Wireline Standard Data Processing
DSDP operator and logging contractor: Scripps Institution of Oceanography
Hole: 624A
Leg: 96
Location: Lower Mississippi Fan (Gulf of Mexico)
Latitude: 25° 45.24' N
Longitude: 86° 16.63' W
Logging date: November 1983
Sea floor depth (drillers' mudline): 3198 mbrf
Sea floor depth (step in GR log): 3198 mbrf
Total penetration: 207.6 mbsf
Total core recovered: 86.76 m (84% of cored section)
Oldest sediment cored: Pleistocene
Lithologies: Clay, silt, sand.
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 October 2003.
| Tool string |
|
|
|
|
Notes |
| 1. DIT/LSS/GR/MCD |
Pass
1
|
50
|
200
|
53
|
|
|
Pass
2
|
0
|
197
|
53
|
Reference run
|
|
|
Pass
1
|
42
|
189
|
54
|
|
|
|
Pass
2
|
0
|
182
|
54
|
|
There was some fill at the bottom of the hole, and some hole bridges between 118-142 mbsf, but good logs were obtained from Hole 624A.
The depths in the table are for the processed logs (after depth matching between passes and the 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 Pass 2 of the DIT/LSS/GR/MCD tool string, and were then shifted to the sea floor (-3198 m). DIT/LSS/GR/MCD Pass 2 was chosen as the reference run because it was the only run to cross the sea floor. The GR logs from the other passes were matched to the GR log from the reference run.
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 at 3198 mbrf. This is the same as the sea-floor depth given by the drillers (see above).
Sonic data: The transit time data were processed using an in-house program that compares the slowness derived from the 8 different transmitter-receiver pairs at each depth, and discards those times that are significantly different from the majority as bad data. It also discards slownesses that fall outside a reasonable range for the sediments (in this case, outside 160-200 usec/ft). 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 mostly free of the artifacts present in the velocities derived directly from DT and DTL.
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).
Gamma ray logs recorded through bottom hole assembly (BHA) and 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 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 3-arm MCD tool (CALI), and by the caliper arm on the FDC tool. In this hole, the MCD caliper gives the better values, but even these appear to be reading too narrow.
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 96. For further questions about the logs, please contact:
Cristina Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: Cristina Broglia