In dealing with the issue of poor seismic imaging in
boundary of salt dome in study area, the sensitivity of
several attributes (amplitude, coherence and edge detection)
to the dome boundary are tested, With the use of a chosen
edge detection technique, The salt dome boundary is
achieved.
Geological setting
The study area is located in a region with abundant
petroleum resources, and potential structure position, In
this area, there are several salt domes of great thickness
ranging from 1000-3000 m, developed in Permian
overlying the target zone of Carboniferous. VSP data show
the velocity of salt varies from 4000 to 6000 m/s, which is
obviously higher than that of surrounding rock. Sub-salt
reflections are pulled up intensely due to the great velocity
difference between salt and surrounding rock and the
thickness variation of salt layer. The thicker the salt is, the
greater sub-salt reflection is pulled up. which makes the
sub-salt events appear as pseudo-structural configuration
so iso-chrone maps of Carboniferous are not able to reflect
its true shape. Therefore, the key to sub-salt structure
interpretation is determine the thickness and boundary of
salt dome .
Different shooting patterns resulted in seismic data with
the character of nonuniform distribution of dominant
frequency band and amplitude strength, low S/N ratio of
horizons related to Permian, and poor imaging quality of
reflection at the top and bottom of salt body. As the result,
it is difficult to define the salt dome boundary in the
seismic data .
Principle
Edge detection is one of common imaging processing
technique to find the position of pixels with intense
variation which can be express the useful information in
the data.
Edge detection is a basic method in image processing, that
detects the position of pixels that vary greatly in the image,
which can express a useful that value to the central sample.
In practice, this amounts to weighting the central sample
based on how similar it is to surrounding traces. If the
surrounding traces are consistently similar, the central
sample will be given a value near zero. If the surrounding
traces show marked variation, the central sample will be
given a non-zero value. Edge detection can thus give you a
clearer image of lateral dissimilarities caused by such
conditions as faulting or stratigraphic changes.
This technique can be applied to conventional seismic data
or to a dataset of similarity attributes. If the input is
conventional seismic data, edge detection preserves
amplitudes but weights them (as opposed to ESP
processing which replaces amplitudes with Manhattan
distance values). If the input is ESP data, edge
detection accentuates the differences you uncovered in
the Manhattan distance values generated from the
original seismic data. For the salt application
LandMark, the Sobel method which uses the first
derivative is used.
Sobel Edge Detection
In Sobel edge detection, the first derivative is
calculated in both the inline and crossline directions
for a 9-trace plane. The average of these two
orthogonal measurements is then assigned to the
center sample
In practice, the Sobel algorithm is implemented by
applying two sets of sample weights and then
combining the weighted samples. The first weighting
mask detects dissimilarity in one direction (at 90° to
the row of zeros).The second weighting mask detects
dissimilarity in the other direction (at 90° to the row
of zeros).By combining all of the weighted samples,
dissimilarity can be detected along any orientation.
Applied effect
Constrained by the plane dip, Sobel method is applied
to detect salt dome boundary in post stack seismic
data, and the time slices through edge detection cut
from shallow to deep level are used to delineate salt
dome boundary.
Conlusions
Not only can the boundaries of salt domes and faults
(especially high angle fault) be identified with the use of
edge detection technique, but the variation of lithological
property also can be detected. With the results of edge
detection interpretation, boundaries of salt domes are tracked,
and the thickness of salt layer is achieved by which the
velocity field built up.True structural configuration
of sub-salt target zone is achieved after time-depth
conversion.
Acknowledgment
We are grateful to Professor Huang Zhong fan for reviewing
this paper and giving many constructive suggestions;thanks
to wang ya lin for her supported.