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Downloaded 10/25/17 to 80.82.77.83. Redistribution subject to SEG license or copyright; see Terms of Use at http://library.seg.org/
The application of a new static correction technique for seismic data in complex surface area: a
case study of PD 3D area in Tarim Basin
Guo Nianmin,Chen meng,Cui Yongfu,Fang Bing, Research Institute of Exploration and Development, Tarim
Oilfield Company, PetroChina, Korla , China
Summary
Static correction technology is the core technology of
seismic data imaging in complex surface area. Based on the
traditional tomography static correction technology, we
propose an improved workflow of tomography static
correction technology, including picking up precise first
break, progressive tomographic inversion from shallow to
deep, picking up isovelocity surface as datum surface for
static correction, etc. We apply the technology in the PD
3D area for practical application. The problem of static
correction is effectively resolved and the seismic image
quality is improved.
Introduction
Tarim Basin has always been a key area of oil and gas
exploration in the Tarim Oilfield. For example, the surface
conditions of the PD 3D area are complex. The PD 3D
work area is high in south and low in north as a whole with
elevation between 1700-3000m. The northern part is mainly
Gobi, which local distributing a small amount of village
farmland. The terrain is relatively flat; and the southern part
is loess hill, with undulating topography. The
characteristics of this area are shown as follows: (1) The
surface structure of this area is complex and the relative
height difference of the surface is up to 1000m from
southwest to northeast; (2) The lateral variation of surface
and near-surface lithology is large; (3) The weathering
layer has a great change in the velocity and thickness. As
shown in Fig. 1, the gathers in typical positions with static
problems (thick loess area, high-speed surface area and
Gobi area) are selected, where the loess is thick and the
near-surface velocity near-offset is high and the first break
is undulating. At the same time, the lateral variation of
velocity near surface makes the static correction becoming
more complex. Such surface conditions bring great
difficulties to the problem of static correction.
Fig. 1 The variation of surface conditions in this work area
© 2017 SEG
SEG International Exposition and 87th Annual Meeting
Fig. 2 The source gathers under different surface conditions
At the moment, there are three methods to solve the
problems of static correction: the model method, the
refraction static correction, the tomography static correction
technology. The model method is a mathematical method of
linear interpolation, which is more effective in areas with
simple surface conditions. Refraction static correction is
based on the hypothesis of horizontal layered model, and it
can obtain the ideal effect with good stability and efficient
in the area where the ground surface is smooth and the
lateral velocity uniformity is good, and the obvious
refraction surface has good stability. However, due to the
simple model assumption, the method of refraction static
correction is generally not ideal in the areas where the
ground surface fluctuates strongly and the high-velocity
layer is exposed. Because the difference of the elevation
and the reverse of speed and the lateral variation of the
velocity of weathered layer.
In the complex mountainous area where the old stratum
exposed, the surface fluctuates violently and the velocity of
the surface layer changes greatly. The surface model does
not accord with the layered assumption, and the refraction
static correction is ineffective, while the tomographic static
correction utilizes all the information of the first-arrival
waves, including direct wave, refracted wave, inflected
wave. It is not necessary to distinguish the type of the first
wave and has no assumptions and limitations on the
variation of the surface elevation and the velocity
distribution of the subsurface. In theory, we can solve the
problem of static correction in complex mountain areas by
reliable inversion to build near surface depth velocity
model.
In order to solve the problem of static correction in this area,
an improved tomography static correction technology
workflow is proposed. Firstly, the initial near-surface LMI
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layered model is obtained by first picking, and then the
near-surface grids model is gradually obtained using
reverse branch tomography inversion. Finally, the static
corrections are calculated based on the final model.
Technology and Method
The first break picking up is the basement for tomography
static correction and the key to get the right result. We
adopt the self-developed neural network to pick up the first
break for records before NMO, to avoid the large
morphological changes in the phenomenon caused by the
large variety of lateral velocity after NMO. The energy
discrimination is made on the basis of the instantaneous
amplitude. At the same time, the shape of the first arrival is
identified by pattern recognition, and then polynomial
fitting and multiple iterations are carried out, which can
accurately and efficiently pick up the first arrival and
greatly improve the efficiency, and ensure the accuracy of
static correction and real reliability.
The first break with offset gradually increasing roughly
reflects the shallow velocity model from shallow to deep.
Then start tomographic inversion from the small offset.
And when the previous inversion of the final model has
good convergence, gradually increase the larger offset for
the tomographic inversion iterations. The last final model
used as the initial model of the next tomographic inversion,
can be relatively fixed the velocity of shallow layers, which
equal to updating the shallow velocity model from shallow
to deep gradually.
In the case of complex near-surface conditions, the surface
longitudinal and lateral velocities change greatly. Using the
traditional isobathic surface as the static correction surface
will bring large errors. In this paper, the isovelocity surface
with replacement velocity of 2500 m/s was used as the
static correction surface. The calculated static corrections
are more accurate than those calculated from the isosceles
surface.
Example
The tomography imaging technique is a reverse branch
inversion method using first break, which treats the surface
model as an arbitrary medium. It does not limit the
difference of surface height, velocity and refraction
interface and it can adapt to the vertical and lateral variety
of velocity of weathering layer. It is more suitable for
regions with complex surface structure, and can obtain
accurate correction amount. First of all, an initial model is
created by splitting the subsurface into grid cells, where the
rays from the source to the receiver pass through the
subsurface cells, where the velocity values for each element
are constant. Then calculate the simulated first arrival time
by ray tracing, and then modify the model to minimize the
difference of the first arrival time between observations and
calculations. The optimization process of the inversion is
achieved by linearizing a large-scale nonlinear least squares
problem and iteratively calculating the near-surface model.
