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Depth adjustments
The main processing task is to remove depth discrepancies between the different logging runs. Such discrepancies are caused by cable stretch, incomplete heave compensation, and by tides. The natural gamma ray log (SGR and HSGR) is generally used to match between the logging runs, as this log is recorded on all toolstrings. One gamma log is chosen as the reference, on the basis of the length of the logged interval and data quality. The other gamma logs are matched to the reference using an automatic routine; the match of each log is checked to make sure distinctive peaks and troughs line up, and the match is adjusted, as necessary, by the log analyst. The resulting depth shifts are then applied to the other logs on the tool strings. The depth reference is then shifted from the rig floor to sea floor, which is determined from the step in the natural gamma log seen at the sediment-water interface.
Environmental corrections
Environmental corrections are designed to remove any effect from the borehole (size, roughness, temperature, tool standoff) or the drilling fluids that may partially mask or disrupt the log response from the formation. Onshore, only the natural gamma (NGT) logs are generally corrected. The logs from the HNGS, HLDS, and APS tools are corrected in near-real time during log acquisition.
Sonic log corrections
Sonic slowness logs from the SDT, LSS, and DSI-2 sonic tools are routinely edited to remove noise and cycle-skips that are often present in the raw log. The travel times are converted into sonic velocities.
Quality control and documentation
The quality of the data is assessed in terms of reasonable values for the logged formation, repeatability between different passes of the same tool, and correspondence between logs affected by the same formation property (e.g., the resistivity log should show similar features to the sonic velocity log). Invalid data at the top (affected by the bottom hole assembly) and bottom of the logs are removed. Depth adjustments, corrections, and data quality are documented in the processing report.
Data delivery
The processed data are saved as ASCII files and transmitted via satellite back to the ship. They are also put in the on-line database, the Initial Reports CD-ROM, and are archived to tape in LIS/DLIS format.
Processing is required to convert the 64 electrical current traces recorded by the FMS into a color-scale image representative of the conductivity changes in the formation.
BorEID corrections
Several corrections are applied using the BorEID module of GeoFrame:
- Speed Correction. The data from the z-axis accelerometer are used to correct the vertical position of the data for variations in the speed of the tool ("GPIT speed correction"), including "stick and slip." In addition, "image-based speed correction" is also applied to the data, based on reducing any offset between the data from two rows of button electrodes on each FMS pad.
- Equalization. The responses of the button electrodes on the pads of the tool are equalized to correct for various tool and borehole effects which affect individual buttons differently.
- Button Correction. If the measurements from a button electrode are unreasonably different from its neighbors (e.g., "dead buttons"), the defective trace is replaced by traces from adjacent good buttons.
- EMEX voltage correction. During logging, the voltage that drives the current is continuously regulated so that current flows even through very resistive formations. The button response is divided by the EMEX voltage so that the response corresponds more closely to the conductivity of the formation.
Depth adjustment
The natural gamma log (SGR) resulting from the BorEID speed correction is matched to the SGR log from the same pass after conventional log depth shifting. The logs are checked for a good match, and then the resulting depth shifts are applied to FMS images and their associated logs (pad azimuth, etc.). The resulting FMS images are then on a comparable depth scale to the conventional logs.
Image normalization
Using the BorNor module of GeoFrame, "static" and "dynamic" normalizations of the image are applied. In the static normalization, the resistivity range of the entire interval of data is computed, and is partitioned into 256 color levels; this image is good for examining large-scale resistivity variations. In the dynamic normalization, the full range of color levels is assigned to resistivity range of short intervals (e.g., 2m); thus the color contrast is increased, enhancing the fine details of the resistivity structure.
Data delivery
Static images are output as GIF files and added to the on-line database and the Initial Reports CD-ROM. In the future, dynamic images will be treated in a similar manner. The FMS data are also saved in DLIS format and archived.
Once the GHMT logs have been depth shifted, the magnetic polarity stratigraphy is determined as follows:
- The Earth’s main field and the field of the metal drill pipe are subtracted from the total magnetic field log (MAGB) to isolate the field anomaly caused by the local formation.
- The local field anomaly is caused by the induced and remanent magnetizations of the local formation (see figure). The induced anomaly can be calculated from the magnetic susceptibility log (MAGS), and so the remanent anomaly can also be isolated. Prior to this step, the logs are smoothed so that they have comparable vertical resolutions.
- The induced and the remanent anomalies are correlated over depth intervals of varying heights ("correlation analysis"). If the induced and remanent anomalies correlate, then the magnetic polarity of the formation is normal; if they anti-correlate, the polarity is reversed (below left). This polarity interpretation can then be related to the geomagnetic polarity timescale (below right).
- The processed data are included along with a summary diagram in the on-line database and the Initial Reports CD-ROM.
Temperature data
Time vs. temperature logs recorded by Lamont’s TAP tool are merged with time vs. depth data recorded during logging by the Schlumberger MCM unit to give the variations of borehole temperature with depth. The temperature data are added to the Initial Reports CD-ROM.
Sonic waveform data
During logging, sonic travel-times are picked from the waveform data acquired by the DSI-2 and LSS sonic tools; these picks are used in the conventional log processing. It is anticipated that the waveform data from the DSI-2 sonic tool can be reprocessed after the leg (using newly acquired GeoFrame waveform processing modules) to derive compressional, shear, and Stonely wave velocities, and seismic anisotropy data.
WST (checkshot and VSP) data
WST data, both individual shot records and the stacks for each station, are archived in DLIS format. First arrival times are picked on the ship and are not generally re-picked onshore. Where there are enough stations for a vertical seismic profile, a corridor stack can be produced and compared to the synthetic seismogram and seismic section.
Other data
Processing of data from tools that are used, or have been used, only occasionally -- Geochemical tool (GLT), Borehole televiewer (BHTV), Azimuthal Resistivity Imager (ARI), third party logging tools, etc. -- is determined on a per-leg basis and may be outsourced.
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