Kazuki Abukawa

Assessing the biomass and distribution of submerged aquatic vegetation using multibeam echo sounding in Lake Towada, Japan

Here we report on an advanced survey system that combines multibeam echo sounding with underwater photography, which was used to collect accurate data on the distribution and abundance of submerged aquatic vegetation (SAV) in Lake Towada. The use of this system enabled us to visualize the cover, height, and biomass of the SAV over the lake bottom, as well as to distinguish between different components of the SAV such as vascular and algal plants. The spatial distributions of these major components of the SAV varied as a function of the depth gradient. The vascular component was mostly represented by Potamogeton species, which accounted for around one-third of the standing mass of SAV, whilst more than half of the SAV was algal (charophytes). This abundance of charophytes may well be responsible for the high water quality and transparency of Lake Towada.

A method of generating 3D views of aquatic plants with DIDSON

DIDSON is a highly definition sonar which uses acoustic lenses to make images in dark, turbid waters where optical systems are ineffective. The acoustic lenses of it focus and form an acoustic image on the transducer array at the rear of the sonar. Then electronics convert that acoustic image into a digital image which can be displayed on a computer screen. The combination of high frequency sound waves, acoustic lens configuration, and high resolution transducer array make it suitable for different kinds of applications such as: positive identification and inspection, navigation in close quarters, monitoring in turbines or being used as a security video camera underwater in dark and turbid conditions etc. However, the images formed by DIDSON are a little bit different from the images generated by Optical video cameras. Sometimes, it is high demanding for users to analyze the DIDSON data. In order to make the images being understand much more easily, one method is to improve the algorithm which was being used in the conversion of acoustic images. We used the standard DIDSON to take the video of the water column at several spots in the Yamanaka Lake, Japan. Considering with the depth of lake, we chose the high frequency sound waves (1.8 MHz) which consists of 96 beams, each beam spanning 0.3 degrees in the horizontal plane. Aimed to work out the whole 3D image of the aquatic plants, we set acoustic beam projections of DIDSON towards the bottom of the lake and turned it 45 degrees up from vertical. By doing that, echoes came back from aquatic plants were began with the root and ended with the top of the aquatic plants. Also, we added a concentrator lens which reduces the vertical aperture from 14 to 1 degree in front of the DIDSON. Therefore, each beam pattern is consisting of 96 beams covering a 30 deg by 1 deg spread. It allowed horizontal DIDSON beams to go farther in the water with reduced surface and bottom reverberation and illuminates thin slices which were very useful in 3D reconstruction of the growth condition of aquatic plants. In this study, we designed a new system for the detection of aquatic plants with suitable equipments which can be widely used in surveys of underwater ecological reserves. We also developed a software which can generate 3D views of the aquatic plants in accuracy of 1 centimeter. For future work, we hope to apply it into the identification of aquatic plants species.

Experimentation for development of underwater acoustic video camera: In experiment dock

Ocean resources exploration and underwater works using Remotely Operated Vehicle and Autonomous Underwater Vehicle increase interest, and a techniques of underwater image with acoustic has become a significant concern. Therefore, a development of new underwater acoustic device adapted to ocean resources exploration and underwater works. In this study, we experiment with the use of acoustic video camera for development of a new acoustic video camera and software to meet ocean resources exploration and underwater works needs. The experiment was performed by means of an acoustic video camera, underwater teleoperated excavator, underwater crawler, and simulated chimney. High spatial imaging and software were developed for this experiment and they were used to generate 3D views, to display an operation views and measurement distance of targets. Our results will be necessary for the ocean resources exploration and underwater works.

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.

Three-dimension diagnostic methods of quay wall using focusing acoustic parametric probe and imaging sonar

Collapse and subsidence of quay wall at a harbor are often reported and a diagnosis method to prevent the accident has become a significant concern. To achieve the more accurate assessment of quay wall, we try to develop the new diagnosis method which enables to estimate the inside and outside state of the quay wall from the underwater site. In this study, the outside mapping of the quay wall was conducted using multi-beam echo sounder, and the focusing acoustic parametric probe was developed for the detection of the air hole inside the quay wall. Outside mapping was performed at the port in Chiba. Three-dimensional image of the quay wall was reconstructed from the bathymetry data and seafloor topographys, and the mean tilt angle of quay wall was quantified. The sediments and tilted quay wall were clearly seen in this experiment. The acoustic characteristic of the probe, which was developed for the inner air hole detection, were evaluated in the experimental aquarium using three transmission signals [Summing1, 2 and linear frequency modulation signals (LFM)]. We have succeeded in the observation of the penetration wave passed through the concrete plate of 7 cm thickness. In our study, LFM signal showed the best results of the acoustic characteristics in three ones.

