This month's column, the first of a two-part series, is titled "The Check Shot Velocity Survey: Is It Enough?"
The Geophysical Corner is a regular column in the EXPLORER, produced cooperatively by the AAPG Geophysical Integration and SEG Interpretation committees, and edited by M. Ray Thomasson.
VSP Is a Check Shot Step Up
By R.J. BREWER
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:
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!
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.