DSDP operator and logging contractor: Scripps Institution of Oceanography
Hole: 504B
Leg: 83
Location: Costa Rica Rift (equatorial NE Pacific)
Latitude: 1° 13.63' N
Longitude: 83° 43.81' W
Logging date: 16-21 December 1981
Sea floor depth ('bottom felt'): 3474 mbrf
Total penetration: 1350 mbsf
Total core recovered: 106.9 m (20.8 % of cored section)
Oldest sediment cored: Upper Miocene
Lithologies: chert and siliceous limestone (sediments), basalt pillows, flows, and dykes (basement).
The logging data was recorded by Schlumberger in LIS format. Data were processed at the Borehole Research Group at the Lamont-Doherty Earth Observatory.
| Tool string | Pass | Top depth (mbsf) | Bottom depth (mbsf) | Casing depth (mbsf) | Notes |
| 1. HRT |
Pass
1
|
222
|
1278
|
274.5
|
|
| 2. BHTV |
|
|
|
|
Failed
|
| 3. DLL/GR |
|
|
|
|
Failed
|
| 4. DIT/GR |
Main
|
273
|
1288
|
274.5
|
|
|
Repeat
|
383
|
452
|
|
|
|
| 5. BHTV |
|
|
|
|
Analog data
only
|
| 6. LSS/GR/MCD |
Pass
1
|
254
|
1285
|
274.5
|
|
| 7. HRT |
Pass 2 Main
|
0
|
1280
|
274.5
|
|
|
Pass
2 Rep.
|
1172
|
1280
|
|
|
|
| 8. FDC/CNL/GR |
Main
|
270
|
1289
|
274.5
|
|
| 9. HRT |
Pass
3 Upper
|
0
|
1196
|
274.5
|
|
|
Pass
3 Lower
|
1171
|
1282
|
|
|
|
| 10. LSS |
Pass
2
|
277
|
1286
|
|
|
|
Pass
3
|
250
|
1279
|
274.5
|
7 sections,
spliced. SWF recorded
|
Hole 504B was logged extensively during Leg 83, and the operations were eventful. Fifty barrels of 12 lbs/gallon mud was pumped and displaced out of the hole with seawater, cleaning and cooling the hole. A HRT (High Resolution Temperature) log was taken first, 5 hours after circulation was completed. The next two tool strings, the Borehole Televiewer (BHTV) and the Dual Laterolog (DLL), failed to record useful data due to tool malfunctions. Therefore, the DIT/GR tool string was run instead of the DLL, though the induction resistivity measurement is not ideally suited to the highly resistive formation. The hole was cooled prior to a second attempt at a BHTV log; very good quality pictures were obtained, but were only recorded on video and Polaroid film. The LSS/GR/MCD tool string was then run to obtain a P-wave log. Unfortunately, the tool became stuck in the bit when pulling out, and attempts to pump out the tool were unsuccessful. The cable was pulled past the breaking point of the cable head (9000 lbs), and the bit and stuck tool string were brought to the surface. 12000 ft of logging cable were lost. A second temperature log was run, 58 hours after circulation was completed. The FDC/CNL/GR was then run successfully. A Schlumberger water sampler was run and collected 650 ml of fluid from 978.5 mbsf. A third temperature log was run, 82 hours after circulation was completed. The hole was cooled and the LSS was then re-run to obtain sonic waveforms. Additionally, S-wave logs were obtained by setting the first-break detection window to detect the slower, higher amplitude S-wave arrival rather than the faster, lower amplitude P-wave arrival. Pass 3 was run in 7 sections. Finally, the hole was re-cooled, and a large-scale resistivity experiment was run. See page 20 of the Initial Reports of DSDP volume 83 for further details.
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 FDC/CNL/GR tool string, and were then shifted to the sea floor (- 3474 m). The DLL (uplog) pass from the ODP Leg 111 logging of Hole 504B was used as the reference run because it had already been used as the 504B reference run for ODP Legs 111, 140, and 148 and for DSDP Legs 69 and 70. The SFLU logs from the DIT/GR passes were matched to the reference LLS log. The NPHI logs from the FDC/CNL/GR passes were matched to the main SFLU log. The GR log values were very low, and could only be used for matching at a few depths. The HRT logs were not depth matched to the other logs.
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.
Because no logs reached the mudline on any DSDP or ODP Legs, the sea floor depth is the 'bottom felt' depth given by the drillers (3474 mbrf). This value 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 velocity logs that are free of the artifacts present in the velocities derived directly from DT and DTL.
On this leg, LSS pass 1 recorded P-wave logs as normal. On the second and third passes, recording of S-wave logs was attempted, by setting the first-break detection window to detect the slower, higher amplitude S-wave arrival rather than the faster, lower amplitude P-wave arrival. This was mostly unsuccessful for pass 2, but pass 3 appeared to record valid S-wave travel times and velocities. For more details, see the paper by Newmark et al., in Volume 83 of the Initial Reports of the DSDP.
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). Hole diameter was recorded by the hydraulic caliper on the FDC tool (CALI) and by the 3-arm MCD tool (CALI). According to the caliper on the first pass of the LSS/GR/MCD tool string, the hole is mostly between 9-11 inches wide from 280-900 mbsf, and between 8-9 inches wide from 900-1280 mbsf. Bridges are apparent in this log as meter-thick narrowings of the hole by several inches. However, they are not apparent in the caliper log from the FDC tool, which reads a fairly constant 10 inches and repeats well in the FDC caliper log repeat section.
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 83. For further questions about the logs, please contact:
Trevor Williams
Phone: 845-365-8626
Fax: 845-365-3182
E-mail: Trevor Williams
Cristina Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: Cristina Broglia