VERY HIGH RESOLUTION 3D SEISMIC IMAGING OF A COMPLEX SHELF
STRUCTURE IN THE ADRIATIC SEA
B. MARSSET, Y. THOMAS, E. THEREAU, S. DIDAILLER, T. MARSSET,
P. COCHONAT
IFREMER, BP 70, 29280 Plouzané Cédex, France
A. CATTANEO
Istituto di Geologia Marina, CNR, via Gobetti, 101, 40129 Bologna, Italy
Abstract
Very High Resolution 3D seismic data acquired on the Adriatic shelf image the internal
geometry of the late-Holocene mud wedge in a transitional zone between shore-parallel
undulated features and small-scale mud relief elongated perpendicular to the
bathymetry. The 3D seismic approach applied to Very High Resolution studies allowed
to image the internal structure of these features possibly related to either sediment
failure or transport by current and to enhance the comprehension of geological
problems of key importance.
Keywords: VHR 3D seismic, Adriatic Sea, Quaternary sediment, seismic processing
1. Introduction
A full 3D Very High Resolution (VHR) seismic data set has been acquired during the
TRIAD survey (2001), covering a surface of 800 x 3600 meters (Figure 1).
The main scientific aim of the
14°26
14°28
cruise was to gain information
on the internal geometry and
superficial
morphology
of
Triad
survey
Triad Survey
deformation
features
that
affect
Ortona
Ortona
the late Holocene high-stand
deposits on the south-western
42°28
portion of the Adriatic shelf. The
scope of this paper is to provide
a description of both the 3D
method and the 3D data set that
help to feed the discussion
addressing the depositional or
slope
failure
hypotheses
(Correggiari et al., 2001;
42°26
Cattaneo et al., this volume;
Marsset et al., this volume).
Figure 1. Location of the TRIAD survey in the central Adriatic
Sea. The black rectangle on the bathymetric map represents the
area covered by the 3D seismic survey.
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Marsset et al.
2. Methodology
From the imagery point of view, the main key constraints for a survey design are the
following: maximum depth of target to image and dip of layers, horizontal and vertical
resolution. These constraints may, in turn, be related to the acquisition geometry
addressing the following parameters: signal frequency bandwidth, trace length, trace
interval, streamer length, streamer spacing, line spacing and shot interval. The formerly
developed 3D acquisition system (Marsset, 2001) is capable of being dimensioned in a
dynamic way for optimal coverage of geotechnical and geological sites of small extent
(2 km by 1 km) in water depths up to 200 meters and penetration up to 100 meters and
a vertical target resolution of decimetric scale.
Figure 2. Artist view of the VHR3D acquisition layout.
During acquisition, movement of the vessel and of the streamers, away from the
nominal geometry, can affect the recorded data and the quality of the processed results
(Figure 3). Vertical variations due to tide, wave motion and swell require static
corrections, i.e. time-shifts. Lateral variations, due to currents and changes in the ship’s
heading require dynamic corrections since they change the source-to-receiver offset. In
conventional seismic acquisition, these variations are small compared to the dimensions
of the system and are generally not considered a major problem. In Very High
Resolution data however, with frequencies above 800 Hz and bin sizes of 1 to 2 metres,
they can severely affect the results.
3. Navigation processing
The success of any water-borne VHR 3D seismic method depends on accurate
positioning of both seismic source and receivers. Processing Very High Resolution 3D
data, the positions of the source and receivers are required to obtain bin size accuracy
in all three directions (X, Y, Z) to ensure the correct seismic processing of the data.
VHR 3D seismic imaging of a complex shelf structure in the Adriatic Sea
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However, the current acquisition technology does not allow this level of accuracy in a
cost-effective way. The proposed solution to handle relative positioning is based on
numerical inversion of direct/reflected travel-times: although this problem is clearly
undetermined, simple assumptions may easily be added to constrain the solution.
The approach to retrieve the acquisition geometry is three sided:
- First Break travel time: to evaluate the different static components (tide, wave motion,
firing jitter) and to correct the offset errors using the redundancy of information
contained in the first break arrival times;
- Water Break travel time: to derive the positions of the source and receivers, using
direct travel times, which is hampered by a number of unknowns for the information
available. Solutions are derived by applying certain restraints to the system which in
turn conditioned the results;
- Water Break and First Break travel time: to derive the positions of the source and
receivers, using direct and first break travel times, the latter adding constraints to the
inversion problem. The assumption that is made is that the water bottom within each
shot is relatively flat; however, considering the small footprint of the acquisition
system, this assumption appears acceptable.
Figure 3. Vertical and lateral variation away from the nominal geometry.
4. Quality control
Merging relative positioning, absolute positioning and sea-bottom reflection travel
times allows to evaluate a bathymetric map (Figure 3) through simple NMO correction.
