We develop an effective method for exploring seafloor hydrothermal deposits using acoustic apparatus. The important thing for seafloor hydrothermal deposits exploration is to carry out the efficiently detailed exploration after narrowing down a promising area from a vast area by wide-area exploration. Here, we aim to establish such a method, by examining methods for obtaining detailed bathymetric data and normalization of backscattering strength. Compared to conventional methods, data density is increased in both the transverse and traveling directions by narrowing the swath width and dropping the vessel speed. Also, normalization of submarine acoustic image is proceeded by applying the least square method from actual measurement focusing on the backscattering strength varying logarithmically with respect to the incident angle. Utilizing these techniques, we carried out a wide-area hydrothermal deposits exploration in the Higasih-Aogashima caldera. Accuracy of acquired wide-area bathymetry data had a difference of less than 0.1% against the area depth (about 700 m). In addition, normalized seafloor acoustic images were able to grasp the features of the sediment in the caldera. By narrowing the hydrothermal deposits candidate point, hydrothermal deposits were discovered from subsequent detailed exploration using an autonomous underwater vehicle and gravity corer. We are confident that these techniques will increase the efficiency of wide-area exploration of hydrothermal deposits.
Some marine mineral resources have been found in the waters surrounding Japan, but exploration techniques are still under development, and the amount of those resources is currently unknown. As part of these efforts, the Ministry of Education, Culture, Sports, Science and Technology launched the system development of their marine mineral resources regional exploration project in 2013. In the Asada laboratory, we are working to establish a system to explore hydrothermal deposit areas on the seafloor. One aspect of this is the development of hydrothermal deposit exploration technology by Interferometric Synthetic Aperture Sonar (InSAS). In the conventional method, manual processing led to variation and a large processing time to obtain the results of analysis. In this study, we attempt to eliminate these problems by replacing interference processing in a general-purpose approach. A detailed survey of coring, etc., is required, one that incorporates synthetic aperture processing in order to be able to generate high resolution data. The results show the characteristics of the hydrothermal deposit terrain of chimneys and mounds in an acoustic image. Furthermore, it was possible to eliminate the time-consuming manual analysis used in the conventional method.
The exploration of deposits buried under sea sediment requires high directivity and low attenuation beams. The parametric sub-bottom profiler (PSBP) meets these requirements. Moreover, it is comparatively small and can be easily mounted on autonomous underwater vehicles (AUVs), which are both highly stabile and able to operate close to the target. A new PSBP system for AUVs has been developed for this purpose, and we present recent results from its development. The performance of the PSBP system was estimated using a sonar equation with the Biot–Stoll model, and the results showed that the secondary/primary wave with low/high frequency was valid for measurements under thick/thin sediment. A field test was conducted at Beppu Bay, and a new data-processing method based on the continuous wavelet transform (CWT) was applied to the data. The method increased S/N, and some layers under the sea bottom could be clearly detected in the acoustic data. One example of such data obtained during the sea trials was compared with core data (BP09-3), and the peaks in the acoustic data agreed with the positions of the layers in the core. Thus, the PSBP system will be suitable for exploring buried deposits using AUVs in deep-sea areas.
Japan is a small island country surrounded by a vast EEZ which is thought to contain rich offshore resources, such as oil and natural gas. Recently, seismic surveys have been carried out to locate such resources. Because of their high resolution, seismic surveys are predicted to increase in the future. However, these surveys utilize air guns, which generate intense and low-frequency impulse noises known to disturb and harm marine mammals. The impact of seismic survey noise on marine mammals began to attract attention in the early 1970s, and acoustic impact thresholds and mitigation measures are still discussed all over the world. Here we propose an assessment method utilizing ARGO data and ocean-bottom topography data from Google Earth. We studied influences by seasonal climate changes and ocean-bottom topography with respect to the results of impact assessment, and found that these gave meaningful differences to our assessment results. This finding suggests that our proposed method is adequate for assessing the impact of seismic survey noise on marine mammals. We also assessed received levels of noises generated by air guns in a cetacean habitat near the Ogasawara Islands, utilizing the proposed method and the newest acoustic impact thresholds as updated in March 2016 by NOAA.
In order to assess hydrothermal deposits within the Japanese Exclusive Economic Zone to secure domestic marine mineral resources, we are developing and using five observation devices: Multi-Beam Echo Sounder (MBES), Sub-Bottom Plofiler (SBP), Remotely Operated Vehicle (ROV), Interferometric Synthetic Aperture Sonar (InSAS), and samplers. Although topography can be determined with an ROV and/or InSAS, the accurate identification of submarine hydrothermal deposits requires a detailed observation of the geological structures underneath the vents. To this aim, we developed an SBP to perform detailed and geocoded surveys of zones previously mapped at a rough scale with MBES. Our SBP can observe vents as large as several tenths of meters. Its airfoils help suppress roll, and its arrays of 8-channel acoustic receptors reduce noise. Chirped signals allow a resolution in depth of 3.75 cm, which we believe should be sufficient to analyze hydrothermal vents.
Surveys over submarine calderas of the Izu-Bonin island arc were carried out in July 2014 with our SBP. On the Bayonnaise reef, we determined the structure down to 30 m below the seafloor.
In the future, our devices could be used to determine the geological structures of hydrothermal vents in other hydrothermal zones.
When marine disasters occur, it is sometimes necessary for search and rescue dive teams to begin rescue and recovery efforts before a complete assessment of the underwater safety situation can be conducted. This makes the job of the Coast Guard personnel in charge of these types of operations very risky. In order to alleviate this type of risk and to be able to operate safely in affected areas, it is desirable for the divers to have as much prior knowledge as possible of the physical and spatial aspects of the area. Furthermore, the rapid presentation of this information in a visual manner is critical in order for the divers to be able to clearly understand and quickly interpret the information being presented. This paper discusses a newly developed optimized search and diver support technique that utilizes high-resolution, forward-looking sonar to allow Coast Guard personnel to improve the accuracy of the search operation while also increasing the efficiency and safety factors involved in these types of operations. Concretely, we developed a special mounting hardware for survey launches to install the R2Sonic 2024/2022 as well as the elemental software necessary to support underwater divers. Additionally, we evaluated the function of the software in indicating sonar images on marine charts, making mosaic images, and tracking underwater divers by using the 2024/2022 sonar data acquired in the actual sea area, though the diver tracking requires further improvement.