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CRS technique for advanced prestack merging and regularisation of vintage 3D seismic dataGuido Gierse* ,Dennis Otto, Arnim Berhorst, Henning Trappe, Juergen Pruessmann, TEEC
Summary
The merging of seismic data of different origin is a
common task in the reprocessing of vintage 3D seismic
data. In contrast to poststack merging, the prestack merging
is rewarded by the much broader possibilities of prestack
migration and analysis techniques but requires a larger
effort to adjust different acquisition and bin geometries,
and to interpolate missing data in the binning grid of the
merged dataset. In this case study, a new strategy is
proposed using Common-Reflection-Surface (CRS) partial
stacking for both, the merging and the regularisation of the
prestack data from two 3D marine surveys. The acquisition
already provided some irregularities in the CMP and offset
coverage of both surveys, which are increased by the
adjustment of the binning grids requiring a smaller grid cell
in one of the datasets. In a addition, the overlap zone of the
two surveys exhibits a general decrease of the coverage.
The data which are missing in the regular CMP/offset grid
of the merged dataset are recovered by partial CRS
stacking of original traces in the CMP/offset vicinity of a
missing regular trace. This data mapping benefits from the
detailed event description in the CRS attributes derived in
the CRS zero-offset stacking workflow. It combines a dip-
consistent interpolation of the prestack data with a
significant increase of the signal-to-noise ratio as part of
the partial CRS stacking.
Introduction
Modern acquisition equipment, and increasing processing
capacities based on high-performance IT technology have
stimulated a steady growth of project sizes in 3D seismic
surveying. This trend to larger units has also influenced the
reprocessing of old 3D seismic data, that often had been
acquired in much smaller patches. The merging of several
small or medium 3D seismic surveys of different vintages
has thus become a common task in seismic exploration
projects. Since contemporary processing sequences have
replaced former poststack imaging by prestack migration
techniques the merge is generally performed in prestack
domain.
Before merging, different acquisition footprints, signal
characteristics, amplitude levels, and static shifts are
commonly adjusted separately in the individual 3D surveys.
The prestack merging then aims at a maximum
homogeneity of the resulting dataset, not only comprising
the similarity of the seismic events in traces from different
sources, but also the structure of the dataset. With a
consistent regularisation throughout the dataset prestack
migration is expected to minimize migration noise, and
produce the best results. This case study concentrates on
the aspect of adapting the dataset structure in a 3D seismic
merge project, and proposes a new workflow based on CRS
regularisation.
CRS interpolation strategy
The CRS method, or Common-Reflection-Surface method,
was originally developed by Hubral et al. (1999), Mann et
al. (1999), and Jaeger et al. (2001) within the concept of
macro-model independent imaging (e.g. Gelchinsky 1988).
CRS zero-offset stacking assumes local reflector elements
with dip and curvature in the subsurface that give rise to the
seismic reflections. The corresponding CRS stacking
parameters, the so-called CRS-attributes, accordingly
comprise the wavefield dip together with wavefront
curvatures observed at the surface. They define hyperbolic
CRS stacking surfaces that extend across several CMP
locations, and thus collect high-fold contributions from the
prestack data.
The CRS attributes are optimized locally for each point of
the image, thus providing a detailed kinematic description
of the seismic events in the data that can also be used for
mapping seismic data to a regular grid of traces. Event data
from original traces in the vicinity of a regular trace is
mapped to that regular trace by dip-consistent partial CRS
stacking, based on the CRS attributes.
This CRS interpolation strategy has proven to be a suitable
tool for regularizing CMP and offset coverage within single
3D seismic datasets (Gierse et al. 2009). Similar
regularization techniques based on the local measurement
of time dips and curvatures have been successfully
performed in various data domains by Hoecht et al., 2009,
but without associated model assumptions of local curved
and dipping reflector elements from CRS zero-offset
imaging.
The merging and regularisation of vintage 3D seismic data
with various acquisition designs generally includes more
complex interpolation tasks than single 3D seismic
datasets. Incompatible binning grids due to different
acquisition parameters lead to large portions of empty grid
cells after regridding.
Merging and regularisation of two 3D seismic surveys
The 3D seismic data to be merged in this case study
comprised two marine surveys from the Norwegian North
Sea which differed not only in subsurface fold but also in
acquisition direction and bin cell size. The fold maps and
CRS technique for advanced prestack merging and regularisation of vintage 3D seismic data
Figure 1 - Original CMP fold maps of two marine surveys to be merged. Both maps cover the same area at the same scale showing the strong
fold variations of the surveys. The corresponding bin cell geometries are added as dotted rectanglar frames at strongly exagerated scales. A merge
example line illustrates the direction of data extraction before and after merging in the gather and stack displays of Figurey 2 and 3, respectively.
the original bin cells are displayed in Figure 1 for these
datasets termed Survey 1, and Survey 2, respectively. The
inline directions of the two surveys are perpendicular to
each other, as well as the original rectangular bin cells. The
inline width of 12.5 m of these cells had been defined
previously after trace decimation from the acquisition width
of 6.25 m. In addition to the different orientations of the
inlines, the surveys exhibit strongly inhomogeneous fold
distributions in the fold maps of Figure 1. Strong feathering
and irregular acquisition required a data regularisation even
for Survey 1 where the bin cell was retained in the merge.
