The idea of lowering
a geophone down a well bore to get a better handle on rock velocity is
hardly a new concept. Geophysicists have engaged in the practice with
increasing precision since the1930s -- around the time when the first
geophones were designed to withstand the rigors of the borehole.
The presence of a drilled well
presents a truly unique opportunity to:
- Investigate a target formation
more closely with acoustic measurements.
- Minimize sub-surface attenuation
phenomena.
- Measure depth accurately.
- Overcome the formidable limitation
of all surface geophysical measurements -- the lack of accurate depth
control.
Sonic Logs and Check Shots
Geophysicists are familiar with
the velocity survey's one-way acoustic travel time as a critical component
that's necessary to help convert surface seismic's two-way travel time
to depth. In the absence of the check shot velocity survey, accurate velocity
information can sometimes be extracted from the tried and true sonic log.
Relying solely on sonic logs however,
may entail considerable risk involving interval velocity errors.
What may not be clearly acknowledged
are how limited check shot data is -- and how very limited sonic logs
travel times are inconsistently aiding the time to depth conversion process.
The sonic log excels as a formation
boundary and indirect porosity measurement log, but it can only see one-two
feet into the formation under good down hole conditions -- and can be
subject to cycle skipping and washed-out zones.
When the sonic log is used to
produce a synthetic seismogram for surface seismic correlation purposes,
one hopes that a check shot velocity survey is available from the same
well to calibrate the sonic log.
Calibration and correction of
the sonic log often may be needed because the production of a synthetic
seismogram from a sonic log is a hybridization and transform process that
can introduce seismic travel time error if cycle skipping, tool sticking
and washed-out zone effects are present in the sonic log.
The sonic log is also of very
limited use in identifying interval velocity inversions, or any abrupt
rock density and velocity change that are an appreciable distance from
the well.
The check shot velocity survey
can be used to produce a corrected sonic log, allowing sonic log pitfalls
to be alleviated by enabling a data processing analyst to effectively
and more accurately correlate through questionable zones that were traversed
by the sonic logging tool down hole.
A check shot corrected sonic log
also makes it easier to determine interval velocities between key formations,
since familiar formation boundaries can be readily recognized from the
sonic log.
If density log information is
also available, a more accurate synthetic seismogram log integration usually
results.
A check shot velocity survey measures
a much larger cylindrical volume of rock compared to the relative soda
straw volume measured by the sonic log. The check shot survey and the
more precise vertical seismic profile (VSP) should at least be considered
in the logging program of every exploration and key development well being
planned to minimize or eliminate the ever-present and costly danger of
surface seismic time to depth conversion error.
Borehole seismic data are the
most effective correlation bridge available between the well bore and
the surface seismic data.
Borehole seismic data that includes
the check shot velocity survey and the VSP can measure large volumes of
rock -- and will indicate the presence of velocity anomalies, which may
be totally missed by the sonic log.
These velocity anomalies must
be measured and dealt with accurately when mapping the velocity fields
that are so critical to an effective surface seismic time-to-drill-depth
conversion process.
Some History on VSP
The vertical seismic profile (VSP)
is a truly remarkable, versatile and, unfortunately, under-utilized innovation.
Under-utilized perhaps because of its greater cost than the more routine
check shot velocity survey -- and the possible industry over reliance
on 3-D surface seismic data.
The effective utility of the VSP
was developed by the Soviets in the 1960s, made its way into Europe and
finally arrived in earnest in the United States in the 1970s.
The VSP was quite an industry
sensation when it started to be used in this country because of its "a
look-ahead" of the drill bit capability -- and its use as an aid in predicting
the depth at which a target formation would be encountered after drilling
continued.
The Look Ahead or Prediction Ahead
of the Bit (PAB) VSP, which is actually an inversion routine performed
during the data processing of ideally- zero offset VSP survey data, has
proven itself as a useful exploration tool over the years. It has been
used to predict the depth of over-pressured zones ahead of drilling offshore
wells and to locate granite sediment and salt sediment interfaces.
A zero or near offset VSP survey
has the energy source positioned as close as possible to the well head
to focus the energy down and ahead of the well bore -- and is the preferred
geometry for well correlation as opposed to the offset VSP survey configuration,
which positions the energy source away from the well head to image a distance
laterally away from the well.
Look ahead offset VSP surveys
also have been used recently to successfully locate subsurface features
such as pinnacle reefs in East Texas. The look-ahead VSP survey may seem
like quite a leap of faith to the uninitiated -- until one realizes that
all surface seismic data (2-D and 3-D) is all a look-ahead, as all measurements
are made at the surface!
VSP Perspective
The VSP is simply a precision level
step change up from the check shot velocity survey.
The basic difference
between the check shot survey and the VSP is that the VSP measures nearly
all seismic waveforms in the well bore (up-going and down-going energy),
whereas the check shot velocity survey measures basically only the down-going
energy (figure 1).
Note that a VSP is also a check
shot velocity survey -- but a check shot velocity survey is not a VSP!
Check shot velocity survey measurements
are typically taken every 250-500 feet down hole and were designed to
measure the down-going waveforms used in velocity determination. VSP measurements
are much more closely spaced (50-100 feet).
The VSP, like the check shock
survey, also measures down-going energy. The smaller measurement interval
(level interval) required by the VSP is necessary to also record the reflected
energy in the well bore.
The basic computed product of
the VSP is known as a corridor stack, which in appearance resembles the
synthetic seismogram. In reality it is a vastly superior well correlation
tool, because it contains actual seismic reflection data as well as the
down-going wave field.
The down-going
wave field is all that a check shot velocity survey records. The corridor
stack made from the VSP is the well bore converted to a full reflection
waveform seismic trace basically free of multiples. (figure
2)
Another significant limitation
of relying only on check shot velocity surveys is that the surface seismic
data that it is being correlated with contains almost entirely reflected
waveforms.
Surface seismic does not measure
down-going energy because all the detectors are at the surface.