Abstract Seismic geomorphology, the extraction of geomorphic insights using predominantly three-dimensional seismic data, is a rapidly evolving discipline that facilitates the study of the subsurface using plan view images. A variety of analytical techniques is employed to image and visualize depositional elements and other geologically significant features. This volume presents key technical papers presented at a recent research conference – the Seismic Geomorphology Conference (10–11 February 2005), co-convened by the Society for Sedimentary Geology and The Geological Society (London). These papers cover a broad range of topics, from detailed depositional element analysis to big picture regional issues, from lithology prediction to diagenetic modification of the stratigraphic section. This discipline is only in its early stages of development and will henceforth expand rapidly in response to the growing availability to researchers of high-zquality three-dimensional seismic data.

Abstract A successful study of seismic geomorphology depends not only on knowledge of sedimentological, geomorphological principles and the local geological setting, but also on quality of the seismic geomorphological imaging. A thorough understanding of how seismic waves respond to geomorphology of depositional sequences and facies is essential prior to developing strategies and selecting tools for field seismic data interpretation. This is especially important in data that are of variable quality or lack marked amplitude anomalies. Studies presented in this paper show that it is more desirable to use stratal slices to display seismic information on geological time surfaces. Multi-slice or movie display of stratal slices is effective for the study of depositional process and is a good quality-control tool for avoiding seismic artifacts. Seismic wavelets adjusted to 90° phase help tie seismic traces to lithofacies with higher stratigraphic resolution. Seismic frequency bands of stratal slices should match the lithofacies thickness of interest for optimal facies imaging. Seismic facies analysis can be improved by automated geomorphological classification.

Abstract The sequence boundary associated with the last glacial–eustatic lowstand was mapped across the northwestern Gulf of Mexico continental shelf. The geomorphology of incised fluvial valleys varies widely across the shelf. These differences are due to differences in shelf physiography and the interval of the eustatic cycle the valleys were occupied. Incision begins during the falling limb of sea level and results in terraced valleys. Rivers that abandoned their valleys during the fall in sea level to cut new valleys during the lowstand generally have u-shaped profiles. Incised valleys connected to turbidite systems only occurred in two valleys (the Colorado and Rio Grande), but this may be because sea level did not fall below the shelf break during the last eustatic cycle. Some valleys deepen in an offshore direction, others become shallower. The timing of fluvial incision was constrained using radiocarbon dates so that incision can be tied directly to the sea-level curve for the last glacial–eustatic cycle. The results show that the fluvial incision occurred throughout the falling limb of sea level and lowstand; however, maximum incision occurred during the lowest position of sea level. The resulting surface has significant relief, extends across the shelf, and has time significance. The associated conformable surface, on the other hand, is much harder to recognize and occurs at different stratigraphic levels relative to different shelf-margin deltas.

Abstract The West African continental margin evolution is preserved in a small source-distant setting (20 × 30 km area) by changes in lobe-channel–levee seismic geomorphological elements within a threefold seismic stratigraphic hierarchy. The c. 32 Ma depositional record of rift, drift and depositional outbuilding of the margin by gravity-driven adjustment, deformation and deposition produced a hierarchy of second- through fourth-order stratigraphic cycles bounded by laterally continuous fine-grained drapes inferred to record prolonged periods of sediment starvation. The margin outbuilding phase, the focus of this contribution, consists of three second-order adjustment bounded cycles (ABCs) that record major adjustment and/or modification of the deep-marine depositional system. Seven third-order cycles also show changes in depositional trend and seismic facies architecture. Ten fourth-order cycles, best resolved within the upper part of the succession, consist of multiple, wedge-shaped and compensating, lobe–channel–levee complexes up to 20 km wide. These complexes show an upward increase in channel–levee and decrease in lobe proportion. They also show an upward change from lobes incised by sinuous channels to channels deflected to lobe flanks. Outcrop and shallow core-calibrated analogues from the Permian Brushy Canyon Formation, and modern Amazon and Zaire Fans help constrain these patterns. Changes in the sediment composition and volume of subaqueous flows at their point of origin, and subsequent gravitational deformation, syn-sedimentary mass-wasting and large-scale fan avulsion punctuating deep-marine sedimentation, adjust deep-marine depositional pattern during basin margin outbuilding. Lobe-channel-levee distributions in this sediment source-distant setting record a progressive increase in local topographic relief and gradient related to the basinward migration of deformation during depositional outbuilding of the continental margin. Two important conclusions derived from this record include (1) the importance of local seabed topography and gradient on producing changes in depositional pattern, and (2) that repeated and cyclic changes in these patterns reflect adjustment/deformation within, and probably restricted to, the deep-marine record. Integrated seismic stratigraphic and geomorphic analysis delineates multiple scales of these adjustment-bounded cycles. The evolving map patterns record adjustment by shifts in geomorphic pattern and orientation. These local geomorphic changes can be used to predict longer-term and larger-scale changes in the depositional record of the continental margin evolution. This analytical approach should have general utility along high shelf-to-basin relief margins with similar gravity-driven deformation.

