Standard Wireline Data Processing

 

Operator and logging contractor: LDEO-BRG

Hole: 16A  (step-out site from Holes 7A/D)

Expedition: NGHP-1

Location: Krishna-Godavari Basin, Eastern India (Bay of Bengal)

Latitude: 16° 35.5986' N

Longitude: 82° 41.0070' E

Logging date: July 16-17, 2006

Sea floor depth (drillers'): 1264.5 mbrf

Sea floor depth (loggers'): 1266 mbrf

Total penetration: 1481.5 mbrf (217 mbsf)

Total core recovered:  165.39 m  (72.2 % of cored section)

Oldest sediment cored: n/a

Lithology: Mostly nannofossil and foraminifer-bearing clays with some sandy intervals (from Hole 7D)

 

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. DIT/HLDS/APS/HNGS Downlog
10
156
62.5
Main
0
217
56
 Reference
2. FMS/DSI/GPIT/SGT Pass 1
95
218
Pass 2
56
126
56

 

Prior to logging the hole was conditioned with a wiper trip and a sepiolite mud sweep, and then displaced with 70 bbl of 10.5 ppg barite mud. The DIT/HLDS/APS/HNGS and FMS/DSI/GPIT/SGT runs both reached the bottom of the hole, but had difficulty passing a bridge between 118-123 mbsf. The bridge increased in thickness for the FMS/DSI/GPIT/SGT first pass, and the second pass could not pass below it. Increased cable tension at the bridge means that logs (even from the same tool string) may not be well depth matched around this interval. 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.

 

Processing

 

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 (-1266 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 1266 mbrf (after depth matching). This differs by 1.5 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 the following modes:

Pass 1: Low frequency monopole, low frequency upper dipole, standard (high) frequency lower dipole, and Stoneley modes.

Pass 2: Low frequency monopole, and low frequency crossed dipole modes.

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 hole is highly variable in diameter (11-18 inches), with a bridge between 118-123 mbsf.

 

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