Toshiaki Nakamura

Numerical Analysis of Underwater Acoustic Lens Using Wide-Angle Parabolic Equation Method

The autonomous underwater vehicle mounted on the forward obstacle avoidance sonar is developed to investigate ocean environments such as that of the Arctic Ocean. In order to obtain real time, high efficiency and clear acoustic images, the acoustic lens sonar system has been adopted. Usually, the design of the acoustic lens as well as that of the optical lens is based on geometrical optics theory. In this paper, the acoustic characteristics of the acoustic lens are studied by using the two- and three-dimensional parabolic equation methods.

Design and Convergence Performance Analysis of Aspherical Acoustic Lens Applied to Ambient Noise Imaging in Actual Ocean Experiment

In this study, an aspherical lens with the aperture diameter of 1.0 m was designed for utilization in an actual ocean experiment of ambient noise imaging (ANI). It was expected that this ANI system would realize directional resolution, which is a beam width of 1° at the center frequency of 120 kHz. We analyzed the sound pressure distribution focused by the designed lens using the 3D finite difference time domain method. The frequency dependence of a -3 dB area was then compared between 120 kHz and the higher or lower frequency. The analysis results suggested that the designed lens has fine directional resolution over the center frequency of 120 kHz. We had measured the directivity of the designed lens in an actual ocean experiment in Uchiura Bay in November of 2010. It was verified that the ANI system with this lens realizes a beam width of 1° at 120 kHz.

Numerical Simulation of Target Range Estimation Using Ambient Noise Imaging with Acoustic Lens

In ambient noise imaging (ANI), each pixel of a target image is mapped by either monochrome or pseudo color to represent its acoustic intensity in each direction. This intensity is obtained by measuring the target objects reflecting or scattering wave, with ocean background noise serving as the sound source. In the case of using an acoustic lens, the ANI system creates a C-mode-like image, where receivers are arranged on a focal plane and each pixels color corresponds to the intensity of each receiver output. There is no consideration for estimating a target range by this method, because it is impossible to measure the traveling time between a transducer and a target by a method like an active imaging sonar. In this study, we tried to estimate a target range using the ANI system with an acoustic lens. Here, we conducted a numerical simulation of sound propagation based on the principle of the time reversal mirror. First, instead of actual ocean measurements in the forward propagation, we calculated the scattering wave from a rigid target object in an acoustic noise field generated by a large number of point sources using the two-dimensional (2D) finite difference time domain (FDTD) method. The time series of the scattering wave converged by the lens was then recorded on each receiver. The sound pressure distribution assuming that the time-reversed wave of the scattering wave was reradiated from each receiver position was also calculated using the 2D FDTD method in the backward propagation. It was possible to estimate a target range using the ANI system with an acoustic lens, because the maximum position of the reradiated sound pressure field was close to the target position.

Extraction of Target Scatterings from Received Transients on Target Detection Trial of Ambient Noise Imaging with Acoustic Lens

We have already designed and fabricated an aspherical lens with an aperture diameter of 1.0 m to develop a prototype system for ambient noise imaging (ANI). It has also been verified that this acoustic lens realizes a directional resolution, which is a beam width of 1° at the center frequency of 120 kHz over the field of view from -7 to +7°. In this study, a sea trial of silent target detection using the prototype ANI system was conducted under only natural ocean ambient noise at Uchiura Bay, in November of 2010. There were many transients in the received sound. These transients were classified roughly into directly received noises and target scatterings. We proposed a classification method to extract transients of only target scatterings. By analyzing transients extracted as target scatterings, it was verified that the power spectrum density levels of the on-target directions were greater than those of the off-target directions in the higher frequency band over 60 kHz. These results showed that the targets are successfully detected under natural ocean ambient noise, mainly generated by snapping shrimps.

Tidal Effect in Small-Scale Sound Propagation Experiment

A sound propagation experiment in very shallow water was conducted at Hashirimizu port in 2009. We transmitted 5 kHz sinusoidal waves with M-sequence modulation. As a result, we found that the travel time concentrated in two time frames. When comparing the travel time with the tide level, the travel time was dependent on the tide level. In terms of the wave patterns, most of the wave patterns have two peaks. As the tide level changed, the biggest peak switched within two peaks. To discuss this, numerical simulation by finite difference time domain (FDTD) method was carried out. The result agreed with the experimental result. Finally, we changed the material of the quay wall in the FDTD simulation and concluded that the first peak is a multireflected combination wave and the effect of its reflected wave at a quay wall has superiority in the second peak.

