fault 4 is too small to be detected in this survey. Fault cut C cannot be correlated to any other, so that it may be regarded as structural 'noise'. The long fault 6 does not intersect any of the proffles. It would therefore be invisible in a unidirectional survey and only appear as a monocline.

Interpreted horizons were digitized and read into GEOFOP, an interactive surface modelling program developed at RCMG. The data were triangulated and a three-stage modelling cycle (edition of a triangulation of the data, gridding and 3D evaluation) was then repeated to correct remaining errors and try out different correlations of the fault traces.

n u s , a quantitative 3D model of clay tectonic faulting could be produced. Rg. 4 represents a part of this model. Fault traces less than 100 m apart and with a length of only 200 m were sufficiently well constrained to allow reliable correlations. These faults are known to have been reactivated during Quaternary times and may therefore still represent likely slip planes under differential loading.

4. CONCLUSIONS

The examples show how (pseudo-)3D reflection seismics can be scaled down to the realm of complex and subtle structural features. These methods open up a vast domain of 3D analysis of small scale sedimentary structures and faulting patterns. Some of these may be of relevance in geotechnical practice.

ACKNOWLEDGEMENTS

The development of (pseudo-)3D seismic acquisition hardware and modelling software has been supported by the CEC Directorate General for Energy (F'mject TH/06036/87), the Belgian Science Policy Office, the Belgian Ministry of Education and the National Fund for Scientific Research. The cooperation of the Master and Crew of the RV Belgica, the Management Unit of the Mathematical Model of the North Sea and Scheldt Estuary (MUMM), the Antwerpse Zeediensten and the Land3D-1 Group at Seismograph Services Ltd. (Raytheon) are gratefully acknowledged.

REFERENCES

(1) CORSMIT, J., VERSTEEG, W.H., BROUWER, J.H. and HELBIG, K.(1988). High resolution 3D reflection seismics on a tidal flat: acquisition, processing and interpretation. First Break, 6,9-23.

(2) HENRIET, J.P., VERSCHUREN, M. & VERSTEEG, W. (1992) - Very high resolution 3D reflection seismic imaging of small-scale structural deformations. First Break, 10,81-88

(3) VERSCHUREN, M. (1992). 3-D modelling of a complex fault pattern on an entry level 2-D workstation. In: PFLUG, R. & HARBAUGH, J. (eds.), Three-dimensional computer graphics in modelling geologic structures and simulating geologic processes. Lecture Notes in Earth Sciences, 41, 83-88.