Wireline Standard Data Processing
ODP logging contractor: LDEO-BRG
Location: Shatsky Rise North (NW Pacific Ocean)
Latitude: 37°47.437' N
Longitude: 162°45.053' E
Logging date: 12-13 September, 2001
Bottom felt: 3111.7 mbrf
Total penetration: 622.8 mbsf
Total core recovered: 60.2 m (12.9 %)
Logging string 1: DIT/HLDS/APS/HNGS/MGT (1 pass DIT/HLDS/APS/HNGS, 1 pass MGT)
Logging string 2: FMS/DSI/GPIT/NGT (2 passes)
Logging string 3: GHMT/NGT (2 passes)
The first pass of the DIT/HLDS/APS/HNGS/MGT tool string reached to within 1 m of the base of the hole. The tool string was lowered back down the hole for the MGT, but a bridge at 383 mbsf prevented further penetration down the hole during subsequent runs.
The wireline heave compensator was used for the GHMT/NGT runs only.
The following bottom-hole assembly depths are as they appear on the logs after differential depth shift (see "Depth shift" section) and depth shift to the sea floor. As such, there might be a discrepancy with the original depths given by the drillers onboard. Possible reasons for depth discrepancies are ship heave, use of wireline heave compensator, and drill string and/or wireline stretch.
DIT/HLDS/APS/HNGS: Bottom-hole assembly at ~125.5 mbsf
FMS/GPIT/DSI/NGT: Bottom-hole assembly at ~117 mbsf (both passes).
Depth shift: The original logs were depth matched to the HNGS from the DIT/HLDS/APS/HNGS/MGT run (pass 1) and were then shifted 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 then the features in the equivalent logs from the other runs are matched to it in turn. This matching is performed automatically, and the result checked and adjusted as necessary. 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 was then determined from the step in gamma radiation at the sediment-water interface to be 3110.5 mbrf. This depth differs by 1.2 m from the water depth determined from the mud-line core (see "bottom-felt" depth above). Depth differences occur because of wireline slip stretch, drill pipe stretch, tides, heave, etc. The log depths were adjusted to be referenced to sea floor.
Gamma-ray processing: NGT data have been processed to correct for borehole size and type of drilling fluid. The HNGS data were corrected for hole size during the recording.
Acoustic data processing: The sonic velocity logs are of moderately quality. Slow formations and rough borehole walls, like the ones encountered in Hole 1207B, generally do not produce good sonic logs. Minor spikes were manually edited from the log. The DSI was run in P&S and dipole modes on both passes, with a medium frequency source for Pass 1 and a low frequency source for Pass 2. For Pass 1 of the FMS/DSI/GPIT/NGT the data were mistakenly acquired with the automatic speed correction setting; for Pass 2, it was switched off. However, the speed correction does not seem to have had an adverse effect on the data.
High-resolution data: Bulk density and neutron porosity data were recorded at a sampling rate of 2.54 and 5.08 cm respectively. The enhanced bulk density curve is the result of Schlumberger enhanced processing technique performed on the MAXIS system onboard. While in normal processing short-spacing data is smoothed to match the long-spacing one, in enhanced processing this is reversed. In a situation where there is good contact between the HLDS pad and the borehole wall (low density correction) the results are improved, because the short-spacing has better vertical resolution.
Geological Magnetic Tool: The Geological Magnetic Tool collected data at two different sampling rates, the standard 0.1524 m rate and 0.0508 m. Both data sets have been depth shifted to the reference run and to the sea floor.
The GHMT data are of very poor quality because of tool malfunction. The total field log (MAGB) alternates between 32000 and 42000 nT, whereas small variations about 42000 nT are expected. The susceptibility logs (MAGS, RMGS) show large negative values near the base of the hole, and a high baseline of about 500 ppm in the upper part of the hole. Both of these observations are impossible, given the known properties of the formation. In the interval of overlap with the core measurements, the log and core susceptibility data show similar patterns and therefore the logs might still provide some information, though they should be used with caution.
null value=-999.25. This value may replace recorded log values or results which are considered invalid.
During the processing, quality control of the data is mainly performed by cross-correlation of all logging data.
The principal control on log quality is hole diameter. Hole diameter was measured by the hydraulic caliper on the HLDS (LCAL) and on the FMS string (C1 and C2). Hole diameter varies between 10 inches at the base of the hole to 17 inches at 155 mbsf. Softer lithologies are more easily washed out of the borehole wall; as a result, each chert layer forms a small ledge (1 or 2 inches). As the cherts occur roughly every meter, the borehole wall can be quite rough - not ideal for the tools that require good contact with the borehole wall (density, porosity). The DRH and STOF logs provide measures of quality for the density and porosity logs, respectively.
On the other hand, the resisitivity logs are relatively insensitive to hole diameter. The SFLU (shallow penetration resistivity) shows an offset from the IMPH and IDHP (medium and deep penetration resistivity) for part of the hole. The offset is too large to be explained by high-salinity formation water. It could possibly be caused by the SFLU being preferentially affected by the cherts, because SFLU is a current-based measurement rather than an induction-based measurement like IDPH and IMPH.
Data recorded through bottom-hole assembly should be used qualitatively only because of the attenuation on the incoming signal.
Additional information about the logs can be found in the "Explanatory Notes" and Site Chapter, ODP IR volume 198. For further questions about the logs, please contact:
E-mail: Cristina Broglia