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doi:10.2204/iodp.pr.320T.2009

Downhole logging

Data quality and comparison with Hole 807A logs

Hole U1330A was cored between 103.6 and 122.8 m DSF but was primarily drilled for logging and testing of the new WHC. Multiple passes were made with each of the three tool strings over a maximum depth range of 424 m (106–530 m WSF) in Hole U1330A. The HLDS caliper log (Fig. F18A) shows that borehole diameter was generally constrained and the borehole wall relatively smooth. The caliper arm maintained good contact with the borehole wall, with the exception of an interval between ~160 and 260 m wireline log matched depth (below seafloor) (WMSF), suggesting that the data are of good quality. Even in the interval with poor contact, resistivity data continue to show amplitudes similar to those within the narrower portion of the hole, indicating that the data reflect real variations in the formation. The caliper logs from the FMS (two per pass) provided another means for assessing borehole condition (diameter and rugosity) (Fig. F19). FMS caliper logs indicated that borehole conditions for the majority of the logged section were good. However, between ~290 m WSF and the top of the logged section, two of the four caliper arms of the FMS were not touching the borehole wall, degrading the quality of the logs.

Logs from Hole U1330A compare well with the data recorded ~150 m away in Hole 807A during Leg 130 (Fig. F18). Comparison between the two HLDS caliper logs in Figure F18A shows that hole conditions were better in Hole U1330A. Hole 807A was cored before logging, resulting in a larger hole, whereas Hole U1330A was drilled as a logging-dedicated hole. Accurate caliper data were not collected during the main pass of the lower ~80 m of Hole U1330A, where the caliper arm was closed because of winch problems. Figure F18B shows that gamma ray values from the first pass with the first tool string were very low in Hole U1330A, particularly in the upper section. The low values are due, in part, to the low natural radioactivity of the nannofossil ooze that constitutes the bulk of the formation. When comparing the holes, one can observe similar features between the two curves. The difference is mainly in amplitude, although below ~500 m WMSF, gamma ray data from Hole U1330A do approach values similar to those from Hole 807A. Gamma ray data were collected using two different tools (the Natural Gamma Ray Spectrometry Tool [NGT] in Hole 807A and the HNGS in Hole U1330A), but both were operating at the lower limits of their sensitivity. The apparent discrepancy may be due to the fact that the HNGS was not calibrated for several months prior to Expedition 320T. The gamma ray data from Hole U1330A will be corrected as soon as the tool is recalibrated for future expeditions. Resistivity logs show good agreement between Holes U1330A and 807A (Fig. F18C), exhibiting an almost perfect match in the common interval.

Logging stratigraphy

At Site 807, the upper 950+ m of sediments was placed in a single lithostratigraphic unit (nannofossil ooze and chalk), with a subdivision at ~290 m CSF identified as the ooze–chalk transition (Kroenke, Berger, Janecek, et al., 1991). Site U1330 sediments can generally be described by low natural radioactivity and resistivity. Although there is some variation in the resistivity log, there are no common events in the limited data collected to clearly define distinct logging units. The lack of distinctive character in the logs reflects the relatively uniform composition of the sediments. Sediments at this site are defined as a single logging unit, and the subunits shown in Figure F18 are predominantly based on changes in resistivity values.

Logging Subunit 1A is characterized by relatively constant low resistivity and gamma ray values. Minor peaks in resistivity in this subunit may correspond to more competent chalk intervals in the surrounding ooze. Logging Subunit 1B displays an increasing trend with depth in resistivity and gamma ray values, starting at ~290 m WMSF. The top of this subunit corresponds to the base of lithostratigraphic Subunit 1A at Site 807, which is characterized by increasing induration (Kroenke, Berger, Janecek, et al., 1991). Logging Subunit 1C is characterized by a decrease with depth in resistivity and gamma ray values. There is a local increase in gamma ray values in Hole U1330A but no corresponding change in resistivity.

FMS images

FMS imagery (Fig. F20) captures the banded and laminated nature of the formation(s) in the borehole wall. When compared with core taken from Hole 807A, features such as bioturbation (Fig. F20B), bedding/laminations (Fig. F20B), and wispy structures (observed as braided color bands, Fig. F20C) can be picked out from the images (Kroenke, Berger, Janecek, et al., 1991). The good correlation between the features seen in the Hole 807A core and the FMS images collected ~150 m south of Hole 807A attest to the lateral extent of the formations. However, sinusoidal features seen in FMS images (Fig. F20A) indicating dipping structures are not apparent in the corresponding core section, likely because of the steep nature of these features (25°–30°) and the large offset between the holes. These features may be due to local sediment instabilities or local tectonic influences, such as microfaulting.

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