Abstract While being very different in term of horizontal and vertical resolution, the three-dimensional seismic and the two-dimensional lines straddling the Mahakam shelf complement each other to refine the classical two-dimensional seismic-based sequence stratigraphy models and the associated depositional environments interpretation by establishing closer three-dimensional genetic relations between fluvial, deltaic and turbiditic sediments using palaeolandscape morphologies. The Mahakam lowstand seismic geomorphology study can be compared with ancient sinuous fluvial to slope and deep marine channels outcropping in the cenozoic of the Spanish Pyrenees. This comparison emphasizes the interaction of various processes such as sea-level cycles, shelf-edge failures and the behaviour of flood/fluvial flows shaping the fluvial conduits that fed turbiditic systems during the lowstand period. Fluvial-derived flows by-passing the depositional shoreline break or the shelf break appear to be one of the main mechanisms for sediment transfer from shelf to deep basin both in the Mahakam and the Sobrarbe delta despite the obvious difference in tectonic setting, slope dip and flow efficiency.

Abstract Seismic facies mapping in large seismic surveys can be time consuming, even if only a basic overview of the facies distribution is needed. Therefore this study outlines an approach for the use of volume-based seismic attributes from 2D surveys for automated seismic facies mapping within carbonate settings. The study area is located in the central Persian Gulf, offshore Iran. The interval of interest is the mid-Cretaceous Sarvak Formation, which is part of the extensive Cretaceous shallow-water carbonate platform of the eastern Arabian Plate. A set of nine volume-based seismic attributes, calculated from time, amplitude and frequency information of post-stacked 2D seismic data, was chosen to characterize geological information within the interval of interest. The volume-based attributes were supplemented by two grid-based attributes to highlight structural elements. The geological significance of each attribute was evaluated by comparing it with results of seismic sedimentological/geomorphological studies. Furthermore, statistical methods were applied to highlight direct relationships amongst the attributes. The results of these tests were then used to choose a limited set of attributes for neural network-based multi-attribute classifications. The results show that seismic attributes derived from 2D surveys can be used to map basic seismic facies types in carbonate settings and that the outlined, general approach might be applied in other studies.

Abstract The parametric quantification of geological bodies from high-resolution seismic data helps in understanding and predicting their occurrences, but is often hampered by layer distortions caused by post-depositional processes. A method called GeoTime cube is presented that overcomes this by creating a seismic volume between two near-isochronous geological markers in which the vertical axis corresponds approximately to relative geologic time. This volume is no longer affected by post-depositional deformations, a feature that greatly facilitates the extraction of sedimentary elements of interest. A case study of a fluvio-estuarine reservoir from Suriname demonstrates how fluvial point bars, channel fills and crevasse splays can be extracted from the GeoTime cube. Their geometries are quantified with the help of recent analogues. Meandering rivers are found to show relatively constant curvatures and a characteristic spacing of their meander loops. Cubic splines are suitable parametric descriptors of such river paths. Point bars are their main depositional product and can be approximated by two intersecting circle segments, representing the initial and the final position of the meander loop. The axis joining the circle centres corresponds to the direction of accretion, and the normals to these axes describe the drainage trend. Knowledge of these parameters from a limited area can be used to stochastically model the meander belt in the up- and downstream direction.

