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3D symmetric sampling introduced in the 1990s is characterized by dense sampling of two of the four spatial coordinates. The two sparsely sampled coordinates determine the periodicity of the geometry and the dimension of the offset-vector tiles that can be used to generate pseudocommon-offset-vector gathers. These gathers turn out to be useful for prestack processing applications, such as regularization, migration velocity analysis, and azimuthal anisotropy analysis. Although single-point acquisition is the ideal acquisition method, it is not necessarily better than array-based acquisition. Field arrays are still useful in suppressing noise and need not harm signal in most practical cases. In hybrid geometries three spatial coordinates are sampled densely. In all published cases at least two of the three are sampled quite coarsely and may not provide the best quality for the given trace density. Coil geometry (sailing in circles) is a special case of wide-azimuth towed streamer acquisition. It is essentially a random geometry that should be modifiable into a geometry with regularly sampled midpoints, absolute offsets and azimuths. Despite recent technological developments, the basic idea of 3D symmetric sampling still is a highly useful principle for the design of land and marine 3D seismic surveys.
- Asia (0.94)
- North America > United States (0.28)
- Europe > Norway > North Sea (0.28)
- Europe > United Kingdom > North Sea > Northern North Sea > East Shetland Basin > Block 211/25 > Statfjord Field > Statfjord Group (0.99)
- Europe > United Kingdom > North Sea > Northern North Sea > East Shetland Basin > Block 211/25 > Statfjord Field > Cook Formation (0.99)
- Europe > United Kingdom > North Sea > Northern North Sea > East Shetland Basin > Block 211/25 > Statfjord Field > Brent Group (0.99)
- (40 more...)
Using a modern wide-azimuth land survey, we demonstrate the power of offset vector tile (OVT) processing and subsequent analysis of offset vector gathers (OVG) to identify potential anisotropy and fracture characteristics of certain reservoirs of interest. Migration of the inherently azimuth-limited OVT gathers and the accompanying velocity updating scheme, based on surface fitting in offset and azimuth, yields robust measurements critical to this analysis. Both the kinematic and dynamic aspects of the processing are considered and contrasted. The results of the processing and analysis are then confirmed by comparison to the values predicted from two wells in the area.
- North America > United States > Texas > Anadarko Basin (0.99)
- North America > United States > Oklahoma > Anadarko Basin (0.99)
- North America > United States > Kansas > Anadarko Basin (0.99)
It appears that there are two very distinct schools of thought with respect to the patch configuration for land 3D seismic surveys; often the like or dislike of one over another is based on which part of the business one is involved in rather than any scientific reason. Oil and gas companies (after all the clients) may prefer a wide recording patch to maximize the truly 3D coverage while acquisition contractors (if given a choice) may opt for a narrow recording patch for operational reasons. There are more reasons than these to consider either recording patch and the benefits of both need to be considered when designing a 3D survey.
- Geophysics > Seismic Surveying > Surface Seismic Acquisition > Land Seismic Acquisition (1.00)
- Geophysics > Seismic Surveying > Seismic Processing > Seismic Migration (1.00)
- Geophysics > Seismic Surveying > Seismic Interpretation > Seismic Reservoir Characterization > Amplitude vs Offset (AVO) (0.93)
- Geophysics > Seismic Surveying > Seismic Modeling > Velocity Modeling (0.68)
Threedimensional seismic surveys have become accepted in the industry as a means of acquiring detailed information on the subsurface. Yet, the cost of 3-D seismic data acquisition is and will always be considerable, making it highly important to select the right 3-D acquisition geometry. Up till now, no really comprehensive theory existed to tell what constitutes a good 3-D geometry and how such a geometry can be designed. The theory of 3-D symmetric sampling proposed in this paper is intended to fill this gap and may serve as a sound basis for 3-D geometry design and analysis. Methods and theories for the design of 2-D surveys were developed in the 1980s. Anstey proposed the stackarray approach, Ongkiehong and Askin the handsoff acquisition technique, and Vermeer introduced symmetric sampling theory. In this paper, the theory of symmetric sampling for 2-D geometries is expanded to the most important 3-D geometries currently in use. Essential elements in 3-D symmetric sampling are the spatial properties of a geometry. Spatial aspects are important because most seismic processing programs operate in some spatial domain by combining neighboring traces into new output traces, and because it is the spatial behavior of the 3-D seismic volume that the interpreter has to translate into maps. Over time, various