Standard Wireline Data Processing
DSDP operator and logging contractor: Scripps Institution of Oceanography
Hole: 438A
Leg: 57
Location: Japan Trench (NW Pacific)
Latitude: 40° 37.79' N
Longitude: 143° 13.90' W
Logging date: October 1977
Sea floor depth ("bottom felt"): 1568 mbrf
Sea floor depth (Gamma Ray log): 1567.5 mbrf
Total penetration: 878.0 mbsf
Total core recovered: 555.3 m (63 % of cored section)
Oldest sediment cored: earliest middle Miocene
Lithologies: clay and claystone with sands, silts, volcanic ash.
The logging data was recorded by Schlumberger in LIS format on tape. The tape, however, was not readable, and the data had to be retrieved from the paper plots made during logging operations. The logs were digitized by Centerline Data in 2004. Data were processed at the Borehole Research Group at the Lamont-Doherty Earth Observatory in June 2004.
| Tool string | Pass | Top depth (mbsf) | Bottom depth (mbsf) | Bit depth (mbsf) | Notes |
| 1. BHC/GR |
main
|
0
|
869
|
107
|
reference
|
|
repeat
|
732
|
833
|
|
|
|
| 2. FDC/CNL/GR |
main
|
98
|
872
|
107
|
|
|
repeat
|
768
|
871
|
|
|
|
| 3. DLL/GR/SP |
main
|
102
|
858
|
105
|
|
|
repeat
|
752
|
852
|
|
|
|
| 4. HRT |
main
|
0
|
862.5
|
|
|
Logging at Hole 438A was successful, with all tool strings reaching close to the base of the hole.
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 BHC/GR tool string, and were then shifted to the sea floor (-1567.5 m). The BHC/GR main pass was chosen as the reference run because it covered all of the logged interval, and was the only pass to cross the sea floor. The GR logs from the main passes of the other tool strings were matched to the GR log from the reference run, and the matches were cross-checked with the velocity and porosity logs. The GR logs from the repeat passes each had a 5-8 m data gap, with the data below shifted downwards. Therefore the RHOB and the ILD logs from the repeat passes were matched to the DT 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 1567.5 mbrf. This differs by only 0.5 m from the sea floor depth given by the drillers (see above).
Sonic data: The DT slowness data have been converted to velocity. The data contain some anomalous positive and negative spikes, but apart from this the velocity log is of good quality.
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 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. (The CNL porosity can sometimes be used qualitatively through the BHA and pipe, but most of the other logs will not give usable data.)
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). Unfortunately there is no caliper log for this hole. The FDC density log, however, appears to be of good quality, so it is likely that the hole was in reasonable condition and not too washed out.
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 57. For further questions about the logs, please contact:
Cristina Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: Cristina Broglia