http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2117.2005.00252.x/full

Basin Research

Volume 17, Issue 1, pages 1–20, March 2005

3D seismic technology: the geological ‘Hubble’

Joe Cartwright, Mads Huuse

Article first published online: 28 FEB 2005

DOI: 10.1111/j.1365-2117.2005.00252.x

Abstract

The proliferation of three-dimensional (3D) seismic technology is one of the most exciting developments in the Earth Sciences over the past century.

3D reflection seismic data provide interpreters with the ability to map structures and stratigraphic features in 3D detail to a resolution of a few tens of metres over thousands of square kilometres.

It is a geological ‘Hubble’, whose resolving power has already yielded some fascinating (and surprising) insights and will continue to provide a major stimulus for research into geological processes and products for many decades to come.

Academic and other research institutions have a major role to play in the use of this data by exploiting the enormous volume of geological information contained in 3D seismic surveys.

This paper reviews some of the recent advances in basin analysis made using the medium of 3D seismic data, focusing on the fields of structural and sedimentary geology, fluid–rock interactions and igneous geology.

It is noted that the increased resolution of the 3D seismic method provided the essential catalyst necessary to stimulate novel observations and discover new geological structures such as mud diapir feeders, km-long gas blow-out pipes, giant pockmarks and sandstone intrusions, and to capture the spatial variability of diagenetic fronts.

The UKs first impact crater was also discovered using 3D seismic data. The potential for future developments in this field of geophysical interpretation is considerable, and we anticipate that new discoveries will be made in many years to come.

Table of Contens

Abstract

INTRODUCTION AND HISTORICAL CONTEXT

SEISMIC STRATIGRAPHY

STRUCTURAL ANALYSIS

FLUID–ROCK INTERACTIONS

IGNEOUS SYSTEMS

DISCUSSION: FUTURE POTENTIAL

Acknowledgements

References

INTRODUCTION AND HISTORICAL CONTEXT

Observationally based scientific disciplines are inherently limited by the resolving power of the tools used to make the primary observation set.

A perfect illustration of this principle is in Astrophysics. Since the launch of the Hubble

Telescope in 1990, Astrophysics has received an enormous boost in its ability to tackle fundamental questions of stellar evolution, galaxy formation and basic cosmogenic questions, such as the amount of dark matter in the Universe, because of the dramatically increased quality of the imagery provided by the Hubble Telescope (http://www.hubblesite.org).

The vast superiority in resolution of the Hubble over earth-based observing systems lies in the reduction of noise because of atmospheric effects that plague all earth-based systems.

In the Earth Sciences, spectacular advances can be attributed to developments in microscopy, particularly with the orders of magnitude increase in resolution achieved by the scanning electron microscopy and X-ray diffraction over the more conventional optical microscope, with its wide impact on petrology, sedimentology, micropalaeontology, palaeobotany and diagenesis (e.g. Melzer et al., 2000; Poole & Lloyd, 2000; Mahaney et al., 2001).

Recent advances in geomorphology and neo-tectonics similarly rely on new technology such as the global positioning system (GPS) to provide extremely accurate location data in three dimensions that allow the fine-scale characterisation of changes in the landscape because of tectonic movements (e.g. Ching et al., 2001) or changes because of erosion and deposition. A combination of laser-scanning technology and GPS-derived location data furthermore allows geological outcrops to be digitally stored and interpreted with hitherto unattainable precision (e.g. Xu et al., 2000).

In Geophysics, as in a number of other Earth Science disciplines, the advent of high performance computing in the 1980s has led to a number of significant developments that have in turn resulted in improved resolution of the observation field. Whole earth tomography, for example, is a new field that has made a major impact on our understanding of global dynamics (Grand et al., 1997; Bijwaard et al., 1998).

However, in the context of basin research it was undoubtedly in the field of reflection seismology that the impact of technological improvements in electronics and computing played a most significant role.

Reflection seismology has its origins in the 1920s, and only developed slowly during the 1930s to 1950s, mainly by increasing the number of channels in the recording spread.

It was not until the advent of the common mid-point (CMP) stacking method and digital recording and processing in the 1960s and 1970s (e.g., Sheriff & Geldart, 1995) that reflection seismology really became a crucial tool for the non-specialist explorationist and the wider academic community alike.

The coming of the digital age transformed the acquisition and processing of seismic data, and improvements in the quality of 2D seismic reflection profiles were rapid through the 1970s and early 1980s, to the point where the 2D seismic method exerted a major influence on the development of the nascent discipline of basin analysis.

The publication of the landmark AAPG Memoir 26 on seismic stratigraphic interpretation (Payton, 1977) represents a watershed in this respect, and launched a systematic new approach to seismic interpretation of sedimentary sequences.

Around the same time, the initiation of major deep seismic exploration programmes by COCORP and BIRPS was to lead to major advances in the relationship between crustal structure and basin development by the early 1980s. It could be argued that many of the fundamental concepts of modern basin analysis (e.g. McKenzie, 1978) owe their development to multi-channel 2D seismic reflection profiling.

Although the 1980s witnessed a steady improvement in data quality in 2D seismic profiles with increasing in-line sampling density and increasing vertical resolution, one fundamental limitation remained with the method: the limits of spatial resolution.

These limits derive from the spacing of the 2D profiles in a survey grid combined with the imaging problems that arise from the lack of constraint of the third (cross-line) dimension. A general principle underlying seismic interpretation using 2D grids is that objects will be spatially aliased whenever their size is less than the spacing of the grid (Fig. 1).

This meant that the line spacing of any 2D survey effectively defined the spatial resolving power, because only features larger than the grid spacing could be mapped uniquely.

The majority of 2D grids acquired during this formative period had line spacings of a few kilometres, hence our ability to map the details of structures or sedimentary units was limited to relatively large-scale features.

Moreover, because 2D migration, no matter how advanced, only collapses the Fresnel Zone in the in-line direction (Berkhout, 1980), the interpretation of complex structures was a matter for highly experienced geophysicists.

Figure 1. Spatial resolution of 3D vs. 2D seismic data sets. The area to the left shows faults and a channel mapped using conventional 3D seismic data with a line spacing of 12.5–50 m whereas the map on the right shows the same structures as mapped using conventional 2D seismic data with a line spacing of 2 km.

