DOWNHOLE MEASUREMENTS

After finishing the wiper trip and releasing the bit, the wireline tools were rigged and deployed (Table T19) (see "Operations"). Because of rough seas, we decided not to use the triple combo to avoid damaging its calipers or losing the radioactive source. Instead, we used the DITE-DSI-NGT (resistivity, velocity, and natural gamma ray) combination which does not include a caliper. When we started pulling the tools up the hole, we lost communications with the tools. The DSI had malfunctioned, rendering the entire tool string inoperative. After replacing the DSI with the LSS, we re-entered Hole 1139A. However, we could not pass a bridge at 593 mbsf, which marks the transition to the basaltic lava flows. Thus, we started to log from 593 mbsf to the end of pipe at 101.7 mbsf. The seas were too heavy to run the wireline heave compensator, but the data were corrected by using acceleration measurements from LDEO-TAP.

Logging data from Site 1139 are generally of good quality (Fig. F97), although the lack of borehole diameter measurements compromises downhole measurement accuracy. However, good correlation between MST and downhole natural gamma-ray data suggest that our data are accurate (Fig. F98). We can clearly see the tephra layer between 190 and 200 mbsf (see "Lithostratigraphy"), indicated by high natural gamma radiation data. Resistivity data resemble the MST density data in quality. For example, changes in density at 188, 295, and 336 mbsf are also evident in downhole resistivity measurements and are related to lithologic changes.

The velocity data are of low quality in the basement section (Fig. F98). While logging, the bowsprings, used for centralizing the tool, produced noise that disturbed the velocity measurements. Shore-based processing enhanced the data quality and eliminated effects caused by cycle skipping. Although the data are noisy, the average values are reliable, and general trends correlate with those in resistivity data. We cannot yet explain the general offset between core velocity and log data in the sediments. In the basement section, the log data are only slightly lower than the core measurements (see "Seismic Stratigraphy").

The logging data delineate lithologic Units II to VI (Figs. F97, F98). As expected, pelagic sediments of Unit II show the lowest natural gamma ray (SGR) (<50 gAPI). At the base of Unit II, SGR values increase and, in Units III and IV, vary at a high frequency (Fig. F99). Volcanic lithics found in Unit IV (see "Lithostratigraphy") are characterized by slightly increased resistivities. In Unit V, SGR values continue to increase (>70 gAPI), primarily because of K and Th. Core recovery is low in this unit, and natural gamma-ray data from the MST are comparably low (Fig. F97). This suggests that sediment enriched in clays or volcanic material was not recovered.

The transition to basement is marked by a thin layer of high K content (458-460 mbsf) (Fig. F99). This layer probably overlies igneous basement. The top of the igneous basement (Unit 1A) (461.7-489.7 mbsf) consists of rounded rhyolite cobbles that are interpreted as a weathered pavement or conglomerate (see "Physical Volcanology").

The upper part of the basement is composed of felsic volcanics (see "Igneous Petrology") enriched in Th and K. We distinguish basement Units 1-4 by differing Th, U, and K contents. Because of poor recovery of basement units, core-log integration is difficult. The radioactive nuclides highlight general chemical differences among igneous rocks, but they are also affected by alteration. Aside from geochemistry, alteration also affects resistivities and velocities, particularly where alteration accompanies changes in structure (e.g., grain-size differences and faulting).

Figure F99 illustrates our attempt to relate the logging data to the different basement units. Subunit 1A can be subdivided into two intervals (460-479 and 479-490 mbsf). At 479 mbsf, the values of total gamma ray, resistivities, and velocity increase, indicating a lithologic change. Subunit 1B, a bioclastic sandstone, was probably completely recovered. The logs suggest a thin bed at 489.5-490 mbsf, where low natural gamma-ray values (~60 gAPI) indicate low K, Th, and U content, as expected for bioclastic sandstone. The region from which Subunit 1C was recovered shows fairly constant resistivity data. However, K increases slightly toward the base (from 2 to 3.2 wt%). At the base of this interval are the highest K and Th (~20 ppm) values. It is not clear whether this layer belongs to Subunit 1C or is a separate unrecovered lithology. Subunit 1D, which is a perlitic felsic glass, is characterized by low resistivity data (~2 m), which might result from rock alteration and structure. Subunit 1E is composed of altered and sheared volcaniclastics and may correspond to a zone of U enrichment (5-10 ppm) seen in the logs. This enrichment might be related to fluid flow and the incorporation of U into secondary phases. Basement Unit 2 is a dark red welded rhyolite that shows relatively constant and high SGR (~100 gAPI), Th (~9 ppm), U (~1.5 ppm), K (~3.5 wt%), and resistivity (~4.5 m) data. In the altered crystal vitric tuff in basement Unit 3, the K content decreases downhole to 1 wt% in the lower third of the layer, and then starts to increase again. Resistivity and velocity are low overall. Basement Unit 4 is similar to basement Unit 2; basement Unit 4 shows comparable K and slightly higher Th and U values. We cannot identify the boundaries of basement Unit 5 on the basis of the resistivity data alone.

Temperature data from the TAP tool clearly show the sediment/basement boundary (Fig. F100). Generally, measurements taken uphole are slightly higher than those taken downhole; the variations in tool speed account for the difference. Also, when attempting to pass the ledge, we disturbed the hole's temperature regime. While logging upward, temperatures are lower at the bottom and higher above 540 mbsf than data from the downgoing measurements. At ~380 mbsf, the temperature starts to increase downhole from 5.2° to ~8°C. This corresponds to the sediment-basement transition. The maximum detected temperature in the hole is 11.2°C at 595 mbsf.