IODP

doi:10.2204/iodp.sp.343.2012

Observatory plan

The highest priority is to gather temperature measurements across the fault zone to determine the amount of frictional heat generated by the earthquake. Observations of a decaying temperature signal will be used to estimate the level of dynamic friction during the large rupture. To obtain good resolution of the thermal signal, calculations show that initial measurements need to be started within about 2 y following the earthquake with measurements continued over several months to years. Temperature and pressure data will be collected in both boreholes with observatory instrumentation as described below.

Hole A

The 4.5 inch diameter casing will extend 100 m below the fault, (~900 mbsf) to provide temperature measurements mainly above the fault. A string will be constructed by attaching ANTARES temperature loggers and RBR temperature and temperature/pressure loggers to a Vectran rope. The string of ~50 sensors will have a miniature temperature logger (MTL) hangar attachment on the top and will be fixed to the wellhead for later retrieval in September 2012 by the ROV Kaiko (Fig. F5). The spacing is closer in the depth range where the fault-related temperature anomaly is expected. The exact length of the temperature string will be decided on board after looking at the results of LWD/MWD.

LWD/MWD is expected to acquire important data in the first stages of drilling and provide the first information on the location of the fault zone (e.g., Expedition 314 [Kinoshita et al., 2008]). LWD-derived resistivity is an effective way to identify the depth and thickness of the fractured zone associated with the plate interface fault zone. Resistivity images with 360 coverage of borehole are also essential for identifying geometry of fractures, stress-induced borehole compressive failures (borehole breakouts), and DITFs. Changes in fracture and in situ stress may be expected to change at the slipped interface.

Of particular concern is the possibility of afterslip from the Tohoku event occurring along the fault at the drill site, which could deform or collapse the casing sufficiently to clamp the MTL string and prevent retrieval. Portions of the string are being connected with weak links, which are expected to break at the time of retrieval if the lower portion of the string becomes stuck.

Hole B

After completing the coring operation, 3.5 inch casing and a circulation obviation retrofit kit (CORK)-type (Becker and Davis, 2005) observatory completion assembly will be run in the second borehole, extending approximately 100 m below the fault (~900 mbsf) to permit temperature and pressure measurements from the fault zone. A temperature and pressure measurement string will be installed in the borehole. This system will comprise an internal string of 20–30 thermistors with data telemetered to a data logger at the wellhead (Fig. F6). Three pressure ports along the outside of the casing will be used to measure pore fluid pressure inside and outside of the fault zone. Pressure lines using a flatpack will be connected to a pressure data logger at the wellhead. The battery life of the data loggers is expected to be about 5 y. Observatory data recorded in this hole also will be retrieved later using the ROV Kaiko. The system will allow data retrieval without physical recovery of the thermistor string for as long as the string remains functional. A robust cable will be used so that if there is a break in any portion of the cable (possibly due to afterslip), data will still recorded for portions above the break.

The exact depth of the sensors will be decided on board before deployment. As is the case for the observatory in the first borehole, it is critical to accurately determine the exact location of the slipped fault in order to place the temperature measurement string in the proper location. Locating the fault in the second borehole will benefit from the geophysical data from the first borehole and from analysis of core samples in the second borehole. Analysis of core sample data, including micropaleontological data, will be carried out during the time associated with preparing and running casing in the second borehole after coring is completed.