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doi:10.2204/iodp.proc.343343T.102.2013

Observatory and downhole measurements

Downhole temperature measurements at Site C0019 were made using independent miniature temperature loggers (MTLs). An observatory of 55 MTLs was installed to study the frictional heat of the fault zone, with the data to be recovered later. In addition, during drilling operations observations of temperature and pressure in the water column and borehole were performed with the temporary deployment of a free-fall MTL string.

MTL observatory

One of the main goals of Expedition 343 is to monitor the fault zone temperature following the 2011 Tohoku-oki earthquake in order to determine the level of frictional stress during the main shock rupture. Monitoring of subsurface temperature is being performed with observatory instrumentation installed in Hole C0019D. Each MTL independently records temperature data (some units also record depth/pressure). The instrument string will be retrieved by the Kaiko 7000II remotely operated vehicle (ROV) in February 2013 to obtain the data. The MTL observatory was installed in July 2012 during Expedition 343T.

Sensors

The MTL observatory consists of 55 autonomous MTLs attached to a rope on a recoverable hanger inside tubing. The tubing has one Exway ball valve at the bottom that allows water to be jetted out during installation but restricts influx of overpressured formation fluids. Three different types of MTLs are used: 10 TDR-2050s and 15 TR-1050s manufactured by RBR Ltd. (Canada; www.rbr-global.com/) and 30 Antares 1357 high-pressure data loggers manufactured by Antares Datensysteme GmbH (Germany; www.antares-geo.de/). The different instrument types are collectively referred to as “the RBRs” and “the Antares.” All instruments have temperature sensors and an autonomous data logger and are enclosed within titanium casings pressure rated for up to 10,000 m water depth. The TDR-2050s also have a pressure sensor that effectively records the sensor’s water depth inside the cased borehole. The RBRs are 230 mm × 38 mm, and the Antares are 190 mm × 20 mm in dimension. The RBR temperature sensors have a resolution of <0.00005°C and accuracy of <0.001°C. The Antares temperature sensors have a resolution of 0.001°C and accuracy of <0.1°C.

Rope

Roughly 1000 m of rope was prepared on which the MTLs are attached. The rope consists of 10 mm dynama-spectra containing spliced eyelets at predetermined locations for attaching the instruments. The rope is slightly negatively buoyant to prevent tangling with ROV cables if the rope breaks during recovery. The overall length consists of nine separate rope sections (R0–R8) whose length and eyelet positions are listed in Table T9. Each section has loops at the end allowing simple connections to each other. The MTLs are connected to the spliced eyelets with 2 or 3 mm spectra rope cord that is looped through holes in the instrument and knotted with a double fisherman’s knot. Rope R8 does not contain prespliced eyelets. On this rope section, eyelets were made using 3 mm cord and a series of prusik knots that were wrapped in electrical tape to prevent movement along the rope.

Coverings

The MTLs are surrounded with rubber coverings to help secure them to the rope, protect them from the effects of banging against the tubing wall, and create a smooth edge between the rope and MTL to help avoid getting caught on tubing joints during installation and recovery. The method of covering and attachment of MTLs is adapted from a similar MTL string deployment during IODP Expedition 327. Figure F21 shows pictures of the sensors and their attachment on the rope within their protective coverings. The coverings for the Antares are made of general-purpose air and water service rubber hose with an ID of 1 inch. The coverings for the RBRs have similar form and are cut from sheets of ¼ inch thick black rubber and wrapped around each instrument and the rope. The covering material is cut with tapered ends to lengths a few inches longer than the MTL. The extra space above and below the instrument helps ensure it is protected. A slit is cut down the center of the hose to easily get the MTL in. For the RBR coverings, this slit location marks where the sheeting material is joined when wrapped around the rope and instrument. The slits are on the far side of the instrument away from the rope. Holes were either drilled into the sides of the hose material or poked through the tape material to allow fluid movement around the instruments as a means to help prevent corrosion.

Each MTL is connected to the splice with pieces of cord using a double fisherman’s knot. The hose is connected to the rope by a zip tie through two ¼ inch holes drilled through each tapered end of the covering. The zip tie junction is made within the inner part of the covering. Both the MTL and rope are enclosed within the covering. The RBR coverings are kept closed by wraps of duct tape. All coverings are further secured by several wraps of 3M Super 88 electrical tape around both tapered ends and near the center of the covering for the Antares. The wraps of tape on at least the upper ends are also covered in the opposite wrap direction with friction tape (either 3M 1755 or similar “hockey tape”). The maximum diameter of the covered sensors on the string is 55 and 34.5 mm for the RBRs and Antares, respectively.

