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doi:10.2204/iodp.proc.343343T.103.2013

Observatory and downhole measurements

MTL observatory

Installation and data recovery plans

The MTL observatory was installed beneath the seafloor in Hole C0019D on 16 July 2012 during Expedition 343T. Sensors were installed into the tubing on the rig floor on 14 July and the lowering of the observatory to the seafloor began that evening. The bottom of the observatory tubing entered the wellhead at ~1045 h on 16 July and was lowered into place until the casing hanger met with the seafloor wellhead at ~1645 h. Detachment of the casing running tool was completed at ~1815 h on 16 July. The final overall observatory length from the top of the casing hanger to the bottom shoe below the float collar is 829.20 m with the top of the casing hanger rising 4.42 m above the seafloor. The final calculated depths of the sensors are reported in Table T9 in the “Methods” chapter (Expedition 343/343T Scientists, 2013).

Dives by the remotely operated vehicle (ROV) Kaiko 7000II to Hole C0019D are scheduled for October 2012 and February 2013 to retrieve the temperature data. In October, the wellhead will be precisely located and checked so that retrieval of the MTL string can be done quickly in February. Two dives by ROV Kaiko are scheduled for each of the October and February visits.

Free-fall MTL string

The free-fall MTL string consisting of 1–3 MTLs inside of an internal core barrel was deployed for three separate runs in Hole C0019E. Run 1 was entirely within the water column, whereas Runs 2 and 3 recorded in the water column and subseafloor. The respective measurements from the sensors were consistent and nearly identical to each other throughout the course of the deployments. For simplicity, the figures within this section show only the results from the lowermost sensor. The absolute recorded depths converted internally from pressure measurements are slightly larger than reported from the drill floor and likely need correction. The subseafloor temperatures recorded were highly disturbed by the drilling process and changed steadily when continuously monitored at a given depth. Estimates of undisturbed formation temperature from the data will require further analysis and modeling. Pressure/depth data are high enough resolution to clearly record oscillations consistent in period with heave motion recorded on board the Chikyu. The magnitude of these vertical oscillations appears to increase with accelerations of the core line winch upon extraction or following a drill stand being added or removed.

Run 1: 12 May 2012

During WOW on 12 May 2012, the free-fall MTL string was deployed with one sensor (serial number 023949) at ~1630 h Japan Standard Time (JST). After reaching the bottom of the drill string, it was retrieved by the core line winch and exited the water at ~1915 h. The bottom of the drill string was in the water column for the duration of the run and never below seafloor. Figure F64 shows the recorded temperatures and depth as a function of time, whereas Figure F65 shows the temperature data as a function of depth. Differences in temperature at a given depth from when the instrument was going up and going down likely reflect the effects of the instrument response time to changes in temperature and the fast speed at which the instrument changed depth during free fall and retrieval.

Run 2: 16 May 2012

During WOW on 16 May 2012, the free-fall MTL string was deployed with three MTLs (from bottom to top, serial numbers 023950, 023949, and 023947) at ~0700 h. It was recovered by core line winch and exited the water at ~1530 h for a total deployment duration of ~8.5 h. Figure F66 shows the recorded temperatures and depth as a function of time, whereas Figure F67 shows the temperature data as a function of depth. Figure F67B is zoomed in to show results while below seafloor and is annotated to describe how temperature changed in conjunction with other operations.

Run 3: 22–24 May 2012

After reaching TD in Hole C0019E and following the circulation of kill mud on 22 May 2012, the free-fall MTL string was deployed at ~1830 h with three MTLs in the same arrangement as Run 2. The free-fall MTL string was left within the drill string; it was not recovered by core line winch but was rather recovered by shortening the drill string stand by stand to the rig floor. The entire deployment lasted ~33 h, ending at ~0400 h on 24 May. Figure F68 shows the recorded temperatures and depth as a function of time. Figure F69 shows the temperature data as a function of depth. Figure F69B is zoomed in to show results while below seafloor. There was no circulation during the deployment. Interpretation of the temperature profile for the period during extraction will require further analysis and comparison with measured rig floor parameters. A sharp decrease in temperature at ~7900 m water depth likely results from an unplugging of the deplugger nozzle or drill bit and a resultant surge of water from the drill string.

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