DSDP operator and logging contractor: Scripps Institution of Oceanography
Hole: 483
Leg: 65
Location: Tamayo Fracture Zone (tropical NE Pacific)
Latitude: 22° 53.0' N
Longitude: 108° 44.9' W
Logging date: February 1979
Sea floor depth (Bottom felt): 3084 mbrf
Sea floor depth (step in GR): 3085 mbrf
Total penetration: 204.5 mbsf
Total core recovered: 110.92 m (54 % of cored section)
Oldest sediment cored: Late Pliocene
Lithologies: hemipelagic clays (sediments); pillow and massive basalt (basement).
The logging data was recorded by Gearhart-Owen in analog form only; it was digitized by Centerline Data in 2004. Data were processed at the Borehole Research Group of the Lamont-Doherty Earth Observatory in October 2004.
| Tool string
|
Pass
|
Top
depth (mbsf)
|
Bottom
depth (mbsf)
|
Bit
depth (mbsf)
|
Notes
|
| 1. CDL/GR
|
Main
|
124
|
197
|
Open
hole
|
Reference run
|
|
|
Repeat
|
136
|
197
|
|
|
|
|
Thru
pipe
|
0
|
125.5
|
|
GR only
|
| Temperature
|
Downlog
|
0
|
197
|
|
|
| 2. LL3/NL/GR
|
|
106
|
171
|
Open
hole
|
|
The CDL/GR tool string reached close to the bottom of the hole and good quality logs were obtained to the bit (placed below the sediment-basement contact). GR only was recorded through the bottom hole assembly (BHA). A BHC/GR tool string was run next, but the BHC failed when it was at the bottom of the hole; no data was recorded. The bit was raised above the sediment-basement contact (110 mbsf) for the LL3/NL/GR tool string.
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 GR logs from the different passes were found to be on-depth with each other, within the limits of the effects of tool string heave. The logs were shifted to the sea floor (-3084 m). The GR logs from the other passes were matched to the GR log from the reference run, cross checking with the caliper and physical property 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.
There is a step to low values at 3085 mbrf in the GR log through the pipe, but as there is only 1m of log data above this, it is not considered a reliable indicator of sea floor depth, and the "bottom felt" value of 3084 mbrf is used to shift the logs to sea floor.
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 (CDL, NL). Hole diameter was recorded by the hydraulic caliper on the CDL tool: the hole is about 10 inches wide in the basalt units and reaches over 14 inches in the interlayered sediments.
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 65. For further questions about the logs, please contact:
Cristina Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: Cristina Broglia