We have installed the Dense Oceanfloor Network System for Earthquakes and Tsunamis (DONET) that includes 20 observatories at Kumanonada (called DONET1) for the purpose of monitoring the seismogenic zone around the Nankai Trough. During the construction of DONET1, a remotely operated vehicle (ROV) was successfully used to lay the optical-fiber submarine cables that connect the observatories with the science nodes; however, the cable-laying operation was onerous because it takes approximately 10 h and is manually performed by ROV operators. For this reason, the cable-laying operation is usually not performed on successive days, which makes it difficult to schedule the construction work. In this paper, we present an automated cable-laying system that can automatically pay out the optical-fiber submarine cable on the seafloor at a rate that keeps pace with the ground speed and also adjusts the cable tautness. The components and control strategies are first described, and the results of some experiments and the first mission are then presented. The experiments and the first mission demonstrate that the automated cable-laying system can reduce the physical and mental burdens on the ROV operator and speed up the installation of DONET2 in 31 observatories. DONET2 is planned to be deployed off the Kii Peninsula (on the western side of DONET1).
