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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,