For the Petroleum Industry this in turn meant that only larger traps could be defined and drilled, and subtle details of trap integrity or reservoir continuity were beneath the effective seismic resolution, as determined by the (interval velocity)/(dominant frequency) relation and migration apertures and techniques.

Research into the building blocks of basins (structures and seismic-stratigraphic units) was similarly limited to larger scale phenomena, which, important as they were, often left important subtleties missing from the analysis.

Thus, although interpreters could map the basic framework of basins, defining for example, basic rift structure or geometries of progradational basin margins, many questions were left unresolved, such as the specifics of fault geometry in three dimensions, or the networks of drainage features feeding major progradational systems.

These problems of limited spatial resolution were solved in dramatic fashion by the introduction and proliferation of three-dimensional (3D) seismic acquisition and processing in the 1980s and 1990s (Sheriff & Geldart, 1995; Nestvold, 1996; Dorn, 1998).

The 3D seismic method differs from the 2D method in two important respects, both of which tackle the root of the resolution problem.

Firstly, the grid spacing was reduced from the kilometre or so typical of 2D seismic profiling, to 25 m or less in 3D seismic surveying thus ensuring dense sampling in the lateral dimension comparable with the vertical resolution (Brown, 2003).

Secondly, the 3D seismic sampling in combination with advanced 3D seismic migration algorithms such as 3D dip move out and 3D migration allows accurate positioning of reflections in all direction, thus collapsing the Fresnel Zone in 3D and allowing complex geological structures to be accurately imaged in three dimensions.

The ultimate limit for lateral resolution in 3D seismic data using wide apertures and sophisticated migration algorithms is nowadays often quoted as equal to the bin spacing (range typically 12.5–25 m) of the 3D data set, although it may be more appropriate to consider the limit somewhere between the bin spacing and the dominant wavelength (typical range 20–200 m, depending on target depth).

The impact of the much improved imaging was impressive in terms of detailed stratigraphic imaging, and in the interpretation of complex structures such as salt domes and thrust fault systems, where the complex 3D structure, steep dips, and lateral velocity variations combined to present processors with formidable imaging problems in the 2D approach.

The multiplicity of data and 3D coverage thus allowed for innovative approaches such as 3D pre-stack depth migration to be developed to solve these problems.

The impact of the early 3D seismic surveys was, however, limited by their small areal extent (typically<100 km2), mainly because of high costs of dense 3D surveys, and hence tended to be located in small areas above producing fields.

Another major limitation was the lack of workstation-based interpretation functionality, which did not become available until the late 1980s, leading to some of the first 3D surveys being interpreted on paper!

These early surveys were therefore only of limited value to the wider basin research community and their impact was primarily within the development and production of hydrocarbon resources. Nevertheless, important research advances were made with these early generation 3D seismic surveys, particularly within the fields of reservoir geophysics and structural geology (Weimer & Davis, 1996).

Over the last decade, technological innovation has enabled survey vessels to tow up to 16 long streamers in up to 1-km wide swaths and acquisition costs per unit area have come down; survey areas have thus increased in size by an order of magnitude or more, to the point where individual surveys are now acquired over areas of >10 000 km2, sometimes so early in the exploration cycle that they eliminate the need for 2D seismic reconnaissance surveys.

At the same time computer-based interpretation and visualisation technology is developing at a breathtaking pace, thanks in large part to advances made within the gaming industry, to an extent where the interpreter can now track horizons and faults and calculate seismic attributes in real time and be virtually immersed in the 3D seismic data.

In addition, in a relatively recent and pioneering geo-political breakthrough, some geophysical companies have developed techniques of merging smaller surveys within a region, to create a homogenised display akin to a single ‘Mega Survey’ (Edwards, in press).

This increase in survey dimensions is driven mainly by the increasing use of 3D seismic data as a primary pre-drill exploration tool, rather than as an aid to more efficient production, as was the case in the early stages of its deployment.

The petroleum industry has now widely embraced 3D seismic technology to the point where few exploration wells are ever drilled without a pre-drill 3D survey being acquired.

These large 3D seismic surveys are now of such a scale relative to basin sizes, that in some cases, almost entire basins are covered by 3D seismic data, and this fact alone means that we are now entering a new phase in basin analysis.

In the North Sea Basin, for example, where 20 years ago there would have been incomplete coverage of variable quality 2D data, there is almost continuous 3D seismic coverage over an area exceeding 100 000 km2 (Edwards, in press).

These aerially extensive surveys allow basin analysis at very high spatial resolution afforded by the 3D grid spacing of 12.5 or 25 m. This means that there is no loss of detail with increasing area and that detailed structural and stratigraphic elements can be placed in a regional basinal context.

When combined with extensive bio- and lithostratigraphic constraints from boreholes, this availability of data gives basin analysis a fundamentally novel tool whose resolving power and precision dwarfs all previous methods.

With this tool, we now have the potential to address regional tectonics, depositional systems and processes, igneous systems, fluid flow, compaction and even diagenesis in a truly 3D context that sacrifices little in the way of detail (although fundamental imaging limitations still apply of course) in exchange for the enormity of the depth interval and area covered by the data.

This enormous and rapidly growing database combined with the computational tools with which to interpret it thus opens up an extraordinary opportunity for basin research that has the potential to equal basic geological mapping as a force for revolution of thinking in our subject.

The main aim of this paper (and of this special issue), therefore, is to raise awareness of this potential for 3D seismic data as a research medium in basin analysis and related disciplines.

Having set the historical context in this introduction, the remainder of the paper is a review of some recent advances made across a range of traditional geological disciplines that have stemmed directly from the interpretation of 3D seismic data.

The intention here is not to provide an exhaustive review, but to provide some highlights of discoveries made using 3D seismic data and give a sufficiently broad summary of past achievements to demonstrate the wide-ranging potential of the method, as well as the enormous potential of the ultimately researchable database.

The examples are grouped into four main themes: Seismic stratigraphy, structural analysis, fluid–rock interactions, and igneous systems. The paper closes with a brief discussion of future research directions that could prove most fruitful using the 3D seismic approach.

