HR: 0800h
AN: G41A-0350    [Abstracts]
TI: Application of Phase Conjugate Underwater Acoustic Wave to the Measurement of a Fix Point Displacement on Seafloor
AU: * Iwase, R
EM: iwaser@jamstec.go.jp
AF: JAMSTEC, 3173-25 Showa-machi, Kanazawa-ku, Yokohama, 236-0001 Japan
AU: Naoi, J
EM: naoij@jamstec.go.jp
AF: JAMSTEC, 3173-25 Showa-machi, Kanazawa-ku, Yokohama, 236-0001 Japan
AU: Kikuchi, T
EM: kikuchit@jamstec.go.jp
AF: JAMSTEC, 3173-25 Showa-machi, Kanazawa-ku, Yokohama, 236-0001 Japan
AU: Mizutani, K
EM: mizutani@esys.tsukuba.ac.jp
AF: University of Tsukuba, 1-1-1 Tennodai, Tsukuba, 305-8573
AB: A method of fix point displacement measurement on seafloor by utilizing phase conjugate underwater acoustic wave is proposed and is examined by a simulation. Phase conjugation is a well-known process in the field of optics. The primary feature is cancellation of propagation distortion. When we consider a phase conjugate mirror which generates phase conjugate wave and assume that the spatial property of the propagating medium is stationary over the time needed for a round trip, a phase conjugate wave reflected at that mirror propagates opposite direction of the original incident wave showing time-reversed signature. In case of a spherical wave transmitted from a point source, the reflected phase conjugate wave converges to the original source point. Recently these phenomena of phase conjugation are also demonstrated experimentally for underwater acoustic waves, such as in Kuperman et al. (1998). In underwater acoustics, a source-receiver transponder is used for an original source, and a vertical transducer array, which retransmits time-reversed acoustic pulses generated from received ones that were transmitted from the source transponder, works as a phase conjugate mirror. In this case, the retransmitted pulses also converge to the original source transponder in original shapes by interfering one another. As the feature of the phase conjugation, this process is not affected by frequency of the pulses and property of the medium, such as water depth, topography of seafloor and thermal structure of seawater, as far as the property is stable enough for a round trip time. In long term, however, the retransmitted pulse may not be converged at the source because of the change of propagating condition that includes distance between the source and the array. The collapse of phase conjugation can be detected by observing the acoustic field at the source. Acoustic amplitude structure at the source is less affected by propagating condition. On the other hand, acoustic phase structure changes linearly. Based on this background, the following method of fix point displacement measurement on seafloor is proposed. At the array, the phase-shift modification is performed to the received pulses in the way the relation of phase conjugation is not broken after they are time-reversed. And then these phase-shifted pulses are retransmitted from each element of the array. The shapes of pulses converged at the source are the same as the original. However, phases of their carrier signal are shifted according to the modification amount at the array. The phase-shift modification is continuous and linear. At the source, in order to detect a slight phase change, a phase interference method is applied for successive converged pulses whose phases are linearly shifted at the array. Under the same condition, the original converged pulse without phase-shift modification is canceled when it is interfered with the one whose phase is shifted by pi. On the other hand, if the distance of return path between the source and the array differs from the original, the amount of phase-shift modification for the cancellation changes from pi, which linearly corresponds to the distance change. The proposed method is examined by the simulation with the coupled-modes method. The propagation model is a fixed water depth (100 m) with simplified sound speed profile. The distance between the array and the source is 5 km. A transmitting pulse at the source is ten-cycle tone burst wave with carrier frequency of 500 Hz. By the simulation, a linear relation of changes among phase, sound velocity and distance is obtained.
DE: 4259 Ocean acoustics
DE: 4594 Instruments and techniques
SC: Geodesy [G]
MN: Fall Meeting 2005