2020 Volume 73 Pages 14-22
Marine mineral resources such as seafloor hydrothermal deposits, cobalt-rich crusts and manganese nodules have been discovered around Japan. The authors have developed new survey tools for exploration of deep-sea mineral resources and examined the efficiency of the survey system. In 2018, an integrated survey including core sampling was conducted in uninvestigated hydrothermally active area. In this paper, we present high-resolution subseafloor image obtained from acoustic survey using a deeply towed hydrophone cable as part of the integrated survey. The spatial resolution (lateral and vertical) of this survey system is on the order of meters. We identified the distribution of fracture structures such as faults and fissures that would be the hydrothermal fluid path from the acoustic survey records. In addition, by comparing with the results of the coring survey, we were able to identify the layer where the hydrothermal sulfide mineral was deposited in the past. In this area, we consider that fault activity continues from the distribution of cracks in the surface sediments, but because the specific terrain suggesting the existence of faults is not found on the seafloor, meaning that the sedimentation rate is fast. These results collectively suggest that the survey area is under the environment where hydrothermal activity is existing, but it is difficult to form a large hydrothermal deposit due to the high deposition rate. From these outcomes from the acoustic survey, it is demonstrated that valuable information indicating the relationship between hydrothermal activity and seafloor sediment including sulfide ores, that is difficult to understand only from the core sampling, can be obtained from our high-resolution acoustic survey.