SEISMIC STRATIGRAPHY

Seismic stratigraphy has been central to petroleum exploration and development since the 1970s, with the formalisation of the discipline in the landmark AAPG Memoir 26 (Payton, 1977).

Not surprisingly, therefore, many of the most exciting developments in 3D seismic-based geoscience have come in this area, partly because of its commercial significance, and partly because the vast majority of data sets and interpreters are based in the petroleum industry.

The techniques developed to analyse depositional systems in the 1970s using 2D seismic data are easily translated into the 3D seismic realm, augmented by many additional attribute-based interpretation techniques that exploit the great spatial resolution afforded by modern 3D seismic data (Bahorovic & Farmer, 1995; Dorn, 1998; Abreu et al., 2003; Brown, 2003).

Because the focus of petroleum-related research is directed at the reservoir part of the depositional system, much of the emphasis has thus far been on refining our understanding of 3D geometry of different reservoir types, and on using seismic data to improve rock property prediction.

Both these themes featured prominently, for example, in one of the first volumes dedicated to the impact of 3D seismic technology in exploration and production (Weimer & Davis, 1996).

The enhanced spatial resolution of 3D over 2D seismic data is graphically seen in the recognition of many previously unappreciated architectural elements in depositional systems (Fig. 2; Lamers & Carmichael, 1999).

Foremost amongst these are the family of deepwater submarine channels and associated facies. First recognised from modern fan systems (Fig. 3; Kolla & Coumes, 1987; Pirmez & Flood, 1995; Droz et al., 1996; Posamentier, 2003; Morgan, 2004), 3D seismic mapping has revealed numerous examples of often spectacularly sinuous channels in the subsurface of many continental margin basins (Wonham et al., 2000; Kolla et al., 2001; Posamentier 2001; Abreu et al., 2003; Prather, 2003; Posamentier, 2004).

Challenges for researchers now are to explain why sinuous channels form on submarine slopes, what controls their evolution, and how different facies are distributed as functions of sediment supply, sinuosity of the channel system, slope angle, channel depth, allocyclic controls, etc. (e.g. Mutti et al., 2003).

Figure 2. Paleocene/Eocene depositional environments in part of the Faroe–Shetland Basin revealed by the amplitude variation of the Top Balder Formation. Only feature visible in the TWT-structure map is the present-day regional dip and velocity pull-up from an overlying channel as revealed by the vertical section. The reflection amplitude reveals a number of relatively straight channels that emanate at the paleo-shelf break and converge down slope.

Most of the features readily visible in the reflection amplitude would be equivocal or completely overlooked in vertical seismic sections such as A-A'. The orientation of the lower right image is rotated 185°. It shows the RMS amplitude for a 60-ms window centred on the Top Balder horizon draped on the horizon TWT-structure map. This display combines overall structure with fine detail and significantly enhances the perception of the depositional environments.

Figure 3. Perspective view of the seabed over a part of the lower western slope of the Niger Delta cone (see Morgan, 2004). View is towards the east. N-S trending linear ridges are developed above km-scale toe-thrusts.

Note how recent deepwater channels have been deflected to the south and partially exploit fault relays in the centre of the area. Image courtesy of Richard Morgan, Veritas DGC.

The great resolving power of 3D seismic has led to the discovery of amazingly detailed seascapes (Fig. 3; Long et al., 2004; Morgan, 2004); and perhaps more importantly to the discovery of detailed paleo-, land- and seascapes (e.g. Smallwood & Gill, 2003; Morgan, 2004; Posamentier, 2004; Bertoni & Cartwright, 2005; Frey Martinez et al., 2005) that are comparable in resolution to those achieved by landsat or multi-beam bathymetry of the present-day seabed and land surface.

Often these amazing landscapes are revealed by amplitude, dip or coherency attributes that enable the detection in great detail of structures that would normally be considered on the margin of the seismic resolution (Fig. 2; Bahorovic & Farmer, 1995; Lamers & Carmichael, 1999).

The proliferation of these landscape-like 3D seismic images spurred the definition of a new geological (sub-) discipline, ‘seismic geomorphology’, which is essentially a surface and volume based approach to unravelling past land- and seascape evolution and the processes that shaped them (Posamentier, 2004).

Interestingly, it has been shown that, in deep water, 3D seismic data may be superior in spatial resolution with respect to multi-beam echo sounders because of the small bin sizes of the 3D seismic survey relative to the large footprints of surface towed echo sounders (Mosher et al., 2004; Bulat, 2005).

Where available, 3D seismic data should thus be a first stop for marine scientists in need of detailed seabed information for purposes such as seafloor stability studies, oceanography, etc. (e.g. Knutz & Cartwright, 2003; Bulat, 2005).

Recently, 3D seismic data have been used to map the detailed morphology and infill stratigraphy of buried Quaternary valleys formed by sub-glacial meltwater erosion in the North Sea Basin (Posamentier et al., 1996; Praeg, 2003), leading to a refined model for their formation involving erosion under a retreating ice margin and deposition by back-filling of the eroded valley (Praeg, 2003).

Iceberg scours and glacial tectonic features can similarly be mapped using 3D seismic data revealing past ice-flow directions in formerly glaciated shelf areas (Posamentier et al., 1996; Rafaelsen et al., 2002).

Whilst the majority of examples of 3D seismic application in stratigraphic analysis are from siliciclastic settings, an increasing number of examples from carbonate environments are emerging, showing the application of 3D seismic analysis to investigate carbonate platform evolution (Masaferro et al., 2003; Zampetti et al., 2004), to map the detailed structure of carbonate build ups (Elvebakk et al., 2002; Masaferro et al., 2003), and to study the stratigraphic evolution of cool-water carbonates (Van der Molen et al., 2005).

In principle, there should be no limits, other than those of seismic resolution, to the types of depositional systems that can be investigated using 3D seismic reflection surveys, and hence we expect 3D seismic stratigraphy and seismic geomorphology to be major areas of discovery, also for the years ahead.

STRUCTURAL ANALYSIS

The most significant advances in structural analysis that have resulted from the application of 3D seismic interpretation are in the description and analysis of fault system geometry and kinematics, and in the field of salt tectonics.

