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GCAcquisition Footprint*
By
Andreas Cordsen1
Search and Discovery Article #40128 (2004)
*Adapted from the Geophysical Corner column in AAPG Explorer, March, 2004, entitled “Acquisition Footprint Can Confuse” and prepared by the author. Appreciation is expressed to the author, to Alistar R. Brown, editor of Geophysical Corner, and to Larry Nation, AAPG Communications Director, for their support of this online version.
1Managing partner, GEDCO, Calgary, Canada.
Any
3-D seismic
survey can have an acquisition footprint. Our problem is to
determine whether we have one -- and if so, whether we can recognize it, how
severe it is and, most importantly, what we can do about it.
What is an
acquisition footprint? It is an expression of the surface geometry (most common
on land data) that leaves an imprint on the stack of our 3-D seismic
data. Often
we recognize it as amplitude and phase variations on time slices, which of
course display the amplitudes within our data set at a specified two-way time.
More seriously, on horizon slices, footprint can interfere with and confuse
stratigraphic patterns.
Many different contributions to the generation of acquisition footprint are possible:
These can be divided into two main categories of geometry effects and non-geometry effects.
Geometry Effects |
Non-geometry Effects |
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uGeneral statementuFigure captionsuGeometry effectsuNon-geometry effects
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Figure Captions
Geometry EffectsMost of the time the acquisition footprint is based on source and receiver line spacings and orientations; the larger the line spacing, the more severe the footprint. In land situations where access is very open and, therefore, the lines are very regularly spaced, we may be able to recognize the footprint very clearly. Because the geometry is regular, the footprint also will have the same periodicity. Fold variations themselves are the simplest form of an acquisition footprint. Fold changes with offset (or rather mute distance from the source point); each offset range, therefore, has differing fold contributions. Because each individual bin of a 3-D survey has changing offset distributions, the CMP stack of all traces in a bin will display bin-to-bin amplitude variations. This variation in itself can produce an acquisition footprint (Figure 1). Some processors compensate for this with simple trace borrowing from surrounding bins to fill in the missing offsets and to provide smooth offset distributions in all bins. Although this may be successful in reducing the footprint, it also may reduce the resolution by degrading the high frequency content.
Generally
it has been thought that acquisition footprint is far worse in the
shallow part of the
Wide
recording patch geometries are far more accepted these days than narrow
patch geometries. The reasons are numerous and range from reduction in
acquisition footprint (particularly that due to back-scattered shot
noise) to improved statics solutions and the availability of large
channel capacities on In addition to the impact of the fold variations, acquisition footprints are made worse by source-generated noise trains that penetrate our data sets (Figure 2). The lower the signal-to-noise ratio is, the worse the footprint will be. Unfortunately, the noise typically has a low frequency content that is much less affected by attenuation. Therefore, the noise becomes more prominent relative to the signal content deeper in the section. Our experiences have shown that acquisition footprint problems can be just as prevalent in the deep section as they are in the shallower section.
Non-Geometry EffectsIf surface access is poor because of topography variations, tree cover, towns, etc., we irregularize the geometry by moving source points to locations of easier access, and, therefore mask the acquisition footprint. It is still present, however. The footprint is just so much harder to identify. Weather and surface conditions may also impact the recorded amplitudes. A swamp in the middle of a 3-D survey can have a significant impact on the amplitude of a stack volume. The interpreter reviewing the 3-D volume has to decide what is a geological anomaly and what is acquisition footprint. Not always an easy task!
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