9.8.1 Input datasets
The following data is required.
- Open hole log data that includes DT and RHOB curves, indexed in measured depth.
- Well deviation data that has a depth index of measured depth (Depth). Required curves are INCL and AZIM. Optional curve is TVD channel. An example is shown below in a CSV format text file.
- VSP time-depth data, that is recorded at about 15 metre spacing. The depth index should be vertical depth below datum (TVD-SRD). Time data is Two-way time. An example is shown below in a CSV format text file.
The depth range of the VSP time-depth data must be greater than the depth range of the open hole log data (when both are in TVD-Datum depth index)
The methodology used for TWT conversion assumes that the TWT data comes from VSP data that is recorded at a regular VSP level spacing of about 15 metres. If the TWT data is from sparse checkshot data, then the TWT conversion will be less accurate since sonic drift correction is not applied, as is required for sparse checkshot processing.
All the open hole log data should be in a single file. This can be done by using merging and splicing as required.
Null values are not tolerated in either of the DT or RHOB curves, and Null values should be removed as discussed in sections 5.5 and 5.6.
9.8.3 Workflow for TWT Conversion
- Pre-processing on the open hole log data, as discussed above in section 9.8.2.
- Load the three datasets discussed in section 9.8.1 into the same well folder.
- Go to the Wellsite data Editor and fill parameters. APD is the elevation of the logging datum above SRD and is required. Well coordinates are not essential if just computing TVD, but they are nice to have.
- Follow the procedure in section 8.3, to convert the Open hole log data to TVD depth-index. If a TVD curve (TVD from logging datum) is already present in the data loaded, then step-1 of the conversion can be skipped. Although it is good to do it anyway, and confirm the results are the same.
- As per section 8.2, switch the TVD open hole data to TVD-Datum.
- Select on the VSP-TWT Conversion process in the Log Processing
- Select the VSP time-depth dataset, then choose the TWT curve.
- Select the TVD_Datum version of the Open hole log dataset, then choose the DT sonic curve.
- All the open hole logs in the same folder as the selected DT will be converted to time index.
- For Synthetics Seismogram generation, a 1 msec sample rate is sufficient, but choosing a smaller rate such as 0.2 msec or 0.1 msec will maintain the resolution of the open hole log data to be similar to the original depth indexed data, should it be nessecary for later interpretation of this data in time-indexed form.
The Drift curve is the difference between the Integrated Sonic Transit Time (ITT) and the VSP one-way time. It is not used for anything, but can be used as a QC curve. If the drift is errartic or very high, then can indicate problems, with either the VSP data, Sonic data or deviation corrrection.
The Interval Velocity curve is from the VSP data, so the stairs in this log represent the VSP level spacing.
The Delta TVD curve is a QC curve and should be zero. It is the difference between the VSP and Sonic Log depths.
9.8.4 Alternative TWT Computation
It is possible to convert logs to a TWT index just by using the Integrated Sonic Transit Time (ITT). This is a last resort method, and suffers from the following shortfalls.
- The absolute value of TWT at the top of the sonic log is unknown, and is an input parameter.
- The ITT will suffer from "drift", so the determined value of TWT will wander. The actual value of drift can only be determined if you already have VSP time-depth data, in which case you wouldnt be using this alternative method.
To use this method, select the ITT-TWT Conversion process.
9.8.5 Synthetic Seismogram
Convert the data to TWT as described above. Preferably, the TWT conversion should be done using VSP time-depth.
If the alternative TWT conversion method was used, then the absolute time at the top of the synthetic seismogram could be in-correct, and the TWT along the computed synthetic could be stretched and squeezed as a result of sonic drift.
Select the Synthetic Seismogram process. The two mandatory input curves are the Time-indexed DT and RHOB curves. Choose the wavelet parameters and click compute.
Polarity convention is an increase in acoustic impedance will give a black peak, for normal polarity.