In both these areas, the additional resolving power of 3D seismic data has revealed much greater geometrical complexity than previously appreciated from 2D-based interpretation, and 3D visualisation technology has allowed complex spatial relationships and multi-phase deformation to be studied in great detail.

The classical illustration of the superior resolution afforded by 3D seismic data is seen in snapshots of fault maps of certain areas constructed on entirely 2D survey grids, followed by increasing amounts of 3D seismic coverage (Fig. 4; Demyttanaere et al., 1993).

The section below reviews some of the key advances in fault analysis. For some examples of recent advances in salt tectonics in areas such as the Gulf of Mexico, the North Sea and West Africa see, for example, Jackson & Vendeville (1994), Rowan et al. (1998, 1999), Davison et al. (2000), and Rank-Friend & Elders (2004).

Figure 4. Three vintages of reservoir structure maps of the structurally complex Cormorant Field Block 4 (from Demyttanaere et al., 1993). Drastic change is noted between fault maps based on 2D vs. 3D seismic interpretation, but also between different vintages of 3D seismic data.

Fault analysis

The refinement of trap geometries, kitchen volumes and migration pathways that resulted from the shift to 3D seismic data has been a major factor in improved drilling success rates (Dorn, 1998; Hart, 1999; Brown, 2003).

However, the new ability to define fault networks with greater accuracy has also created opportunities for structural analysis of fault systems that extends traditional outcrop-based methods and makes full use of the 3D database.

This has led to fundamental changes in the way we interpret faults, and to the development of more advanced techniques for fault analysis.

Perhaps the most significant development in the study of faults is that aimed at defining how faults grow. Pioneering work on this problem by Walsh & Watterson (1987, 1988) was based on 3D mapping of fault surfaces and displacement distributions on normal faults in coal mines using mine plan data.

The basic principles derived from this approach were, however, tested and extended with 3D seismic data during the 1990s and now form the basis for much applied work related to fault systems.

3D seismic data revealed the complexity of relay structures and showed the topology of fault linkage in both strike and dip senses (Childs et al., 1995; Mansfield & Cartwright, 2001; Walsh et al., 2003) (Fig. 5).

3D mapping has also allowed fault planes and fault intersection geometries to be defined in their entirety, a feat that has never been possible with previous data types and methodologies (Nicol et al., 1996).

This has revealed some surprising details of the tip regions of normal faults that have an important bearing on our appreciation of fault propagation processes (Walsh et al., 2003).

Figure5. Fault intersections and relay structures are readily interpreted from 3D seismic data (A) in a level of detail comparable to the best outcrop examples available in areas such as the Utah Canyonlands (B).

The relationship between fault plane geometry and associated wall-rock deformation has been studied for decades, but prior to 3D mapping, it has not been possible to show the full extent of the coupling between these two structural elements (Cartwright et al., 1996).

This type of study is in its early stages, but it seems clear that more insights into fault propagation will emerge once the role of wall-rock folding and other forms of near-field ductile strain have been better constrained.

The study of growth faults in areas of major tertiary deltas such as the Gulf of Mexico or NW Borneo has also received a major boost from the wider availability of 3D seismic data.

Studies aimed at analysing the 3D kinematics of growth faults have made considerable progress in revealing the nature of the relationship between fault block motions and gross basin geometry (Rouby et al., 2000). 3D mapping of growth strata has also yielded insights into propagation and linkage of growth faults (Rowan et al., 1998).

The more recent acquisition of larger semi-regional 3D surveys in mature rift-basin petroleum provinces such as the North Sea, has made it possible to examine the kinematic evolution of the faults controlling entire half-graben basins and hence to gain insights into the behaviour of the upper crust during rifting (Dawers & Underhill, 2000).

The analysis of fault kinematics at this kind of basinal scale also opens up the possibility of testing current models of strain evolution in extensional basins in which there is progressive localisation of strain onto the larger faults in the array (Gupta et al., 1998).

At the smaller end of the scale range of fault-related processes, the limits of seismic resolution still pose significant problems for the coupling between what we know of rock deformation from classic outcrop-scale approaches versus the 3D seismic approach.

Direct kinematic indicators will never be revealed by seismic data, and the subtle details of fault plane structure and localised deformation will generally be undetectable. Nevertheless, some progress has been made in filling this ‘scale gap’ using seismic attributes, notably in the recognition of ‘sub-seismic’ faulting using coherence and amplitude attributes (Bahorovic & Farmer, 1995; Hesthammer & Fossen, 1997).

Despite this obvious limitation, the overriding benefits of the full 3D coverage of faulted structures mean that new insights will continue to be derived from 3D seismic interpretation, no doubt assisted by validation with numerical and analogue models (e.g. Rouby et al., 2000; McClay et al., 2004).

Future research will certainly extend the insights gained into the evolution of normal fault systems to the study of thrust and wrench fault systems (Fig. 6) (e.g. Heffernan et al., 2004; Scheidhauer et al., 2005), particularly since deep water fold and thrust belts are the locus for intense petroleum exploration at present (White et al., 2003; Fowler et al., 2004; Ingram et al., 2004; Morgan 2004; Weimer & Slatt, 2004).

Figure 6. Strike-slip fault and associated horse-tail splay exposed on the seabed offshore Israel. Image courtesy of Ben Hall, BG-Group.

FLUID–ROCK INTERACTIONS

One of the subject themes to have benefited least from studies based exclusively on 2D seismic data, is perhaps benefiting the most from the advent of 3D seismic data, namely the theme of fluid–rock interactions.

This is a broad theme that has been based mainly on outcrop studies and which includes diagenesis, fluid flow, compaction and all varieties of soft-sediment deformation (Maltman, 1994).

Because much of the commercial focus in acquiring 3D seismic data is deeper in the section in most basins (typically >1 km subsea), the uppermost part of the data volume is often viewed as redundant and of no commercial interest.

This makes it less problematic to access for research purposes, so geological problems that are well represented in the shallower sections of sedimentary basins are particularly amenable to the 3D approach.

In addition, the shallow section contains the highest frequency data and hence the seismic resolution is often greatest in this upper interval of the typical 3D seismic volume.

