Previous   |    Next

doi:10.14379/iodp.sp.360.2015

Operations plan

Drilling and coring strategy

The JOIDES Resolution will move directly to proposed Site AtBk6, lower the subsea camera, and do a short survey to find a flat and solid-looking plutonic rock surface suitable for deployment of a reentry system. Prior site survey data indicates a flat bare-rock gabbro pavement at this precise location (proposed Site JR31-BR-8) (Figure F4). If the weather is suitable (<1 m heave), we will deploy a reentry system and complete the primary hole as described below. If the weather is initially unsuitable for deployment of a reentry system but acceptable for drilling, we will instead proceed to contingency proposed Site AtBk4 at the southern end of the platform, where prior seabed rock drilling during the JR31 site survey cruise cored serpentinite. There we would initiate a bare-rock spud in and drill a single-bit hole until weather improves or until bit destruction. In the event that the weather remains unsuitable for the deployment of the reentry system, we will proceed to the second contingency proposed Site AtBk2 immediately east of the Atlantis Bank wave-cut platform and drill a single-bit hole where the original, uneroded detachment surface was down-dropped by a 1000 m offset east-facing normal fault. This would recover the uppermost section of the core complex that is missing at proposed Site AtBk6 due to erosion. If unsuitable weather continues, we will proceed to the third contingency proposed Site AtBk5 and drill another single-bit hole until the weather improves.

At the primary proposed Site AtBk6, deployment of a reentry system will be initiated as soon as weather permits. Reentry system Plan A begins with drilling-in a reentry cone with a 15 m long conductor casing (13.375 inches) using a mud motor. Two such systems will be carried on board. The conductor casing will be cemented in place and the cement plug will be drilled out before coring begins. If Plan A fails, we will proceed with Plan B: drilling an 18.5 inch hole, lowering a 16 inch casing into the open hole (15 m), and then deploying a free-fall hammer funnel (hard rock reentry system [HRRS]-type) over the 16 inch casing. Following deployment of the reentry system, we will core with the rotary core barrel (RCB) to 220 m, log the hole, open the hole to a 12.5 inch diameter, and install 200 m of 10.75 inch casing. We will then drill as deep as time allows, reserving ~2 days for logging prior to departure.

From 220 m downhole, our intention is to core continuously. However, the primary objective of SloMo Project Phase I is to drill to 3 km, requiring that we drill as deep a hole as possible on Expedition 360 to enable a second expedition (SloMo-Leg 2) to achieve this objective. A conservative estimate of what can be drilled with continuous coring during Leg 1 is 1300 m, which is short of what is needed on Leg 1 to achieve the objectives of Leg 2. Accordingly, once penetration to a few hundred meters below the magnetic transition is achieved, and if time is an issue to achieve the primary objective, we will consider alternating four-cone coring bit runs with tricone drilling bit runs to achieve greater depth, alternating coring and drilling without coring between bit runs. A rough estimate is that drilling without coring would be nearly twice as fast due to the heavier weight that can be placed on a more robust tricone drill bit and no wireline time to retrieve core. If the stratigraphy of the lower kilometer is similar to Hole 735B, this would not greatly affect the overall science objectives, as the lower half of Hole 735B exhibited far less variability than the upper portion. If and when the hole reaches 1400 m, we would resume continuous coring as we entered depths deeper than Hole 735B.

Logging/downhole measurements strategy

Wireline logging will form an essential component of the operations at Atlantis Bank during Expedition 360. Downhole measurements of the in situ physical properties of the formation will complement and extend those measured on cores, assist in characterizing the nature of incompletely recovered intervals, and thereby help in refining the igneous, metamorphic, and structural stratigraphies of the borehole(s). Borehole wall imaging will be of great importance in establishing the true geometry of features such as lithological contacts, veins, and fractures; furthermore, by matching distinctive features in cores with their representations on borehole wall images, core pieces may subsequently be reoriented to geographical coordinates. This can in turn allow spatially anisotropic properties of cores, such as magnetic remanence directions, igneous petrofabrics, fault kinematics, and so on, to be considered in the geographic reference frame and allow upscaling and integration of data from the drill site with regional-scale geological and geophysical information.

