surveys are routinely performed in many parts of the world -- especially
in Europe, because of the recognized superiority and versatility
of the VSP over the simpler and less expensive check shot survey.
More and more
VSP surveys are being done -- especially offset surveys and a more
detailed variation of the offset survey called the walk away VSP
survey -- as the advantages become clearer and survey reliability
ray trace modeling has gained wide acceptance and is used to more
accurately design offset VSP surveys and offset energy source placement.
The computed product of the offset VSP is known as a VSP/CDP transform
-- basically a high-resolution, mini-seismic section resembling
a surface seismic CDP (Common Depth Point) stack display.
The VSP/CDP transform has been converted or "transformed" from its
original recorded one-way time to two-way time, and displayed at
a convenient scale to match the surface seismic data it is to be
correlated with (figure 1). The VSP/CDP
transform data set can be migrated, filtered and processed just
like any surface seismic data set.
Because VSP data has a broader bandwidth and contains high frequency
events, subtle features like small faults, stratigraphic changes
and amplitude anomalies can be observed in the vicinity of the well
bore that were not discernable on the surface seismic coverage in
the same area (figure 2).
Note the marked
improvement in resolution that the VSP/CDP transform yields in this
example: It is a VSP/CDP transform display made from a vertical
incident VSP survey, recorded to provide better resolution over
a 2-D reconstruction line from a 3-D seismic volume prior to deepening
this directional well bore.
incident VSP survey requires the energy source to be positioned
at the surface directly over the downhole geophone tool. Vertical
incident geometry is generally preferred over the rig source option,
and has proven to be a more accurate way to obtain velocity control
and image a highly deviated bore hole.
the VSP/CDP Transform and the seismic section together yields a
far more useful product for interpretation.
Downhole VSP Tools
design has improved significantly over the last 20 years.
geophone configurations are routine -- the tools have evolved from
single component analog designs to digital multi-tool designs or
actual downhole geophone arrays comprising up to 24 or more individual
tools or satellites.
tools greatly reduce the historic bane of bore hole seismic surveys
-- rig time consumption -- and record higher quality data.
and 11/16-inch O.D.) downhole geophone tools have proven their versatility
and made it possible to record high quality VSP data in producing
Drilling (LWD) sonic, check shot and VSP tools are available to
meet the real-time demands of directional drilling. LWD tools designed
to record borehole seismic data are becoming increasingly more sophisticated
as LWD logging replaces conventional wireline logging on many directional
Reduced Risk and Saved Money
The two most
important benefits of running a VSP survey are reduced risk and
saved drilling dollars.
The VSP survey
reduces risk by measuring the seismic velocities accurately in the
well bore, which allows accurate time-to-depth conversion of the
surface seismic data. VSP data also has been used to help reprocess
older seismic data to yield more clearly interpretable results.
velocity information from the VSP helps make the velocity analysis
involved in processing and stacking surface seismic data more precise.
VSP data also
can be used to remove multiples from surface seismic data by providing
parameters for an earth filter inversion process known as signature
deconvolution. Accurate time-to-depth conversion is a must in producing
reliable drilling prospect maps -- and helps avoid missed drilling
Money is saved
early on with VSP surveys done in the first wells drilled in a play
by increasing the accuracy of the interpretation and mapping process
-- and later, as more wells are drilled and the velocity field is
Most of the
VSP surveys performed are of the zero or near offset type, which
is primarily used for velocity determination and surface seismic
surveys are gaining wider acceptance, as they have proved successful
in locating stepout locations to discovery and producing wells.
The sonic log
and the check shot velocity survey have been the standard seismic
correlation tools for many years and have proven their utility --
but today's exploration and production challenges require more precision.
The VSP survey
meets that challenge, and is currently considered to be the ultimate
and most effective tool available for matching the well bore to
the sizable investment of surface seismic data that each exploration
More VSP surveys
will be needed in the future -- and may become a standard logging
service -- as we strive to meet the accuracy demands of our industry.