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Logging Summary

IODP Expeditions 304-305:

Oceanic Core Complex Formation, Atlantis Massif

Expedition 304 and 305 Scientific Parties


    Figure 1. (a) Tectonic and morphologic setting of Atlantis Massif.(b) Basemap of Atlantis Massif showing prior geological and geophysical data coverage and the location of IODP drill sites

    The principle objective of Expeditions 304 and 305 was to determine the conditions under which oceanic core complexes develop. A total of 3 sites were drilled, two in the hanging wall (U1310 and 1311) and one in the footwall (U1309) of a major detachment fault system. The deepest hole, Hole U1309D is located on the central dome of Atlantis Massif, 15 km west of the median valley axis of the Mid-Atlantic Ridge, where the seafloor coincides with a gently sloping, corrugated detachment fault surface (Figure 1).

    Two drill holes at this site (U1309B and U1309D) penetrate a multiply-intruded crustal section. During Expedition 304, Hole U1309B (101.8 mbsf) was drilled and Hole U1309D was spudded using a hammer drill with casing, to provide stable reentry for a deep hole. Hole U1309D was cored 401.3 mbsf with excellent recovery. During Expedition 305, Hole U1309D was deepened to a final depth of 1415 mbsf. It mainly comprises gabbroic rocks ranging from troctolite, olivine gabbro, gabbro and gabbronorite to oxide gabbro. In addition, several ultramafic intervals were recovered in sections ranging from 1 to 20 m thick at various depths between 60 mbsf and 1240 mbsf.

Logging Operations

    Figure 2. Detail of the logging operations in U1309B and U1309D. Red lines indicate Expedition 304 runs, blue and green lines Expedition 305 runs.

    Wireline logging operations were carried out in two boreholes (Figure 2). Depths are shown in meters below seafloor (mbsf). Although Hole U1309B is of shallow depth it was logged (21.6 - 95 mbsf) to image the structural variation. Hole U1309D was logged in three separate stages (covering in total 54 - 1415 mbsf).

    Hole U1309B (Expedition 304)

    (1) The triple combo (HNGS [Hostile Environmental Gamma Ray Sonde], APS [Accelerator Porosity Sonde], HLDT [Hostile Environmental Lithodensity Sonde], DLL [Dual Laterolog], TAP [Temperaturea/Acceleration/Pressure tool]) tool string was lowered down to 94.9 mbsf without any problems. Two complete passes were recorded from open hole up to the seafloor. (2) The FMS/Sonic (SGT [Scintillation Gamma Ray tool], DSI [Dipole Sonic Imager], FMS [Formation MicroScanner]) tool string was lowered to 95.1 mbsf for two passes. (3) A third run was devoted to the heave compensator tuning with a short tool string (GPIT [General Purpose Inclinometry Tool], and DLL insulating tube).

    Hole U1309D

    First stage, Interval 54-400 mbsf (Expedition 304)

    (1) The triple combo (HNGS, APS, HLDT, DLL) tool string was lowered to 400 mbsf. Tight spots were encountered at 74, 79, 96 mbsf during the first run. A short repeat was made at the base of the hole. The second tool string was the FMS/Sonic. No problems were encountered for reaching the bottom of the hole and two logging passes were accomplished. After a period when the Schlumberger heave compensator was being tuned in the open hole, the tool string became stuck while entering the pipe and it took approximately 30 minutes to get it free. Any further attempts to log the hole were cancelled.

    Second stage 400-836 mbsf (Expedition 305)
    A total of five tool strings were successfully deployed to the bottom of the hole at 836 mbsf The pipe was set at 170 mbsf to avoid an interval with bad borehole conditions.