survey geometries have bee devised for the acquisition of 3-D seismic data. All geometries constitute some compromise with respect to full sampling of the 5-D prestack wavefield four spatial coordinates describing shot and receiver position, and traveltime as fifth coordinate. It turns out that most geometries can be considered as a collection of 3-D subsets of the 5-D wavefield, each subset having only two varying spatial coordinates. The spatial attributes of the traces in each subset vary slowly and regularly, and this property provides spatial continuity to the 3-D survey. The spatial continuity can be exploited optimally if the subsets are properly sampled and if their extent is maximized. The 2-D symmetric sampling criteriaequal shot and receiver intervals, and equal shot and receiver patternsapply also to 3-D symmetric sampling but have to be supplemented with additional criteria that are different for different geometries. The additional criterion for orthogonal geometry geometry with parallel shotlines orthogonal to parallel receiver lines is to ensure that the maximum crossline offset is equal to the maximum inline offset. Threedimensional symmetric sampling simplifies the design of 3-D acquisition geometries. A simple checklist of geophysical requirements spatial continuity, resolution, mappability of shallow and deep objectives, and signaltonoise ratio limits the choice of survey parameters. In these consideration, offset and azimuth distributions are implicitly being taken care of. The implementation in the field requires careful planning to prevent loss of spatial continuity.
- North America > United States (0.28)
- Europe (0.28)
Summary With the recent developments in marine streamer acquisition, all three main types of acquisition geometry – orthogonal, areal, and parallel – can now be used to acquire wideazimuth seismic data. On top of this, marine streamer acquisition may also be used to acquire multi-azimuth data. In this paper these geometry types are compared with each other on basis of offset-vector tile (OVT) gathers. Contrary to popular belief that reciprocal azimuths do not contribute to better illumination, reciprocal OVTs are highly desirable to compensate for coarse sampling. These reciprocal OVTs are not always integrated in the design criteria of wide-azimuth streamer acquisition. Instead, the parameters of this acquisition technique tend to suffer from serious compromises. Introduction Over the last two decades, 3D marine streamer acquisition has seen an enormous progress in efficiency based on the use of multisource multistreamer configurations. The data acquired with these configurations remained essentially narrow azimuth. It was even shown that increasing the width of the configurations would lead to more and more illumination irregularities (Vermeer, 1994). Yet, these narrow-azimuth configurations served the industry quite well, and high-quality results have been achieved with multisource multistreamer configurations. However, it gradually transpired that narrow-azimuth acquisition is not optimal for complex geology, notably not for illumination and imaging around and below salt. O’Connell et al. (1993) demonstrated with a dual-azimuth experiment that shooting parallel to the salt produced better images than shooting across the salt. Houllevigue et al. (1999) acquired four different azimuths around a salt structure; their results showed that shooting in a single direction misses about 15% of the information provided by the other three directions. Many other multi-azimuth experiments have been discussed in the literature, culminating recently with Keggin et al. (2006) reporting on multi-azimuth streamer acquisition using as many as six different azimuths. In 2005 BP conducted a so-called wide-azimuth towed streamer (WATS) field trial). Shell followed suit in 2006 with another WATS configuration ). In contrast to multi-azimuth, the sailing direction remains the same throughout the WATS survey but the range of crossline offsets between sources and receivers is increased drastically. Wide-azimuth acquisition is quite common for orthogonal and areal geometry; now that wide-azimuth is also becoming feasible for parallel geometry it is interesting to compare the various types of geometry. The analysis starts with a review of some relevant properties of orthogonal geometry, notably the existence of crossspreads and offset-vector tile (OVT) gathers as characteristic features of this geometry. Based on the use of OVT gathers, it is shown that inherent spatial discontinuities caused by the limited extent of cross-spreads can be mitigated provided the geometry is regular. This methodology is then extended to areal geometry and subsequently to parallel geometry. This paper concludes with a discussion of factors influencing the quality of WATS acquisition and proposes some ways of improving this quality. OVT gathers in orthogonal geometry The data of regular orthogonal geometry can be considered as a collection of cross-spreads, all having the same maximum inline offset and the same maximum crossline offset.