Standard Wireline Data Processing
Operator
and logging contractor: LDEO-BRG
Hole: 5D (proposed site KGGH02-A)
Expedition: NGHP-1
Location: Krishna-Godavari Basin, Eastern India (Bay of Bengal)
Latitude: 16° 1.72' N
Longitude: 81° 2.67' E
Logging date: June 14-15, 2006
Sea floor depth (drillers'): 955 mbrf
Sea floor depth (loggers'): 954 mbrf
Total penetration: 200 mbsf
Total
core recovered: 39.75 m (19.8 % of cored section)
Oldest sediment cored: n/a
Lithology: Nannofossil-rich clay with occasional thin silt beds and abundant carbonatic nodules (from Hole 5C)
The logging data was recorded by Schlumberger in DLIS format. Data were processed at the Borehole Research Group of the Lamont-Doherty Earth Observatory.
| Tool string | Pass | Top depth (mbsf) | Bottom depth (mbsf) | Bit depth (mbsf) | Notes |
| 1. DIT/HLDS/APS/HNGS |
Downlog |
0 | 163 | 67 | |
| Main | 0 | 125 | 71 | Reference |
Prior to logging the hole was displaced with 10.5 ppg (barite) mud. The DIT/HLDS/APS/HNGS runs went smoothly,and reached the bottom of the hole. On the next run, there were initial software problems communicating with the VSI tool and after the tool was pumped out of the pipe its progress downhole was blocked by a bridge. At that time it was dark, and it was decided to terminate logging Hole 1A. The LDEO Wireline Heave Compensator compensated ship heave throughout the operation.
The depths in the table are for the processed logs (after depth matching between passes and depth shift to the sea floor). Generally, discrepancies may exist between the sea floor depths determined from the downhole logs and those determined by the drillers from the pipe length. Typical reasons for depth discrepancies are ship heave, wireline and pipe stretch, tides, and the difficulty of getting an accurate sea floor from the 'bottom felt' depth in soft sediment.
Depth match and depth shift to sea floor: The DIT/HLDS/APS/HNGS Main pass was used as the depth reference, and the other passes were matched to it using the gamma ray and caliper logs. All passes were then shifted to the sea floor (-954 m), based on the step in gamma radiation at the sea floor in the DIT/HLDS/APS/HNGS Main pass.
Depth matching is typically done in the following way. One log is chosen as reference (base) log (usually the total gamma ray log from the run with the greatest vertical extent and no sudden changes in cable speed), and then the features in the equivalent logs from the other runs are matched to it in turn. This matching is performed manually. The depth adjustments that were required to bring the match log in line with the base log are then applied to all the other logs from the same tool string.
The sea floor depth was determined by the step in gamma ray values in the DIT/HLDS/APS/HNGS Main pass at 954 mbrf. This differs by 1 m from the sea floor depth given by the drillers (see above).
Environmental corrections: The HNGS data were corrected for hole size during recording. The APS and HLDS have been corrected for standoff and hole diameter respectively during the recording.
High-resolution data: Bulk density and neutron porosity data were recorded at a sampling rate of 2.54 and 5.08 cm, respectively. The enhanced bulk density curve is the result of Schlumberger enhanced processing technique performed on the MAXIS system onboard. While in normal processing short-spacing data is smoothed to match the long-spacing one, in enhanced processing this is reversed. In a situation where there is good contact between the HLDS pad and the borehole wall (low-density correction) the results are improved, because the short spacing has better vertical resolution. SGT gamma ray was recorded at 15.24 and 5.08 cm sampling rates.
The quality of the data is assessed by checking against reasonable values for the logged lithologies, by repeatability between different passes of the same tool, and by correspondence between logs affected by the same formation property (e.g. the resistivity log should show similar features to the acoustic log). The log data of Hole is of good quality. The hole is typically around 12 inches in diameter.
Gamma ray logs recorded through bottom hole assembly (BHA) and drill pipe should be used only qualitatively, because of the attenuation on the incoming signal. The thick-walled BHA attenuates the signal more than the thinner-walled drill pipe.
A null value of -999.25 may replace invalid log values.
For further questions about the processing, please contact:
Cristina Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: Cristina Broglia