Accurate specification of interval velocity is key to the success of depth
migration. It is no surprise, then, that most of WIT's energy in processing
goes toward velocity analysis. Since 2008, we have filed two US Patent
applications on velocity analysis technology. Our two-phase program for depth
migration velocity estimation relies on two unique technologies:
- Migration Velocity Focusing Analysis (MVFA) - This is the first
phase of our two-phase velocity analysis program. Depth Focusing Analysis
predates ray-based reflection tomography algorithms that are now the industry
norm, and fell out of favor primarily because 3D wave equation migration (WEM)
was not computationally feasible in the early 1990's. Radically improved
computational capabilities enable iterative application of 3D WEM. In 2008,
we filed a US Patent
Application on a novel implementation of Depth Focusing Analysis called
MVFA. MVFA relates the time shift parameter from the time-shift imaging
condition for one-way WEM to a velocity error in a novel way.
In practice, we apply MVFA at more than 1000 shot locations throughout the model
and have implemented technology to automatically pick velocity semblances on this
multi-gigabyte volume of information. The picked velocities are then embedded
into a 3D layer model which is smoothed in an intelligent, layer-consistent
fashion. We find that MVFA offers the following advantages over conventional
velocity update techniques:
- Robust when the velocity is "far off" from the true velocity. In many
cases, an offset or angle gather may not show a pretty parabolic curve up or
down, but MVFA focusing panels indicate the direction of velocity error.
- Robust when acquisition geometry is sparse or irregular.
- Robust in certain situations, such as under salt, when angular
illumination is limited.
- WEM Angle Gather Update - This is the second phase of WIT's two-phase
velocity analysis program. WIT filed a patent in 2009 on a method to measure
the 3D propagation direction of a wavefield in a one-way WEM algorithm, and
then to use the propagation direction vectors for the source and receiver
wavefields in a shot record WEM to determine incidence angle, azimuth angle,
and dip angle. In this fashion, we efficiently and directly form true angle
gathers -- usually incidence angle gathers for the purposes of velocity
analysis. WIT's method is vastly more efficient than many other methods in
the literature. Running a WEM with angle decomposition produces a full volume
of angle gathers. For instance, the user might desire incidence angles from 0
to 48 degrees, sampled at 4 degrees, or 12 angle volumes.
The full volume angle gathers can be analyzed for residual curvature and used
to update the velocity. Currently, we perform a parabolic velocity scan on
the gathers and apply a vertical velocity update. The vertical update method
is suited to resolve velocity inversions and sharp boundaries in areas with
high S/N and lots of usable reflectors.
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Examples
This image depicts a South Texas WEM image overlain with the velocity field
obtained by WIT's two-phase procedure. 8 iterations of MVFA were run to
estimate the background velocity field, followed by 3 iterations of WEM Angle
Gather update to refine the details. Faults are shown on the image to
underscore how WIT's workflows are able to resolve sharp velocity contrasts,
both vertical inversions and lateral variations across faults.
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This image depicts the seismic velocity measured on a South Louisiana example.
The seismic velocity is plotted with a nearby checkshot. While the depth of
the velocity inversions is different between the checkshot and seismic, notice
how WIT's two-phase velocity workflow is able to correctly pick up three major
pressure-induced velocity regressions.
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Interested in reading more about WIT's velocity estimation technology?
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