해양 환경은 양상태 능동 소나 운용에 지배적인 영향을 미치는 요소이며 특히, 해양 경계면과 체적 내에 존재하는 산란체들에 의한 잔향음은 표적 신호의 식별을 저해하는 주된 요인 중 하나이다. 능동 소나의 효율적인 운용과 정확한 예측을 위해서는 잔향음 신호 발생 ...

해양 환경은 양상태 능동 소나 운용에 지배적인 영향을 미치는 요소이며 특히, 해양 경계면과 체적 내에 존재하는 산란체들에 의한 잔향음은 표적 신호의 식별을 저해하는 주된 요인 중 하나이다. 능동 소나의 효율적인 운용과 정확한 예측을 위해서는 잔향음 신호 발생에 영향을 미치는 음 전달 및 산란 기작에 정확한 이해뿐 아니라 시스템 인자들에 대한 포괄적인 이해가 요구된다. 본 논문에서는 이러한 잔향음 신호의 예측을 위해 천해 환경용 고주파 양상태 잔향음 모델을 개발하였다. 양상태 잔향음 모델의 음 전달을 모의 하기 위해서는 고주파 대역 (1~100 kHz) 에서 정상 모드 이론이나 포물선 방정식에 비해 사용이 용이한 음선 이론을 사용하였다. 잔향음 모델의 정확도를 좌우하는 중요한 요소인 산란 모델은 크게 경험식과 이론식으로 나뉜다. 경험식의 경우, 많은 실험을 통해 정리 되었으며 이론식은 경계면을 이해하는 방식(고체/유체 탄성체, small/large 거칠기)에 따라 다양한 접근이 가능하다. 본 논문에서는 1)경험식(Empirical formula), 2)APL-UW (Applied physics laboratory of the University of Washington) 3) SSA (Small slope approximation) 등 기존의 3가지 산란 모델을 살펴 보았다. 하지만 이러한 모델들은 실험 자료의 부족과 이론적 제한점 등으로 인해 5~10 kHz 사이의 중고주파 대역에서 한계를 가지고 있다. 본 논문에서는 이를 보완하고자 경험적 산란식과 APL-UW 산란 모델을 결합하고 해수면 근처의 파도에 의한 공기 방울층의 영향을 고려한 hybrid MID-FREQ (mid-frequency) 산란 모델을 제안하였다. 해수 중 음속 구조는 미세 공기 방울에 의해 매우 민감하지만 기존의 산란 모델들은 공기 방울들을 음속 구조의 변화 요인으로써가 아닌 산란체로만 이해함으로써 미세 공기 방울에 변화에 민감한 음속 구조의 변동성을 간과 하였다. 공기 방울층 내에서의 음선 구조 변화는 경계면에 도달하는 고유 음선의 수와 산란 강도를 변화 시키며 결과적으로 잔향음 신호의 변동성에 영향을 미치게 된다.
개발된 고주파 양상태 잔향음 모델은 거리 종속 환경 하에서의 다양한 환경 변수 (해저면 구성 물질, 해상 상태, 음속 구조 및 해저면 기울기 등) 및 시스템 변수 (펄스 길이, 송수신기의 위치 등)의 변화에 따른 모의를 통해 유효성을 검증하였다.
모델의 검증을 위해서는 거리 종속 환경 하에서 선저 부착형 소나를 이용하여 측정된 실측 신호 (6 kHz)가 사용되었으며 결과적으로 제안된 hybrid MID-FREQ 모델이 가장 낮은 수평 입사각이 우세한 실측 신호와 가장 유사한 결과를 보였다.
본 논문에서는 (1) 양상태 잔향음 모의 알고리즘 (2) 양상태 산란 강도 모델 (3) 잔향음 모델 검증 (4) 실측 자료와 모델 자료 비교를 수행한다.

The performance of bistatic active sonar is profoundly affected by the ocean environment in which it operates. The boundary interactions and volume scatterers give rise to reverberant returns that typically exceed ambient noise and may mask target ech ...