Preliminary analysis of background noise distribution in the ocean experiment 2010 of Ambient Noise Imaging

We previously designed and made an aspherical lens with an aperture diameter of 1.0 m for Ambient Noise Imaging (ANI). The prototype system was developed by mounting the hydrophone array on the image surface of this lens. In the ocean experiment 2010 of ANI, this system was deployed through the barge OKI SEATEC II moored in Uchiura Bay on November 8-13, 2010. First, we measured the directivity of this lens using a pinger sound of 120 kHz. It was verified that the prototype ANI system with this lens realized the directional resolution with a beam width of 1 degree at the center frequency of 120 kHz over the field of view from -7 to +7 degrees. In the data analysis results of the silent target detection trials, we successfully detected target scatterings using only ocean natural background noise, which was generated mainly by snapping shrimps. In this study, background noise distribution was observed at the same time as this experiment was conducted. We used a pair of tetrahedral arrays in this observation. Here, four hydrophones were mounted at distances of 1 m on each of two arrays which themselves were located about 10 m apart and about 5 m below the surface. The noise source positions were estimated by the arrival time differences of transient noises received by hydrophone arrays. In the preliminary results for the spatial densities of noise sources, the noise sources were spread when the noises arrived from the sea bottom. Some of the sources were around the sea bottom just under the barge, and other sources were around the sea bottom just under fish preserves. The sources were coincident with the barge when the noises arrived from the sea surface. Considering the directivity of the target scattering, it was suggested that the locations of noise sources, where the ANI system can capture target scatterings having high intensities, were at the barge around the sea surface.

Small scale reciprocal sound propagation analysis in Hashirimizu port

Small scale reciprocal sound propagation was carried out at Hashirimizu port in front of Tokyo Bay, Japan. A pair of the transducers with the distance about 120 m was set at the bank of the port. The average depth at the area is about 4 m. There were so many surface and bottom reflections. In this study, authors investigate the effects of ocean changes such as temperature, tidal level and current, to the reciprocal sound propagations. The 7th order M-sequence was sent every 5 minutes with carrier frequency of 80 kHz. The travel time mainly varied according to the water temperature. But sometimes, it shifted rapidly which could not be considered the effect of the water temperature. As there was almost 1.5 m depth changes because of the tide, the strength of receiving signals also changed according to the interferences of surface and bottom reflections. The biggest peak of the correlated signal was shifted under the conditions of the depth and temperature. It was also confirmed by calculations by finite-difference time-domain (FDTD) method. Because of these complicated interference, it was difficult to estimate current along the propagation path although it was improved by the peak tracing method. But there is still possibility to monitor water flows with few transducers. This method will be possible to monitor the average changes along the sound propagation area. Moreover, it will enable to monitor more accurate temperature or flow distributions using more transducers to create tomography system in the future.

Relationship between Spatial Distribution of Noise Sources and Target Scatterings Observed in the 2010 Sea Trial of Ambient Noise Imaging

An aspherical lens with an aperture diameter of 1.0 m has been designed and fabricated to develop a prototype system for ambient noise imaging (ANI). A sea trial of silent target detection using the prototype ANI system was conducted under only natural ocean ambient noise at Uchiura Bay, in November of 2010. It was verified that the targets are successfully detected under natural ocean ambient noise, mainly generated by snapping shrimps. In this study, we surveyed the relationship between the spatial distribution of noise sources and the target scattering captured by the ANI system. The observation using a pair of tetrahedron arrays was conducted at the same time as the sea trial. The estimated source positions were spread when the noises arrived from the sea bottom. Some of the sources were around the barge, and other sources were around fish preserves. On the other hand, the source positions were coincident with the barge when the noises arrived from the sea surface. The calculated scattering fields of the target showed sharp directivities. The locations of noise sources, where the ANI system can capture target scatterings with high intensities, were roughly determined at the barge around the sea surface.