Akira Asada

Manganese crusts (Mn-crusts) are a type of mineral deposit that exists on the surface of seamounts and guyots at depths of >800 m. We have developed a method to efficiently map their distribution using data collected by autonomous underwater vehicles and remotely operated vehicles. Volumetric measurements of Mn-crusts are made using a high-frequency subsurface sonar and a 3-D visual mapping instrument mounted on these vehicles. We developed an algorithm to estimate Mn-crust distribution by combining continuous subsurface thickness measurements with the exposed surface area identified in 3-D maps. This is applied to data collected from three expeditions at Takuyo Daigo seamount at depths of ∼1400 m. The transects add to ∼11 km in length with 12 510 m2 mapped. The results show that 52% of the surveyed area is covered by Mn-crusts with a mean thickness of 69.6 mm. The mean Mn-crust occurrence is 69.6 kg/m2 with a maximum of 204 kg/m2 in the mapped region. The results are consistent with estimates made from samples retrieved from the area, showing more detailed distribution patterns and having significantly lower uncertainty bounds for regional-scale Mn-crust inventory estimation.

Interplate megathrust earthquakes have inflicted catastrophic damage on human society. An extremely hazardous megathrust earthquake is predicted to occur along the Nankai Trough off southwestern Japan, an economically active and densely populated area with historical records of megathrust earthquakes. Megathrust earthquakes are the result of a plate subduction mechanism and occur at interplate slip-deficit (or coupling) regions. Many past studies have attempted to capture slip-deficit rate (SDR) distributions for assessing future earthquake disasters. However, they could not capture a total view of the megathrust earthquake source region because they had no seafloor geodetic data. The Hydrographic and Oceanographic Department of the Japan Coast Guard (JHOD) has been developing a highly precise and sustainable seafloor geodetic observation network deployed in this subduction zone to broadly obtain direct information related to offshore SDR. Here, we present seafloor geodetic observation data and an offshore interplate SDR distribution model. Our data suggests that most offshore regions in this subduction zone have positive SDRs. Specifically, our observations indicate previously unknown high-SDR regions that are important for tsunami disaster mitigation and low-SDR regions that are consistent with distributions of shallow slow earthquakes and subducting seamounts. This is the first direct evidence suggesting that coupling conditions are related to these seismological and geological phenomena. These observations provide new fundamental information for inferring megathrust earthquake scenarios and interpreting research on the Nankai Trough subduction zone.

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.

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.

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.

SAS (synthetic aperture sonar) has been used in sea bed imagery. Indeed high resolution images provided by SAS are of great interest, especially for the detection, localization and eventually classification of objects lying on sea bed. But as any image obtained with a coherent system, SAS images are highly corrupted by a granular multiplicative noise, called speckle noise which reduces spatial and radiometric resolutions. The appraisal of despeckling methods or in more general denoising methods is a never ending discussion. Since, these techniques are applied on images, one can somewhat merely look at the image and have a good idea of the general outcomes. Yet, in practice this would take too much time and overall would not be objective at all. Thus, people doing research work on filter wanted to assess their developed methods and tried to find out objective assessing processes. In this paper, we present some of the scientific literature proposals and, by the way of experiments on real data, discuss about them.

On the Japanese coasts, there are so many important facilities which can be exposed to the threats of terror attacks. So, we built an acoustic video system, modifying a dual-frequency acoustic camera DIDSON and developing an optimum underwater surveillance method and software. In succession, a 100 kHz-frequency Wharf Installed Security Sonar was developed. Its transmitting beam is variable in width and can be steered up and down depending on the installation environments, making it possible to easily detect targets. Recently, we have developed an effective moving surveillance method employing a Shipboard Installed Security Sonar consisting of three transducers, each with a different frequency (110 kHz, 240 kHz and 455 kHz) and all sharing a receiving array to monitor both far and near fields. In the 110 kHz mode, the sonar can monitor a far distance and wide area. With the embedded software we developed for overlaying aerial photo maps on sonar images, we obtained data useful for further research to develop an effective surveillance method. We are now proceeding with this development while implementing in this system the moving surveillance method employing the acoustic video camera.

Intending to capture sea-bottom crustal deformations with high accuracy, we have been developing new technologies for seafloor geodetic observations with an AUV. This has led to the development of a small high-performance geodetic observation system and an ideal spherical acoustic transducer, both to be mounted on an AUV, and the tuning and evaluation tests of them have yielded good results.