Previous   |    Next

doi:10.2204/iodp.proc.340.104.2013

Downhole logging

A 100 m section of Hole U1394B was successfully logged with two tool strings over a period of ~16.5 h. The downhole logging measurements obtained include total and spectral gamma ray, electrical resistivity and images, magnetic susceptibility, and velocity. The borehole remained in good condition for the duration of the logging operations.

Operations

Downhole logging operations in Hole U1394B commenced on 12 March 2012 at 0520 h (UTC – 4 h) following APC/XCB coring operations, which were terminated at a total depth of 181.4 mbsf. The hole was prepared for logging by sweeping with high viscosity mud, displacing the hole with heavy mud, and pulling the pipe to a bit depth of 82.7 mbsf.

Three tool strings were deployed in Hole U1394B (Fig. F12): (1) the triple combo–Magnetic Susceptibility Sonde (MSS), (2) the VSI, and (3) the FMS-sonic (see Fig. F11 in the “Methods” chapter [Expedition 340 Scientists, 2013]). The first deployment was of the triple combo–MSS tool string, made up of the Hostile Environment Natural Gamma Ray Sonde (HNGS), Hostile Environment Litho-Density Sonde (HLDS), High-Resolution Laterolog Array (HRLA), and MSS (see “Downhole logging” in the “Methods” chapter [Expedition 340 Scientists, 2013] for tool string details). Owing to concerns regarding hole stability, the ¹³⁷Cs source was omitted from the HLDS, so no density data were recorded. The tool string was lowered into the hole at 0710 h on 12 March, completing a downlog to a total depth of 180 mbsf. An initial uplog to 110.5 mbsf was completed, followed by a second and final uplog from 180 mbsf terminating above the seafloor. The triple combo was rigged down by 1145 h.

The second deployment was the VSI tool string. Rig up for this VSP experiment started at 1145 h, and the associated protected species watch commenced at 1130 h. The VSI tool string was lowered into the hole at 1230 h. Unfortunately, the tool string never cleared the BHA. Following various failed attempts at pumping the tool down through the BHA, the experiment was abandoned at 1450 h and the tool string rigged down by 1545 h.

The final deployment in Hole U1394B was the FMS-sonic tool string. The tool string was lowered into the hole at 1800 h, reaching a total depth of 180 mbsf at 1902 h. Two passes of the open hole section were then conducted, after which the tool string was returned to the surface, where rig down was completed by 2140 h.

Data processing and quality assessment

Logging data for Hole U1394B are summarized in Figures F13, F14, and F15. Initially logging data were depth-shifted to the seafloor as identified by a stepwise increase in the gamma ray value. Depth shifts were then applied to the logging data by using the total gamma ray log from the main pass of the triple combo as a reference log. Features in gamma ray logs from the other tool string passes were aligned to the reference log to produce a complete depth-matched data set.

Hole diameter was recorded by the hydraulic caliper on the HLDS and by the FMS calipers (Figs. F13, F14). All caliper measurements indicate an in-gauge hole with a diameter ranging from 11.2 to 15.8 inches (mean = 12.7 inches). The widest part of the hole was observed in the upper portion just below the pipe, between ~87 and 93 mbsf. These measurements were relatively consistent throughout the course of the logging program. Comparisons between repeat passes for each of the tool strings indicate a good degree of repeatability for all measurements (Fig. F16).

The Dipole Shear Sonic Imager (DSI) was operated in the following modes: P&S monopole and upper and lower dipole for both passes. To optimize measurements in this soft sediment, the monopole and upper dipole utilized a standard (high) frequency and a lower frequency was used for the lower dipole. The resulting slowness data were subsequently converted to acoustic velocities (compressional wave velocity [V_P; monopole] and shear velocity [V_S; upper and lower dipole]). The high coherence in sonic waveforms indicated by distinctive red areas in the V_P and V_S tracks (Fig. F14) suggests that the DSI was able to capture both compressional and flexural arrivals. However, in the interval between 90 and 105 mbsf the automatic labeling of the wave arrivals (black curve) failed to recognize the shear wave, and thus the data in this interval should not be interpreted. Additional postcruise processing will refine the V_S profile and provide a better estimate of shear velocity in the poorly labeled interval.

