Ju Lin | Ocean University of China (original) (raw)
Papers by Ju Lin
Remote Sensing, 2022
The first coastal acoustic tomography (CAT) experiment site of the Neko-Seto Channel was revisite... more The first coastal acoustic tomography (CAT) experiment site of the Neko-Seto Channel was revisited to elucidate the propagation and generation characteristics of the M2 and M4 tidal currents with a second CAT experiment, which was conducted from 3–6 April 2018 (local time). Two-dimensional flow fields of the M2 and M4 tidal currents and the residual current were reconstructed using a coast-fitting inversion model with the reciprocal travel-time data of five acoustic stations. The M2 tidal current is a progressive-type wave that propagates eastward at a speed of 0.7 ms−1, much slower than expected for free progressive tides in this region (19 ms−1). The M4 nonlinear current constructed an out-of-phase relationship between the western and eastern halves of the tomography domain, implying the generation of standing-type waves. Such nonlinear processes led to flood- and ebb-dominant tidal current asymmetries for the western and eastern halves of the model domain, respectively. The two-d...
Coastal Acoustic Tomography, 2020
Abstract The horizontal-slice inversion is formulated for both the cases of grid and function exp... more Abstract The horizontal-slice inversion is formulated for both the cases of grid and function expansion methods. The superiority of tapered least squares method for the Gauss–Markov method is provided. Finally, various methods for validating the inversion results are proposed.
Coastal Acoustic Tomography, 2020
Sensors, 2020
Coast-fitting tomographic inversion that is based on function expansion using three types of norm... more Coast-fitting tomographic inversion that is based on function expansion using three types of normal modes (the Dirichlet, Neumann, and open boundary modes) is proposed to reconstruct current fields from the coastal acoustic tomography (CAT) data. The superiority of the method was validated while using CAT data that were obtained in 2015 in the Dalian Bay. The semidiurnal tidal and residual current fields were accurately reconstructed over the entire model domain surrounded by coasts and open boundaries. The proposed method was effective, particularly around the peripheral regions of the tomography domain and the near-coast regions outside the domain, where accurate results are not expected from the conventional inverse method based on function expansion by Fourier function series with no coast fittings. The error velocity for the semidiurnal tidal currents was 2.2 cm s−1, which was calculated from the root-mean-square-difference between the CAT-observed and inverted range-averaged c...
Acta Oceanologica Sinica, 2014
ABSTRACT Nontidal sea level changes generated in Hiroshima Bay of the Seto-Inland Sea in Japan ar... more ABSTRACT Nontidal sea level changes generated in Hiroshima Bay of the Seto-Inland Sea in Japan are studied over various time scales, from the sub-tidal (2 d to 1 month) to inter-annual scales (>2 years). The total sea level variation produces a standard deviation (STD) of 12.5 cm. The inter-annual component of the sea level variation in Hiroshima Bay oscillates with a STD of 3.4 cm, forming a long-term trend of 4.9 mm/a. The STD of the sea level variation is 9.8 cm for the seasonal component (8 months to 2 years) and 4.7 cm for the intra-seasonal one (1 month to 8 months). Significant sea level variations with a STD of 4.2 cm also occur in the sub-tidal range. Special attention is paid to the sub-tidal sea level changes. It is found that the upwelling and associated transient sea level changes generated along the north coast of Hiroshima Bay (opened southward) by the strong northerly wind, play a significant role in sub-tidal sea level changes. The transient sea level changes are over 10 cm in most cases when caused by typhoons that pass through the Pacific Ocean off the Kii Peninsula, located at about 400 km east of Hiroshima Bay. Reasonable sea level changes are evaluated by the balance of pressure forces at the onshore and offshore boundary of the study domain.
The Journal of the Acoustical Society of America, 2012
ABSTRACT
The Journal of the Acoustical Society of America, Oct 1, 2016
An acoustic reciprocity theorem is generalized, for a smoothly varying perturbed medium, to a hie... more An acoustic reciprocity theorem is generalized, for a smoothly varying perturbed medium, to a hierarchy of reciprocity theorems including higher-order derivatives of acoustic fields. The standard reciprocity theorem is the first member of the hierarchy. It is shown that the conservation of higher-order interaction quantities is related closely to higher-order derivative distributions of perturbed media. Then integral reciprocity theorems are obtained by applying Gauss's divergence theorem, which give explicit integral representations connecting higher-order interactions and higher-order derivative distributions of perturbed media. Some possible applications to an inverse problem are also discussed.