At the primary proposed Site AtBk6, we intend to conduct wireline logging operations in two phases. The first phase will be after coring the hole to 220 mbsf and before deploying 200 m of 10.75 inch casing. This first set of logs from ~30 to ~220 mbsf is projected to take <1 day.

The second phase of downhole logging will take place at the end of Expedition 360 and will cover the interval from 200 mbsf to the bottom of the hole, projected to be at ~1300 mbsf. This should take ~2 days.

At the beginning of each logging phase, the hole will be filled with freshwater; based on experience from previous expeditions this enhances the quality of Formation MicroScanner (FMS) electrical borehole wall imagery (Blackman, Ildefonse, John, Ohara, Miller, MacLeod, and Expedition. 304/305 Scientists, 2006).

In both phases of downhole logging, we anticipate making three separate logging runs with the following tool configurations, in order: (1) triple combination (triple combo), (2) FMS-sonic, and (3) Ultrasonic Borehole Imager (UBI). In the second logging phase, the Versatile Seismic Imager (VSI) will be added as a fourth run.

The triple combo tool string consists of six separate probes:

• The Accelerator Porosity Sonde (APS) uses an electronic neutron source to measure the porosity of the formation.
• The Hostile Environment Litho-Density Sonde (HLDS) measures bulk density. The tool employs a single-arm caliper to push the sonde against the borehole wall and, by so doing, giving a measurement of borehole size.
• The Hostile Environment Natural Gamma Ray Sonde (HNGS) measures the natural radioactivity of the formation, including indications of the concentrations of Th, U, and K.
• The High Resolution Laterolog Array Tool (HRLA) measures the electrical resistivity of the formation at two different invasion depths.
• The Phasor Dual-Induction Spherically-Focused Resistivity Tool (DIT).
• The Magnetic Susceptibility Sonde (MSS-B) measures borehole magnetic susceptibility at two different vertical resolutions and depths of investigation.

The FMS-sonic tool string combines the FMS, Dipole Sonic Imager (DSI), and General Purpose Inclinometry Tool (GPIT) tools. It also carries an HNGS sonde to allow direct correlation with the data from other logging runs. The FMS consists of four orthogonal pads with 16 electrodes on each pad mounted on arms that are pressed against the borehole wall. Resistivity measurements from the electrodes are combined to generate high-resolution electrical images of strips of the borehole wall (a single pass images ~22% of the circumference of the hole). In practice FMS images are particularly useful for the identification of (the orientation of) planar features such as faults, fractures, veins, borehole breakouts, and lithologic variations in the borehole wall. The images can resolve millimeter-scale features if they have sufficient resistivity contrast. They are oriented to the geographical reference frame by means of the GPIT. If distinctive planar features in the core can be matched directly to their representations on the FMS imagery, the cores may be reoriented to geographical coordinates. The DSI records a full set of acoustic waveforms to measure the compressional and shear-wave velocity of the formation (V_P and V_S). V_P can be combined with the density log to generate synthetic seismograms and provide high-resolution seismic-borehole integration.

The UBI tool measures the amplitude and transit time of an acoustic wave propagated into the formation. It provides medium–high resolution acoustic images of the borehole wall, which are geographically oriented by means of a GPIT sonde. Although of lower resolution than the FMS images, UBI data have the advantage of providing 100% coverage of the borehole wall. UBI and FMS images are utilized in a similar manner.

The VSI is a well seismic geophone used in conjunction with an air gun source at the surface to help constrain the velocity structure of the vicinity of the borehole. Measurements are made with the tool held at different depths within the borehole, typically 50 m apart.

In addition to the above, it is possible that borehole fluid temperatures may be measured on one or more occasions by deploying the Sediment Temperature Tool (SET), holding the drill bit just above the bottom of the hole, and allowing temperatures to equilibrate. This may be considered near the end of the expedition to ascertain formation temperatures at the base of the hole. We may also consider deploying the core orientation tool ; although useless in hard rock/RCB coring operations for orienting the core itself, the information would give a direct measure of borehole deviation.

More information about the capabilities of the downhole logging tools is available at iodp.tamu.edu/tools/logging/index.html.

Top of page   |    Previous   |    Next