Abstract We illustrate the use of curvature attributes for defining stratigraphic features of interest on horizons mapped in three-dimensional seismic data. Curvature is a two-dimensional property of a curve that quantifies how much the curve deviates from a straight line. Many different types of curvature may be defined for a surface, and these can be more useful than dip, azimuth or even 'conventional' (i.e. second derivative) curvature analyses for defining subtle structural or stratigraphic features of interest. In our workflow, we drape curvature over rendered three-dimensional surfaces and adjust lighting to highlight stratigraphic and structural features of interest. The examples we present are derived from clastic and carbonate settings of various ages, and include applications of curvature analyses to multibeam bathymetry and digital elevation model data.

Abstract Former ice-stream activity is shown from industry three-dimensional (3D) seismic data from the south-western Barents Sea. Although designed for deeper targets, the data allow, due to high spatial sample rate and three-dimensional migration techniques, construction of detailed plan view images. The integration of sea-floor geomorphology with stratigraphy documents the importance of glacial processes in the seascape evolution of this area. Fast-flowing ice streams occupying the cross-shelf troughs during the Late Weichselian glaciation caused large-scale erosion, and also left their imprints in the form of mega-scale glacial lineations on the sea floor as indicators of ice-flow direction. Various types of 3D seismic attributes, combined with detailed geomorphology and seismic stratigraphy, are used to investigate the 2–3 km of stratigraphic record that corresponds to over a million years of ice-stream activity. The appearance of mega-scale glacial lineations on various 3D seismic attribute maps indicates, together with other characteristics of ice streams, that they are formed by erosion beneath fast-flowing grounded ice. Bedform records of former ice streams may, however, be related only to the final stages of ice-streaming, immediately prior to shut down. Because we here have preserved up to several hundred metres of sediments between the buried, glacially eroded surfaces, we have the opportunity to study ice-stream imprints and associated processes covering longer time spans than just the last stages. Seismic volumetric attribute maps reveal that megablocks and rafts, often aligned in chains, commonly occur within the till units, implying that glaciotectonic erosion by fast-flowing ice streams was an important process in the transfer of sediments from the continental shelf to the Bjørnøya Trough Mouth Fan and the deep sea during the Plio-Pleistocene glaciations.

Abstract Across much of the Gulf Coast basin of the USA, the Smackover Formation consists of a nearly 100 m-thick shoaling upward cycle capped by oolitic/oncolitic packstones and grain-stones. It has long been interpreted as a homoclinal ramp succession, which was analogous to the modern example in the southeastern Arabian Gulf. In a three-dimensional seismic survey in north Louisiana, the shoaling-upward cycle is imaged as basinward progradational clinoforms (4–7° inclination) with well-defined toplap and downlap surfaces. In map view, amplitude slices show that the clinoform bodies are strike-oriented and continuous. The inclination and width of the clinoform bodies indicate that water depths of up to 90 m were present within 1 km of the shoreline. Such characteristics indicate that the Smackover Formation cannot be classified as a homoclinal ramp in north Louisiana and that the Arabian Gulf is not analogous to the Smackover.

