Willem Versteeg

Gas hydrate crystals may help build reefs

During a recent cruise in the Porcupine Basin, off southwest Ireland, we discovered two extensive and hitherto largely unsuspected deep-water reef provinces, including a giant cluster of hundreds of buried mounds. The ring shapes of many reefs suggest that they are caused by an axial fluid expulsion at the sea bed, a transient flow well confined in space and time. We are exploring various hypotheses, but a stimulating avenue for research is opened by a glacially controlled growth pulse and subsequent decay of a shallow layer of gas hydrates as a methane buffer and probably indirectly as a ground for overlying biological communities.

Sediment undulations on the Llobregat prodelta: Signs of early slope instability or sedimentary bedforms?

A field of sediment undulations has been mapped by means of high resolution multibeam bathymetry and seismic reflection profiles in the Llobregat River prodelta, off the city of Barcelona, Catalonia, Spain. Similar features had previously been recognized in other prodelta environments and interpreted either as downslope sediment deformation or sedimentary structures induced by bottom currents or hyperpycnal flows. Since the study area is undergoing significant offshore development, proper interpretation of such sediment undulations is needed for a correct risk assessment. The occurrence of the sedi- ment undulations is restricted to the prodelta front on slope gradients between 3 and 0.2o. The undulations have developed at the edge and atop an area of gas bearing sediments within the Late-Holocene high-stand mud wedge. An evaluation is made of the character- istics of the sediment undulations in order to determine the most likely process for the origin of these structures. Amongst these characteristics are the continuity of the reflec- tions and lack of diffractions in between different undulations, their size distribution (large to small) both from shallow to deep and with depth in section, the asymmetry (de- creasing from proximal to distal), the crest to trough vertical distance on the landward side of the undulations (up to 0.5 m), and the lack of features that could indicate a pro- gressive movement such as growth structures and drag folds. These characteristics indi- cate that the sediment undulations on the Llobregat River prodelta do not result from sediment deformation, but rather from the interaction of bottom currents generated by hyperpycnal flows from the Llobregat River with regional sea water circulation. Their identification as sediment waves implies that such features do not pose a major hazard for further offshore development.

Seismic stratigraphy of the Mesozoic and Cenozoic in northern Belgium: main results of a high-resolution reflection seismic survey along rivers and canals

This paper presents the results of high-resolution reflection seismic surveys carried out between 1989 and 1996 along rivers and canals in northern Belgium. The seismic data penetrate down to 900 m in the sedimentary cover or to the Paleozoic basement. The reflection response of the acoustic basement provides clear indications with regard to the top of the Paleozoic: crystalline basement and Lower Paleozoic metasediments and volcanics of the London-Brabant Massif and NE-dipping Devonian and Carboniferous strata. The subhorizontal Mesozoic and Cenozoic sedimentary cover comprises 20 unconformity-bound seismic units: 5 in the Cretaceous and 15 in the Cenozoic. Based on borehole information, these units are correlated with lithostratigraphically defined formations or groups. Some of the unit-bounding unconformities are of regional importance. They are attributed i) to eustatic sea-level changes causing regional flooding during the Late Cretaceous or incision of deep valleys during the Late Oligocene and Late Miocene, ii) to regional tectonic tilting between Late Eocene and Early Oligocene, or iii) to a combination of eustasy and tectonics causing valley incisions during the Lutetian. Faults of the Roer Valley Graben have offset different stratigraphic levels by sometimes considerable amounts (up to 230 m in the Oligocene to Quaternary succession). Although the main tectonic phase took place during the Miocene, the activity has varied considerably through time, and also from fault to fault. Most faults seem to have a 10 to 30-m displacement since the Late Pliocene.

First sighting of active fluid venting in the Gulf of Cadiz

The Mercator mud volcano, located in the Gulf of Cadiz off the coasts of Spain, Portugal, and Morocco (Figure 1), may provide an accessible field laboratory for studying local active venting and its possible internal and external controls. The recent discovery of the first active deep ocean ‘brown smoker’ chimney in this area can possibly be linked with the disintegration of a gas hydrate layer between the seafloor and a subsurface level that is dependent on pressure and temperature. For more than a decade, the international marine scientific community has deployed considerable efforts in exploring the Gulf of Cadiz. Since the discovery of the Gulfs first mud volcano in 1999, research cruises have steadily unveiled one mud volcano after another [Gardner, 2001; Pinheiro et al., 2003; Somoza et al., 2003]. These mud volcanoes are clustered in several fields on the Portuguese, Spanish, and Moroccan margins (Figure 1). Extensive geophysical evidence of shallow gas and subsurface fluid flow has been reported in the Gulf of Cadiz [Baraza and Ercilla 1996; Pinheiro et al., 2003].

