Standard Wireline Data Processing
DSDP operator and logging contractor: Scripps Institution of Oceanography
Hole: 35
Leg: 5
Location: Escanaba Trough (NE Pacific Ocean)
Latitude: 40° 40.42' N
Longitude: 127° 28.48' W
Logging date: May 3, 1969
Sea floor depth ("bottom felt" or echo sounding"): 3280.5 mbrf
Sea floor depth (step in GR log): unclear
Total penetration: 390 mbsf
Total core recovered: 95.3 m (68 % of cored section)
Oldest sediment cored: Pleistocene
Lithologies: sand and silt
The logging data was recorded by Schlumberger and Dresser Atlas as analog data only. The data was digitized by Centerline Data in 2004 and processed at the Borehole Research Group at the Lamont-Doherty Earth Observatory in July 2004.
| Tool string | Pass | Top depth (mbsf) | Bottom depth (mbsf) | Bit depth (mbsf) | Notes |
| 1. AL/CL |
Uplog
|
100.5
|
379.5
|
Data
recorded openhole
|
Dresser-Atlas
|
| 2. GNT/EL/SP |
Uplog
|
0
|
383.8
|
87.5
|
Schlumberger
|
The top of the drill bit was positioned at 50 m above the bottom of the hole during the first pass and at 150 m above the bottom during the second pass. Therefore gamma ray and neutron logs were recorded open hole through pipe up to the mudline.
The depths in the table are for the processed logs (after depth match 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. Typical reasons for depth discrepancies are ship heave, wireline and pipe stretch, tides, and the difficulty of getting an accurate sea floor.
Depth match and depth shift to sea floor: According to the log header, the original GR log was about 8 m higher than the resistivity-SP logs recorded on the same run. Therefore, the GR was depth matched to the resistivity-SP log and then shifted to the sea floor (- 3280.5 m). Also, according to the log header, the caliper log was 4 m deeper than the acoustic log and therefore it has been shifted upward. The acoustic data (AL) recorded by Dresser-Atlas are of very poor quality and do not show any correlation with the other logs. Because no GR log was recorded on the acoustic log run, the data could not be depth matched to the other logs.
The sea floor depth is usually determined by the step in gamma ray value on the logs. This is not clearly shown by the GR recorded at Hole 35, thus the sea floor depth given by the drillers (3280.5 mbrf) has been used instead.
Sonic data: The acoustic data are of very poor quality, due mostly to the degraded hole conditions. Nonetheless, a velocity log has been derived from the acoustic slowness.
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 or density log). At hole 35, there is a reasonably good match between the resistivity and gamma ray log (and locally, the spontaneous potential), whereas the acoustic data do not show any correlation.
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, as is the case of the acoustic data collected in Hole 35. Hole diameter varies from 8 to 18 inches
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 5. For further questions about the logs, please contact:
Cristina Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: Cristina Broglia