Abstract Modern multi-trace geometric attributes produce three-dimensional volumes that can facilitate the recognition of karst geomorphology by avoiding the need to pre-interpret irregular horizons and by enhancing subseismic lateral variations in reflectivity. These geometric attributes include the well-established coherence technology, coupled with recent developments in spectrally limited estimates of volumetric curvature. Coherence measures lateral changes in waveform, and as such, is often sensitive to joints, small faults, sinkholes and collapse features. The many components of reflector curvature, including the most negative, most positive, Gaussian curvature and related shape indices (e.g. valleys, saddles, domes), are complimentary to coherence measures. Short wavelength estimates of curvature will illuminate small-scale lineaments while longer wavelength estimates of curvature illuminate more subtle flexures and compaction features. We show the results of applying a variety of multi-trace geometric attributes to a three-dimensional seismic volume from the Fort Worth Basin, where a collapse system extends vertically some 800m from the Ordovician Ellenburger carbonates through the dominantly siliciclastic Mississippian–Pennsylvanian interval. The collapse features in our data set appear as rounded, sinkhole-like appearances on time and horizon slices in the Pennsylvanian Marble Falls Limestones and the Ellenburger horizon displays features that can be interpreted as cockpit karst, dolines and frying pan valleys. Although a variety of palaeocave breccia facies in core and image logs indicate that the Ellenburger surface has been karsted, these breccias are not confined to the mega collapse features visible in seismic. The large (up to 700 m diameter) collapse chimneys can be shown in multi-spectral curvature attributes to have elongate rhombohedral shapes associated with intersections of Pennsylvanian age, field-scale to basin-scale, basement lineaments and faults. Isochores indicate greatest tectonic growth on faults from Mississippian until early Pennsylvanian, coincident with thickest fill of collapse features. Thus we interpret the origin of the chimneys to be primarily tectonic. The multi-trace geometric attributes permit better imaging of the three-dimensional shapes of the collapse features, provide better constraints on timing of their formation, allow us to begin to separate karst processes from tectonic processes and provide a means of predicting most likely locations of fluid movement along faults.

Abstract Three-dimensional seismic data enable geoscientists to image the stratigraphic record along selected stratal or time slices. These slices provide detailed images of the planform geometry of ancient depositional systems and environments. In this presentation we attempt to provide a partial answer to the question: what kind of information do nonmarine channel and valley patterns and parameters convey regarding sediment load, channel stability, structural or climatic history of the area, and palaeochannel and reservoir architecture? Geomorphologists have long recognized four basic channel patterns: straight, meandering, braided and anastomosing. They have also developed a classification based on aerial pattern and sediment load. This relationship seems straightforward; however the patterns form a continuum with a great deal of complexity related to degree and character of sinuosity, braiding and anabranching. For example an equal-width sinuous pattern suggests moderate to high lateral stability, and low bed load to suspended load deposits. A wide bend sinuous pattern suggests low to moderate lateral stability, and higher bed load to suspended load deposits. Because of convergence and divergence, interpretation of controls on palaeochannel patterns is difficult. Nevertheless, a better understanding of the various controls on modern river morphology and dynamics can produce useful information regarding palaeo-systems. For example, river type (straight, braided, meandering and anastomosing) depends upon upstream controls such as geologic history (glaciated v. non-glaciated systems), tectonics (relief), lithology (sediment type) and climate (hydrology and vegetation), and downstream controls such as base-level (up and down) and length (avulsion). In addition, factors such as bedrock, active tectonics, floods and vegetation cause considerable pattern variability. Only by taking into account the effect of these controls and by using all available data can there be more predictive and interpretive explanations of seismic data.

Abstract The deep-water subsurface offshore Angola is characterized by many linear, high-gradient submarine channels typically only tens of metres wide and deep. Larger channel systems ( C. 3–5 km wide, <300 m deep) with highly sinuous channels at their bases are also common, although they appear to have evolved from small, linear, high gradient systems. Generally, such small linear channels become enlarged by sediment gravity flows and therefore are rarely preserved except in examples where avulsion occurs. These small linear systems are often associated with relatively continuous levees 1–3 km wide flanking the channel. Results presented here suggest that small, linear channels evolve from erosional lineations on the slope generated by large, infrequent turbidity currents. Results also indicate that linear, high-gradient channels also exhibit the most significant and distinctive geometry changes where there is complex topography, such as near salt structures. Sedimentary bodies associated with linear, high-gradient channels often deposit within slope depressions as discreet J- or S-shaped structures in plan view. The dominant control on these sedimentary bodies is interpreted to be seafloor gradient and topography. This paper examines a number of these relatively young channels in terms of their geometry, gradient, levee development and seismic facies. The results improve our ability to predict subsurface channel geometries and recognise key evolutionary trends.