Reconstructed real-time 3D image of chimneys using underwater acoustic video camera in the experimental field

Ocean resources exploration and underwater works using Remotely Operated Vehicle and Autonomous Underwater Vehicle increase interest, and a techniques of underwater image with acoustic has become a significant concern. Therefore, a development of new underwater acoustic device adapted to ocean resources exploration and underwater works. In this study, we reconstructed real-time 3D image of chimneys using underwater acoustic video camera in the experimental field. The experiment was performed by means of an acoustic video camera, underwater crawler, and chimneys. High spatial imaging and software were developed for this experiment and they were used to generate 3D views, to display an operation views and measurement distance of targets. Our results will be necessary for the ocean resources exploration and underwater works.

Prototype of underwater acoustic video camera

Authors have developed 3D acoustic video camera which is small and light weight in order to mount on ROV for ocean floor resources exploration and/or underwater construction. The objective of this camera is forward looking, surveying and especially monitoring underwater work. Here, a prototype of the camera was developed in short range and high resolution, because it monitors a robot arm attached to a vehicle for subsea operations. The main specifications of the prototype; sector angle is 40 deg. (V) × 80 deg. (H), beam width is both 0.25 deg. In this case, updated rate of image is ignored because it has not yet been completed (currently assembling). Then authors were conducted a tank experiment, Prototype was successful to acquire a clear picture of a target in a tank.

Observation of aquatic biota in eutrophied pond using stationary acoustic monitoring system

Active acoustic monitoring system using a dual frequency identification sonar (DIDSON) is applied for the follow-up observation of aquatic biota after Hydrilla verticillata planting. The field experiment was performed in a small pond adjacent to Lake Izunuma (northern latitude of 38.43 degrees and east longitude of 141.04 degrees; surface area, 1,584 m2), and 24 Hydrilla verticillata were prepared and planted at the pond bottom. The DIDSON unit was mounted on an original frame with a 3 degree concentrator lens. The data were collected at 1.8 MHz (high-frequency mode) and at a maximum range of 3.0 m from the imaging sonar. The frame rate was 2 fps and the recording time was about 20 hours. Successfully, we found out the cause of plants disappearance by the acoustic imaging data. All Hydrilla verticillata were disappeared in a day and strongly affected by the predation pressure of crayfish, and whole scene of feeding was recorded. Other aquatic creatures, such as snail, frog, and several species of fish, were also appeared in the acoustic images. Then, we have originally developed an image processing program and quantified the biological features of the aquatic creatures for the better understanding of aquatic biota in freshwater environment.

Species classification of submerged aquatic plants using acoustic images in shallow lakes

Acoustic imaging sonar is used for species classification of the submerged aquatic plants at lakes. We used a measurement system that combined a high resolution acoustic imaging sonar, i.e., a Dual-frequency IDentification SONar (DIDSON) with two types of concentrator lens, motion sensors, and GPS receivers to quantify the underwater status of the lake. Our survey system could efficiently accumulate information from beneath the lake surface and it could be used to reconstruct the spatial state of the lake as a three-dimensional (3D) image at any time. In addition, we developed an image processing program to classify the aquatic plants using acoustic features of each aquatic plant. Two types of the field experiments were performed at Lake Yamanaka and Lake Yunoko in Japan. In the first experiment at Lake Yamanaka, 3D views of Myriophyüum spicatum were generated. In the second experiment at Lake Yunoko, we conducted a species classification of three aquatic plants, Myriophyüum spicatum, Chara globularis, and Elodea nuttallii.

Survey of submerged aquatic vegetation with Multi-beam Echo Sounder in Towada Lake of Japan

Submerged aquatic vegetation (SAV) is a potent water-purifying agent enhancing integrity of lake ecosystems. Therefore, mapping the distribution of SAV in lakes is very important for managing the water quality and serves to conservation purposes. Current method for mapping SAV, i.e. collecting plant samples from shore, is not applicable to mapping wide areas. Here we report the novel method for mapping and quantifying the volume of SAV based on the use of Multi-beam Echo Sounder. The new method was used in Towada Lake, northeast Japan during the summer of 2010. This method includes a Multi-beam Echo Sounder, the Antennas of RTK-GPS and Azimuth Measurement. SAV was scanned with Multi-beam Echo Sounder after taking pictures with an underwater camera and sampling with the plant body extraction device. Using vegetation echo data and lake bottom echo data, we calculated height, coverage, and volume of SAV and produced a distribution map of SAV. In Towada Lake SAV distributed at bottom depths from 2 to 20 m with an area of 1948m2 and volume of 1324m3. The volume changed with the depths, being 393m3, 64m3, 424m3, 308m3 and 134m3 at 4–6m, 6–8m, 8–10m, 10–12m and 12–18m depths, respectively. Our study demonstrated that Multi-beam Echo Sounder is an effective tool for mapping and quantifying SAV distribution including its visualization in three dimensions.