    (1) Triple combo (HNGS, APS, HLDT, DLL, TAP). Two passes were made and excellent data recorded, covering the interval between 836.5-170 mbsf. However, the TAP failed and no data were recorded. (2) FMS/Sonic (SGT, DSI, FMS). Two passes were recorded with the first pass covering the interval from 836.4 to 350 mbsf and the second pass logging the entire open hole up to the pipe. (3) SGT/UBI (Ultrasonic Borehole Imager). A short first pass was completed from 824 to 724 mbsf to acquire high-resolution images at a speed of 400ft/h. The tool string was lowered again to make the full main pass at normal speed (~800 ft/h) but no reasonable results were acquired because the software could not find a consistent signal to define the travel time window. Consequently, only a depth interval of particular interest and good borehole conditions (between 700 and 500 mbsf) was logged at the slow speed. (4) WST-3 (Well Seismic Tool, three components) Following the IODP marine mammal protocol, the WST-3 was lowered. Nine stations obtained viable interval velocities, seven of which were in line with sonic velocities. The other two stations were adjacent to each other and one gave a high Interval velocity (>7.5 km/s) and the other a somewhat low value (5.0 km/s), relative to the corresponding sonic measurement. (5) Third-party magnetometer (GBM, Goettingen Borehole Magnetometer). The tool was initialized, taken to the rig floor, connected to the wireline and oriented along the ship-axes. Down- and up-going passes were recorded in real-time without problems.

    Third stage 836-1414.5 mbsf (Expedition 305)
    (1) Triple combo (HNGS, APS, HLDT, DLL, TAP). The first pass covered the interval from the bottom of the hole at 1415 mbsf to the pipe (194 mbsf). For data quality check a short repeat pass was run in an interval of low core recovery (1270-1096 mbsf). (2) FMS/Sonic (SGT, DSI, FMS). The FMS/sonic tool was lowered to the bottom of the hole, but telemetry problems with the lower part of the tool were encountered. A broken isolation joint between transmitter and receiver section in the DSI was identified when the tool string was pulled back to the rig-floor, and the DSI was subsequently removed. The remaining SGT, GPIT, and FMS tools were lowered back into the hole. A successful first pass was recorded from TD to 734 mbsf and a second pass was run from TD to 629 mbsf. (3) The WST-3 failed after reaching the bottom of the hole and it was replaced by the WST-1. During the WST-1 descent, weather conditions deteriorated and logging operations were terminated. The third-party GBM magnetometer tool was not deployed because the borehole temperatures (>80°C) were above the safe operating range of the instument electronics.


    Figure 3. Results of selected logging measurements from Hole U1309B and their correlation with discrete core measurements.

    Figures 4a, b and c. Results of selected logging measurements from Hole U1309D.

    Figures 5. Example of the excellent borehole wall coverage by the FMS passes.

    Figures 6. Detailed FMS and UBI image displaying A: the transition from a patchy looking coarse-grained olivine gabbro to an olivine gabbro, and B: a steep fracture indicated by low resistivity (dark)

    Figures 7. Detailed Formation MicroScanner (FMS) image displaying an oxide-rich layer (192-195 mbsf).
    Figures 8. Dips measured in Holes U1309B and D between 50 and 400 mbsf, and 400 to 830 mbsf.
    Figures 9. Comparison of GBM and GPIT vertical components.
    Figures 10. Temperature profile recorded by the TAP tool while final stage logging (Legend, see figure 4a).

    Overall, the logging data expand upon core-basedobservations and provide in situ measurements at Site 1301. The triple combo was the only tool string that provided reliable data of the entire borehole revealing ideal places for Packer experiments and allowing for the interpretation of several logging units that correlated to physical and lithological changes identified from core-based observations. The downhole coverage obtained with the other tool string deployments consisted of only 1/4 of the borehole’s total depth because of a borehole obstruction. The VSP experiment obtained the best results of any subsequent tool deployment allowing for the estimation of the shallow basement velocity profile.

