Standard Wireline Data Processing
DSDP operator and logging contractor: Scripps Institution of Oceanography
Hole: 497
Leg: 67
Location: Mid-America Trench (tropical NE Pacific)
Latitude: 12° 59.23' N
Longitude: 90° 49.68' W
Logging date: June 1979
Sea floor depth ("bottom felt"): 2358 mbrf
Total penetration: 396.5 mbsf
Total core recovered: 224.57 m (56.6 % of cored section)
Oldest sediment cored: Nannofossil mudstone (Early Pliocene)
Lithologies: Mud interbedded with ash layers, sand, mudstone and limestone.
The logging data was recorded by Gearhart-Owen in GO format and subsequently translated into LIS format. Data were processed at the Borehole Research Group of the Lamont-Doherty Earth Observatory in December 2004.
| Tool string | Pass | Top depth (mbsf) | Bottom depth (mbsf) | Bit depth (mbsf) | Notes |
| 1. CDL/GR |
Main
|
118.4
|
356.6
|
108
|
Reference
|
| Temperature |
Downlog
|
0
|
363.1
|
|
|
|
Repeat
|
116.2
|
161.2
|
|
|
Logging with the CDL/GR/Temperature tool string was performed before cementing the hole due to the presence of gas hydrates. Only the temperature log was recorded up to the mudline.
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 CDL/GR repeat run was depth matched to the RHOB log from the main pass of the CDL/GR tool string, and were then shifted to the sea floor (- 2358 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 used to shift the data to the sea floor corresponds to the "bottom felt" depth provided by the drillers. The Temperature log is the only log that reached the sea floor but does not display the typical step observed on such logs as the gamma ray.
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 the case of Hole 497, little repeatability is observed between the main and repeat run, particularly in the gamma ray curve, which should be used with extreme caution. Some similarities are observed in the density and caliper curves, but in general there are not many elements to provide an accurate assessment of the quality of the logs. Caution is therefore recommended.
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. (The CNL porosity can sometimes be used qualitatively through the BHA and pipe, but most of the other logs will not give usable data.)
A wide (>12") and/or irregular borehole affects most recordings, particularly those that require eccentralization and a good contact with the borehole wall (CDL). Hole diameter was recorded by the hydraulic caliper on the CDL tool (CALI); it shows an irregular and enlarged hole that becomes more regular in the lower part of the logged interval.
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 67. For further questions about the logs, please contact:
Cristina Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: Cristina Broglia