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Logging Summary

IODP Expedition 336:

Mid-Atlantic Ridge Microbiology

Expedition 336 Scientific Party


    Figure 1. Bathymetric map showing Expedition 336 drill sites.

    The North Pond region of the Atlantic Ocean (2245'N, 4605'W) is in water depths between 4414-4483 m (Figure 1). It is an area that has been visited on previous ocean drilling expeditions (e.g. DSDP Leg 45, ODP Legs 109 and 174B) and is known as a site of particularly vigorous circulation of seawater in permeable 8 Ma basaltic basement underlying a <300 m thick sedimentary pile. IODP Expedition 336 aimed to examine the microbiology of this sediment pond and the underlying young, cold, and hydrologically active flank of the Mid-Atlantic Ridge. Drilling operations at three sites included sediment/basalt coring, basement logging, and installation of two long-term subseafloor observatories (at U1382A and U1383C).

    The Expedition 336 drilling program was designed to investigate (1) the origin of deep-seated microbial communities, and (2) the nature of microbial communities in young ridge flanks and their role in crustal weathering. Downhole measurements were taken at three sites: 395, U1382 and U1383.

Logging Operations

    The logging program on Expedition 336 was designed to obtain continuous in situ physical properties data needed to assist in defining structural and lithological boundaries as a function of depth. Additionally, wireline logging data were compared to results of laboratory analyses of discrete samples to help delineate alteration patterns, fracture densities, and structural orientations and determine how these correlate with fluid flow. Downhole measurements compliment those made on core by determining the thickness and structure of lithologic units in intervals where core recovery is poor. These logs were also critical for both shipboard hydrologic (packer) tests as well as for the precise depth placement of the CORK experiments. Wireline tool strings were deployed in all basement holes and provided measurements including: temperature, natural gamma ray, density, porosity, resistivity, sonic velocity, and microresistivity. Descriptions of the wireline tools and their applications are available here.

    A series of three adapted triple combination tool strings were deployed (Microbiology Combo, Adapted Microbiology Combo I and Adapted Microbiology Combo II). These included sondes to measure natural gamma radiation, borehole diameter, density, resistivity, temperature and density. Additionally, on each of these tool strings the basal sonde was the Deep Exploration Biosphere Investigative tool (DEBI-t). This tool was specifically designed and built for Expedition 336 to assess the natural fluorescence of microbial communities exposed on the borehole wall. Two other tool strings incorporating the Formation MicroScanner (FMS) were deployed in order to acquire electrical images of the borehole wall and two-axis borehole diameter information (at Sites U1382 and U1383). The sonic sonde was also deployed as part of one of the FMS combinations (at Site U1383).

Logging Results

    Figure 2. Summary of Hole 395A logging results.
    Figure 3. Summary figure of borehole temperature, Hole 395A.

    Hole 395

    Following the successful removal of an old CORK from Hole 395A (installed during ODP Leg 174B), the hole was logged with a Microbiology Combo toolstring comprising the Hostile Natural Gamma Ray Sonde (HNGS), the General Purpose Inclinometry Tool (GPIT), the Modular Temperature Tool (MTT) (see Figure 2 and Figure 3) and the new in situ deep UV fluorescence tool for detecting microbial life in ocean floor boreholes - DEBI-t. A rock ledge in the borehole at around 180 m below seafloor (mbsf) had to be bridged by lowering the logging bit to ~198 mbsf, however following this an open hole section of 405.7 m was logged (total depth reached was 603.5 m). The logging results are consistent with the data obtained by Bartetzko et al. (2001) and allow the distribution of massive basalt, pillow basalts, altered lava flows, and rubble zones (sedimentary breccia and hyaloclastite) (Figure 2) to be distinguished.




    Hole U1382

      Figure 4. Summary of Hole U1382A logging results.
      Figure 5. Composite of features imaged by the Formation MicroScanner (FMS), Hole U1382A.

    Hole U1382A was successfully logged with two different tool strings (Adapted Microbiology Combination I [EDTC, HLDS, HRLA, DEBI-t], FMS-HNGS). An open hole section of 105.61 m was measured over a period of ~19.5 hours. Downhole log measurements include natural total and spectral gamma ray, temperature, density, electrical resistivity, electrical images and deep UV-induced fluorescence (DEBI-t) (Figure 4). The borehole remained in good condition throughout logging and no obvious tight spots were encountered in open hole. Preliminary interpretation of the downhole data divided Hole U1382A into eleven log units (using gamma ray, resistivity and density). Integration of core and log measurements and observations showed excellent correspondence between potassium concentrations provided by shipboard NGR, spectral gamma ray logging tool and whole rock geochemical analyses. FMS data (Figure 5) were combined with images of the external surfaces of whole round cores. Prominent veins with alteration halos in core of the massive flows can be matched up with fractures in the FMS images. Also, logging results constrain the depth of the peridotite interval from 165 to 167 wmsf (based on density and low K/U ratios).

    Figure 6. Summary of Hole U1383C logging results.
    Figure 7. Composite of features imaged by the Formation MicroScanner (FMS), Hole U1383C.

    Hole U1383

    Wireline logging data collected in Hole U1383C include natural total and spectral gamma ray, density, compressional velocity, electrical images and deep UV-induced fluorescence (DEBI-t) of an open hole section of 274.5 m (Figure 6). Three main Lithologic Units were identified. Lithologic Unit I is characterized by variable caliper, density and sonic velocity values. Gamma ray intensities are generally low, but increase in the bottom part of the unit. Lithologic Unit II has a uniform caliper, high densities and apparent sonic velocities and shows high-resistivity massive flows with fractures in the FMS images (for examples of FMS imagery see Figure 7). Lithologic Unit III has an upper section (153-166 wmsf) which is characterized by a drop in density, apparent resistivity and velocity and an increase in gamma ray intensity. This interval corresponds to thin flows with inter-pillow/flow sediments and tectonic breccias. From 166 wmsf to the bottom of the hole, the logging data reveal fairly uniform values for density and apparent velocity and resistivity. Areas with peaks in gamma ray intensity correspond to intervals with abundant hyaloclastite in the recovered core (in particular around 175 wmsf, and from 220 to 250 wmsf).


    Bartetzko, A., Pezard, P., Goldberg, D., Sun, Y.-F., Becker, K., 2001. Volcanic stratigraphy of DSDP/ODP Hole 395A: An interpretation using well-logging data. Mar. Geophys. Res. 22, 111-127.

    Louise Anderson: Logging Staff Scientist, Borehole Research Group University of Leicester, University Road, Leicester, LE1 7RH.