Standard Wireline Data Processing
DSDP operator and logging contractor: Scripps Institution of Oceanography
Hole: 477
Leg: 64
Location: Guaymas Basin (tropical NE Pacific)
Latitude: 27° 1.85' N
Longitude: 111° 24.02' W
Logging date: December 1978
Sea floor depth ("bottom felt"): 2020 mbrf
Sea floor depth (step in GR log): 2013 mbrf
Total penetration: 191 mbsf
Total core recovered: 52.46 m (27.5 % of cored section)
Oldest sediment cored: Late Pleistocene
Lithologies: diatomaceous turbidites (sediments) and and dolerite sill (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. LL3/NL/GR |
Main
|
0
|
190
|
|
In pipe and
BHA
|
|
Repeat
|
160
|
188.5
|
|
In pipe and
BHA
|
|
| Temperature |
Downlog
|
0
|
192
|
|
In pipe and
BHA
|
Logging through pipe was carried out in Hole 477 when extra time became available owing to repairs to the gas-chromatograph. There were also concerns about high temperatures in the hole (87.8°C at the base). Good quality logs were obtained , which show the sill from 56 to 100 mbsf. Hole 477 is located 165 m away from Hole 477A, and the sill appears at different depths in each 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 GR log from the repeat pass was matched to the GR log from the reference run, cross checking with the neutron logs; both NL/GR and Temperature logs logs were then shifted to the sea floor (-2013 m).
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 2013 mbrf. This differs by 7 m from the "bottom felt" sea floor depth given by the drillers (2020 mbrf), but is the same as that derived from echo sounding.
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.
Neutron and GR log values are attenuated when logged through the pipe (above 56 mbsf) and even more through bottom hole assembly (56 mbsf to total depth). Some features, however, are still clearly visible, such as the sill between 56-100 mbsf and some more radioactive levels from 107-150 mbsf.
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 64. For further questions about the logs, please contact:
Cristina Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: Cristina Broglia