The iterative reverse branch tomography inversion with
offset increasing can be used to constrain the shallow model.
Aiming at the difficulty of first break picking up in
complex area (Southern Loess Plateau, Middle Foothill
Regions, Northern Gobi Desert), the method of automatic
picking plus two manual modification is adopted to
complete all offset first break picking up as shown in Fig. 3.
① Automatic pick up the first break from the beginning to
4500 offset and manual modification; ② Calculate the
initial tomographic static correction using the first break in
4500 offsets; ③ Apply the initial tomographic static
correction to the original single shot and manual pick up the
first break of all offsets; ④Minus the initial tomographic
static correction and obtain the true first break of all offset;
⑤Using the final first break to compute the tomography
statics.
Fig. 3 First break pick up
After two manual picking up and two tomographic
inversion iterations, it is a good solution to the correction
problem in this work area due to the complex terrain, the
© 2017 SEG
SEG International Exposition and 87th Annual Meeting
large undulating surface, the unevenness of the low velocity
band and the large lateral velocity variation. The advantage
of reverse branch tomography imaging technique is that the
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tomographic inversion is not affected by topographic
fluctuation, lateral velocity variation and underground
interface inclination. It can better fit the original first arrival
time and more fully excavate the first arrival time. Also it
can accurately reflect the variety of the lateral velocity and
make the stratification in the vertical direction more precise.
It can solve the primary static correction problem of
complex data and accurately retrieve the shallow velocity
model. As a result, the accuracy of the seismic data is high,
which can maintain the authenticity of small-scale
structures and improve the imaging quality of seismic data.
The thickness of the loess layer in this project area is not
uniform and the variety of lateral velocity of the loess layer
is large. The isovelocity surface with replacement velocity
of 2500 m/s is more precise than that of the traditional
isobathic surface.
As shown in Fig. 4, the traditional 4500m offset singletomographic inversion is compared with the final model
after the 15th-iterative tomographic inversion from 500m to
4500m offset in this paper. The velocity description of
shallow low-velocity loess layer is more clearly described
by the 15th-iterative tomographic inversion than the result
of traditional single-tomographic inversion. Fig.5 shows the
comparison of ray density with conventional tomographic
inversion and 15th iterative tomographic inversion. The
result shows that the ray density distribution using iterative
tomographic inversion is more reasonable in southern Loess
Plateau.
(a) conventional method
(b) modified method of this paper
Fig. 5 The comparison analysis of ray density
Figure 6 shows the comparison between the stacking
section after the conventional tomographic inversion and
the application of static correction using the 15 times
iterated tomographic inversion. It can be seen that the
stacking section after final static correction obtained by the
improved tomographic inversion obtained better results in
the southern shallow loess area (box area) and the PD
tectonic zone (oval area). Figure 7 shows the single-shot
comparison before and after tomography static correction.
It can be seen that the first arrival is well improved and the
static correction problem is effectively solved.
(a) conventional method
(a) applied the statics of conventional tomographic inversion
(b) modified method of this paper
Fig. 4 The final velocity model comparative analysis
© 2017 SEG
SEG International Exposition and 87th Annual Meeting
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the traditional isobathic surface. And the imaging quality
will be better. The practical application of the data from the
three-dimensional work area in PD proves that the
improved workflow of the static correction technique is
very effective and this technique can be applied to the static
correction of area with complex surface.
(b)applied the statics of modified method of this paper
Fig. 6 The comparison between the stacking section
after the conventional tomographic inversion (a) and the
application of static correction used in this paper (b)
(a) before
(b) after
Fig. 7 The seismic shot data comparison before (a) and after
(b) tomography static correction used method of this paper
Conclusions
Accurate first break picking up is the basis of tomographic
inversion in mountain area. The method of automatic
picking based on network plus two manual modification
used in this paper solves this problem. The proposed partial
offset multi-iterations tomographic inversion imaging
technique can not only reflect the lateral velocity change
finely, but also make the stratification in the vertical
direction more precise compared with the conventional
tomographic inversion static correction technique. For the
complex mountain area data, picking up stable isovelocity
surface as the static correction datum is more precise than
© 2017 SEG
SEG International Exposition and 87th Annual Meeting
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EDITED REFERENCES
Note: This reference list is a copyedited version of the reference list submitted by the author. Reference lists for the 2017
SEG Technical Program Expanded Abstracts have been copyedited so that references provided with the online
metadata for each paper will achieve a high degree of linking to cited sources that appear on the Web.
REFERENCES
Marsden D., 1993, Static corrections — A review, Part 1, Part 2, Part 3: The Leading Edge, 12: 43–49,
https://doi.org/10.1190/1.1436912; 115–20, https://doi.org/10.1190/1.1436936; 210–216,
https://doi.org/10.1190/1.1436944.
Jing X.-L., Tomography modeling for complex media and tests on models: Oil Geophysical Prospecting,
45: 66–71.
Jing Y. H., 2009, Seismic first arrival wave travel-time tomography and near surface velocity model:
Chang’an University.
Lin B. X., 2002, Tomography for LVZ velocity inversion and statics: Geophysical Prospecting for
Petroleum, 41: 136–140.
Wang X, and Z. H. He, 2010, Technology of static correction for multi-information constrained
tomographic inversion and its application: Natural Gas Geoscience, 21: 316–320.
Chen S. J., 2006, The application of seismic first arrival ray tracing tomography static in the complicated
near surface area: Geophysical Prospecting for Petroleum, 45: 34–39.
© 2017 SEG
SEG International Exposition and 87th Annual Meeting
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