After completion of drilling operations at Hole 1131A, the borehole was prepared for logging (see "Operations"). The end of the BHA was placed at 99 mbsf. The WHC was used during all logging runs and coped well with the moderate heave conditions. Three different logging strings were deployed in the following order: (1) triple combo including the Lamont-Doherty Earth Observatory high-resolution temperature/acceleration/pressure tool (LDEO-TAP), (2) FMS/sonic, and (3) WST (see "Downhole Measurements" in the "Explanatory Notes" chapter). The base of the hole became progressively shallower during logging operations because of fill accumulation (Table T19).
Before the main run with the triple combo, a short quality-control run was made from base of hole at 585 mbsf (31 m short of drilled depth) to 494 mbsf. The main run covered the interval from 578 mbsf to the mudline. A single pass with the FMS/sonic was made from 572.5 mbsf to 144 mbsf. The WST was used to record eight check-shot stations (stacks of seven shots) at 30- to 100-m intervals between 560 and 140 mbsf. Check-shot locations were located at log breaks and at the estimated depths of significant seismic reflectors (see "Seismic Stratigraphy" in the "Site 1127" chapter).
Caving was significant in the upper 240 m of the open-hole logged interval, where hole diameter reached 45 cm, occasionally saturating the hostile environment lithodensity sonde and FMS calipers. Despite caving, FMS images were generally of high quality. With the exception of the accelerator porosity sonde, which in some intervals was affected by large borehole diameter (see "Logging Unit 2: 27-529 mbsf"), the triple combo string performed well. Hole-centered tools (sonic digital tool and dual induction tool) were less sensitive to borehole conditions and produced high-quality sonic and resistivity logs. The check-shot data contained consistent background noise (~35 Hz) of undetermined origin, and clipping of the recorded signal was observed at some stations. However, the recorded wavelets show a distinct first break suitable for check-shot purposes.
Despite low core recovery in some intervals, there is good agreement between the gamma-ray log (HSGR) and NGR from core (see "Physical Properties"). Core velocity is consistently lower than sonic velocity measured downhole. This difference increases with depth, as discrete velocity measurements on cores were not measured at in situ pressures.
Sediments at Site 1131 are generally high in calcium carbonate (see "Organic Geochemistry"). The most striking feature of the logs at Site 1131 is the distinct "Milankovich-like" cyclicity of the gamma radiation log (Fig. F23). This cyclicity dominantly results from variations in uranium concentrations (Fig. F23). These changes in uranium may be associated with organic matter and/or diagenetic horizons, although blackened grains observed in the recovered core may be another potential source. Cyclicity in gamma radiation at Site 1131 correlates well with cyclicity at Site 1127. Three major logging units have been defined at Site 1131, mainly on the basis of variations in gamma-ray and sonic logs.
Unit 1 is defined by an increasing trend of cyclic variations in gamma radiation (Fig. F24). The base of Unit 1 is defined at a marked drop in gamma-ray values caused by a drop in the uranium content of the formation (Fig. F24). Logging Unit 1 coincides with lithostratigraphic Unit 1, which is interpreted as a succession of bryozoan mounds (see "Lithostratigraphy").
Unit 2 was logged through pipe to 99 mbsf, with low variability in gamma-ray values (Fig. F24). The open-hole logged interval displays more variation, with gamma-ray values fluctuating between 20 and 60 American Petroleum Institute (API) units (Fig. F23). Throughout logging Unit 2, density values show a steady increase, with periodic excursions to higher values associated with peaks in sonic velocity and resistivity (Fig. F25). These intervals probably represent firmgrounds in the sedimentary sequence and are often accompanied by increased gamma radiation. These lithified horizons are also observed in FMS images as thin (~1-2 m), highly resistive intervals (Fig. F26). A few lithified horizons (e.g., at 260 and 323 mbsf) in Unit 2 are associated with crossovers (negative separation) of porosity and density and low gamma-ray values and may be interpreted as chert horizons. As with bulk density, sonic velocity also shows a steadily increasing downhole trend typical of a compaction profile (Fig. F25). Resistivity values are nearly constant throughout Unit 2, with the exception of excursions found within firmgrounds (Fig. F25). Porosity values are highly variable above 340 mbsf in Unit 2, result partly from variations in borehole diameter. Below 340 mbsf, porosity values remain moderately constant (45%) (Fig. F25). Photoelectric effect (PEF) values within logging Unit 2 show a slight downhole increase with low to moderate variability. The base of logging Unit 2 coincides with a major hiatus between the Pliocene and the middle Miocene, the boundary between lithostratigraphic Units II and III, and the base of seismic Sequence 2 (see "Biostratigraphy" and "Lithostratigraphy", also see "Seismic Stratigraphy" in the "Site 1127" chapter).
Logging Unit 3 correlates with lithostratigraphic Unit III (see "Lithostratigraphy"). This portion of the sedimentary section had only minimal recovery; thus, the downhole logs of logging Unit 3 provide important information on the sediments in this interval. Unit 3 is characterized by a shift to uniformly low gamma-ray values (<10 API units) and a higher variability of all other logs except density (Fig. F25). Photoelectric effect values fluctuate between 3.5 and 4.5 barn/e-, and the porosity curve shows frequent cross-overs with density. Within logging Unit 3, there are numerous excursions in shallow resistivity, indicating fluid invasion into the sequence (Fig. F25). Peak sonic velocities reach 2.9 km/s (Fig. F25). Downhole measurements indicate a change to a succession of thinly bedded cherts (porosity-density cross-over and low PEF values; high resistivity and sonic values), alternating with layers rich in calcium carbonate (no cross-over and high PEF values). The sediments of Unit 3 are well imaged by the FMS, which clearly identifies individual chert layers as thin (~ 20 cm), highly resistive intervals interbedded with thicker (1-2 m), less resistive grainstones (Fig. F27). This conclusion is supported by the limited core recovery in this interval.
The measurements of the LDEO-TAP tool define a very smooth temperature gradient (~17°C/km) below the pipe (Fig. F28), indicating that flow into the borehole is insignificant. This is dissimilar to the situation at Hole 1127B, immediately downslope from Hole 1131A.