The Journal of the Acoustical Society of America, 2012
ABSTRACT Sonar detection performance is related to ocean environmental parameters, such as the so... more ABSTRACT Sonar detection performance is related to ocean environmental parameters, such as the source position, the ocean depth, the sound speed profile and geoacoustic parameters, etc. These parameters have strong spatial and temporal variability, which result to environmental uncertainty. The sonar detection system can be limited by the presence of environmental uncertainty. Based on a statistical model of the environmental uncertainty, the optimal Bayesian predictor by L. Sha has been applied in this paper to analyze the effects of environmental uncertainty on detection performance using vertical array data collected in two experiments. The first experiment took place in shallow water off the Italian west coast by the NATO SACLANT Center in 1993(SACLANT Sonar Data). The second experiment took place in shallow water in China in 2008(LOFAR'08 data). Quantitative effects of various uncertain parameters on detection performance have been illustrated to evaluate which one is the most sensitive and which one is insignificant. The present work is supported by the National Defense Fundamental Fund of China (No.613xxxxx).
IEEE Journal of Oceanic Engineering, 2000
A coastal acoustic tomography (CAT) experiment with seven acoustic stations was successfully carr... more A coastal acoustic tomography (CAT) experiment with seven acoustic stations was successfully carried out to measure multisubtidal internal modes, generated in Hiroshima Bay, Japan, in spite of severe data gaps due to many oyster aquaculture rafts, accompanied by an array of vertical wire strings for growing oysters and distributed widely over the bay. The travel-time data acquired along five transmission lines with better data acquisition rates in one-way or reciprocal directions are analyzed, focusing on subtidal variations of sound speed (proportional to temperature). Significant multisubtidal internal modes are observed in the period range of 1.0-4.0 h. To specify the source region of internal modes and to estimate their propagation speed and direction, the power-spectral and cross-spectral analyses are applied to the time-series data of range-averaged sound speed along the successful transmission lines. Waves of period ranges 3.8-4.2, 2.0-2.3, and 1.4-1.5 h are identified as the second, third, and fourth modes of long internal waves, respectively, and the first internal mode is attributed to the semidiurnal tide. It is suggested that the source region of internal modes is located in the narrow channel at the northeastern corner of the tomography site and the modes radiate out from the source region, satisfying the propagation parameters (period, phase speed, and propagation direction) determined by this study. The propagation parameters of the subtidal internal modes can also be measured by point temperature sensors, attached to aquaculture rafts with an appropriate arrangement. However, it should be noted that the tomographic technique can confirm temperature variations through a spatial average along each ray path.
Geophysical Research Letters, 2005
1] The Kanmon Strait acoustic tomography data acquired at a 5-min interval are assimilated sequen... more 1] The Kanmon Strait acoustic tomography data acquired at a 5-min interval are assimilated sequentially into a 2D ocean model on the basis of the ensemble Kalman filter scheme to image strong tidal current structures occurring in the strait. When the accurate range-averaged currents obtained for the sound transmission lines connecting eight tomography stations are used as assimilation data, the complicated vortex-imbedded currents are imaged with horizontal resolution and accuracy much better than the result of tomographic inversion. The assimilated currents are well compared to the shipboard ADCP data with a RMS difference of about 24 cm s À1 for both the horizontal velocity components. The assimilated volume transport across the strait also shows a good agreement with the transport estimated from the range-averaged current on a pair of transmission lines crossing the strait, making a RMS difference of 3700 m 3 s À1 . Citation: Lin, J., , Accurate imaging and prediction of Kanmon Strait tidal current structures by the coastal acoustic tomography data, Geophys.
