Standard Wireline Data Processing

 

DSDP operator and logging contractor: Scripps Institution of Oceanography

Hole: 415

Leg: 50

Location: Agadir Canyon (NE Atlantic)

Latitude: 31° 1.72' N

Longitude: 11° 39.11' W

Logging date: September 1976

Sea floor depth ("bottom felt"): 2817 mbrf

Total penetration: 283 mbsf

Total core recovered: 31.3 m (70 % of cored section)

Oldest sediment cored: Nannofossil marl and chlak (Early Miocene)

Lithologies: nannofossil marls, oozes and chalks

 

Data

 

The logging data was recorded by Schlumberger in LIS format. Because the data on tape was incomplete and corrupted, the original logs were digitized by Centerline and then processed at the Borehole Research Group of the Lamont-Doherty  Earth Observatory.

 

Logging Runs

 

Tool string Pass Top depth (mbsf) Bottom depth (mbsf) Bit depth (mbsf) Notes
1. BHC/GR
Main
101.5
275
107
Repeat
204.8
270.6
2. FDC/CNL/GR
Main
99.2
272.9
107
CNL invalid. Caliper malfunctioning
Repeat
205.1
267.9
CNL invalid. Caliper malfunctioning
3. DIL/LL8/GR
Main
105.4
267.9
107.5
Repeat
173
268.6

 

 

Logging operations at Hole 415 were successful, with the exception of the caliper and CNL tools; the former would read only up to 8 inches and the latter became saturated, thus producing flat readings throughout the 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.

 

Processing

 

Depth match and depth shift to sea floor: The original logs were depth matched to the GR log from the main pass of the BHC/GR tool string, and were then shifted to the sea floor (- 2817 m). The water depth corresponds to the Òbottom feltÓ depth provided by the drillers: as none of the logs reached the sea floor, this value has been used to depth shift the logs to the sea floor.

 

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.

 

Sonic data: Velocities were calculated from the delay times.

 

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). The gamma ray logs recorded with each of the three logging strings show excellent repeatability; on the other hand correlation between resistivity, density, and velocity logs is not as good. A wide (>12") and/or irregular borehole affects most recordings, particularly those that require eccentralization and a good contact with the borehole wall (FDC). The caliper on the FDC tool did not work properly, as it became saturate; therefore, there is no indication of the borehole true conditions, which might be responsible for the lack of good correlation between the aforementioned logs.

 

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.  Less than 10 m of logs were acquired through the BHA.

 

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 50. For further questions about the logs, please contact:

 

Cristina Broglia

Phone: 845-365-8343

Fax: 845-365-3182

E-mail: Cristina Broglia