Mud, sand, and other substances that flow into the ocean from the land via rivers and the atmosphere accumulate on the seafloor and form sedimentary layers. The materials generated in the ocean by biological production processes or chemical reactions are also deposited at the same time, and the global environment at that time is recorded in the sediments of the seafloor from moment to moment. Therefore, it is believed that the analysis of seafloor sediments can be used to elucidate the evolution of the global environment from the past to the present and to predict climate change in the future.
On the other hand, there are places on the seafloor where the crust is exposed, such as ocean ridges. The distribution characteristics of the rocks that make up the crust can be traced to faults and plate motions, and analysis of crust and rocks collected from the seafloor is expected to elucidate the mechanisms of earthquake generation.
Since geological samples collected vary in particle size, hardness, and viscosity depending on the purpose of the research and the area of the sea, various instruments have been developed to efficiently collect geological samples. In addition, acoustic instruments such as multinarrow-beam bathymetry (seafloor topography survey equipment) and sub-bottom profilers are sometimes used to investigate the shape and characteristics of the seafloor prior to observation and to ensure safe and accurate sample collection. In recent years, it has become clear that useful resources such as natural gas and methane hydrate exist on the seafloor, and geological samples are also collected for the purpose of seafloor resource surveys.
Fig.1 Schematic of geological sample collection observation
ピストンコアラー Piston corer
マルチプルコアラー Multiple corer
ボックスコアラー Box corer
オケアングラブ採泥器 Okean grab sampler
スミス・マッキンタイヤ採泥器 Smith Maclintyre grab sampler
ロックコアラー Rock corer
Because most of the equipment used to collect geological samples shown in Figure 1 is large and heavy, observation work is performed using a gantry crane (A-frame) equipped at the rear of the observation vessel.
This section introduces columnar samplers, grab samplers, other samplers, and crust and rock samplers that are currently used for geological sampling.
A columnar sampler is a generic term for an instrument that collects sediment samples by piercing a cylindrical cylinder into the seafloor. Since this type of instrument can collect samples without disturbing the stratigraphy of the sedimentary layers, the collected samples are analyzed and classified according to their ages. Typical columnar samplers include piston corers, multiple corers, G.S.type core sampler（ASYURA）, Freger core sampler, and HR type core sampler.
The piston corer is a gravity free-fall type sediment sampler. A metal cylinder connected under the main weight is thrust into the seafloor to collect sediments in a cylindrical shape from several meters to a maximum of 20 meters below the seafloor. It is the deepest mud sampler of its type, and the collected samples can be used to estimate the earth's environment hundreds to hundreds of thousands of years ago.
The observation workflow for the piston corer is as follows. First, the main body of the piston corer is connected to the trigger of the balance type and lowered to the seafloor using a winch. When the pilot weight contacts the seafloor, the trigger operates and the corer drops freely, penetrating the seafloor by its own weight and momentum. The characteristic of the piston corer is that negative pressure is generated by the piston rising inside the cylinder during penetration, allowing the sampler to penetrate smoothly and deeply. This negative pressure also prevents the loss of samples from the bottom of the pipe (Figure 2).
On the other hand, since the sediment surface samples are disturbed when the sampler is thrust into the seafloor, a small corer is used for pilot weights and the sediment surface is sampled separately.