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SEISMIC
RESOLUTION
: A KEY ELEMENT*
By
Robert E. Sheriff1
Search and Discovery Article #40036 (2001)
*Adapted for online presentation from article of the same title by the same author in Geophysical Corner, AAPG Explorer, October, 1997. Appreciation is expressed to the author and to M. Ray Thomasson, former Chairman of the AAPG Geophysical Integration Committee, and Larry Nation, AAPG Communications Director, for their support of this online version.
1University of Houston, Houston, Texas ([email protected]).
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Click here for sequence of Figure 2A (F-37) and 2B (F-39).
Click here for sequence of Figure 3A and 3B. Click here for sequence of Figure 3C and 3D.
“ If we consider two similar features (Figure 1), the measurable anomalies that they produce may show as separate, distinguishable anomalies when the two features are well separated – a condition we call “resolved.” When they are close together, however, their effects merge and it is impossible (or at least difficult) to tell that two rather than just one feature is present – and this is a condition we call “unresolved.” The
problems of The
practical problem of To
quantify the separation of resolved from unresolved domains, mathematical
criteria have to be set; yet, different criteria in the literature lead to
different definitions of “resolvable limit.” The first definition by Lord
Rayleigh with respect to optical In seismic
work one usually distinguishes between To improve
the Horizontal
The
stratigraphic situation illustrated in Figure 2 is taken from some South Texas
studies by the Bureau of Economic Geology (supported by the Gas Research
Institute and the U.S. Department of Energy). In Figure 2, a succession of
channels, which are commonly productive, lie on top of each other. The different
channels are isolated from each other so that each is a separate reservoir. This
is clear from production records as the different channel reservoirs have
different pressure histories. The evidence of the stacked channels blends
together to create a single low frequency reflection that wanders up and down
across the section depending on which of the stacked channels is present at a
given location. The Figure 3 shows reflection F-11, which appears to be relatively uniform over the entire area and unaffected by the channels shown as F-37 and F-39, which meandered over the area. It is assumed that the F-11 reflector indicates the surface of the earth at the time of deposition and that, except for overall subsidence, the surface did not change very much between F-11 and the time of the productive channels (F-37 and F-39). Thus, after we flatten data on F-11, any slices through the 3-D volume of seismic data parallel to F-11 but displaced below it represent surfaces of the earth at times earlier than F-11 time. A display of such data is called a horizon slice. Horizon slices are used to remove the effects of deformation subsequent to deposition. This area was subjected to both faulting and tilting since deposition, but these effects have been largely removed by the horizon slicing process. Figure 2
shows the F-37 and F-39 horizon slices that are spaced only 4 ms (0.004 seconds)
apart. These two horizon slices show different features, especially the channels
in the lower right but also elsewhere. The Figure 3
shows portions of two east-west lines across the channel shown in the lower
right quadrant of Figure 2. It would be very difficult to separate the channel
features on the Spatial
sampling is clearly a factor in horizontal
Sheriff, R.E., 1991, Encyclopedic Dictionary of Exploration Geophysics, Third Edition: Society of Exploration Geophysicists, 384 p. Sheriff, R.E., 2001, Understanding the Fresnel zone: Search and Discovery Article #40014. |