    The logging data reflect the overall variability of the drilled lithologies comprising diverse kinds of gabbroic rocks, diabase, and dunitic troctolites. Figures 3 and 4 present the results of selected logging measurements from Holes U1309B and D. In the gabbroic rock intervals, log bulk density varies between 2.8 and 3.2 g/cm3, resistivity ranges from 50 to 2000 Ohm.m. In general, the PEF is below 4 barns/e- and it averages around 3.1 barns/e-. The compressional velocity log ranges between 5.5 and 6.5 km/s. Most intervals of oxide gabbro, as identified in the visual core descriptions, can be recognized in the logging data. They are generally characterized by elevated values of density (3.0-3.2 g/cm3), PEF (4-8 barns/e-), Sigma (>30 cu) and low electrical resistivity (<100 Ohm.m).

    Logging data also reflect structural changes and alteration modes. Structural features like discrete, open faults and fracture zones are portrayed by enlarged borehole diameter (> 11 in), which causes sudden apparent drops in density (1.5-2 g/cm3), resistivity (10-50 Ohm.m), and velocity (4-5 km/s) and an increase in neutron porosity. FMS images show structural variations as well as textural variations of gabbroic rocks. In most intervals the coverage of the borehole wall by the FMS is excellent and is in limited intervals complemented by the UBI images (Figures 2, 5). FMS sections with patchy appearance correspond to intervals of coarse-grained gabbro (resistive patches) (Figure 6) or oxide-rich gabbro (conductive patches) (Figure 7).

    There is not only a good correlation of logging data with cataclasis and vein occurrence but also with alteration intensity. Alteration most strongly affects the neutron porosity. Most olivine-rich rocks, such as troctolite, dunitic troctolite or olivine gabbro show high levels of serpentinization and they contain more structurally bound H2O than olivine-poor gabbros. Based on this relation, intervals with neutron porosities of less than 5% as the least altered gabbro. In concert with low neutron porosity are high resistivities (>500 Ohm.m). The dunitic troctolites at 689-691, 1092-1170 and 1185-1195 mbsf are highly altered and chemical analyses on core samples indicate H2O contents of around 8%. For these intervals, neutron porosity is on average 20% and electrical resistivity decreases to below 100 Ohm.m. In Hole U1309B, within the interval 57.6 to 61.5 mbsf, high porosity values correspond to interval where serpentinized peridotite was recovered. High neutron porosity in this particular interval could be explained by the high content of bound water in the serpentine minerals (10% H2O).

    The continuous structural information gained from the FMS images with respect to dip and azimuth of conductive fractures is a crucial contribution to the understanding of the tectonic evolution of the Atlantis Massif. Structural analyses of FMS images indicates a change in direction of the dominant azimuth for conductive features from the upper 400 m to the lower depth interval between 400-800 mbsf (Figure 8). The dominant azimuth changes from preferentially southeast dipping structures to a combination of north dipping shallow structures and south dipping steep structures.

    Magnetic field intensity and direction were recorded by the GBM and GPIT (Figure 9). The vertical field component z shows a high level of repeatability for the downhole and uphole logs. In addition to the GBM fluxgate sensors, the angular rate of the GBM tool around the x, y, and z spin axes was measured using three fiber optic gyros. Rotation data will be used for reorientation of the magnetic data, the processing is still in progress.

    During the final logging run the temperature of Hole U1309D was recorded using the TAP tool. Log curves show a slight change in the temperature gradient below the 375 mbsf, at 720 mbsf, and 1100 mbsf (Figure 10). These changes are recorded in each pass. The depth intervals coincide with changes in lithology (occurrence of dunitic troctolites) or structural features (fault zone). The maximum recorded borehole temperature is 118.9°C at 1415 mbsf; it is a minimum temperature as the borehole fluid was not in full equilibrium so shortly after the drilling operation had finished.

    Florence Einaudi: Logging Staff Scientist, Expedition 304:, LGHF, Université de Montpellier II, France

    Heike Delius : Logging Staff Scientist, Expedition 305:, Department of Geology, University of Leicester, United Kingdom

    Margarete Linek: Logging Trainee, Angewandte Geophysik Rheinisch-Westfälischen Technischen Hochschule, Aachen, Germany