The Journal of the Acoustical Society of America, 2006
ABSTRACT
The Journal of the Acoustical Society of America, 2013
Ocean current profiling using ocean acoustic tomography (OAT) was conducted in the Kuroshio Curre... more Ocean current profiling using ocean acoustic tomography (OAT) was conducted in the Kuroshio Current southeast of Taiwan from August 20 to September 15, 2009. Sound pulses were transmitted reciprocally between two acoustic stations placed near the underwater sound channel axis and separated by 48 km. Based on the result of ray simulation, the received signals are divided into multiple ray groups because it is difficult to resolve the ray arrivals for individual rays. The average differential travel times from these ray groups are used to reconstruct the vertical profiles of currents. The currents are estimated with respect to the deepest water layer via two methods: An explicit solution and an inversion with regularization. The strong currents were confined to the upper 200 m and rapidly weakened toward 500 m in depth. Both methods give similar results and are consistent with shipboard acoustic Doppler current profiler results in the upper 150 m. The observed temporal variation demonstrates a similar trend to the prediction from the Hybrid Coordinate Ocean Model.
Journal of Oceanography, 2005
Geophysical Research Letters, 2010
Internal waves are one of the most pronounced oceanic phenomenons to the oceanographer. During pa... more Internal waves are one of the most pronounced oceanic phenomenons to the oceanographer. During past decades much effort has been made to investigate the effect of internal waves on shallow water acoustic propagation. Even though many field observations, such as SWARM '95, have provided fruitful information about the relation between internal waves and acoustic propagation, it is necessary to conduct more numerical simulations due to their extensive feasibility. In this study, the shallow water internal wave environment is constructed by using a non-hydrostatic ocean model, the open boundary forcing is set by considering single or several internal wave modes at the M2 tidal frequency. In order to show the mode coupling caused by the internal wave field more clearly, the acoustic starting field with different single normal modes is adopted. The acoustic simulation can be used to check whether a specific combination of internal wave modes is related to the mode coupling, and which mode pair will be affected. The combination of internal wave modes can be separated into several groups. Even though the internal wave fields are different among every case in each group, the acoustic field structure and the mode coupling are similar. Different acoustic normal mode coupling occurs due to the different combinations of internal wave mode forcing. When the parameters of internal wave mode are modified gently, the acoustic mode coupling becomes quite different. It is interesting and important to investigate the sensitivity of acoustic fields to the variability of the internal mode combination.
Remote Sensing, 2022
The first coastal acoustic tomography (CAT) experiment site of the Neko-Seto Channel was revisite... more The first coastal acoustic tomography (CAT) experiment site of the Neko-Seto Channel was revisited to elucidate the propagation and generation characteristics of the M2 and M4 tidal currents with a second CAT experiment, which was conducted from 3–6 April 2018 (local time). Two-dimensional flow fields of the M2 and M4 tidal currents and the residual current were reconstructed using a coast-fitting inversion model with the reciprocal travel-time data of five acoustic stations. The M2 tidal current is a progressive-type wave that propagates eastward at a speed of 0.7 ms−1, much slower than expected for free progressive tides in this region (19 ms−1). The M4 nonlinear current constructed an out-of-phase relationship between the western and eastern halves of the tomography domain, implying the generation of standing-type waves. Such nonlinear processes led to flood- and ebb-dominant tidal current asymmetries for the western and eastern halves of the model domain, respectively. The two-d...
Coastal Acoustic Tomography, 2020
Abstract The horizontal-slice inversion is formulated for both the cases of grid and function exp... more Abstract The horizontal-slice inversion is formulated for both the cases of grid and function expansion methods. The superiority of tapered least squares method for the Gauss–Markov method is provided. Finally, various methods for validating the inversion results are proposed.
Coastal Acoustic Tomography, 2020
Sensors, 2020
Coast-fitting tomographic inversion that is based on function expansion using three types of norm... more Coast-fitting tomographic inversion that is based on function expansion using three types of normal modes (the Dirichlet, Neumann, and open boundary modes) is proposed to reconstruct current fields from the coastal acoustic tomography (CAT) data. The superiority of the method was validated while using CAT data that were obtained in 2015 in the Dalian Bay. The semidiurnal tidal and residual current fields were accurately reconstructed over the entire model domain surrounded by coasts and open boundaries. The proposed method was effective, particularly around the peripheral regions of the tomography domain and the near-coast regions outside the domain, where accurate results are not expected from the conventional inverse method based on function expansion by Fourier function series with no coast fittings. The error velocity for the semidiurnal tidal currents was 2.2 cm s−1, which was calculated from the root-mean-square-difference between the CAT-observed and inverted range-averaged c...
