
VSP
Vertical VSP Processing
Vertical seismic profiling is a vital tool in subsurface imaging and reservoir characterization. The technique allows the geophysicist to infer critical information that cannot be obtained otherwise.
Vertical seismic profiling is a technique of simultaneously recording the upgoing and downgoing wavetrains. This represents a major advantage over the conventional surface reflection seismic technique, which records only the upgoing waves. An analysis of the upgoing and downgoing components permits the detailed study of the change of the seismic wavetrain with depth. The acoustic properties of the earth can then be directly linked to and interpreted in terms of the subsurface lithology.
Applications
Some of the Zero Offset VSP / Normal Incidence VSP applications are listed below:
Seismic tie between borehole measured seismic data and surface seismic or OBC
Establishment of a precise link between the surface seismic results and well logs, since the VSP is a high resolution measurement.
Improved resolution, since the seismic waves pass the highly absorptive low-velocity layers near the surface only once.
Look-ahead-of-the-bit; Predict distance to reflectors below the current total depth of the well.
Measure average and interval velocities
Ability to accurately distinguish multiple reflections present in the surface seismic data.
Estimation of Q-factor
Offset VSP Processing
The Rig Source VSP is defined as source on the rig and receiver in a deviated well. The Fixed Offset VSP is defined as offset survey geometry with the source fixed and the receiver array is moved along in a deviated or vertical well. A constant offset / constant angle survey is used for horizontal wells where the receivers have 3 component geophones. The source is kept on an offset which create the maximum separation between downgoing and upgoing wave modes. The offset geometries will achieve lateral coverage up to one half of the source offset distance in the direction of the source.
Applications
Some of the Offset VSP applications are listed below:
Seismic tie between borehole measured seismic data and surface seismic or OBC
High-resolution seismic data, since the seismic waves only pass the highly absorptive low-velocity layers near the surface once.
Ability to accurately distinguish multiple reflections present in the surface seismic data.
Delineate faults and other stratigraphic features
Structural imaging
Reservoir delineation and characterization
Anisotropy estimation
AVO
Check Shot Survey
The purpose of the check shot survey / report is to define a unique relationship between time and depth. The survey uses the well logging technique to measure the receiver depths and then the one-way travel time of a pulse generated by a surface source to the down hole receiver is measured.
The average velocities are then calculated and corrected to the seismic datum. Interval velocities are calculated between check shot levels. If the hole is deviated or if there is a significant source offset, the transit times obtained must be converted to true vertical depth (TVD) transit times. Correction to the seismic reference datum is also necessary if the source is above or below the seismic datum.
The check shot report will consist of a time/depth listing including measured depth, true vertical depth, transit times corrected to vertical, average velocity, interval velocity and all corrections. The same information will also be graphically represented in the report. A display of the stacked VSP data with the time picks marked is included for Quality Control. All data are available in digital format.
Applications
Some of the check shot survey applications are listed below;
Provide time-to-depth relationships for formation top identification on seismic time sections.
Determine interval and average velocities versus depth
Sonic calibration; Yield a reference time to the seismic datum, which allow sonic integrated times to be tied to the seismic section.
Accurate synthetic seismograms can be calculated based on the correction for i.e. dispersion and borehole effects on transit times.
Convert any well log data from functions of depth to functions of two way reflection time.
Provide velocity information for processing or reprocessing of conventional seismic data or OBC.
Provide well constraint on velocity for tomographic inversion
Sonic Calibration
The seismic defines a unique relationship between time and depth derived from the check shot survey / report. This relationship is used to correct the velocities obtained by integration of the sonic interval transit times. The adjusted sonic may then be used for the translation of surface seismic times into depth and in the calculation of formation acoustic impedance necessary for the generation of a synthetic seismogram and for other application such as presenting other depth logs in the time domain (i.e. dipmeter results presented in time domain).
Formation velocities from the integrated sonic times are expected to differ from the check shot derived velocities because of velocity dispersion with frequency. Borehole effects, such as those caused by formation alteration (i.e. fluid intrusion) on the other hand, may decrease the apparent sonic log velocities. One should remember that the sonic transit time measurements is fundamentally different from the surface seismic measurement. The sonic log velocity is measured in a continuous manner alongside the borehole, while the seismic waves reaching the geophones take the most direct seismic path.
The sonic calibration adjustments consists of computing the raw drift between the check shot times and the integrated sonic times, selecting a drift curve, adjusting the sonic log, and checking the validity of the result.
The report will consist of a time/depth listing of the calibrated sonic data, displays of the drift curve, raw and calibrated sonic data alongside any chosen depth domain logs. Velocity logs at any scale on film to overlay seismic display. This data will also be available in digital form.
Applications
The sonic calibration can be used for the following application but not limited to;
Sonic calibration; Produce time / depth relationship, which allow sonic integrated times to be tied to the seismic section.
Accurate synthetic seismograms can be calculated based on the correction for i.e. dispersion and borehole effects on transit times.
Convert any well log data from functions of depth to functions of two way reflection time.
The calibrated sonic log can provide well constraint on velocity for tomographic inversion.
Calibrated sonic logs can be utilized for pseudo log creation using Wyllie, Gardner, or Faust equations.