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doi:10.2204/iodp.proc.319.102.2010

Observatory

Sensor dummy run test

After casing operations at Site C0010 (proposed Site NT2-01J), we conducted a simulation of the planned future borehole sensor installation as part of the preparation for installation and fabrication of long-term borehole observatories in future NanTroSEIZE expeditions. This test had two main objectives: (1) to evaluate environmental conditions, such as shock, acceleration, and vibration during installation and (2) to confirm sensor installation operational procedures, such as shipboard assembly of the sensor tree, ship maneuvers to reenter the hole, and hole reentry. In addition, nine miniature temperature loggers (MTLs) were attached to the dummy instrument package to provide temperature data.

The sensor tree is illustrated in Figure F24. The tree was attached to the end of the drill pipe before running into the hole. The tree consists, from the bottom up, of one 6 ft pup joint, two 3½ inch tubing joints, a borehole strainmeter (Fig. F25), a multiinstrument carrier housing a self-recording accelerometer-tiltmeter (Fig. F26), two broadband borehole seismometers (Guralp CMG1T and CMG3T), and four joints of 3½ inch tubing to connect the tree to the drill pipe (Fig. F24). Specifications of the sensors and instrument carrier are summarized in Table T20. The orientation of the sensors is shown in Figure F27. The tree was similar to the structure of the bottom section of planned future observatories (Fig. F28), with the exception that no cables or hydraulic lines were attached, a shorter total length of tubing was used, and there was no packer seal above the tree for hydraulic isolation.

A self-recording accelerometer-tiltmeter equipped with internal lithium batteries was included with the sensors to record vibration and shock data throughout the experiment. The accelerometer measures acceleration in three orthogonal directions (x, y, and z). The orientation of the x- and y-axes are shown in Figure F27, and the z-axis is vertical. The tiltmeter senses two components (x and y) of tilt; their orientations are the same as for the accelerometer (Fig. F27).

The strainmeter is composed of nine sensors. It runs on internal batteries and records strain changes of the instrument during the experiment, with a positive value indicating compressional strain. The directions for each sensor are shown in Figure F27. The strainmeter has a very weak deformable surface to make contact with the formation and record strain changes without interfering with the surrounding strain field. In order to withstand high pressures in the borehole during installation, the strainmeter is designed to balance its internal pressure with the hydraulic pressure outside of the instrument. In pressure cycling tests, deformation of the strainmeter sensing surface was confirmed to be well within the full scale of the instrument (~2 mm) even under hydrostatic pressure changes up to 60 MPa.

The two borehole seismometers were not set to record any data during testing but were included to test the ability of the internal sensor mechanics to withstand vibration. Performance of the broadband seismometers is highly dependent on the integrity of fragile pivots that suspend the moving mass. Before the experiment, performance of each broadband seismometer was verified by 2 weeks of continuous observation in the Matsushiro vault of the Japan Meteorological Agency. The plan for the dummy run originally included onshore inspections of the strainmeter and seismometer packages after the expedition to evaluate any performance changes.

One MTL was set inside a pup joint at the bottom part of the sensor tree, and eight MTLs were attached to the instrument carrier in the first dummy run (Fig. F24; Table T21). Two sets of configurations for sampling interval and observation period were adopted (1) to observe temperature over the entire period of the dummy run experiment and (2) to observe temperature at as high a sampling rate as possible (Table T21).

Temporary monitoring system

As part of operations at Site C0010, the mechanically set retrievable packer (Baker-Hughes A3 Lok-Set) to be installed inside the 9⅝ inch casing string was modified to attach a small instrument package to monitor pore pressure and temperature within the shallow megasplay fault zone (Fig. F29). The instrument package includes one "upward looking" and one "downward looking" pressure sensor to monitor pressure both below the packer seal in a screened interval that is open to the fault zone and above the packer seal to serve as a hydrostatic reference open to the overlying water column. The temperature sensor is located within the instrument package itself and thus records temperature at a depth just below the packer (Fig. F30). The retrievable packer was set inside casing above two screened casing joints that span the splay fault (Fig. F29). The instrument package is threaded to the bottom of the bridge plug and includes a self-contained temperature sensor and data logger, as well as a pressure gauge and data logger package. These instruments will monitor formation pore pressure and temperature from the time the bridge plug is set until they are retrieved at the beginning of permanent riserless observatory installation operations.

General description

The smart plug instruments built for Expedition 319 are designed for deployment immediately beneath a casing packer seal. Structurally, each unit includes a hollow-bore 3.5 inch EU 8RD box end–threaded coupling at the upper end, which mates with the lower end of the Baker-Hughes packer supplied by CDEX, and an outer O-ring–sealed structural shell that is designed to withstand the loads encountered during hole reentry operations (Figs. F31, F32). Housed inside are a high-precision (~10 ppb of full-scale pressure or ~0.7 Pa) A/D converter and data logger (designed and built by Bennest Enterprises, Ltd., Minerva Technologies, Ltd., and the Pacific Geoscience Centre, Geological Survey of Canada), two pressure sensors (Paroscientific, Inc., USA), and an MTL (Antares, Germany). Four independent temperature readings are made by (1) the MTL, (2) a platinum thermometer mounted on the primary data logger end cap, and (3) each of the two pressure sensors as an internal compensation for the pressure measurements. The inside of the structural shell is exposed to the cased borehole above through the internal open bore of the casing packer seal. One of the pressure sensors is in communication with this volume to provide a hydrostatic reference, and the second sensor is in communication with the sealed screened borehole interval below via hydraulic tubing that passes through the bottom end of the smart plug (Fig. F32). RS-422 communications with the pressure logging package for setting recording parameters and downloading data are conducted via a multisegment Seacon AWQ connector on the logger pressure case. Communications with the MTL are conducted with a special Antares interface. The instrument frame is shock-mounted within the structural shell, and the pressure sensors are mounted with secondary shock pads within the frame (Fig. F32). Structural components are constructed with 4140 alloy steel, and pressure sensor housings and hydraulic tubing are constructed from 316 stainless steel.

Settings

The instruments were set to begin recording data at the time they were shipped from the Pacific Geoscience Centre to the Shingu, Japan, port on 11 April 2009 and were stored at Shingu until they were transported to the Chikyu via supply boat on 1 June 2009. Logging intervals for the formation and hydrostatic pressure sensors and the internal platinum thermometer were set to 1 min; at this rate and with other operational parameters as set, battery power (provided by six Tadiran TL-5137 DD primary lithium cells) is the limiting factor for operational lifetime, which is roughly 7 y including a derating factor of 75% applied to full power withdrawal. The instruments are equipped with 512 Mb (low power) flash memory cards, which provide ~40 y of storage at a 1 min sampling rate. The independent MTLs in Instruments 8A and 82 are set to sample temperature at 30 and 60 min intervals, respectively. The main logger clocks were synchronized to Universal Time Coordinated (UTC) on 11 April 2009, and the MTL clocks were set at approximately the same date.

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