Wireline Standard Data Processing
DSDP operator and logging contractor: Scripps Institution of Oceanography
Hole: 612
Leg: 95
Location: Baltimore Canyon (NW Atlantic)
Latitude: 38° 49.21' N
Longitude: 72° 46.43' W
Logging date: August 1983
Sea floor depth (drillers' mudline): 1414.3 mbrf
Sea floor depth (step in GR log): logs did not reach the mudline
Total penetration: 675.3 mbsf
Total core recovered: 580.66 m (86 % of cored section)
Oldest sediment cored: late Campanian
Lithologies: muds and sands, oozes and chalks, mudstones and shales.
The logging data was recorded by Schlumberger in LIS format. Data were processed at the Borehole Research Group at the Lamont-Doherty Earth Observatory in November 2003.
| Tool string | Pass | Top depth (mbsf) | Bottom depth (mbsf) | Bit depth (mbsf) | Notes |
| 1. DIT/LSS/GR/MCD |
main
|
97.5
|
678
|
|
Reference run
|
|
repeat
|
96.5
|
154
|
|
|
|
|
upper
|
91
|
317
|
|
LSS/GR only
|
|
| 3. FDC/CNL/NGT |
main
|
71
|
678
|
73
|
|
|
repeat
|
95
|
167
|
|
|
The two tool strings were run to the bottom of the hole without problems. After the main and repeat DIT/LSS/GR/MCD runs, the tool string was lowered again with the DIT tool turned off in order to acquire sonic waveforms. The available tapes contain waveform data only for the upper part of the hole (see data files), though the blueprints show data for the entire logged section.
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 original logs were depth-matched to the GR log from the main pass of the DIT/LSS/GR/MCD tool string, and were then shifted to the sea floor (-1414.3 m). The DIT/LSS/GR/MCD main pass was chosen as the reference run because it covered the whole open hole interval. The GR logs from the other passes were matched to the GR log from the reference run, apart from the upper DIT/LSS/GR/MCD run, which was matched by the DT slowness log. A very large shift was required to bring the upper DIT/LSS/GR/MCD pass into line with the reference: it was shifted downwards by 51.5 m.
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 could not be determined from the logs because they did not extend to the sea floor. The driller's seafloor depth of 1414.3 mbrf was used to shift the logs to the sea floor.
Sonic data: The transit time data were processed using an in-house program that compares the slowness derived from the 8 different transmitter-receiver combinations at each depth, and discards those times that are significantly different from the majority as bad data. The 'points' column in the LSS data files is a measure of confidence: it records the number of transmitter-receiver pairs retained - a value of 8 means that no data was discarded. This processing leads to improved compressional wave velocity logs that are free of the artifacts present in the velocities derived directly from DT and DTL.
Spectral gamma-ray data. No environmental corrections were applied, because the old NGT tool used in DSDP is not listed in the Geoframe environmental correction module, and because of uncertainties about the absolute value of the caliper (hole diameter) data.
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 sonic velocity log).
A wide (>12") and/or irregular borehole affects most recordings, particularly those that require eccentralization and a good contact with the borehole wall (FDC, CNL). Hole diameter was recorded by the hydraulic caliper on the FDC tool (CALI) and by the 3-arm MCD tool (CALI). The hole was in good condition, reading about 10 inches for the DIT/LSS/GR/MCD runs (the bit size was 11.4375 inches). The caliper on the FDC tool reads a near-constant value and is probably not a reliable indication of hole diameter.
A null value of -999.25 may replace invalid log values.
Additional information about the drilling and logging operation can be found in the Operations section of the Site Chapter in DSDP Initial Reports volume 95. For further questions about the logs, please contact:
Cristina Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: Cristina Broglia