Successful installation of temperature sensors

　The depth of the fault was determined by comparing the drilling information at the time and the LWD data of the JFAST expedition. Based on this depth a total of 55 high-accuracy temperature sensors were placed.

　Initially the intention had been to drill two holes during the JFAST expedition, and install different observation equipment in each. One would have temperature sensors only, and the other one would have observation equipment ranging from temperature sensors to pressure gauges. They were designed so that the former would be retrieved as a whole after a specific period of observations, while the latter would be left in place with only the data being collected.

　At the time of the JFAST2 expedition however there was only one hole for observation equipment, and the decision on what to employ was eventually made in favor of the temperature sensors alone. This involves a rope of over 850m length with individually fitted tempera ture sensors. “We used a method which ensures data collection. There are 55 individual temperature sensors, so even when one or two of them fail, we can still obtain data from the other sensors,” tells Masanoru Kyo of the Technology Development Group. The observatory consisting of temperature sensors and pressure gauges which wasn’t used, has memory fitted to the part sticking out of the seafloor, and data from the various sensors are sent to and stored in that memory. This means that if there is a problem with the wiring or the memory those data can’t be collected anymore. For that reason the simple and fault-tolerant observation equipment with temperature sensors only was used, even though retrieving it takes time.

　The temperature sensors which have been positioned onboard ship are inserted into a pipe (casing pipe) together with a rope. This rope, extending to 850m, is attached to the tip of a pipe which stretches from the CHIKYU instead of the drill pipe. It then gets installed by insertimh it into the well head and cut loose from the pipe from the CHIKYU. The 55 temperature sensors in the pipe will be recording changes in temperature over the next few months, focussing on the area around the fault.