The uppermost section of sedimentary basins is the zone of maximum compactional strain i.e. where the rate of change of porosity is greatest (Karig & Morgan, 1994), so compaction and fluid expulsion are likely to be dominant aspects of the phase of early burial of sediments.

There have been a number of important discoveries made in the field of fluid–rock interactions in the past decade that could not have been made without the benefit of 3D seismic coverage.

Polygonal fault systems

One of the clearest illustrations of how 3D seismic technology has illuminated previously unrecognised geological features is the discovery of polygonal fault systems. These fault systems consist of arrays of small normal faults that are organised into polygonal networks, and are confined to specific stratigraphic intervals (Fig. 7).

They were first recognised as having polygonal geometry using 3D seismic data from the central North Sea Basin (Cartwright, 1994).

Although they had been observed on 2D seismic data by numerous interpreters working in this major petroleum province, and in some cases correctly attributed to early dewatering of clay-rich sediments, it was only with 3D seismic data that their complex geometry could be correctly established as truly polygonal. With typical 2D seismic grids of 1-km spacing, the fault system with its intrinsic spacing of ca. 500 m could not be mapped without spatial aliasing, and hence early maps based on 2D grids misinterpreted their planform geometry (Higgs & McClay, 1993).

Time slice imagery revealed the multidirectional nature of the fault system (Cartwright, 1994), and time structure and attribute mapping showed the huge variety of polygonal planforms that were to be found within a single polygonal fault system (Cartwright & Lonergan, 1996).

Figure 7. The true spatial geometry of polygonal fault systems were first discovered in 3D seismic data over the Alba Field located in the Outer Moray Firth, UK North Sea (Cartwright, 1994).

Since their first recognition in the tertiary of the North Sea Basin, many more polygonal fault systems have been observed on 3D seismic surveys from many of the world's passive continental margins and interior cratonic basins, where they cover vast areas in densely faulted networks (Cartwright & Dewhurst, 1998).

Polygonal fault geometry can generally only be established with certainty using a 3D survey. However, profile-based criteria derived from 3D seismic mapping may also be used to identify likely polygonal fault systems using 2D seismic data (Cartwright & Lonergan, 1997).

Their importance as structures lies in their development at an early stage of burial, i.e. during the initial high strain stage of compaction. They are restricted in occurrence to dominantly fine-grained depositional systems, and it is these systems that experience the greatest compaction strain because of their high initial porosities (Aplin et al., 1995). Cartwright & Lonergan (1996) developed a model for polygonal fault systems based on volumetric contraction during early burial in which they argued that compaction strain was not one-dimensional (vertical) as is classically assumed for clay-rich sediments, but acted in three-dimensions as a form of bulk shrinkage of the sediment volume as pore fluid was expelled.

Other models have subsequently been applied to explain polygonal fault systems, and their precise origin(s) is not yet established with certainty (see reviews by Goulty, 2003 and Cartwright et al., 2003). However, regardless of their origin, polygonal faults systems have now been established as globally widespread, with major implications for compaction processes and for the bulk permeability of fine grained depositional systems (top seals) and for early fluid expulsion in basins (Dewhurst et al., 1999).

Polygonal faults affect the reservoir geometry of at least one major North Sea oil field (Lonergan & Cartwright, 1999). More recently, they have been found to intensely deform deepwater sandstone reservoirs in the major Ormen Lange Gas Field, offshore Norway, suggesting that they may have a more significant role in reservoir geology than previously thought (Stuevold et al., 2003).

Blow-out pipes

Another potentially fundamental discovery made in recent years using 3D seismic is that of gas blow-out pipes (Løseth et al., 2001). These features are tall, cylindrical structures in which the continuity of reflections is disrupted often over a vertical extent of up to or greater than 1000 m (Fig. 8).

Their planform is generally circular to sub-circular, and their diameter is often less than 200 m. The limited lateral extent and sub-vertical orientation explains why they have not been recognised previously using 2D seismic data.

Their appearance is superficially similar to many well-known seismic artefacts related to the scattering/attenuation effect of anomalous velocity bodies such as gas accumulations or reefs, particularly when relatively short cable lengths are used in the seismic acquisition.

However, the better imaging provided by modern 3D seismic data and detailed mapping that reveals a complex 3D geometry of the pipes sets them apart from imaging artefacts caused by near-surface anomalies.

Figure 8. The spatial geometry and extent of blow-out pipes caused by gas escape from a buried reservoir through sealing mudstones was first discovered in high-quality 3D seismic data from the south Niger Delta (from Løseth et al., 2001).

The association of these pipes with surface pockmark craters led Løseth et al. (2001) to propose that they form because of rapid migration of gas-charged pore fluids from a deeper reservoir.

They argued, from field analogues, that the pipes most likely consist of zones of distributed fractures within otherwise impermeable clay-rich sediments, and hence represent vertical pathways for fluid flow from the time of their formation.

Since their first identification by Løseth et al. (2001), similar pipe-like structures have been observed in several other case studies where focused fluid flow can be invoked as the driving mechanism in their formation (e.g. Berndt et al., 2003; Bünz et al., 2005; Ligtenberg, 2005).

A brief review of published data on pockmarks shows that many other pockmarks and pockmark fields are underlain by pipe-like zones of disturbed reflections, which have often been dismissed as scattering or migration artefacts because of their association with pockmarks, shallow gas anomalies and underlying shallow gas reservoirs (Hovland & Judd, 1988; Cole et al., 2000).

The wider significance of this discovery of pipe-like fluid conduits is that they occur in sedimentary units that would otherwise be considered as aquicludes because of their extremely low matrix permeability.

The presence of fractured pathways for fluids extending across 1000 m of otherwise sealing lithologies thus opens an important route for analysing basinal fluid flow that could be particularly relevant for hydrocarbon migration and trap retention (e.g. Ligtenberg, 2005).

Thus far, the detailed structure and the process by which these pipes form is poorly understood, although it is likely that there will be much in common with the early stages of development of conduits for mud volcanoes (Davis & Stewart, 2005), where gas migration through fine-grained sequences is also invoked as a driving mechanism (Kopf, 2002).