Abstract The architectural framework of the Mauritanian continental slope is characterized by a complex mixture of gravity and bottom current deposits that modify the pelagic background sedimentation. Since the Neogene, the Mauritanian passive margin bottom-currents have been the main control on the construction and topography of the slope. On the lower slope, numerous sediment waves and contourites occur. Turbidity-current channel incisions and slope failures are responsible for the destructive remodelling of the slope. The upper-slope incisions have a dendritic pattern, and converge into highly sinuous main channels in the lower slope, where the channels become constructional with levees and lateral accretion packages. Subsurface data exhibit older slope failures, which guide the location of later turbidity-current channel pathways. The seabed and shallow seismic expression of deep-water slope deposits generated by variable gravity-transport and bottom-current processes in offshore Northern Mauritania can be compared with analogous settings in lower resolution, deeper subsurface seismic data to gain a better understanding of the slope environment and deposits.

Abstract Three-dimensional seismic data are used to document the geometry, scale and distribution of soft-sediment deformation features in the post-rift succession of the Lomre Terrace, offshore Norway. In the Cretaceous to Upper Oligocene succession a polygonal fault network, developed in the in response to compaction and dewatering of the interval, was mapped using dip and azimuth grid-based attributes. In the same stratigraphic interval a series of chaotic seismic reflection packages are developed which are visualized using a volume-based seismic coherency attribute and interpreted as the seismic expression of mobilized mud masses. Immediately overlying the mobilized mud masses are a series of fault-bounded depressions that are interpreted to have formed in response to deflation of the mobilized mud masses caused by loading of the overlying succession. A series of shallow, curvilinear erosion surfaces are present on the seismic horizon bounding the top of the Pliocene succession and represent iceberg-keel plough marks. This study demonstrates that interpretation and visualization of three-dimensional seismic data coupled with attribute analysis provide valuable insights into soft-sediment deformation features in sedimentary basins, in particular the scale, geometry and distribution of such features and their temporal and spatial inter-relationships.

Abstract We are poised to embark on a new era of discovery in the study of geomorphology. The discipline has a long and illustrious history, but in recent years an entirely new way of studying landscapes and seascapes has been developed. It involves the use of 3D seismic data. Just as CAT scans allow medical staff to view our anatomy in 3D, seismic data now allows Earth scientists to do what the early geomorphologists could only dream of - view tens and hundreds of square kilometres of the Earth's subsurface in 3D and therefore see for the first time how landscapes have evolved through time. This volume demonstrates how earth scientists are starting to use this relatively new tool to study the dynamic of a range of sedimentary environments.

Abstract In recent years, 3D seismic has become an essential tool for the interpretation of subsurface stratigraphy and depositional systems. Seismic stratigraphy in conjunction with seismic geomorphology, calibrated by borehole data, has elevated the degree to which seismic data can facilitate geological interpretation. 3D seismic data has enabled interpreters to visualize details of complex depositional systems, which can be incorporated into borehole planning for exploration as well as development needs to improve risk management significantly. Common techniques for geological visualization include (1) imaging stratigraphic horizons, (2) time slicing and flattened time slicing, (3) interval attribute analysis, (4) voxbody interpretation and mapping, (5) 3D perspective rendering and (6) opacity rendering. One of the key benefits of modern 3D seismic interpretation is that stratigraphic horizons can be interpreted and horizon attributes (such as reflection amplitude, dip magnitude, dip azimuth, and curvature) can then be imaged directly in 2D or 3D space. Techniques such as variable illumination can enhance geomorphological interpretations, and, when integrated with stratigraphic analyses, can yield insights regarding distribution of source, seal, and reservoir facies. Stratigraphic intervals bracketing sections of geological interest can be evaluated for amplitude and frequency content and can contribute to geological interpretations. Time slices and flattened time slices (also referred to as horizon slices) can bring to light map patterns and geological features that other techniques might overlook. Voxel picking can further bring out features of geological interest. This method involves autopicking of connected voxels of similar seismic character, a technique that can illuminate discrete depositional elements in three dimensions. Similarly, opacity rendering, which makes opaque only those voxels that lie within a certain range of seismic values, can further bring out features of stratigraphic interest. Examples of fluvial, shallow marine, and deep marine depositional environments are shown. A variety of visualization techniques are applied to these examples in an effort to illustrate the variety of interpretation techniques available to the geoscientist. These examples will highlight the integration of seismic stratigraphic and seismic geomorphological analyses essential for maximum benefit to be derived from geological analyses of 3D seismic data.