Adequate data conditioning
The purpose of data conditioning is to enhance the signal-to-noise ratio by following a workflow that starts with the stacking of azimuthal sectored (6 sectors of 30o each) pre-stack data which yields the prestack migrated gathers. These are then put through a series of steps for signal-to-noise enhancement, comprising bandpass filtering, generating supergathers, applying random noise attenuation and trim statics, etc. as shown to the right of Figure below. All this is done in the offset domain, wherein the traditional processing of seismic data is carried out. In doing so, amplitude distortion due to anisotropy is overlooked. It is believed that traces exhibiting azimuthal velocity variations due to anisotropy when stacked deteriorate the quality of the far stack which is essential for extracting the fluid information from seismic data. Hence, we recommend another workflow as shown to the left of Figure below.
Flowchart for conditioning azimuthally-sectored seismic data for AVO analysis or simultaneous impedance inversion. The azimuthal amplitude variations in the data if stacked as such can deteriorate the quality of the far stack data.
Instead of stacking the azimuthally-sectored NMO corrected traces at every CMP, we suggest generating supergathers using adjacent CMPs and organizing those supergather traces in a snail gather for every CMP. In doing so we don’t expect to see azimuthal velocity variation at shorter offset traces. However, as we get to the larger offsets, we begin to see the azimuthal velocity variations in the form of undulations. Before the individual azimuth traces are stacked within every CMP trace, the azimuthal variation should be removed so that the traces are aligned for a better-quality stack. It may be mentioned here that in the azimuthal AVO analysis such an azimuthal variation is quantified into attributes such as fracture intensity and orientation. But for preparation of pre-stack seismic data for traditional AVO analysis or impedance inversion, the azimuthal variation does not need to be quantified. One of the methods for aligning the azimuthal variation on individual azimuth traces is to pick some horizons at appropriate intervals on the stacked data, and then overlaying them on the CMP gathers. Using a cross-correlation procedure within individual intervals (bounded by horizons indicated with colored arrows), the reflection events can be aligned. Now if the individual azimuthal traces are stacked for every CMP, the resulting gather traces appear flatter after conditioning and alignment. Furthermore, a comparison of far angle stacks when conditioning is followed in offset domain and offset-azimuth domain, is also shown below in Figure a and b, respectively. Notice the improved reflection events at the Montney and Ireton marker levels. The strong seismic events corresponding to Wabamun, Ireton and Swan Hills markers have been further strengthened after following the proposed workflow for data conditioning. Such conditioned data when taken into the impedance inversion show improved quality and detail.
Segments of far-angle stacked seismic sections (a) before, and (b) after offset/azimuth domain conditioning. The highlighted areas in orange and magenta outlines represent the zones of interest. The reflection events between the block arrows in pink, yellow and green seem strengthened after the proposed conditioning. Such enhanced definition of reflection events contribute to better characterization of the reservoir properties. (Adapted from Sharma & Chopra, 2019)
References
Sharma, R. K. and S. Chopra, 2019, Conditioning prestack seismic data in the offset-azimuth domain, published in the AAPG Explorer, October’19 issue, 18-19.