The ubiquity of highly focused fluid flow regimes in the shallow parts of sedimentary basins is further suggested by the increasing recognition of fields of pockmarks on the seabed and in subsurface maps generated within 3D seismic volumes (Gemmer et al., 2002; Hovland et al., 2002; Gay et al., 2003, 2004), since pockmarks are by themselves direct evidence for such a flow regime (Hovland & Judd, 1988).

Other examples of fluid–rock interactions imaged by 3D seismic

Many additional examples of fluid–rock interactions have been documented from 3D seismic data at scales or with geometries that would not have been suspected from outcrop-based studies. These include an extensive set of studies on large-scale sandstone intrusions (e.g. Dixon et al., 1995; Lonergan & Cartwright, 1999; Lonergan et al., 2000; Molyneux et al., 2002; Huuse & Mickelson, 2004; Huuse et al., 2004; Shoulders & Cartwright, 2004).

The occurrence of large-scale sandstone intrusions has been known from outcrops for over a century, but the scale and basinal extent of sandstone intrusions in areas such as the North Sea (Huuse & Mickelson, 2004) was only realised with the advent of dense basinal coverage by 3D seismic data.

Moreover, 3D seismic data led to the discovery of previously unknown intrusion types, including ‘wing-like’ and ‘conical’ intrusions each several tens of metres thick and hundreds of metres in lateral and vertical extent (Fig. 9; Molyneux et al., 2002; Huuse et al., 2004).

Other examples of large-scale soft-sediment deformation phenomena include basin-scale development of density inversion and differential loading (Davies et al., 1999; Davies, 2003), large-scale slumps and slides (Huvenne et al., 2002; Bünz et al., 2005; Frey Martinez et al., 2005), mud diapirism and mud volcanism (Van Rensbergen et al., 1999; Graue, 2000; Van Rensbergen & Morley, 2003; Hansen et al., 2005).

3D seismic data have proven to be instrumental for the detailed analysis and genetic interpretation of the intricate structures presented by these phenomena, and there is still plenty of scope for additional studies within all of these areas.

Figure 9. Conical sandstone intrusions were first discovered in 3D seismic data from the Outer Moray Firth, UK North Sea (Molyneux et al., 2002). Later studies have shown that these structures are ubiquitous within the lower Eocene of the northern North Sea Basin (Huuse & Mickelson, 2004). Conical sandstone intrusions form tens of metres thick downward tapering cones that may crosscut up to 300 m of strata at angles of 10–40° (figure from Huuse et al., 2004).

One of the most unexpected avenues for research using 3D seismic data to study fluid–rock interactions, is the study of diagenetic fronts, specifically that relating to the transformation of silica from the Opal A to the Opal C/T phase (Davies & Cartwright, 2002).

This transformation results in a dramatic porosity reduction in the host sediments, and this in turn is expressed acoustically as a boundary with a major increase in acoustic impedance. The diagenetic front is thus visible on reflection seismic data as a discrete reflection, that is often instantly recognisable because of its high amplitude and discordant geometry, often cross-cutting stratal reflections (Davies & Cartwright, 2002).

3D mapping of this boundary through large parts of the Faeroe-Shetland Basin has revealed a complex geometry that appears to be influenced by a number of competing factors (Fig. 10). The ability to map a diagenetic front on a basinal scale (>10 000 km2) thus offers a novel method for investigating rock–fluid interactions during a major diagenetic transformation. This is likely to lead to many new insights of wider relevance to other diagenetic processes.

Figure10. (a) Seismic section and (b) map view showing the intriguingly complex spatial geometries of the Opal A/CT diagenetic front discovered in 3D seismic data from the Faroe–Shetland Basin (modified from Davies & Cartwright, 2002).

The coupling between fluids and rock deformation in general is widely appreciated as a critical linkage with implications for the mode of deformation and for the flow of fluids in the crust, and 3D seismic data is now beginning to make an impact on this topic by providing evidence of direct interactions in three dimensions, thus defining the structural controls on fluid plumbing in the upper crust (e.g. Heggland, 1997, 2004; Hurst et al., 2003; Huuse & Cartwright, 2004; Ligtenberg, 2005).

Recently, academic studies have begun to employ 3D seismic techniques to understand the fluid–rock interactions in accretionary prisms demonstrating the role of detachment faults in fluid flow (McIntosh & Silver, 1996; Wallace et al., 2003). Increasing focus on shallow hazards along continental margins have shown linkages between fault slip and fluid flow during growth faulting (Dugan & Flemings, 2000).

Similarly, the association between gas hydrate dissociation and slope stability has been discussed for many years (Kennett et al., 2000 and refs therein), but 3D seismic is now showing a direct physical connection between hydrate bodies and slump masses (Buenz & Mienert, 2004; Bünz et al., 2005).

All these recent examples show how far our perception of soft sediment deformation and fluid flow has changed in only the last decade, and point to far greater potential impact of 3D seismic data on our understanding of linkages between basin processes.

IGNEOUS SYSTEMS

The study of igneous systems is rooted in many generations of field- and laboratory-based studies in which petrological and geochemical disciplines have typically played the dominant role.

Historically, geophysical studies have made only a relatively minor contribution, but with the realisation that many of the most voluminous igneous provinces are located along volcanic continental margins (White & McKenzie, 1989) the contribution, for example, of wide-angle reflection seismology has been increasingly to the fore in our overall appreciation of the distribution and genesis of major igneous systems.

As hydrocarbon exploration has spread to encompass petroliferous volcanic margins such as the Atlantic margins of the UK, Norway, Brazil and West Africa, large 3D seismic surveys acquired in these basins are increasingly revealing a diverse range of igneous products (Planke et al., 2000).

Although not of direct commercial significance, the petroleum industry is nevertheless interested in these igneous systems because: (1) they impact seismic imaging and drilling of deeper reservoir targets (Planke et al., 2000; Archer et al., 2005), (2) the hydrothermal systems interact with reservoirs and lead to diagenesis (Davies et al., 2002; Svensen et al., 2004), (3) they form trap structures through forced folding of cover rocks during intrusion episodes igneous rocks (Trude et al., 2003).

Igneous sills in particular are an ideal illustration of the optimum use of 3D seismic data in a research context (Smallwood & Maresh, 2002; Hansen et al., 2004; Trude, 2004: Archer et al., 2005).

