Full-swath bathymetric survey system with synthetic aperture and triangle-arrayed interferometric techniques for autonomous underwater vehicle

Abstract

We have carried out a three-year project from 2008 for developing a seafloor bathymetric survey technology at cm-level resolution, when an AUV skims several tens meters above the seafloor. The technologies could be expected a practical application of searching for submarine minerals, such as thermal vents a few meters high, to a depth of 3,000 meters. In order to develop exploration technologies for submarine resources, new sensor technology needs to be developed for use with autonomous underwater vehicles (AUV) or remotely operated vehicle (ROV). Since the seafloor is so large, it is important that sufficient data is collected in a single precise dive, and that all potential resources are measured reliably. Therefore, we began our research aiming to develop the most advanced high-precision measurement technology for observation of seafloor bathymetry. The most accurate seafloor bathymetric measurement and sonar image research is realized when an AUV skims 20-50m above the deep seafloor. This would contribute largely to the measurement and discovery of deep-sea chimneys several meters in size, i.e. hydrothermal deposits. Since there are many hydrothermal deposits that have already ceased hydrothermal activity and that cannot be found with chemical sensors, it is important to develop a new method to search for these resources. For seafloor surfaces featuring little geomorphic change and covered with mineral resources, such as a cobalt rich crust, we expect acoustic images to be greatly helpful in grasping the distribution of potential resources, and the type and distribution of sea bottom materials could be investigated through analysis of the images. Combining the advantages of standard high-precision multi-beam echo sounder technology, synthetic aperture technology with high-resolution focusing function, and interferometric sonar able to take bathymetric measurements of large areas, the new bathymetric system is the world-advanced technology capable of full-swath measurement. For AUV exploration of an unknown deep seafloor, powerful observation with wide swath in a single pass is crucial. Therefore, we aim to cover 400m at a height of 50m from the seafloor. We created a new projector and five rows of new 1.2m-long receiving arrays with eight embedded hydrophones at an interval of 9 wavelengths, and located them in a triangular position. As a result, a total of 40 hydrophones are arranged at an interval of 1.8 wavelengths in horizontal. The projector was designed to have a mainlobe of 15 degrees width in horizontal, and to have no sidelobe within 0 to 90 degrees in vertical beam pattern. We also have used this triangle array and combine the interferometric system and synthetic aperture processing. Operating frequencies of 109.375 kHz and 93.75 kHz are used for right and left sides of sonar arrays respectively. So far, we have carried out operation tests of the interferometric synthetic aperture sonar equipped on a subsurface AUV. Based on these test data, we have designed and developed new technology to efficiently measure wide swath bathymetry with synthetic aperture and interferometric analysis. The new sonar algorism was designed to respond to each ping and estimate the vertical angle of incoming echoes with 1.2m-long five-row hydrophones. We then separate the echo horizontally with multi-beam forming using a total of 40 hydrophones. After this, the synthetic aperture processing is recreated for each row of the 8 hydrophones using the echoes from more than one ping, and it is focused to obtain the best resolution of the target echoes. Next we estimate the vertical angle of the target echoes again by using the five-row hydrophones. The simulation result of the algorithm shows that to estimate the vertical angle based on phase analysis with five hydrophones arranged at an interval of wider than 3 wavelengths in a triangular shape. In usual interferometric depth measurement, the hydrophones are placed linearly at an interval of one wavelength. But in this new system, the interval was increased to several times the wavelength to achieve high-resolution vertical angle measurement with a single echo. The new system also has a great advantage to measure the vertical angle using each of the three linear sides. In January 2011, we will conduct a practical test of the interferometric and synthetic aperture bathymetry sonar to verify the effectiveness of a submarine resource survey using ROV.