2022 年 75 巻 p. 13-28
Obtaining marine geophysical data, particularly from seismic surveys and observations, is difficult when using ocean-bottom seismographs (OBSs) in coastal areas without causing a conflict with fishery activities. To minimize conflict, we have designed and operated anchor-recovery-type OBSs in coastal areas shallower than 200 m. The OBSs shallower than 650 m were required to recover with their anchors to minimize such conflict during our seismic survey cruise off Yamagata Prefecture by the research vessel (R/V) Kairei in August 2019. Therefore, we improved the OBS anchor recovery system, which can operate from the continental shelf to the upper continental slope at depths shallower than 650 m. Two operation trials of the improved anchor-recovery-type OBS were conducted using a multi-purpose experiment tank at the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) Yokosuka Headquarters, to evaluate its performance before the cruise. In addition, we prepared a sweep-line with grapnel anchors in case unexpected problems occurred during the OBS recovery operation. The seismic survey cruise used a tuned air-gun array system and 39 OBSs (including eight anchor-recovery-type OBSs). The OBS recovery operation was initiated following the air-gun shooting. To prevent the propellers of R/V Kairei from tangling in the recovery rope, a two-ship operation was planned for the recovery of the anchor-recovery-type OBSs. The two-ship operation was performed as follows: First, the R/V Kairei sent an acoustic release command to the OBS. Second, the surfaced OBS was recovered by the ocean tugboat, motor vessel (M/V) Hirokai. Finally, the anchor recovery rope was wound using the winch on M/V Hirokai to recover the anchor. We successfully recovered seven OBSs (Sites 1-7) and their anchors; however, one OBS (Site 8) did not respond to acoustic communication, including the release command. We attempted to recover this unreleased OBS using the prepared sweep-line with grapnel anchors. The sweep-line was deployed from the stern of the R/V Kairei and towed around the OBS position with a radius of approximately 100 m. The OBS and its anchor were caught by the sweep-line and successfully recovered. Our improved anchor-recovery-type OBS can reduce conflict with fishery activities and enable to conduct marine seismic surveys in coastal areas from the continental shelf to the upper continental slope.