They are very well imaged on 3D seismic data because of their large impedance contrasts with the host sediments, and hence are relatively straightforward to interpret.

This has meant that the 3D geometry of sills can be defined with considerable accuracy, and this has led to several novel conclusions about the interactions between sills and their host rocks as well as the fundamental mode of emplacement itself.

One of the most important new findings derived from 3D seismic mapping of sills is that many sills have a bowl-shaped geometry, with flattish or gently concave bases and steeper, highly transgressive margins (Fig. 11) (Davies et al., 2002; Trude, 2004; Hansen et al., 2004; Thomson & Hutton, 2004).

Field-based studies of igneous sills even from areas of relatively good outcrop such as the Karoo Basin (Du Toit, 1920; Chevallier & Woodford, 1999) were not able to fully define this geometry because of the incompleteness of the outcrop, although early workers such as Du Toit (1920) came close to drawing this conclusion.

Figure 11. Saucer-shaped igneous sill from the Cretaceous–Paleocene of the Faroe–Shetland Basin. Width is about 3 km and vertical relief is about 200 m distributed over a broad almost concordant base and transgressive edges inclined by about 10° (from Hansen et al., 2004).

In addition to defining what amounts to an standard sill geometry of concave upwards bowls, 3D seismic also allows the inter-relationships between individual sills to be mapped in detail.

In a recent study of sill–sill interactions, Hansen et al. (2004) made the important interpretation that sills intersect in a systematic way using a number of distinctive intersection geometries to form completely interlinked arrays of sills within a larger sill complex (Fig. 12).

They argued that because each component sill in the complex links upwards with at least one other sill into a continuous network, then it can be concluded that shallower sills are fed by deeper sills.

The complex can thus be seen to build by upwards propagation of the sill network. This geometrical argument with its kinematic constraints, effectively settled a long-standing debate in the igneous literature over whether sills young upwards or downwards within igneous provinces such as the Karoo (Du Toit, 1920; Bradley, 1965).

Figure 12. (a) Igneous sill complex in the Cretaceous-Paleocene of the Faroe-Shetland Basin. (b, c) Inter-linked upward concave sills in cross section and perspective view (from Hansen et al., 2004).

3D seismic has also shown that small-scale features can be resolved on sill surfaces that have considerable value as kinematic indicators for the propagation of sills, and this too is proving to be an invaluable aid in establishing the hierarchy of intrusion within large sill complexes.

For example, Trude (2004) has identified flow ridges on the surface of a large sill in the Faeroe-Shetland Basin (Fig. 13), and has used the geometry of these ridges to establish the propagation path of the sill, and to locate the feeder position.

Thomson & Hutton (2004) have identified branching patterns in a sill complex in the Rockall Trough, West of Britain, and used these to infer a mechanism of magma transport through systems of tubes, analogous in some respects to lava tubes. Both these studies have demonstrated the value of being able to define quite subtle features and map them fully in three-dimensions.

Whilst subtle features are evident at outcrop, and are often of a scale that is beneath seismic resolution, the lack of geometrical control often prohibits full value being derived from their identification.

A good example of this is provided by small steps on sill surfaces, which are easily seen at outcrop (Bradley, 1965), but which cannot be mapped because sills are so rarely exposed in planform for any distance.

Steps are quite commonly identified on sill surfaces by using the amplitude attribute, because the sharp topographic irregularity of the step combined with the high acoustic impedance contrast of the upper sill boundary acts as a scatterer of seismic energy.

This leads to a reduction in amplitude, and linear anomalies are easily detected on amplitude displays. Mapping these steps provides an additional source of kinematic indicators for sill propagation (Trude, 2004; Hansen et al., 2004).

Figure 13. Perspective view of a ridged igneous sill in the Faroe–Shetland Basin. The ridges are thought to reflect an interplay of relative magma and host sediment rheologies and may be suggestive of a very shallow depth of emplacement (<400 m). Image from Trude (2004).

One other important research theme that has emerged from the seismic interpretation of sill complexes is the recognition and analysis of hydrothermal vent systems that are associated with the emplacement of the sills.

Hydrothermal vents with a mound-like cross-sectional geometry were first inferred to be linked to underlying sills from sparse 2D seismic data (Joppen & White, 1990; Skogseid et al., 1992), but they could not be mapped because the mounds were only a kilometre or so in diameter, whilst the grid spacing was much larger than this.

These mounds were first mapped with 3D seismic in the Faeroe–Shetland Basin, and the internal geometry and external form of these mounds were defined as layered and conical, respectively, (Davies et al., 2002).

Based on these characteristics they were interpreted as volcanic or volcaniclastic in composition, but subsequent studies have shown that they can also be formed by remobilisation of sediments overlying the sills by the rapid upwards flow of hydrothermal fluids focused at the tips of the underlying sills (Svensen et al., 2004).

The importance of these studies of sill-related vents is that they represent an important component of the magma-sediment interaction, and until now, field-based studies have not been able to provide any volumetric constraints because the full extent of vent complexes in any sill complex has not been established.

With the benefit of extensive 2D and local 3D seismic coverage, Svensen et al. (2004) have been able to do just that: estimate the total number of vents along a large portion of a volcanic margin (Norway), and from this estimate the potential contribution to atmospheric methane.

This is thus an excellent example of what is potentially achievable when 3D seismic coverage has been extended over such large regions of basins.

DISCUSSION: FUTURE POTENTIAL

The use of 3D seismic technology as a research tool is in its infancy, despite the relatively widespread use of the technology by the Petroleum Industry over the last two decades (e.g. Davies et al., 2004).

The reasons for this are three-fold:

(1) the data are difficult to manipulate, and require dedicated software and hardware (although this is becoming more straightforward with the transition to PC platforms),

(2) relatively few research geoscientists are familiar with interpretation techniques, and

(3) there is a lack of awareness within the research community of the sheer resolving power and extent of the global 3D seismic database.

One of the aims of this paper has been to address this latter point. As more and more research results based on 3D seismic data are published, however, it seems reasonable to expect usage of this medium to grow, so what are the areas for greatest potential research advances in the next decade or so?

