Leg 168 Logging Summary
Shipboard Scientific Party
The primary objective of the ODP Leg 168 was to characterize the hydrothermal regimes that occur on the eastern flank of the Juan de Fuca Ridge, northeast Pacific. The studies were focused on exploring the causes and consequences of ridge-flank hydrothermal circulation by drilling, sampling, downhole measurements, and post-drilling observations of the fundamental physics and fluid chemistry of hydrothermal circulation, and the consequent alteration of the upper igneous crust and sediments that host the flow. Ten relatively shallow sites were drill during Leg 168 and they penetrated mostly sediments and upper oceanic crust along a transect of crustal ages varying from 0.6 to 3.0 Ma. The sites were divided in three main areas. The first three sites (1023, 1024 and 1025) constituted the Hydrothermal Transition Transect. The second group of sites (1026 and 1027) formed the Permeable Penetrator Transect and finally, Sites 1028 to 1032 were drilled within the Lithospheric Heat Flow Transect.
Due to time constraints following the deployment of four instrumented CORK systems during the cruise and other wireline downhole measurements, it was only possible to log one hole (1032A) during Leg 168. Although the hole had been drilled well into basement with the hope of obtaining high quality downhole measurements across the sediment-basement contact, hole fill and bridging problems prevented passage of the tools to this depth. A complete suite of downhole logs obtained in Hole 1032A, including depth intervals, is presented in Table 1.
Table 1: Logged Depth Intervals in Hole 1032A
The Triple Combo string includes the phasor induction tool (DITE) and the integrated porosity-lithology tool string (IPLT). The latter includes the new spectral gamma-ray tool (HNGS) the new porosity tool (APS) and a litho-density tool (HLDS). The Formation MicroScanner-Sonic string includes the Array Sonic tool (SDT) and the Formation MicroScanner (FMS) together with the natural gamma-ray tool (NGT). Finally the Geochemical string includes the Geochemical Spectroscopy tool (GST), the Aluminum Activation tool (AACT) and the NGT. The compensated neutron tool (CNT-G) is also deployed to provide a source for aluminum activation.
The quality of the log data acquired during Leg 168 was generally very good. The logged sequence consisted of a turbidite sequence (more than 200m thick). Core recovery within the logged interval was less than 30%, therefore logging became an important tool for reconstruction of the facies and bedding patterns within the sequences. FMS images together with geophysical and geochemical logs integrated with physical property data from cores provide an important complement in order to characterize the sedimentary section.
The sedimentary sequence is best observed in gamma-ray and FMS data. Natural gamma-ray data show the same cyclicity observed in core data not only in Hole 1032A but also along the other holes within the Heat Flow transect. A soft sand layer, around 40 meters thick is observed between 105-145 mbsf. Low gamma-ray values together with an increase in resistivity and sonic velocity are observed for this interval. For the rest of the hole, downhole geophysical data such as resistivity, density, porosity and sonic should provide good stratigraphic control and means for core-log integration. These data show a relatively steady behavior downhole with minor changes given by the alternation of muddy and sandy layers (Figure 1).
FMS data became very important in characterizing and counting the mud-sand alternation. Due to good borehole conditions, FMS images provided a closer look at these bedding structure, corresponding to highly resistive (yellow) sand layers against a relatively conductive (brownish) mud layers (Figure 2).
Preliminary synthetic seismogram results have been obtained, using the density log and sonic Vp log (Figure 3). In Figure 3, the density log is plotted against core-measured GRAPE density and Index Property (IP) measurements, where measurements on core samples are available. Neutron porosity and sonic Vp logs are also shown against available IP measurements. Synthetic seismic trace (thick line) is compared with the real seismic trace at the drilling location of this hole (1032A). The synthetic seismic trace is generated by using density and sonic Vp logs with minor corrections for instrument errors. The agreement between synthetic and real seismic traces is good for depth interval 75. - 200.0 mbsf. For depths below 200 mbsf, two large seismic events on the synthetic trace which are absent in the real seismic trace are due to the effects of unfavorable hole conditions on the density and velocity logs. Further corrections on log data are needed in this deeper interval. The seismic reflection from basement on the real seismic trace is not available on the synthetic trace as the logging tools did not pass through it.
Yue-Feng Sun (LDEO-BRG)
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