After successfully reentering the wellhead on the edge of the Japan Trench on the seafloor 6926 meters (4.3 miles) below the ship, we began drilling. The goal: to drill ~850 meters (2800 feet) below the seafloor across the plate boundary and through the fault that slipped more than 50 meters (164 feet) at this location during the March 2011 Tohoku earthquake causing the enormous tsunami. We will then try installing a temperature observatory down into the hole to measure the remaining frictional heat across the fault.
Instead of using the standard top-drive drilling system on the ship to rotate the entire drill stem and create the torque on the drill bit 7-8 kilometers below, as in the previous drilling at the site, this time we used a mud-motor located just above the bit to create the torque at the bit. The mud-motor causes rotation of the bit from pumping drilling mud (in our case the mud is actually sea-water) through the motor and out of the jets at the bit.
Drilling with the mud motor has been incredibly effective and we quickly reached our target depth. We could see from the drilling parameters measured on board that we started to drill through a hard chert we had previously encountered, confirming that we had successfully crossed the plate boundary fault and well into the down-going Pacific Plate.
The great water depth here is much deeper than conventional wells, and the total depth drilled for the observatory joins our other holes as part of the JFAST project in being the deepest below the sea ever drilled for scientific ocean drilling. Our TD (total depth), as shown by the driller’s console in the picture below: 7780.81 meters below the ship’s rig floor!
In the first picture, I’m giving the “thumbs up” for the great depth we were able to obtain. The depth of this deep borehole will provide space for us to install the observatory that will include temperature sensors that straddle the fault zone.
Next, the hole will be cleaned out and all the drill pipe will be returned to the surface. We can then start assembling the observatory and lowering it down to the seafloor for the final, most difficult task of carefully installing it all the way down into the hole.
- Patrick Fulton
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