Clay and gas forming a hindrance

　The first experience to drill into the mud volcano was a continuous challenge. First of all, when they pulled up the pipe which had pierced the seafloor to collect deposits, walls inside of the pipe were completely sprayed with mud. This looked like the methane gas’ doing. The gas expands when the mud is brought up and it seems that it escaped from the sample material inside the pipe like a fizzing soda pop. Then, the method to collect sample material by plunging the pipe down using hydraulic power (hydraulic piston corer) resulted in a high sample collection rate, but as the gas expanded while it was brought up to the ship the inside of the samples was often hollow.

　For the drilling a hybrid pressure-coring system was used for the first time, which brings up samples maintaining the pressure to the level it is on the seafloor. Says Iijima, “We weren’t really able to get any samples with this system at first. It seemed to keep throwing bad stuff at us, like gravel with clay in it.” Clay, especially in strata of dehydrated clay minerals has a high level of viscosity and forms an obstruction to drilling. Water is used for the removal of drill cuttings, but the clay absorbs the water and expands, and for this reason samples don’t really get into the pipe.

　Eventually they did however finally manage to obtain a very beautiful sample. By examining the sample with X rays without opening it so that the pressure didn’t drop, they could see the clay that had spouted the mud volcano, fragments of the rock and the methane hydrate running through it like a web. That is the inside of the mud volcano, its original form. Collecting and examining the mud from a mud volcano, in its original form, is a world’s first.

　Mud volcanoes are a tricky business, with their mud obstructing the drilling throughout and with expanding gas mixed besides. But mud volcanoes are information pipelines that tell us about what it is like deep in the earth. Says Iijima, “The technology and expedition members, the pride of the CHIKYU, succeeded in a difficult mud volcano drilling expedition. In future, we want to install measuring equipment inside a borehole in a mud volcano and obtain a variety of information.”