The goal we are working towards on the JFAST2 expedition is to install an observatory of temperature sensors across the fault zone that slipped more than 50 meters during the March 2011 Tohoku Earthquake. The temperature sensors will allow us to measure the frictional heat and determine the strength of the fault. To accomplish a critical step of the installation, we must first find the wellhead we installed last May on the seafloor 7 kilometers (4.3 miles) below the ship, reenter it with the drill bit, and then drill down about 850 meters (2800 feet) through the plate boundary fault.
The only way we are able to reenter the wellhead on the seafloor, which will allow us to install the observatory into the hole after drilling across the fault, is by finding it with an underwater television camera system (UWTV), slowly moving the 8.5 inch drill bit above the 20 inch opening, and then lowering the pipe down into it. An added challenge is that the only way to move the drill bit towards and above the well head opening is slowly moving the boat a few meters at a time and then waiting for the 7 kilometer long string of drill pipe to swing beneath us.
Once we were positioned above the GPS location of the wellhead, we slowly lowered pipe down the nearly 7 kilometers below our drilling ship, Chikyu, one 40 meter (130 foot) stand at a time. We then attached the UWTV around the drill pipe and lowered it down (see photograph) to just above the drill bit. Last May, the stress of raising and lowering the fiber optic cable connecting the UWTV to the ship in such deep water created some damage to the cable, and prevented us from seeing anything or installing the observatory. Now, at the start of this expedition, the cable was repaired and we attempted using it at depth again for the first time.
It was a long, tense night in the Doghouse (the small enclosure on the rig floor shown in the photograph) trying to find and position ourselves above the wellhead and wondering whether the UWTV would work okay. Luckily things went well, we found the wellhead with the help of the cameras and a small sonar device on the UWTV, and successfully reentered the hole.
The next critical step is to recover the UWTV, and then drill about 850 meters through the fault zone and into the subducting plate below…
- Patrick Fulton
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