Course News 3
Aug 17th 2018In the “Advanced Seismic Data Acquisition and Processing” Course two new topics have been introduced: 1) Blended acquisition and 2) Penta-source acquisition.
In blended acquisition sources are fired closely in time such that the records do overlap. To separate the responses of the subsurface two methods can be applied.
One method is called “dithering” in which a “random” variation is applied to the auxiliary sources with the result that their responses are non-coherent and can be considered as random noise and be removed/suppressed in processing. As the “dithering” for each auxiliary source is known, it can be removed resulting in coherent records for each, leaving the other source records incoherent.
The other method uses “seismic apparition”. In this method a periodic variation, be it time delay or amplitude scaling, is applied to each source and each source response can be made to “appear” at the expense of the responses of the other sources this is often done in the KF domain.
The Penta-source (Polarcus), consists of 6 arrays (spaced 12.5 m apart), which are used in more than one source configuration resulting in 5 single sources. For operational reasons they are fired in the following sequence: (1,2), (3,4), (5,6), (2,3), (4,5). They can be used with a shot-point interval of 12.5 m and “dithered” over 1 s, or with a shot interval of 62.5 m and “dithered” over 5 ms, allowing deblending of the shot records. Note that the first part (1 or 5 s) of the record is “clean” and do not require deblending.
In both cases also the streamer spacing might vary, where an increasing separation from centre spread outwards is beneficial for a high-resolution shallow image.
Course News 2
to the “Effective Media and Anisotropy” an extensive AVA modelling
exercise has been implemented. It allows investigating the various
aspects of AVA analysis, namely what rock and fluid properties can be
derived from observing the Amplitude changes with Angle of incidence on
the interface between two rocks. It not only deals with density, P- and
S-wave velocities, but also with anisotropy. From the anisotropy, not
only average properties of sub-seismic-scale fine layering, but maybe
even more important fracture orientation and density can be derived.
These are important in relation to where to drill production wells
relative to injection wells (think of fractured carbonates).
In the exercise the AVA response of different rock interfaces (shale, sand, salt, limestone, etc) can be modelled using Vertical Transvers Isotropy (VTI) for fine layering and Horizontal Transvers Isotropy (HTI) related to fractures. Although more than one fracture set do occur in nature, the exercise is limited to one fracture set, based on the idea that most often it is the present stress regime that keeps fractures open in the minimum horizontal stress direction.
Course News 1For the “Quantitative Reservoir Characterisation” (QRC) course a new module has been designed concerning Effective Media and Anisotropy. The Earth is often too complicated in terms of inhomogeneities to fully honour it in modelling wave propagation and inversion. The solution has been to replace the real earth by a so-called effective medium. This effective medium allows an “appropriate approximation” of the wave propagation and inversion. Although an approximation, it is still appropriate for solving the issue at hand.
One main application is related to the concept of anisotropy. Anisotropy occurs when, for example, we deal with thin sequences of different rocks or in case of fractures. If these “inhomogeneities” are on a sub-seismic wave-length scale the inhomogeneous medium can be replaced by a homogeneous medium with “anisotropic" character. The anisotropy is then an appropriate replacement of the sub-seismic scale inhomogeneities and allows derivation of the rock properties using seismic observations. These anisotropic properties tell us that the wave propagation is not only a function of location (the commonly considered large scale inhomogeneity), but also at each location a function of the direction of propagation.
In the exercise, you will learn how effective media velocities are calculated. There are namely two ways and how in modelling these effective anisotropy parameters can be calculated. You also will derive anisotropy directly from seismic data.
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