Standard Wireline Data Processing
Operator
and logging contractor: LDEO-BRG
Hole: 13A (between Holes 10A and 12A)
Expedition: NGHP-1
Location: Krishna-Godavari Basin, Eastern India (Bay of Bengal)
Latitude: 15° 51.70' N
Longitude: 81° 50.18' E
Logging date: June 23-24, 2006
Sea floor depth (drillers'): 1044 mbrf
Sea floor depth (loggers'): 1046 mbrf
Total penetration: 1244 mbrf (200 mbsf)
Total
core recovered: none
Oldest sediment cored: n/a
Lithologies: Unknown
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 |
148 |
57 |
|
| Main | 0 |
146 |
41 |
Reference |
After an abandoned logging attempt at Hole 12A, the ship was moved 150 m NW to drill dedicated logging Hole 13A without coring. Prior to logging the hole was conditioned with a wiper trip and a sepiolite sweep, and then displaced with 80 bbl of 10.5 ppg barite mud. The DIT/HLDS/APS/HNGS initially could not exit the pipe into the open hole, and an XCB core barrel was sent down to clear any obstruction at the base of the pipe. The toolstring was then sent down, and reached 148 mbsf before being blocked, about 50 m short of the bottom of the hole. The LDEO Wireline Heave Compensator compensated ship heave during logging.
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 downlog was matched to it using the gamma ray log. All passes were then shifted to the sea floor (-1046 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 1046 mbrf. This differs by 2 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 hole is typically around 11-13 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: Crisitna Broglia