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Western Onshore Basin, ONGC, Vadodara,
Email: [email protected]
10th Biennial International Conference & Exposition
P 407
Effective Surface wave attenuation in cross spread domain
– A case study from Cambay Basin
Anjan K. Bakshi*, Puneet Saxena
Summary
Surface wave noise generally obscure seismic signal during recording of data by single point sensor in land seismic reflection
exploration. Key problem is how to effectively attenuate surface waves and remnant surface waves during the noise attenuation
in processing. This work demonstrates a technique of attenuating surface waves in the three dimensions frequency-
wavenumber domain on cross spread based on a grid of modified Inline and Crossline without altering the actual shots and
receivers positions. Here the new grid we call it as pseudo grid. The method is applied to the raw 3D shot gather of seismic
data acquired in the Western Onshore Basin, Vadodara. For the remnant surface wave mute guided multichannel spectrum
editing technique is applied. Finally, comparing the time slices and gathers before and after using the strategy, it is observed
that the method, described here, attenuates surface waves and remnant surface waves effectively and improves the signal to
noise ratio without weakening the desired reflected signal.
Keywords: Cambay Basin, Surface Wave Attenuation
Introduction
Strategy of noise attenuation with respect to signal depends
upon the fact how the characteristics of noise differs from
signal in terms of a particular physical quantity in a
specific domain This work deals with surface noise
attenuation in cross-spread domain.
Due to several reasons it may not be possible to take shots
with regular orthogonal geometry (fig.1a). In such
situation layout of a particular cross-spread with respect to
a receiver line is not be an ideal cross
(fig.1b).Effectiveness of 3D Fk filter to attenuate the
Figure 1: Cross-Spread layout. a) All cross-spreads along a
receiver line 911. b) Single cross-spread along the same receiver
line with respect to shot line 1651.
surface noise seems to be not reaching to its optimum level.
On time slice surface waves appear as concentric circles
but due to irregular shot disposition it does not remain so
(fig.2a). In the pseudo grid of modified Inline and crossline
numbers corresponding to regular orthogonal geometry it
becomes concentric (fig.2b) 3D FK Filter becomes
Figure 2: Cross-Spread Time Slices at 646 ms. a) Actual grid. b)
Pseudo grid.
effective in the pseudo grid. As the positions of source and
receivers are not changed therefore, in subsequent stages
of processing integrity of data is maintained
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Theory
The data acquired with orthogonal geometry can be
considered as a collection of cross-spreads (Vermeer,
2002). Each intersection of a source line and a receiver line
forms the center of a cross-spread. The dense sampling of
the sources along the source line and of the receivers along
the receiver line creates a dense single-fold areal coverage.
The midpoints of traces with the same absolute offset are
located on a circle with diameter equal to that offset.
Therefore, the first arrivals of the ground roll are lying on
the surface of a circular cone. Hence, the ground roll in a
cross-spread behaves as a truly three-dimensional event
and can best be removed by a 3D FK filter. Karagül et al.
(2004) demonstrated that this 3D filter performs
considerably better than the cascaded application of two
2D FK filters. But due to irregular shot locations those
circles are not perfect circles. In order to make them perfect
for optimum performance of 3D FK filter, a new grid was
created that had its X-axis along the inline direction and its
Y-axis along the crossline direction. In that new system
sources were binned using their X-coordinates so as to
obtain new source line header that would have a number
falling on the ideal source line by calculating the deviation
of shots from its ideal shotline in terms of no. of crossline
depending upon the binning of geometry and then by
making correction in crossline. The similar procedure can
be applied in case of irregularity in receiver position.
However, in the present study correction is applied to the
crosslines only as the receiver lines are almost straight.
Procedure
Surface wave attenuation procedure was carried out on a
3D seismic volume which was taken from Western
Onshore Basin. Time slices of shot gather in both actual as
well as in pseudo grid has been shown in fig.3.
Figure 3: Cross-Spread Time Slices at 3600 ms. a) in original
grid. b) in pseudo grid.
The slices show that the circles are more regular in pseudo
grid. Application of 3D FK filter was showing better result
when applied in new grid system in comparison to that
applied in original grid (fig.4). It is obvious that the surface
Figure 4: Application of 3D FK filter. a) in original grid. b) in
pseudo grid.
noise was not fully attenuated in both the grid system but
time slice in pseudo grid is comparatively less noisy. When
above time slices were compared with the input dataset, it
reveals the effectiveness of psudo grid(fig.5). All the data
set were shown in the same grid that is in the original grid
so that one can have the idea how much surface noise was
attenuated when the data was brought back into the original
grid. In order to attenuate remnant surface noise, frequency
dependent multi channel spectral editing was carried out in
original grid. Application was restricted to the surface
wave noise zone only by using an internal mute. Noise
band was kept below 14 Hz. Time slices in fig.6 shows the
comparison of remnant surface noise attenuation in both
Figure 5: Comparision of 3D FK Filtering. a) Input. b) in original
grid. c) in pseudo grid
Figure 6: Comparision of residual surface noise attenuation. a)
Input. b) 3D FK filtering c) 3D FK filtering and frequency
dependent multichannel editing.
the grids and then those were compared with the input data
set. Further, to validate the result a comparative study was
carried out in shot gather mode also (fig.7).
3
Figure 7: Comparision of 3D FK filtering in Shot gather mode.
a) Input. b) in original grid c) in pseudo grid
Figure 8: Comparision of data set after surface noise
attenuation. a) Input. b) 3D FK filtering c) 3D FK filtering and
frequency dependent multi channel editing.
Input data in shot gather mode was also compared with the
data set after applying 3D FK filter followed by frequency
dependent multichannel editing (fig.8). Care was taken in
designing of filter so that no signal is lost. That has been
shown in fig.9 by taking differences between input and the
filtered data sets. The display gain of the difference section
(fig.9b) was slightly increased in comparison to ensure that
no signal should lost.
Figure 9: a) Input data set. b) difference with denoised data set
Conclusions
Surface wave attenuation in cross spread domain after
reorientation in pseudo grid can lead to better imaging of
final output in case of data acquired with orthogonal
geometry. Quality is further increased by applying mute
guided frequency dependent multi channel editing in
cascade with 3D FK filtering.
Acknowledgements
The authors are thankful to ONGC for providing
infrastructure to carry out this work. The authors are
thankful to Shri S.K.Das, ED-Basin Manager, WON Basin
Baroda for permission to publish this work. Thanks are due
to Shri U.S.D. Pandey, GGM ,Head Geophysical services,
Vadodara for his keen interest and providing support for
acquisition of the multicomponent data. The authors are
thankful to Shri G. Sarvesam, Ex ED-OSD, Project
Coordinator, 3D-3C and Shri B K Barve, GM (GP), 3D-
3C for their initiatives and valuable suggestions during the
project. Authors are also thankful to Shri M K Jain,
DGM(S) and Shri S S Singh, DGM(S) for the critical
analysis during study.
Last but not the least, thanks to all the members of 3D-3C
project who were directly or indirectly involved in the
analysis of the data.
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The views expressed in the paper are solely of the authors
and do not necessarily reflect the views of the organization
which they belong to.