In the presence of HTI (horizontal transverse isotropic) media seismic wave velocity will vary with shot-receiver azimuth. This velocity variation with azimuth (VVAZ) causes several problems with imaging and AVO analysis, but it is also a source of extra information that we can use to infer characteristics about a reservoir.
If the goal is to improve imaging and AVO analysis by making the gathers flatter then VVAZ analysis can be done and the data then pre stack migrated or stacked using the VVAZ field.
To derive a velocity field to flatten the gathers for AVO analysis and improve the imaging
Input is isotropic or VTI PSTM gathers in either COV or COA domain
Analysis (HAMMER) produces a fast velocity field, slow velocity field and azimuth of the fast velocity
The data is again Pre stack migrated using VVAZ field
HAMMER is a standalone interactive velocity analysis tool for picking Anisotropic (HTI) velocities. The program first identifies the major horizons and estimates an isotropic velocity that best flattens the data. The program then estimates residual move-out on the horizon by a short window cross correlation between the trace and a near offset stack trace. These residual shifts are then fed into a least squares curve fitting algorithm that estimates the fast and slow velocities and their azimuth for that specific horizon. These attributes are calculated independently for each horizon and for each control point.
If the goal is to measure the anisotropy of a zone then our preferred method is to directly measure the azimuthal variation of the travel time from the top of the zone to the bottom.
The approach used by Absolute Imaging is an adaptation of the layer-based method first described by Ye Zheng in section 3.5 of his PhD thesis (Zheng, Y., 2006) ‘Seismic azimuthal anisotropy and fracture analysis from PP reflection data’, and subsequently revisited by Wang et. al. in 2007 (Wang, J., Y. Zheng, M. Perz, 2007, ‘VVAZ vs. AVAZ: practical implementation and comparison of two fracture detection methods’)
Zheng’s approach is to isolate the HTI effects ascribable to a particular interval (layer) by measuring the shifts required to flatten the horizons corresponding to the top and bottom of the fractured layer and modeling, for each trace, the difference between the top and bottom shifts as a function involving properties of the layer, and the ray geometry.
A powerful aspect of the method is that by taking differences between correcting shifts, any possible HTI ripple propagated down from further up in the overburden is largely cancelled out by the subtraction.