Akira Asada

Evidence of viscoelastic deformation following the 2011 Tohoku‐Oki earthquake revealed from seafloor geodetic observation

The GPS/acoustic seafloor positioning has detected significant postseismic movements after the 2011 Tohoku-Oki earthquake (M9.0), just above the source region off the Pacific coast of eastern Japan. In contrast to the coastal Global Navigation Satellite Systems sites where trenchward-upward movements were reported, the offshore sites above the main rupture zone in the northern part of the source region exhibit landward displacements of tens of centimeters with significant subsidence from almost 3 years of repeated observations. At the sites above around the edge of the main rupture zone, smaller amount of trench-normal movements was found. Although the terrestrial movements were reasonably interpreted by afterslip beneath the coastal area, these offshore results are rather consistent with effects predicted from viscoelastic relaxation in the upper mantle, providing definitive evidence of its occurrence. On the other hand, the results in the southern part of the source region imply superposition of effects from viscoelastic relaxation and afterslip.

Three dimensional mapping of aquatic plants at shallow lakes using 1.8 MHz high-resolution acoustic imaging sonar and image processing technology

A new integrated measurement system that combines the acoustic imaging sonar of DIDSON (Dual-frequency IDentification SONar) with concentrator lenses, motion sensors, and differential global positioning system (DGPS) was used to classify different species of aquatic plants and make three-dimensional (3D) mapping of them at shallow lake. In the field experiment at the Lake Yunoko, the image was captured with a 3° concentrator lens. Difference of Gaussian (DoG) filtering was used to classify two species of aquatic plants, Chara globularis and Elodea nuttallii appeared in 2D acoustic images. After the classification, 3D acoustic image was reconstructed from the consecutive 2D processed images. The high-resolution acoustic images obtained allowed us to identify individual aquatic plants with high accuracy. The integrated DIDSON measurement system will contribute to the protection of endangered species in rapidly changing underwater environments.

Application of a high-resolution acoustic video camera to fish classification: An experimental study

In this study, we try for fish classification using high resolution acoustic video camera at 3.0 MHz center frequency (ARIS). Newly observation method with wide in vertical and narrow in horizontal beam (WVNHB) was proposed for getting high quality acoustic image of swimming fish, and tested an image processing algorithm basing on Normalized Cross Correlation (NCC) for fish classification. This high-frequency acoustic video camera was applied to fish classification for the first time. Using the proposed observation method, we successfully obtained high quality acoustic images of fish and performed an image processing algorithm on the images.

Sub-bottom synthetic aperture imaging sonar system using an AUV and an autonomous surface tracking vehicle for searching for buried shells of toxic chemicals

On Kanda port construction in Fukuoka prefecture, no fewer than 2,800 harmful chemical bombs have been discovered beneath the sea bottom so far. In advance of dredging in the port, we have carefully carried out magnetic inspection for looking for chemical bombs. Since a true target often gets mixed in a lot of suspicious metallic objects, we must cautiously draw up detected objects one by one. Use of a magnetic sensor only is uneconomical and inefficient. On account of these, we started a new three-year project of developing a sub-bottom synthetic aperture imaging sonar (sub-bottom SAS) system by using an autonomous underwater vehicle (AUV) and an autonomous surface vehicle for tracking the AUV. The AUV Tri-Dog 1, which was developed by T. Ura and H. Kondo, is going to be used. The AUV will be installed with a new sub-bottom SAS system and could perform stable and suitable navigation for SAS imaging in shallow water. The surface autonomous tracking vehicle can compensate for positioning error of the AUV and monitor sub-bottom imaging. The synthetic aperture sonar technology for sub-bottom imaging is now in the worldwide limelight, and several projects are ongoing. Since most of chemical bombs were buried up to 5 meters beneath the sea bottom, operating frequency 10 kHz to 30 kHz is expected to be useful for sub-bottom SAS. At the beginning of the project, we measured acoustical characteristics of sub-bottom sediments in Kanda port in 2010 May using a 33kHz single beam echo sounder RESON-Navisound 410 and a parametric sub-bottom profiler Innomar SES2000 Compact. Based on the acoustical characteristics, we will design a sonar projector and hydrophone arrays and a new surface autonomous tracking vehicle in this year.

Assessing the biological process of Hydrilla verticillata predation in a eutrophic pond using high-resolution acoustic imaging sonar

Most Hydrilla verticillata disappeared soon after planting in unsuccessful restoration efforts carried out at Lake Izunuma (northeastern Japan). Furthermore, the disappearance of aquatic plants has been speculated to be a result of crayfish predation. However, the precise cause remains unclear because of the low transparency of the lake water. In this study, to identify the source of Hydrilla verticillata predation and characterize the potential behaviors of predators, we used an acoustic monitoring system based on an image-processing program that we developed. A field experiment was performed in a small eutrophic pond adjacent to Lake Izunuma, and 24 Hydrilla verticillata were planted on the pond bottom. A DIDSON unit was mounted on an original frame with a 3° concentrator lens. Data were collected at 1.8xa0MHz and at a maximum range of 3.0xa0m. All Hydrilla verticillata disappeared within a day of planting. Using the image tracking approach, we estimated the walking speeds of two potential predators, namely crayfish (Procambarus clarkii) and pond snails (Cipangopaludina japonica), and distinguished them accordingly. Using this method, we identified crayfish as the main source of predation and characterized their foraging behavior in terms of the decreasing amount of Hydrilla verticillata. The foraging behavior of crayfish mostly ended after 17xa0h of planting, possibly because all plants had been eaten by that time.

Inspection methods for underwater structures of ports and harbors

In Japan, problems about deterioration of infrastructure become prominent recently. Inspections for outside states of underwater structure in ports and harbors are dependent on the visual observation by divers. Furthermore, in aged quay wall, the infill sand behind the wall escape into the sea, and sometimes subsidence or collapse of top surface of quay wall can occur. Against these back-ground, the Civil Engineering Research Institute for Cold Region (CERI) and the Institute of Industrial Science (IIS, the University of Tokyo) study and develop inspection methods for underwater structure by using acoustic measurement device. This paper introduces the inspection methods for outside and inside states of underwater structures of ports and harbors.

Development of a handy mobile 4-hydrophone array system for in-situ census of the Ganges river dolphins (platanista gangetica)

Around 15-20 Ganges river dolphins (platanista gangetica) inhabit the 12-km stretch from Karnavas to Narora Barrage in the Ganges river system. However, more than 10 dolphins were carried away downstream after the flood of June-July 2013. In order to urgently conduct a quick census to secure the dolphins in the original habitat, the existing compact four-hydrophone array system (J-Array) was remodeled for towing, and a function to display the real-time acoustic data on a GUI (Graphical User Interface) on an operational PC at the site was added. Using the new J-Array system, the initial in-situ census in combination with acoustic and visual census was conducted for two days from 19 to 20 November 2013. The functions that were added to the new J-Array performed well. Immediately following the towing survey, using both the acoustic and visual census data, an estimate of the number of the dolphins was made at the site. It was estimated that a minimum of six dolphins were still in the habitat. Off-line acoustic data analysis results were also found in accordance with the in-situ census results. From this, the effectiveness of the new J-Array system for conducting in-situ census has been demonstrated.