Standard Wireline Data Processing

 

DSDP operator and logging contractor: Scripps Institution of Oceanography

Hole: 485A

Leg: 65

Location: Tamayo Fracture Zone (tropical NE Pacific)

Latitude: 22° 44.92' N

Longitude: 107° 54.23' W

Logging date: March 1979

Sea floor depth ("bottom felt"): 2996.5 mbrf

Total penetration: 331 mbsf

Total core recovered: 136.1 m (49 % of cored section)

Oldest sediment cored: Quaternary

Lithologies: hemipelagic clays, and silt claystones (sediments), massive and pillow basalts (basement).

 

Data

 

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.

 

 

Logging Runs

 

Tool string Pass Top depth (mbsf) Bottom depth (mbsf) Bit depth (mbsf) Notes
1. CDL/GR Main 164.5 306.5 Open hole Reference run
Temperature Pass 1 0 328   Downlog
2. BHC/GR Main 155 303.5 161  
  Repeat 250 308.5    
3. LL3/NL/GR Main 159 269 Open hole Bad GR data
  Repeat 208 269   Bad GR data
4. Temperature Pass 2 0 258 Open hole  

 

 

Good quality logs were obtained in Hole 485A. The hole was filling with cavings as it was being logged, so the bottom depth for successive tool strings became shallower. The GR logs from the LL3/NL/GR tool string gave erratic values in the basalt units and is therefore considered invalid.

 

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 Œbottom feltı depth in soft sediment.

 

Processing

 

Depth match and depth shift to sea floor: The GR logs from the BHC/GR tool string were depth matched to the GR log from the main pass of the CDL/GR tool string, and the Neutron logs from LL3/NL/GR tool string were matched to the RHOB log from the CDL/GR tool string. The logs were then shifted to the sea floor (-2995.5 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.

 

With the exception of the temperature logs, the logs did not extend to the sea floor, so the drillerıs sea floor depth of 2995.5 mbrf was used.

 

Sonic data: Sonic velocity was calculated from the DT slowness log. The velocity logs are of moderate to good quality, however, the absolute value of the DT log seems to be approximately 2-times too high (seawater slowness is around 200 msec/ft, but the log values reach 400 msec/ft), leading to underestimated velocity values.

 

Quality Control

 

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 about 10-12 inches wide in the basalt units and reaches over 14 inches wide 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