Iwao Nakano

AN OCEAN ACOUSTIC TOMOGRAPHY SYSTEM WITH A 200 HZ GIANT MAGNETOSTRICTIVE SOURCE

This paper describes tomography equipment for measuring ocean phenomena in a 1000 km2 area. A 200 Hz giant magnetostrictive source was developed to increase the transmitting level and decrease the source size. Five 200 Hz transceiver systems were constructed with these sources. These systems have surface buoys for real-time data transmission via the INMARSAT-C. Sea tests of the five transceiver systems were conducted east of the Izu-Ogasawara Trench to observe the Kuroshio Extension during July and September 1997.

Tomographic measurements of the Kuroshio Extension Meander and its associated eddies

A 1000-km scale three-dimensional (3D) acoustic tomography experiment has been conducted in the Kuroshio Extension region from July to September 1997. The 3D temperature fields show the energetic meander of the Kuroshio Extension with high time resolution. Mesoscale eddies associated with the meander with horizontal scale greater than 200-km are resolved in the tomographic inversion. These results are consistent with other measurements such as ADCP, XCTD and TOPEX/POSEIDON satellite altimetry. Variable oceanic structures in a 1000-km domain, which are difficult targets for conventional point measurement techniques, can be measured well by ocean acoustic tomography with real-time data telemetry.

Optimum design of the ocean acoustic tomography system for the Sea of Japan

An ocean acoustic tomography system covering the region of 800×1000 km with the spatial resolution of eddy-resolving scales has been designed on the basis of computer experiments using the hydrographic data collected in the Sea of Japan. The optimum number of acoustic sources required for 20 acoustic receivers was estimated as 13 by changing the source number. The spatial resolution for the optimum system was 41 km smaller than the dominant size of meso-scale eddies in the Sea of Japan. The effect of travel-time errors on tomographic maps is also quantified.

Simultaneous Transmission of Tomography Transceivers Using Multiple M-Sequence Signals

A tomography array which consists of five 200 Hz transceivers was deployed in the Central Equatorial Pacific in December 1998 to monitor the variability of water temperature and current in relation to El Nino. In this experiment, we intend to perform precise measurement using simultaneous transmission from five transceivers. As each transceiver receives an overlapped signal from other transceivers, multiple M-sequence signals were selected carefully and used to discriminate mutual signals among transceivers.

Estimation of the stability of acoustic reciprocal transmission in long-range propagation at the Central Equatorial Pacific

We present the stability estimation results of the reciprocal acoustic transmission data collected during the tomography experiment performed by Japan Marine Science and Technology Center (JAMSTEC) in 1999 in the Central Equatorial Pacific. Travel-time perturbations due to ocean currents are correspondingly one to two orders of magnitude smaller than travel-time signals due to sound-speed perturbations. It is important to estimate the stability of signals of reciprocal transmission. The standard deviation of the phase of the ray signal is very stable within 130 s. Between reciprocal transmissions, the overall structures of the signal are similar, however, the fine structures are different. The standard deviation of the effective phase is a useful index for determining whether or not the ray exists. The received signal is judged as an arrived ray when the standard deviation of the effective phase of 13 consecutive shots is less than 1 rad. The combination of the amplitude and phase information is effective for observing the ocean structure change.

Estimation of the Stability of Acoustic Multipath in Long-Range Propagation at the Western Equatorial Pacific.

In this paper, we present the stability estimation results of the data collected during the sound transmission experiment performed by Japan Marine Science and Technology Center (JAMSTEC) in 1996 at the western equatorial Pacific. The amplitude and the phase of a correlated signal were very stable within 50 s. There was a small component of the acoustic changes with a period of less than 50 s due to the ocean-structure change. The amplitude and phase, changed significantly within about 3 h. The travel time could be estimated with an error of less than 0.3 ms. The combined information of the amplitude and phase was effective for identifying a ray and observing the ocean-structure change.

Estimation of the Stability of Acoustic Multipath in Long-Range Propagation

We present the stability estimation results of the data collected during the sound transmission experiment performed by Japan Marine Science and Technology Center (JAMSTEC) in 1996 in the western equatorial Pacific Ocean. The combined information of the amplitude and the phase was effective for identifying a ray and observing the ocean-structure change. The phases of correlated signals were very stable within 50 s not only for the paths propagated near the sound fixing and ranging (SOFAR) axis but for ones turning at about 2500 m depth. The travel time could be estimated within an error of less than 0.3 ms.

Numerical Analysis of Acoustical Propagation Characteristics in Central Equatorial Pacific Ocean with Ridges of Seabed Used by Three-Dimensional Parabolic Equation Method

Ocean acoustic tomography (OAT) using sound propagation times in the ocean is a useful observation system to determine the actual temperature of water. OAT needs an adequate numerical method of sound propagation to obtain the prior information about the ocean. We reported about the wide-angle parabolic equation (PE) calculation method for long-range propagation. However, if the ridges on a seabed exist near the SOund Fixing And Ranging (SOFAR) axis, it blocks sound pulses passing near the SOFAR axis. Sound pulses are reflected not only in the vertical direction such as sea surface, but also in the horizontal direction because of ridges of seabed. Two-dimensional (2-D) analysis of the sound pressure distribution many have a slight error behind ridges or islands. Therefore, it is necessary to consider the sound propagation characteristics using the three-dimensional parabolic equation (3-DPE) calculation method but the 2-DPE calculation method. We obtained accurate results for sound pressure distribution on the SOFAR axis using 3-DPE calculation method, when the sound pulse was blocked by the seabed. The calculation results of underwater characteristics by 3-DPE calculation were compared with those of the 2-DPE calculation method.

Long-term acoustic measurement of temperature variations in the Pacific North Equatorial Current

A 247-km path length sound transmission experiment using a single sound source sending to a remote receiver was performed during February-June 1992 along the latitudinal line of 10°N in the North Equatorial Current off Mindanao Island, Phillipines. The range-averaged temperature was reconstructed at three layers (65-500 m, 500-1500 m, and 1500-3300 m) through the inverse analysis of travel time data obtained between the source and receiver. The temperature remained nearly constant during February-March and increased by 0.8°C for the upper and middle layers and 0.2°C for the lower layer in the subsequent months. A 0.8°C temperature increase was in good agreement with expendable bathythermograph results obtained at the start and finish of the experiment. National Oceanic and Atmospheric Administration advanced very high resolution radiometer images also show evidence of a warming of the surface water by 2.0°C during the same period. It is concluded that this acoustic experiment observed a warm event which took place in the western equatorial Pacific with the decay of the 1991-1992 El Nino.

Static and Dynamic Characteristics of Giant Magnetostrictive Materials under High Pre-Stress

We describe the static and dynamic characteristics of a giant magnetostrictive material under high pre-stress, using the pre-stress given to the giant magnetostrictive material as a parameter, and also evaluate material constants. Seven evaluation units, each having an optimum magnetic bias applied to vary the pre-stress given to the material from 15 MPa to 54 MPa, were used to measure the static and dynamic characteristics. As a result, the trends of equivalent circuit constants such as stiffness, internal resistance, and force factor in response to the increase in pre-stress for enhancing the output, were clarified. Furthermore, considering the linearity to the input level, the necessity of examining the optimum pre-stress region, in addition to the simple increase in pre-stress, was suggested.