Acta Oceanologica Sinica, 2014
ABSTRACT Nontidal sea level changes generated in Hiroshima Bay of the Seto-Inland Sea in Japan ar... more ABSTRACT Nontidal sea level changes generated in Hiroshima Bay of the Seto-Inland Sea in Japan are studied over various time scales, from the sub-tidal (2 d to 1 month) to inter-annual scales (>2 years). The total sea level variation produces a standard deviation (STD) of 12.5 cm. The inter-annual component of the sea level variation in Hiroshima Bay oscillates with a STD of 3.4 cm, forming a long-term trend of 4.9 mm/a. The STD of the sea level variation is 9.8 cm for the seasonal component (8 months to 2 years) and 4.7 cm for the intra-seasonal one (1 month to 8 months). Significant sea level variations with a STD of 4.2 cm also occur in the sub-tidal range. Special attention is paid to the sub-tidal sea level changes. It is found that the upwelling and associated transient sea level changes generated along the north coast of Hiroshima Bay (opened southward) by the strong northerly wind, play a significant role in sub-tidal sea level changes. The transient sea level changes are over 10 cm in most cases when caused by typhoons that pass through the Pacific Ocean off the Kii Peninsula, located at about 400 km east of Hiroshima Bay. Reasonable sea level changes are evaluated by the balance of pressure forces at the onshore and offshore boundary of the study domain.
The Journal of the Acoustical Society of America, 2012
ABSTRACT
The Journal of the Acoustical Society of America, Oct 1, 2016
An acoustic reciprocity theorem is generalized, for a smoothly varying perturbed medium, to a hie... more An acoustic reciprocity theorem is generalized, for a smoothly varying perturbed medium, to a hierarchy of reciprocity theorems including higher-order derivatives of acoustic fields. The standard reciprocity theorem is the first member of the hierarchy. It is shown that the conservation of higher-order interaction quantities is related closely to higher-order derivative distributions of perturbed media. Then integral reciprocity theorems are obtained by applying Gauss's divergence theorem, which give explicit integral representations connecting higher-order interactions and higher-order derivative distributions of perturbed media. Some possible applications to an inverse problem are also discussed.
The Journal of the Acoustical Society of America, 2012
ABSTRACT Sonar detection performance is related to ocean environmental parameters, such as the so... more ABSTRACT Sonar detection performance is related to ocean environmental parameters, such as the source position, the ocean depth, the sound speed profile and geoacoustic parameters, etc. These parameters have strong spatial and temporal variability, which result to environmental uncertainty. The sonar detection system can be limited by the presence of environmental uncertainty. Based on a statistical model of the environmental uncertainty, the optimal Bayesian predictor by L. Sha has been applied in this paper to analyze the effects of environmental uncertainty on detection performance using vertical array data collected in two experiments. The first experiment took place in shallow water off the Italian west coast by the NATO SACLANT Center in 1993(SACLANT Sonar Data). The second experiment took place in shallow water in China in 2008(LOFAR'08 data). Quantitative effects of various uncertain parameters on detection performance have been illustrated to evaluate which one is the most sensitive and which one is insignificant. The present work is supported by the National Defense Fundamental Fund of China (No.613xxxxx).
IEEE Journal of Oceanic Engineering, 2000
A coastal acoustic tomography (CAT) experiment with seven acoustic stations was successfully carr... more A coastal acoustic tomography (CAT) experiment with seven acoustic stations was successfully carried out to measure multisubtidal internal modes, generated in Hiroshima Bay, Japan, in spite of severe data gaps due to many oyster aquaculture rafts, accompanied by an array of vertical wire strings for growing oysters and distributed widely over the bay. The travel-time data acquired along five transmission lines with better data acquisition rates in one-way or reciprocal directions are analyzed, focusing on subtidal variations of sound speed (proportional to temperature). Significant multisubtidal internal modes are observed in the period range of 1.0-4.0 h. To specify the source region of internal modes and to estimate their propagation speed and direction, the power-spectral and cross-spectral analyses are applied to the time-series data of range-averaged sound speed along the successful transmission lines. Waves of period ranges 3.8-4.2, 2.0-2.3, and 1.4-1.5 h are identified as the second, third, and fourth modes of long internal waves, respectively, and the first internal mode is attributed to the semidiurnal tide. It is suggested that the source region of internal modes is located in the narrow channel at the northeastern corner of the tomography site and the modes radiate out from the source region, satisfying the propagation parameters (period, phase speed, and propagation direction) determined by this study. The propagation parameters of the subtidal internal modes can also be measured by point temperature sensors, attached to aquaculture rafts with an appropriate arrangement. However, it should be noted that the tomographic technique can confirm temperature variations through a spatial average along each ray path.
