Standard Wireline Data Processing
DSDP operator and logging contractor: Scripps Institution of Oceanography
Hole: 473
Leg: 63
Location: Tres Marias Island (NE Pacific)
Latitude: 20° 57.92' N
Longitude: 107° 3.81' W
Logging date: November 1978
Sea floor depth (drillers'): 3267.5mbrf
Total penetration: 287.5 mbsf
Total core recovered: 142.07 (49 % of cored section)
Oldest sediment cored: Siltstone and silty sand (Upper Miocene)
Lithologies: Clay, diatomaceous and calcareous claystone, claystone (sediments); basalt (basement).
The logging data was recorded by Gearhart-Owen in GO format. The logs were digitized by Centerline Data in 2004. They were processed in February 2005 at the Borehole Research Group of the Lamont-Doherty Earth Observatory.
| Tool string | Pass | Top depth (mbsf) | Bottom depth (mbsf) | Bit depth (mbsf) | Notes |
| 1. BHC/CL/GR |
|
126.5
|
230.5
|
133.2
|
Invalid BHC
|
| 2. TEMP |
Downlog
|
0
|
187.5
|
117.5
|
|
|
Uplog
|
0
|
187.5
|
127.5
|
|
Logging operations started with the BHC/CL/GR tool string: at 185 mbsf the string encountered a bridge, which prevented further penetration, was retrieved, and then lowered again after a stand of pipe took weight at the bridge. The string was lowered again to about 230 mbsf, where it could not proceed any further; it missed the sediment-basement contact by only 15 meters. The Temperature tool was lowered next, but could not penetrate the bridge at 187 mbsf; data were collected both downhole and uphole.
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 drillers' water depth in soft sediment.
Depth shift to sea floor: All logs were shifted to the sea floor (- 3267.5 m). The sea floor depth used for depth shifting was the drillers' water depth), as the temperature logs do not provide a clear indication of the mudline depth.
Sonic data: The sonic velocity log equals the water velocity and is therefore useless.
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). In Hole 473 only the gamma ray, caliper, and temperature logs show reliable readings. The large hole diameter (mostly > 18 inches), affected the acoustic measurement, which is therefore useless.
Any gamma ray log 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 BHC/CL/GR tool string entered the bottom hole assembly at 133.2 mbsf, the temperature tool at 117.5 and 127.5 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 63. For further questions about the logs, please contact:
Cristina Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: Cristina Broglia