Return to the Japan Trench

　The Japan Trench Fast Earthquake Drilling Project(JFAST) expedition which took place from April to May 2012 left one issue unresolved. One of the main aims, the installation of temperature sensors for long-term borehole observation, was not achieved because of a fault in the underwater camera cabling.

　It was exactly this installation of temperature sensors which formed the reason that this expedition went ahead with such unprecedented speed. The frictional heat in the fault generated at the time of the earthquake of March 11, 2011 is lost with the passing of time. Observations needed to start at an as early stage as possible to measure these temperatures.

　From the moment the underwater camera cable problem occurred, people involved started to develop plans for another attempt. Repeat attempts for one expedition normally hardly happen in scientific drilling. This time however there was the urgent purpose of analysing the massive earthquake. Through the efforts of all participants in the CHIKYU expedition, the operation planning was revised to enable a renewed attempt, JFAST2, and the decision was taken to make it happen.

　And thus, on July 5th, the CHIKYU, after maintenance work in the dock, set sail once again for the Japan Trench.

Return to the JFAST site

　The JFAST expedition had in fact succeeded in establishing a well head in the seafloor, forming the drill site. Temperature sensors are installed in a hole dug out by a drill. A well head is an equipment at the entrance of the borehole when inserting the temperature sensors.

　The repaired underwater camera is also fitted with a sonar device. On arriving at the drill site, the CHIKYU crew lowered a drill pipe with an attached drill bit, and started searching with a sonar device alongside the underwater camera. They were looking for the well head that was installed in the JFAST expedition.

　The CHIKYU is naturally fitted with GPS and can drop anchor in the same location as the JFAST drill site using the latitude and longitude information displayed on board. However, the rendez-vous is on the seafloor at 7,000m depth. Depending on the direction and strength of the ocean currents, the tip of the pipe 7,000m below the ship doesn’t necessarily arrive in the same spot as the previous time.

　Searching using a sonar device isn’t a cure-all either. If it gets a response that looks like it has found the well head the CHIKYU is moved in that direction. But this response is sometimes noise, and not the target well head. The underwater camera’s range of vision is about 10m. Relying on sonar and a small range of vision in the darkness of the deep sea, they are looking for a hole of 76cm diameter. As previously explained, you can compare this to dangling a thread from the roof of a 20 storey building in the middle of the night, and look for a hole the size of a five-cent coin on the ground using a sensor and a camera at the end of the thread. And although the position of where to stand on the roof is fixed, a breeze is blowing so it isn’t just a matter of lowering the thread straight down. The person holding the thread relies therefore on the sensor and camera, and waits for the five-cent coin to be captured by the camera while moving in different directions.

　Nevertheless, the well head was captured on camera on July 10th, and drilling through a water depth of 6,897.5m and up to 854.81m below the seafloor was achieved on the 12th. The length of the subseafloor drill pipe represents a world record.