Good FMS image quality relies on a number of factors, including an in-gauge hole, regular borehole walls, and good contact between the tool’s pads and the borehole wall. Hole U1394B was in gauge, and the FMS pads seem to have maintained good contact with the borehole walls while the calipers were open. However, there was a 6 m section between ~99 and 106 mbsf where the calipers were closed because of excessive sticking (evidenced by increased tension on the wireline cable) over which no images were recorded. Processing of the FMS image data allows a speed correction to be applied that takes account of variations in the speed of the tool including stick and slip. The shifts applied on this basis were within acceptable limits (<1 m).

Logging stratigraphy

Although there are characteristic trends in the logging data from Hole U1394B, there are not enough large-scale differences over the 100 m interval of the open hole to warrant subdivision into multiple logging units. The entire logged interval was assigned to one logging unit (Figs. F13, F14, F15). Logging Unit 1 extends from 80 to 180 mbsf. Variations in logging measurements within the unit are described in detail below.

Logging Unit 1

The gamma ray profile in logging Unit 1 is characterized by low-amplitude, high-frequency changes in the uppermost 10 m and relatively higher amplitude, lower frequency changes below that. Consideration of lower amplitude variations below 100 mbsf suggests that there are peak-to-peak changes in gamma ray across 5–10 m intervals (Fig. F13). Overall gamma ray increases with depth and has an average value of 19 gAPI with a mean potassium concentration of 0.32% (standard deviation = 0.08). There is some suggestion from the spectral data that uranium (mean = 0.97 ppm) and thorium (mean = 1.80 ppm) are anticorrelated, particularly in the lower portion of the logged section (Fig. F15). A net increase in resistivity from 80 to ~100 mbsf is followed by a subtle decrease to the base of the hole, a trend that is echoed in the magnetic susceptibility data set. True resistivity values range from 0.01 to 2.45 Ωm, with a mean of 1.33 Ωm (Fig. F13). Through the logged section there are a number of resistivity highs on the scale of ~1–2 m. These, like the general trend, correspond with magnetic susceptibility highs (Fig. F17), and, as expected, the resistivity logs relate well to the FMS images (Fig. F14, F18).

The magnetic susceptibility data set provides the best coverage of Hole U1394B because of its position at the base of the triple combo (see Fig. F11 in the “Methods” chapter [Expedition 340 Scientists, 2013]). The data are given in uncalibrated units and are therefore most useful when used in tandem with a calibrated data set, such as the core magnetic susceptibility data from the multisensor loggers. Downhole magnetic susceptibility logs indicate two intervals of higher amplitude variations, the first between ~93 and 105 mbsf and the second between ~160 mbsf and the base of the hole. Individual high-amplitude features in magnetic susceptibility and resistivity correspond, in some cases, to debris deposits and volcaniclastic sand and turbidites (Fig. F17). This correspondence is generally consistent with magnetic susceptibility loop data from the whole core. The interval from ~105 to 160 mbsf exhibits lower amplitude variations, and overall magnetic susceptibility decreases downhole.

Similar to the gamma ray data, V_P measurements from sonic log data generally increase downhole, ranging from ~1650 to 1900 m/s. These downhole V_P measurements are generally higher than those measured on the cores recovered from Hole U1394B (typically ~1600 m/s; see “Physical properties”). There are distinctive localized peaks in V_P between ~90 and 120 mbsf that coincide with increased resistivity and magnetic susceptibility (Figs. F13, F14). These features likely correspond to similarly scaled turbidites identified in the cores (see “Lithostratigraphy”). In the interval below 105 mbsf, V_P and V_S show similar characteristics.

Formation MicroScanner images

Examples of the FMS images from Hole U1394B are shown in Figure F18. The images show local small-scale changes in the sediment’s electrical properties that may be related to subtle changes in sediment hardness and/or clay content. In addition to subtle changes, there are also some very definite boundaries evident in the images that are marked by strong resistivity contrasts (Figs. F17, F18). Most of the boundaries identified are subhorizontal in orientation; however, a few appear to be dipping.

Correlation of the logging data and images to the corresponding core measurements suggests that the data sets are offset on the order of a few meters (Fig. F17). Although some offset is expected because of the different measurements of depth (wireline versus core), this larger offset may be due to coring artifacts/expansion or related to wireline cable stretch.

Top of page   |    Previous   |    Next