Comparison of seismic and multibeam bathymetry assessed the validity of the proposed
approach.
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5. Seismic processing
In order to design 3D seismic processing sequences, modelling was carried out during
the VHR3D project to compute and inverse synthetic seismograms (Marsset, 2001). 3D
seismic processing in the oil industry has a history of more than 60 years, but VHR 3D
processing does not simply reduce to a mere downscaling, it also requires specific
strategy (Henriet et al., 1992, Marsset et al 1994, Pulliam et al., 1996, Abdulah et al.,
1997). The water bottom and its near subsurface are by nature 3-dimensional structures
and should be treated as such, which exclude NMO-like algorithms: the proposed prestack migration routine (Noble et al., 1996), involves an adapted Kirchohff depth
migration as this algorithm easily handles the particular VHR 3D acquisition geometry
(irregularly sampled spatial grid). It should be pointed out that this algorithm needs
exact position of source and receivers to calculate propagated and retro-propagated
wave fields and therefore cannot be used with pre-processed data correcting offset
deviation upon which 3D NMO should be applied.
Figure 4. Comparison of multibeam and seismic bathymetry on the TRIAD data set.
6. Application to the TRIAD 3D data set
The TRIAD 3D data set was collected in August 2001. The 3D data acquisition
required 9 days of navigation (daylight only); 139 seismic profiles, i.e. 350000 shot
points or 8.5 million traces or 20 Gbytes of data were recorded during the survey. The
processed 3D block consists in a cube of 1827 * 395 elementary cells (or bins), the
acquisition bin size being reduced by processing to 2m * 2m, each cell extending
through depth at a 20 cm sampling interval.
In-line and cross-line sections as well as depth slices or cubes have been extracted for
interpretation. Semi-automatic picking of seismic horizons in the 3D cube allows to
easily elaborate numerical models of the main geological surfaces.
In-line (Figure 5) illustrates the continuous reflectors and the different sedimentary
patterns (undulations, relief).
Cross line (Figure 5) documents the presence of relief (antiform) and highlights their
internal structure (see detailed description in Marsset et al., this volume).
VHR 3D seismic imaging of a complex shelf structure in the Adriatic Sea
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Depth slice (Figure 5) allows to distinguish the different features (undulations,
elongated reliefs, small rounded reliefs), their orientation and their spacing. Moreover
depth slices help to resolve geological problems as the gas distribution (Figure 6).
Figure 5. Extraction of In-line, Cross-line and Depth slice through the TRIAD data set, units are named after
Marsset et al., this volume.
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Figure 6. Depth slices extracted from the Triad 3D volume and revealing the non-randomly distribution of
gas across seismic units. a) in-line profile; b) slice showing absence of gas within U3, that seems to play the
role of a gas-tight layer, at this resolution c) slice showing alignments of gas pockets within U2, thus
evidencing weak lines along which gas concentrated preferentially; d) slice showing that U1 is partially
affected by gas making difficult to distinguish the internal geometry and the seismic facies in profiles.
7. Discussion : 2.5D or 3D ?
Taking into account the resolution achieved in VHR 3D survey, it is worth asking the
question if conventional VHR 2.5D, i.e. parallel 2D lines, is appropriate in complex
geological area. This 2.5D method allows to survey large targets in a cost effective
way, but the interpreter is left with a “cross-line” sampling rate inadequate with the “inline” and Z resolution.
Figure 7 proposes two simulation of 2D lines, acquired 300 metres apart, compared to
small 3D cubes extracted from the TRIAD data set: although in the upper example the
2D approximation appears legible, in the lower example the interpretation from the two
VHR 3D seismic imaging of a complex shelf structure in the Adriatic Sea
447
in-lines will miss the three dimensional morphology revealed by the cube (see details in
Marsset et al., this volume, and Cattaneo et al., this volume.
Figure 7. In lines, simulating. 2D acquisition lines 300 metres apart, and 3D cubes underlying the interest of
the 3D approach in complex sedimentological environment.
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8. Conclusion
The 3D VHR seismic method is a new surveying tool which allowed, during the
TRIAD cruise, to determine the detailed sedimentary pattern of a complex area made of
2 classes of features characterised by perpendicular elongation. These results confirm
that water bottom and its near subsurface are by nature 3-dimensional structures and
should be treated as such; the use of 2D approximation for extrapolation may result
misleading in some cases. Nevertheless, due to the large amount of data to collect and
to process, this method is preferably applied for targets of limited extent and may not
be used as a routine tool.
9. Acknowledgement
This work is part of IFREMER and IGM-CNR contribution to the EU project COSTA,
EVK3-CT-1999-00006 (IGM-CNR contribution number is 1303)
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