Standard Wireline Data Processing

 

Operator and logging contractor: LDEO-BRG

Hole: 5E (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'): 956.5 mbrf

Total penetration: 200 mbsf

Total core recovered: none

Oldest sediment cored: n/a

Lithology: Nannofossil-rich clay with occasional thin silt beds and abundant carbonatic nodules (from Hole 5C)

 

Data

 

The logging data was recorded by Schlumberger in DLIS format. Data were processed at the Borehole Research Group of the Lamont-Doherty Earth Observatory.

 

Logging Runs

 

Tool string Pass Top depth (mbsf) Bottom depth (mbsf) Bit depth (mbsf) Notes
1. DSI/SGT Pass 1
46
193
55.5
Pass 2
46
193
55.5
Pass 3
106
120
55.5
2. DIT/APS/HLDS/HNGS Downlog
0
168
65
Main
0
197
56
 Reference
3. FMS/GPIT/HNGS Pass 1
75
197
open hole
Pass 2
37
186
54
4. VSI
20 stations of good data

 

Prior to logging the hole was displaced with 10.5 ppg (barite) mud. The DIT/HLDS/APS/HNGS and FMS/ DSI/ GPIT/SGT 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.

 

Processing

 

Depth match and depth shift to sea floor: The FMS/DSI/GPIT/SGT 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 (-2667 m), based on the step in gamma radiation at the sea floor in the DIT/HLDS/APS/HNGS Main Pass. The FMS/DSI/GPIT/SGT Pass 2 was chosen as the reference run because it covered the whole open hole interval.

 

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 2667 mbrf (after depth matching). This differs by 7.2 m from the sea floor depth given by the drillers (see above).

 

Environmental corrections: The HNGS and SGT 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.

 

Acoustic data: The dipole shear sonic imager (DSI) was run in P&S monopole, upper and lower dipole, and Stoneley modes on both main and repeat pass. Because of the slow formation, the automatic picking of wave arrivals in the sonic waveforms did not provide reliable results. Reprocessing of the original waveforms was required to extract meaningful compressional and shear velocities. The most reliable shear velocity value is the one derived from the upper dipole (VS2) during the first pass, where the lower source frequency used  generated more coherent waveforms.

 

Quality Control

 

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 1A 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