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Seismic Processing

Surface seismic data processing (Land & Marine)

REX Geo Solution focus on offshore and onshore surface seismic processing in the vicinity of the well / reservoir, and aim at seamless integration with any other available seismic data as VSP or OBN (offshore). Well seismic data provides a unique tie between the subsurface and surface seismic data and by using this knowledge we can improve the quality of the processing of both the surface seismic and well seismic data. VSP data provides a link between the ultra-high resolution wireline logs with the more limited surface seismic. The VSP data and the additional information it provides gives a unique opportunity to understand and improve the surface seismic.

Seismic Anisotropy

Walkaway and Offset VSP's are excellent tools for quantification of anisotropy. Using walkaway VSP's both the average anisotropy (from surface to receiver) and local anisotropy (at the receiver) can be quantified. REX Geo Solutions migration algorithms include compensation for anisotropy.

Some of the key information that can be derived from well seismic data and later applied to the processing and interpretation of surface seismic data are:

Seismic anisotropy.

Anisotropy is caused by internal layering, grain orientation, thin-bedding and cracks in rock formations i.e. the seismic velocity changes with the direction of propagation in the medium. Anisotropy again leads to non-hyperbolic moveout for reflections and if not accounted for gives inaccurate velocity estimates and deteriorates the quality of stacked sections.

Anisotropy symmetry axis (TTI)

In fractured reservoirs another important factor is the symmetry axis of the anisotropy. A medium with a non-vertical symmetry axis will have variable anisotropy depending on the propagation direction of the seismic signal. REX Geo Solution can estimate the symmetry axis orientation and apply this to migration algorithms which includes TTI anisotropy compensation. A tilted anisotropy symmetry axis can result in a significant positioning error of the image if not accounted for in the migration. 

Attenuation factor (Q)

Seismic wavetrains are attenuated as they propagate through the earth. Intrinsic attenuation is caused by grain-to-grain friction in sedimentary rocks and high frequencies are more attenuated than lower frequencies. This reduces the frequency content of the seismic section and results in lower resolution. REX Geo Solutions software includes migration algorithms that compensates for the attenuation factor.

Multiple control

Surface seismic data can sometimes be hampered with multiples and especially intra-bed multiples which can be difficult to identify. VSP data is an excellent tool for multiple control since the downgoing wavefield is recorded in the subsurface and can effectively be removed. VSP data can therefore be used in combination with high-resolution well logs and synthetic seismograms to identify and more effectively remove multiples from surface seismic.


Wavelet & phase control. High-confidence tie of well data to surface seismic

VSP data represents an important link between the high-resolution wireline well logs and the regional image of the surface seismic. To make a high-confidence tie between the surface seismic and the well data it's essential to have the same wavelet and phase characteristics in both the surface seismic and the VSP data. REX Geo Solutions have several tools for wavelet estimation and the matching of VSP data and surface seismic data.

OBN / OBC data processing

With a processing sequence adapted from VSP methodology our techniques differ quite significantly from conventional OBS data processing. First, to achieve a tie between the final P-P and P-S images in the depth domain, we apply depth processing instead of time processing.  Secondly, instead of assuming that Vz contains all P-P and that Vx contains all P-S, we make use of the 4C elastic decomposition algorithm proposed by Amundsen and Reitan (Geophysics 1995) to extract separate down going and up going P-P and P-S wave modes. Third, due to use of this wave equation based decomposition algorithm, we expect to be able to extract scalar down going wavefields even from data at large offsets. This is not possible if the corresponding conventional tool (Vz–P summation) is used. Further, compared to the use of conventional OBS data processing (Vz-P summation), we are able to better utilize the benefits of having 4C measurements, by deconvolving the up going wave modes with the extracted down going P wave mode. The main objective in using the down going wavetrain deconvolution is to remove all free surface related multiples (i. e. also upgoing multiples) from the wavefields.

All the processing is performed on common receiver gathers, meaning that with large node/geophone separation we do not have the problems conventional processing has.

Additionally, we use an event-correlation procedure for P-S receiver static correction. This procedure should work independently of the complexity (i. e. dip variations) of the horizon selected for the analysis. Thus, instead of deriving the time shifts by using only the P-S data, we also identify (pick) the horizon on the P-P section (in depth). The static correction is then defined as the shifts required to move the horizon from the location on the P-S section to where it appears on the P-P section (in depth).

The methods are implemented and valid in both 2D and 3D with the only restriction being those of standard tau,p domain processing. 

From our experience with VSP processing we are also able to fully benefit from parameters extracted from well data. In particular vertical P- and S-velocities, anisotropy parameters, Qp and Qs (with seabed shear source), wavelet extraction & fitting, shear wave splitting and quantification of P-S conversion with depth. By extraction/separation of scalar downgoing wavefields in both VSP & OBS data and with use of deterministic deconvolution we know the phase of the data and can shape the output as desired.

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