Standard Wireline Data Processing
DSDP operator and logging contractor: Scripps Institution of Oceanography
Hole: 504B
Leg: 70
Location: Costa Rica Rift (tropical NE Pacific)
Latitude: 1° 13.611' N
Longitude: 83° 43.818' W
Logging date: December 1979
Sea floor depth ("bottom felt"): 3474 mbrf
Sea floor depth (step In GR log): no log reached the mudline
Total penetration: 836 mbsf
Total core recovered: 91.5 m (26.3 % of cored section)
Oldest sediment cored: Chert and Siliceous limestone (Late Miocene)
Lithologies: massive and pillow basalts (basement).
The logging data was recorded by Gearhart-Owen in GO format. The data was digitized by Centerline Data and subsequently processed at the Borehole Research Group of the Lamont-Doherty Earth Observatory in January 2005.
| Tool string | Pass | Top depth (mbsf) | Bottom depth (mbsf) | Casing shoe depth (mbsf) | Notes |
| 1. BHC/CL/GR |
Main
|
238.4
|
827.8
|
274.5
|
|
|
Repeat
|
728.6
|
828.4
|
|
|
|
| 2. CDL/GR/TEMP |
Pass
1
|
282.2
|
811
|
|
CDL/GR failed
|
|
Pass
2
|
282.2
|
811
|
|
CDL/GR failed
|
|
| 3. LL3/NL/GR |
Upper
|
89.6
|
369.2
|
274.5
|
Reference
|
|
Lower
|
275.2
|
824.2
|
|
|
The CDL/GR component of the CDL/GR/TEMP tool string did not operate properly; only temperature data were acquired in two passes.
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 initially depth shifted 3483.8 m to match the casing shoe at 274.5 mbsf. Using the DLL curve from ODP Leg 111 as reference, they were then depth matched to it. The Leg 111 log was chosen as the reference run because all subsequent ODP logging of Hole 504B (Legs 140 and 148) had been previously tied to it.
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 given by the drillers ("bottom felt" depth) was 3474 mbrf; none of the logs recorded during Leg 70 crossed the mudline. In order to match the casing shoe at 274.5 mbsf, the logs had to be shifted 3483.5 m, a discrepancy of 9.5 m with the depth given by the drillers.
Sonic data: Velocity was calculated from the acoustic slowness for both main and repeat runs.
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). The overall quality of Hole 504B logging data acquired during Leg 70 is good; most logs show good correlation with ODP Leg 111 data. The massive basalt units are clearly identified opposite the pillow basalts. Eight meters of LL3/NL/GR data (main run) were lost at about 479 mbsf due to computer problems. The GR from the LL3/NL/GR main and repeat runs was scaled in count per seconds (cps).
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 wide (>12") and/or irregular borehole affects most recordings, particularly those that require eccentralization and a good contact with the borehole wall (NL). Hole diameter was recorded by the caliper on the BHC/CL/GR tool string; the caliper shows excellent repeatability in the main and repeat run and was used to depth match the repeat run of the BHC/CL/GR logs to the main run.
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 70. For further questions about the logs, please contact:
Cristina Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: Cristina Broglia