Downhole logs are used to directly determine the in situ physical, chemical, and structural properties of sediments penetrated by drilling. Continuous downhole measurements complement discrete measurements obtained from cores and serve as a proxy for physical and sedimentological properties where core recovery is incomplete. In addition, by correlating core and downhole measurements, the true stratigraphic depth of core material can be determined. By mapping the sediments to the borehole depth, as determined by the wireline, it is also possible to estimate postcoring sediment expansion.
During Leg 177, we used the triple combination and the geological high-sensitivity magnetic tool (GHMT) strings to log Hole 1093D. In addition, the Lamont temperature logging tool (TLT) was deployed to recover borehole temperature changes. The tools were run at the highest resolution allowable to record cyclicity on the shortest possible time scale, in accord with the paleoceanographic objectives of the leg (Table T7).
ODP wireline logging services aboard the JOIDES Resolution are provided by the LDEO-BRG and Schlumberger Well Logging Services. The tools have been modified for use in a 3.8-in drill-string bore.
A brief description of logging tools used by LDEO-BRG is found in the Wireline Logging Service Guide (1994). A more thorough discussion of logging sensor principles and their geologic applications can be found in Serra (1984), Timur and Toksöz (1985), Ellis (1987), and Schlumberger (1989). A description of the GHMT sensors can be found in the ODP Leg 162 Initial Reports volume (Jansen, Raymo, Blum, et al., 1996) and Schlumberger (1994). A summary of the typical vertical resolution of the tools used during Leg 177 is provided in Table T7.
The triple combination tool string consists of a hostile environment natural gamma-ray sonde (HNGS), an accelerator porosity sonde (APS), a hostile environment litho-density sonde (HLDS), and a dual induction tool (DIT). In addition, the TLT is run as part of the triple combination tool string. The GHMT string is composed of the nuclear magnetic resonance tool (NMRT), which measures the total magnetic field, and the susceptibility magnetic tool (SUMT). A natural gamma-ray spectrometry tool (NGT) is also run as part of the GHMT string.
After coring is completed, the hole is flushed with heavy viscous drilling fluid to clear sediment fill and condition the hole for logging. The bottom-hole assembly is then pulled up to logging depth. If obstructions occur on the upward trip, or if bridging is expected to cause a problem, a wiper trip may occur where the pipe is lowered to the bottom of the hole and then brought up to logging depth. Finally, the tool strings are lowered into the hole by a seven-conductor wireline cable that transmits the data back to the Schlumberger Multitask Acquisition and Imaging System (MAXIS 500) logging computers where logging data are monitored and recorded in real time. The wireline heave-motion compensator may be employed in rough seas to minimize the effect of ship heave on tool position within the hole. After logging and initial shipboard processing, the data are transmitted back to LDEO-BRG for shore-based processing and corrections. A shore-based log processing report is provided in the "Site 1093" chapter.
The quality of the log data is dependent primarily on borehole conditions. Large-diameter or irregular boreholes can lead to problems with measurements that require good contact with the borehole wall, such as density and porosity. Natural gamma-ray measurements can also be affected by the size and variability of the borehole. Measurements that have deeper investigation depths, such as resistivity, are less sensitive to variations in borehole size. The quality of the borehole is generally better if circulation of drilling fluid can be minimized and if a young hole or a dedicated hole, which has been drilled immediately before logging, can be used.