Hanger

MTL string Rope R7 was payed out from a spool during installation and cut to length to set the configuration of sensors around the target fault zone depth. A loop splice was made of the cut end of rope and attached to a hanger that sits within the wellhead. The hanger has a ring at the end to allow the ROV Kaiko to grab and retrieve the entire sensor string several months later. The top of the observatory, including the hanger which sits within the casing hanger wellhead, is illustrated in Figure F22.

Sinker bar

A 0.41 m sinker bar weighing 10 kg (8.73 kg in water) is attached at the bottom of the MTL string to ensure the rope hangs straight and does not bunch up during installation. The configuration at the lower end of the observatory is illustrated in Figure F23 and shown Figure F21E.

Weak links/Rope connections

An issue concerning the eventual retrieval of the MTL string is the potential risk of afterslip along the fault that may constrict the tubing walls and restrict passage of the MTL string out of the borehole. To maximize MTL recovery, three weak links are used to connect some segments of rope. The weak links were obtained from TLR, Inc. (Carmel, California, USA; www.tlrinc.biz/) and were specifically designed for this expedition in order to have a small diameter and relatively smooth profile to avoid getting caught in the tubing, as well as having breaking strengths below 300 kgf of tension, which is the maximum safety limit for pulling by the ROV Kaiko during recovery. The breaking strength is controlled by the number of polyvinyl strands sewn through the connecting members of the two sides of the weak link. The weakest weak link has three strands, and the strongest has six. The positions of the weak links and their tested strengths are listed in Table T9.

Where weak links are not used, most rope sections are connected by passing the rope through the end loop of the adjacent section creating a square knot with the two section loops. Ropes R5 and R6 are connected with a locking carabiner that is then wrapped in tape to ensure a smooth profile along the rope.

Sensor programming

The spacing between instruments is variable, with 1.5 m spacing across the target fault zone area and increased spacing further away. The positions relative to both the target fault zone and seafloor, and accounting for rope stretch, were determined during installation. The calculated positions are listed in Table T9. With a few exceptions, all instruments are programmed to record for at least 6 months until the planned retrieval by the ROV Kaiko in February 2013. The instruments are programmed to begin logging at 00:00:00 h Japan Standard Time (JST) on 13 July 2012, with the exception of the RBR sensors on Ropes R4, R5, and R8, which are programmed to begin logging at 00:00:00 h JST on 15 April 2012. All sensors have had their clocks reset to “PC time,” which was updated against an Internet timeserver and set to JST. The Antares are programmed to record every 10 min, the TDR-1050s are set to record every 10 s, and the TDR-2050s are set to record every 20 s. The exception to this programming schedule is two TDR-2050s scheduled to record at 1 s intervals for ~16 days; Sensor 32 will start at 00:00:00 h on 15 October 2012 and Sensor 47 will start at 00:00:00 h on 8 February 2013. These times cover the expected duration of the two possible recovery expeditions and will allow profile logs to be taken during extraction of the MTL string. The duration of recording is limited by the memory capacity of the instruments.

Observatory construction

The observatory is constructed of 4½ inch ID steel tubing with a 20 inch casing hanger wellhead on top in which the MTL sensor string was inserted and lowered. The hanger holding the sensor string rests within the casing hanger wellhead. The sensor string was lowered into the entire constructed length of the tubing while it extended beneath the casing hanger wellhead on the rig floor. All ropes with the exceptions of Ropes R6 and R7 were raised and lowered with tuggers that grab sections of the rope with the help of loops of rope temporarily attached to the sensor string using prusik knots. After the lower ropes with dense sensor spacing were lowered, Ropes R6 and R7 were lowered from a spool, and the uppermost sensors were attached on the rig floor as the rope was lowered. When the total estimated length of the sensor string was approached, lowering was completed by tuggers again and the weight of the sensor string was monitored with a load cell. A reduction in weight comparable to the weight of the sinker bar indicated that the sensor string reached the bottom of the tubing. The position of the rope at the top of the wellhead was then marked and cut to make a looped eyelet at a position 4.54 m shorter that will hang from the hanger. This length of shortening allowed 1.18 m for the length of the hanger that lies below the top of the wellhead and an allowance of 3.36 m of free space below the sinker bar and the top of the float collar marking the bottom inside of the tubing. The overall marked length of rope was then compared with the true length of the tubing and casing hanger wellhead to calibrate estimates of rope stretch along the sensor string and to calculate the positions of the sensors relative to each other and the seafloor (Table T9).