Converted waves in a shallow marine environment: Experimental and modeling studies

Seismic waves converted from compressional to shear mode in the shallow subsurface can be useful not only for obtaining shear-wave velocity information but also for improved processing of deeper reflection data. These waves generated at deep seas have been used successfully in hydrocarbon exploration; however, acquisition of good-quality converted-wave data in shallow marine environments remains challenging. We have looked into this problem through field experiments and synthetic modeling. A high-resolution seismic survey was conducted in a shallow-water canal using different types of seismic sources; data were recorded with a four-component water-bottom cable. Observed events in the field data were validated through modeling studies. Compressional waves converted to shear waves at the water bot-tom and at shallow reflectors were identified. The shear waves showed distinct linear polarization in the horizontal plane and low velocities in the marine sediments. Modeling results indicated the presence of a nongeometric shear-wave arrival excited only when the dominant wavelength exceeded the height of the source with respect to the water/sediment interface, as observed in air-gun data. This type of shear wave has a traveltime that corresponds to the raypath originating not at the source but at the interface directly below the source. Thus, these shear waves, excited by the source/water-bottom coupled system, kinematically behave as if they were generated by an S-wave source placed at the water bottom. In a shallow-water environment, the condition appears to be favorable for exciting such shear waves with nongeometric arrivals. These waves can provide useful information of shear-wave velocity in the sediments.

Near surface 2D and 3D data processing - Beyond stack

Very high resolution (VHR) marine seismics offers a wide variety of applications, such as: engineering, geological or environmental site investigations. This technology progressed quickly, and the constant evolution towards cheaper and more powerful computers is making VHR multichannel recording in 2 dimensions more and more popular. 2D surveys are now common use whereas, 3D acquisition has been demonstrated to be feasible. But up to now, all these new technologies have had little impact on the processing sequence required for such data. This processing sequence seems to stagnate around the 1-D assumption of the Common Mid Point. The sea bed and its near subsurface are by nature 2, often 3 dimensional structures and should be treated as such using prestack depth migration algorithms. These imaging techniques, directly issued from the oil industry, have the well established and bad reputation of being difficult to use, costly and requiring expensive computers. However this is no longer true, due not only to faster processors but also to new numerical algorithms. Prestack depth migration applied to VHR data does enhance the interpretability of the final section, and CPU time on small computers is far from being excessive. We will illustrate this for two sets of VHR seismic data (2D and 3D).

Acquiring Shear-wave Information In Shallow-water Environment From Field Data Near Ghent, Belgium

A multi-component seismic experiment was conducted by deploying a 4-C ocean-bottom cable in a shallow water canal. The aim of the experiment was to investigate the feasibility of acquiring shear-wave information in shallow marine environment. In the recorded data, we observe airgun generated shear-wave reflections. This observation is validated by a model study. The presence of shear-wave reflection is caused by the low frequency of the source in relation to the water depth. Furthermore, converted waves have also been identified based on two “attributes” namely the particle polarization and the normal moveout velocity. Comparison with synthetic data suggests the presence of waves converted at a reflector as well as at the water bottom.

Comparative Seismic Source Study for Buried Palaeolandscape Investigations in the Southern North Sea

The southern North Sea has been an attractive region for human settlement throughout the late Quaternary. Unfortunately, to this date, little attention has been paid to the rich submerged archaeological potential in Belgium. Marine seismic techniques have been used for more than 20 years to investigate buried landscapes but in general the archaeological community often has to work with data acquired for other purposes, meaning they are not well adapted for archaeological studies. In this study, we aim to develop an efficient survey methodology to image buried palaeolandscapes for the purpose of archaeology, through the comparison of a wide range of high-resolution seismic source/receiver configurations. This should finally allow to accurately assess the geo-archaeological potential of the Quaternar

Studying converted waves in shallow marine environment

For many years, waves converted from compressional to shear mode have been successfully applied in hydrocarbon exploration to image and characterize the subsurface. Since shear‐waves propagate with a velocity that is dependent on the shear modulus and are thus directly related to the shear strength of the sediment, they are very useful for geotechnical purposes. Generally, P‐wave reflection amplitudes contain S‐wave information but Riedel et al. (2001) showed that these reflections are not very sensitive to this. An alternative approach is to obtain S‐wave information directly from converted waves. However, it is not clear whether these waves can be applied for geotechnical aims. The main focus will be on understanding the dependence of mode conversion on the seismic properties. In this study, we investigate the possibility of acquiring converted waves in marine unconsolidated sediments. From our numerical experiments we found that the conversion is maximal at two angles where the smallest angle appears t...