Seismic techniques as applied to hazard surveys are essentially the same as those used for deep exploration, differing primarily in scale. Results suggest that the temporal bandwidth commonly specified for shallow profiling can probably be relaxed when areal data recording methods are applied Routine application would reasonably be expected to reduce drilling costs whilst significantly improving safety. The Short-Offset field technique that we propose for this work can be economically applied to hazard surveys. Commonly, these will remain unnoticed or be dismissed as noise in a vertical seismic profile. Even when 3-D methods have been applied, conventional seismic cross sections are inappropriate for displaying responses resulting from fractures or channels that have a small spatial footprint. In many cases small yet potentially important targets could remain unsampled by the customary grid of 2-D survey lines, or would be virtually uninterpretable without the benefit of 3-D migration. The pictorial quality of intensity-modulated time slice displays is particularly striking. Examples of such images are presented showing features of small physical dimensions, as well as a number of responses that are presumed to represent gas hazards. The 1-km-wide channel is unmistakable on this view of the data, and both edges of the channel are readily interpretable.Using results obtained from existing 3-D marine seismic surveys acquired with a compact recording geometry we show how true-amplitude migration processing can yield remarkably well-defined images of the shallow geology in plan view. This surface is shifted 120 ms down, and the seismic amplitudes are extracted where the shifted surface intersects the 3-D volume of data.

The shape of the horizon slice is defined by the shape of the Top Paleocene horizon. Most of the amplitude patterns on the time slice are associated with structure, not stratigraphy.įigure 3 is a horizon slice 120 ms below the Top Paleocene horizon. On both the vertical section ( Figure 1) and the time slice, the channel is difficult to interpret, even though the feature is quite large. Approximately 120 ms below this, at about the level indicated by the arrows, the section crosses a 1-km-wide Paleocene deep-water turbidite channel.įigure 2 is a time slice that intersects a portion of the channel. The interpreted horizon, at approximately 2 seconds, is the Top Paleocene. Figure 1 is a portion of a vertical seismic section from a 3-D seismic survey in the North Sea. Copyright: Dorn courtesy SEG.įigures 1, 2, and 3 illustrate the value of the horizontal slice view of the data. Within the time interval where reflections are approximately conformable to the interpreted horizon in three dimensions, the shape of the horizon surface is a reasonable description of the shape of the paleodepositional surfaces.įigure 3 . The interpreted reflection (horizon) is an approximation of a paleodepositional surface. One way to improve the imaging of the paleodepositional system is to create horizon slices through the 3-D volume. In fact, as the structural relief increases, the anomalies on the time slice associated with the structure quickly dominate the image.

Subsequent structural deformation of the depositional surface can mean that only a small portion of a depositional system is imaged on a time slice. A time slice provides at best an image of a small portion of a depositional system.

Timeslice seismic consultants