Installation

With the sensor string and hanger in the tubing, the casing hanger wellhead was joined to a casing running tool at the bottom of the drill string. Circulation tests were conducted before lowering of the observatory to the floor, and the weight of the entire sensor string was measured before and after to confirm that the weak links survive circulation forces. The casing running tool and drill string was then connected to the casing hanger wellhead again, and the observatory was lowered to the seafloor. Near the seafloor, an underwater television camera system with two television cameras and a sonar device connected to the Chikyu by fiber-optic cable was sent down the drill string to the bottom of the observatory to find the seafloor wellhead and guide reentry. Upon reentry of the wellhead by the tubing, the observatory was lowered until the casing hanger wellhead rested securely atop the seafloor wellhead. A dart and sinker bar attached to the core line winch was then lowered down the drill string to plug the casing hanger running tool, allowing a buildup of pressure within the running tool and causing the running tool to release and the observatory to remain in place.

Free-fall MTL string

Background

Temperature and pressure (depth) measurements in the water and subsurface were conducted using instrumentation deployed within the drill pipe while in Hole C0019E. These measurements allow immediate monitoring of the temperature disturbance caused by drilling and may allow estimation of formation temperatures at different depths. The “free-fall MTL string” consists of 1–3 autonomous MTLs covered in rubber protectors that are tied to one another with spectra cord and installed at the bottom of an empty core barrel with a deplugger nozzle at the end instead of a core catcher. The whole inner core barrel assembly containing the MTLs was dropped to the bottom of the drill string by free fall from the rig floor. The core barrel containing the free-fall MTL string was then recovered as the pipe was recovered or independently by latching onto the inner core barrel with a sinker bar and pulling the assembly up with the core line winch. Pulling it back to the surface allows for a continuous temperature-depth profile that may highlight anomalous intervals.

Sensors

The instruments used were TDR-2050s, as described in “MTL observatory.” The sampling interval was set to record once every 1 s.

Coverings

The MTLs are covered with rubber sleeves to aid in securing them to one another and protect them from the effects of shock. The sleeve coverings are made of general-purpose air and water service rubber hose with 38 mm (1½ inch) ID. The maximum diameter of the sensors with rubber sleeves is 51 mm (2 inches). The rubber hose is cut to lengths a few inches longer than the sensors and are tapered on each end (Fig. F24). Extra space within the covering above and below the MTL helps ensure the MTL is protected. Each hose covering is longitudinally split to allow easy insertion and recovery of the MTLs between deployments. The hose covering has six ½ inch holes drilled into the sides to allow fluid movement around the sensor. To ensure the coverings lie flat length-wise and do not curl and get stuck within the core liner at an arbitrary depth above the bottom, extra slits are cut on the backside of the hose perpendicular to the length and ending in the side holes. The tapered ends are rounded to prevent them from clogging the deplugger nozzle at the bottom of the core barrel or jamming within the core liner.

Connections

Each MTL is connected to the rubber covering with separate loops of 2 and 3 mm Spectra cordage looped through holes in the sensor and two ¼ inch holes drilled into the top of the top taper of rubber hose. The second loop is for backup in case a loop breaks. The cord loops are tied using a double fisherman’s knot on the inside of the rubber hose covering. The coverings and MTLs are further secured by several diagonal wraps of 3M Super 88 electrical tape around the covering (Fig. F24).

The sensors covered in rubber are connected directly above each other with loops of 3 mm cordage looped through two ¼ inch holes drilled into the tapered ends of coverings, which are oriented in opposing directions. The cord loops are tied within the joint created by the two sleeves with a double fisherman’s knot. The final arrangement looks like a snake (Fig. F24).

Deployment

The connected MTLs are placed within the bottom of a plastic core liner installed in an empty core barrel (Fig. F24B). The MTL string is not secured to anything. Instead of a core catcher at the bottom, the core barrel is closed at the bottom with a narrow tipped deplugger nozzle (Fig. F24C). The whole core barrel assemblage is deployed by free fall and recovered with the core line winch or removed from the bottom of the drill string on the rig floor.

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