Standard Wireline Data Processing
DSDP operator and logging contractor: Scripps Institution of Oceanography
Hole: 1
Leg: 1
Location: Sigsbee Abyssal Plain (Gulf of Mexico)
Latitude: 25° 51.50' N
Longitude: 92° 11.00' W
Logging date: August 15-16, 1968
Sea floor depth (sea level, echo-sounding, corrected): 2827 mbsl
Sea floor depth (step in GR log): 2841 mbrf
Total penetration: 770.5 mbsf
Total core recovered: 50 m (64.6 % of cored section)
Oldest sediment cored: Pleistocene
Lithologies: mudstone
The logging data was recorded by Schlumberger as analog data only. Three runs were recorded but only selected data were digitized by Centerline Data in 2004. Data were processed at the Borehole Research Group at the Lamont-Doherty Earth Observatory in November 2004.
| Tool string | Pass | Top depth (mbsf) | Bottom depth (mbsf) | Bit depth (mbsf) | Notes |
| 1. GNT (gamma ray and neutron log) | Uplog | 0 | 742.1 | 740 | Through BHA and drill pipe |
| 2. GGD (gamma ray and density log) | Uplog | 116 | 753.4 | 113 | Reference |
| 3. EL | Uplog | 121 | 749 | 113 |
The gamma ray and neutron logs (GNT) were recorded through bottom hole assembly and drill pipe up to the mudline.
Gamma-gamma density logs were recorded open hole during the second run; they included gamma ray and density logs. The density log is expressed as cps (counts per second) and should be used qualitatively only.
The third run (EL) included Short (16') and Long (64') Normal and Lateral (18'8') resistivities. Only the amplified short normal has been digitized.
The depths in the table are for the processed logs (after 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, in this case, from echo-sounding.
Depth match and depth shift to sea floor. The GNT neutron did not match the evenly spaced (every 9.5 m) drill pipe collars on the GNT gamma ray; depth matching was performed before comparison with the other logs. Also, the GGD gamma ray did not match the GGD density log; again, depth matching was performed before comparison with the other logs. Finally, using the GGD run as the reference, the GNT logs were depth matched, while the EL did not need any depth shift.
The sea floor depth was determined by the step in gamma ray value at 2841 mbrf observed on the GNT log. A water depth of 2827 m, corrected for sound velocity and transponder depth, was reported in the DSDP Reports. Neither "bottom felt" depth or information about the distance between the rig floor and the sea level (~10 m?) were provided in the report. Therefore, it was decided to use the water depth as it appears on the logs.
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). The logs of Hole 1 show a good correlation despite some of them were recorded through bottom hole assembly and pipe.
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. Both gamma ray and neutron logs show attenuation by drill pipe collars every 9.5 m down to 619 mbsf and bottom hole assembly down to 740 mbsf.
No hole diameter measurement was performed in Hole 1.
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 1. For further questions about the logs, please contact:
Cristina Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: Cristina Broglia