To a large extent, the true value of 3D seismic data as a research tool lies in our ability to map large areas of basins in unprecedented detail.

It may not be too much of an hyperbole to claim that we are entering a new era of basic geological mapping, where computer power allows us to visualise vast areas of the earth's surface and subsurface with no compromise on resolving power.

This capability undoubtedly opens many new avenues albeit along traditional lines of attack for basin analysis: stratigraphic correlation and calibration of surfaces and sequences over large areas, definition of major and minor structural elements and the exploration of the relationships between stratigraphy and structure.

We should be able to refine existing models for tectonics and sedimentation in a host of contrasting basin settings, and thus explore relationships between crustal deformation and accommodation space.

We should also be able to explore the systematics of large-scale depositional systems such as coupling between deltas and fans, for example, to examine exactly how relative sea level leads to organisation of submarine fans systems.

We should equally be in a position to make more precise volumetric estimates of sediment flux to the ocean margins from the continents along vast stretches of the modern continental margins, and link these fluxes to climate, topography and geodynamics of continental interiors.

In short, 3D seismic data will allow us to increase the degree of quantification of many of the inter-relationships in basin evolution.

Undoubtedly, 3D seismic technology will make a significant impact on many specific elements of structural geology, simply because it is first and foremost a geometrical tool.

A few of the themes have been mentioned in this paper, but more complex deformational systems may prove to be fruitful areas of future research, such as strike-slip and thrust systems and all aspects of salt and shale tectonics.

Salt tectonics has already been advanced considerably but there is much scope for additional quantification of salt and shale tectonic processes through better resolution of mobilised and intrusive bodies and their overburden and substratum.

Both thrust and strike-slip systems present major imaging challenges to reflection seismology, but the 3D seismic method is overcoming these problems and with increasing quality of imaging many new insights are possible in the years to come.

As already suggested in the review of existing research, basin hydrodynamics and diagenesis could benefit greatly from future research using 3D seismic data.

Although not discussed in this paper, increasing sophistication in geophysical inversion techniques means that lithological and fluid calibration of seismic data is increasingly accurate, and there is thus considerable potential for any discipline that is based on detailed analysis of physical properties of sedimentary rocks.

It is now possible to invert seismic cubes to provide litho-cubes (calibrated by boreholes) and from these to examine diverse phenomena and processes, such as compaction, lithification and cementation.

It should also be possible to derive relationships between seismic attributes and petrophysical characteristics so that lateral prediction is feasible not only for reservoir rocks (the current focus), but also for other lithological groups, such as the fine-grained sediments.

Recognition of the plumbing system of basins with direct seismic evidence for past and present fluid escape routes, will open the way for a new generation of basinal fluid flow models where cross-formational flow is incorporated explicitly into the models by means of actualistic mapping and risking.

As the total volume of 3D seismic data and the volumes of individual surveys continue to increase there is a real danger that the sheer volume of data will become overwhelming to individual interpreters and their organisations.

Interpreters, including academic researchers, thus need to be aware of efficient computer-based interpretation tools and techniques, and their pitfalls and shortcomings, in order to efficiently interpret the vast quantities of data provided by 3D seismic volumes (Dorn, 1998; Hart, 1999; Brown, 2003).

Because 3D seismic data fundamentally provide us with a volumetric coverage of basins and their fills, basic mapping is the means to subdivide this volume into tractable units.

Hence, computer-aided mapping is aimed at making the basic mapping process more efficient and thus leaving more time to analyse the data and mapped horizons and fault surfaces.

An immensely valuable by-product of surfaces mapped in great detail is that new features are often best recognised from attribute displays of mapped surfaces, and this principle is the foundation for the new discipline of seismic geomorphology.

Some of the many serendipitous, but no less important discoveries originating from basic 3D seismic mapping have already been discussed (Figs 7–10).

Some remarkable recent additions include giant paleo-seafloor craters offshore Norway (Fig. 14; Riis et al., in press), and Britain's first impact crater (Fig. 15; Stewart & Allen, 2002; Stewart, in press). The latter has recently been the subject of intense debate (Underhill, 2004; Smith, 2004; Stewart & Allen, 2004), showing that even if near-perfect 3D imaging is available, the interpretation of these data is often far from trivial.

As more and more data become available for academic exploration and as more academics adopt 3D seismic technology we are most certain that the stream of discoveries resulting from basic 3D seismic mapping will continue.

Hypothesis-testing approaches tend to dominate the research agenda at present, but given the nature of 3D seismic data, the types of advances outlined in this paper demonstrate the invaluable dividend of adopting a more exploratory approach in which basic mapping and curiosity-driven analysis are combined to exploit the processing power of the modern computer.

Figure 14. Giant evacuation crater filled with Pleistocene slump deposits offshore mid Norway (from Riis et al., in press). The seismic profile is located in the Møre Basin, and crosses a major inversion structure of Miocene age, the Havsule Dome. The crest of the dome is drilled by Well 6404/11-1, which proved a major unconformity juxtaposing Pleistocene clays on diatomaceous oozes of Oligocene age. The map of this unconformity shows the form of a major crater measuring some 12 × 20 km, and ca. 150-m deep. This crater is linked to a deposit of ooze at a shallow level (ooze mound), which is interpreted by Riis et al. (in press) to represent the products of a catastrophic evacuation of ooze from depth and extrusion onto the sea floor. This is the first such evacuation structure to be recognised in the stratigraphic record.

Figure 15. TWT-dip map showing the Silverpit crater as it is expressed at the Top Chalk (Top Cretaceous) surface offshore Northumberland, UK North Sea. This is interpreted as the first impact crater to be discovered on UK territory (figure from Stewart, in press).

Acknowledgements

The majority of the papers in this issue were presented at session TS17 of the joint EGS-AGU-EUG conference held in Nice, April 2003, dedicated to the application of 3D seismic technology in basin research.

Compiling this special issue would not have been possible without the diligent refereeing of individual papers by numerous geological and technical experts from around the world to whom we are grateful.

We would like to thank Anne Schwab and Chris Elders for their insightful reviews of this paper. Simon Stewart kindly supplied an up to date image of the Silverpit crater.

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