Survey 1 incorporated a bin cell of 12.5 m X 25.0 m which
was adopted for data interpolation and regularisation in the
merge project. This merge geometry strongly contrasted
with Survey 2 showing a bin cell of 37.5 m X 12.5 m due to
the different orientation and separation of the streamers.
The regridding in the merge project intended to refine the
grid interval in the first dimension from 37.5 m by a factor
1/3 to 12.5 m, and to coarsen the interval in the second
dimension from 12.5 m by a factor 2 to 25.0 m.
The prestack merge and interpolation procedure is
illustrated along a line following the inline direction of
Survey 1 like the example line in Figure 1. Simple re-
binning of Survey 2 to the bin grid of Survey 1 produced an
irregular and sparse data distribution in the CMP gathers of
the new inline and crossline grid, with a large proportion of
empty grid cells, and of grid cells with large offset gaps.
Figure 2a (top) shows some CMP gathers after rebinning in
the overlap zone, and in the adjacent regions of both
surveys. Survey 1 which enters the merge procedure
unchanged provides a reasonable offset distribution but also
exhibits some missing offsets due to the irregular
acquisition fold as shown in Figure 1. In the overlap zone,
only partial offset ranges are covered within the CMP
gathers, or there is hardly any data at all in these gathers. In
Survey 2 the rebinning fills each CMP gathers at several
small offset ranges only in which the data happens to fall
into the redefined smaller grid cells.
CRS technique for advanced prestack merging and regularisation of vintage 3D seismic data
Figure 2b (bottom)
displays the correspon-
ding CRS gathers in
which missing data
were reconstructed by
partial CRS stacking of
original data from the
vicinity of each desired
new CMP / offset loca-
tion. The partial CRS
stacking completely
filled the large data
gaps in the low-fold
overlap zone and in the
regridded Survey 2,
and also compensated
for the irregular offset
distribution of Survey
1. In addition, the
partial stacking strong-
ly increased the signal-
to-noise level.
In this CRS data
interpolation strategy,
the missing data were
reconstructed from the
kinematic event infor-
mation supplied by the
CRS attributes. Unlike
conventional grid map-
ping and regularisation
methods, this CRS
technique did not
require any interpolat-
ion between neighbor-
ing shots, or flexible
binning techniques that
degrade dip and
resolution.
NMO stacks of the
prestack data before
and after this regulari-
sation served as a
quality control as
shown in Figure 3. The
near-surface data gaps,
and some fully missing
traces were consistent-
ly filled by the CRS
interpolation strategy.
As expected, the CRS
technique did not
Figure 2a – CMP gathers after data regridding to the merge grid. Note the irregular offset coverage.
Figure 2b - CRS gathers corresponding to CMP gathers of Figure 2a after partial CRS stacking.
Survey 1 Survey 2Overlap zone
Survey 1 Survey 2Overlap zone
CRS technique for advanced prestack merging and regularisation of vintage 3D seismic data
imply any loss of resolution. On
the contrary, the image was even
improved due to the compensation
for some irregularities and fold
variation in the original data, and
due to the noise suppression
during the partial CRS stacking.
Conclusions
This case study demonstrates the
successful prestack merging of
two 3D marine datasets using a
CRS interpolation and
regularisation strategy. The
strategy is based on the CRS
attributes from the CRS zero-
offset stacking workflow that
represent a detailed database of the
local kinematic behaviour of
seismic events in multicoverage
data. These attributes allow a local
mapping of existing seismic data
to a regular merge grid in the
CMP/offset domain. The strength
of this CRS interpolation and
regularisation technique is
emphasized in areas where the
merge grid implies a refinement of
the original bin grid, and an
associated deterioration of the
original CMP/offset coverage.
Partial CRS stacking of original
data in the vicinity of each regular
CMP/offset location results in so-
called CRS gathers. These CRS
prestack gathers show a uniform
CMP/offset coverage with a
complete recovery of missing
prestack data, and a significant
increase of the signal-to-noise
ratio. This merged and regularized
prestack data is considered as an
ideal input for prestack migration
which is presently performed.
Acknowledgements
We thank Wintershall Norge ASA
for their kind permission to
present their data.
Figure 3a – Zero-offset stack of CMP gathers as in Figure 2a after data regridding to merge grid.
Figure 3b – Zero-offset stack of CRS gathers as in Figure 2b after partial CRS stacking.
Survey 1 Survey 2Overlap zone
Survey 1 Survey 2Overlap zone