Geophysical Research Letters, 2005
1] The Kanmon Strait acoustic tomography data acquired at a 5-min interval are assimilated sequen... more 1] The Kanmon Strait acoustic tomography data acquired at a 5-min interval are assimilated sequentially into a 2D ocean model on the basis of the ensemble Kalman filter scheme to image strong tidal current structures occurring in the strait. When the accurate range-averaged currents obtained for the sound transmission lines connecting eight tomography stations are used as assimilation data, the complicated vortex-imbedded currents are imaged with horizontal resolution and accuracy much better than the result of tomographic inversion. The assimilated currents are well compared to the shipboard ADCP data with a RMS difference of about 24 cm s À1 for both the horizontal velocity components. The assimilated volume transport across the strait also shows a good agreement with the transport estimated from the range-averaged current on a pair of transmission lines crossing the strait, making a RMS difference of 3700 m 3 s À1 . Citation: Lin, J., , Accurate imaging and prediction of Kanmon Strait tidal current structures by the coastal acoustic tomography data, Geophys.
The Journal of the Acoustical Society of America, 2006
ABSTRACT
The Journal of the Acoustical Society of America, 2013
Ocean current profiling using ocean acoustic tomography (OAT) was conducted in the Kuroshio Curre... more Ocean current profiling using ocean acoustic tomography (OAT) was conducted in the Kuroshio Current southeast of Taiwan from August 20 to September 15, 2009. Sound pulses were transmitted reciprocally between two acoustic stations placed near the underwater sound channel axis and separated by 48 km. Based on the result of ray simulation, the received signals are divided into multiple ray groups because it is difficult to resolve the ray arrivals for individual rays. The average differential travel times from these ray groups are used to reconstruct the vertical profiles of currents. The currents are estimated with respect to the deepest water layer via two methods: An explicit solution and an inversion with regularization. The strong currents were confined to the upper 200 m and rapidly weakened toward 500 m in depth. Both methods give similar results and are consistent with shipboard acoustic Doppler current profiler results in the upper 150 m. The observed temporal variation demonstrates a similar trend to the prediction from the Hybrid Coordinate Ocean Model.
Journal of Oceanography, 2005
Geophysical Research Letters, 2010
Internal waves are one of the most pronounced oceanic phenomenons to the oceanographer. During pa... more Internal waves are one of the most pronounced oceanic phenomenons to the oceanographer. During past decades much effort has been made to investigate the effect of internal waves on shallow water acoustic propagation. Even though many field observations, such as SWARM '95, have provided fruitful information about the relation between internal waves and acoustic propagation, it is necessary to conduct more numerical simulations due to their extensive feasibility. In this study, the shallow water internal wave environment is constructed by using a non-hydrostatic ocean model, the open boundary forcing is set by considering single or several internal wave modes at the M2 tidal frequency. In order to show the mode coupling caused by the internal wave field more clearly, the acoustic starting field with different single normal modes is adopted. The acoustic simulation can be used to check whether a specific combination of internal wave modes is related to the mode coupling, and which mode pair will be affected. The combination of internal wave modes can be separated into several groups. Even though the internal wave fields are different among every case in each group, the acoustic field structure and the mode coupling are similar. Different acoustic normal mode coupling occurs due to the different combinations of internal wave mode forcing. When the parameters of internal wave mode are modified gently, the acoustic mode coupling becomes quite different. It is interesting and important to investigate the sensitivity of acoustic fields to the variability of the internal mode combination.