The performance of bistatic active sonar is profoundly affected by the ocean environment in which it operates. The boundary interactions and volume scatterers give rise to reverberant returns that typically exceed ambient noise and may mask target echoes at a receiver. Thus, to accurately predict the active sonar performance in real ocean environments, it is necessary to comprehensively account not only for system parameters but also for the complex processes of acoustic propagation and scattering.
This dissertation describes reverberation model for shallow-water environments which is designed for bistatic geometry with high-frequency. The ray approach is used as an acoustic propagation model for high-frequency reverberation modeling. Ray-based reverberation modeling is more practical than normal mode theory and parabolic equation for high-frequency band. High-frequency is identified as 1~100 kHz band in this study.
Scattering strength models are handled as main issues of reverberation modeling, which are divided into empirical and theoretical model. Many researchers have measured scattering strengths as a function of grazing angle, which were fitted by the empirical expressions (e.g. Chapman-Harris formula, Lambert’s law). Theoretical model is separated by originality of physical approach of boundaries (e.g. solid/fluid elastic medium, small/large roughness). Three different bistatic scattering strength models are reviewed to propose new scattering strength model: 1) Empirical formula, 2) APL-UW (Applied physics laboratory of the University of Washington) and 3) SSA (Small slope approximation). These models have ambiguity between 5 and 10 kHz because of the limitation of theoretical approximation and short of measured data. Therefore, we suggest hybrid MID-FREQ (mid-frequency) scattering strength model which is useful for mid-high-frequency (5~10 kHz) bistatic reverberation model. To characterize for mid-high-frequency band, empirical formula and APL-UW model are combined. In addition to, this model includes the effect of bubble layer at near surface which is caused by wave breaking. In existing models, bubble layer has been treated as not causes of variation of sound speed but scatterers. But the speed of sound in water is sensitive to the inclusion of a small fraction of air. And ray-paths are also affected by the variation of sound speed within bubble layer. The fluctuation of ray-path effects to the change of scattering strength and number of eigen ray at boundaries. In consequence, these changes cause the fluctuation of reverberation signals.
In order to validate the model, we have simulated bistatic reverberation with various environmental parameters (bottom type, wind speed, sound speed and bathymetry) and system parameters (pulse length, geometry of source and receiver) for range independent and dependent environments.
Measured and predicted reverberation signals are compared to verify the reverberation model. The measured reverberation signals (6 kHz) were obtained using hull-mounted sonar in range-dependent environments. As a result, the proposed hybrid MID-FREQ scattering strength model for low grazing angle is turned to be a suitable means to generate reverberation signals in mid-high-frequency band.
This dissertation includes (1) overall description of the algorithm for bistatic reverberation modeling, (2) theoretical review on bistatic scattering strength models, (3) the validation of reverberation modeling and (4) model and data comparison.

The performance of bistatic active sonar is profoundly affected by the ocean environment in which it operates. The boundary interactions and volume scatterers give rise to reverberant returns that typically exceed ambient noise and may mask target ech ...

The performance of bistatic active sonar is profoundly affected by the ocean environment in which it operates. The boundary interactions and volume scatterers give rise to reverberant returns that typically exceed ambient noise and may mask target echoes at a receiver. Thus, to accurately predict the active sonar performance in real ocean environments, it is necessary to comprehensively account not only for system parameters but also for the complex processes of acoustic propagation and scattering.
This dissertation describes reverberation model for shallow-water environments which is designed for bistatic geometry with high-frequency. The ray approach is used as an acoustic propagation model for high-frequency reverberation modeling. Ray-based reverberation modeling is more practical than normal mode theory and parabolic equation for high-frequency band. High-frequency is identified as 1~100 kHz band in this study.
Scattering strength models are handled as main issues of reverberation modeling, which are divided into empirical and theoretical model. Many researchers have measured scattering strengths as a function of grazing angle, which were fitted by the empirical expressions (e.g. Chapman-Harris formula, Lambert’s law). Theoretical model is separated by originality of physical approach of boundaries (e.g. solid/fluid elastic medium, small/large roughness). Three different bistatic scattering strength models are reviewed to propose new scattering strength model: 1) Empirical formula, 2) APL-UW (Applied physics laboratory of the University of Washington) and 3) SSA (Small slope approximation). These models have ambiguity between 5 and 10 kHz because of the limitation of theoretical approximation and short of measured data. Therefore, we suggest hybrid MID-FREQ (mid-frequency) scattering strength model which is useful for mid-high-frequency (5~10 kHz) bistatic reverberation model. To characterize for mid-high-frequency band, empirical formula and APL-UW model are combined. In addition to, this model includes the effect of bubble layer at near surface which is caused by wave breaking. In existing models, bubble layer has been treated as not causes of variation of sound speed but scatterers. But the speed of sound in water is sensitive to the inclusion of a small fraction of air. And ray-paths are also affected by the variation of sound speed within bubble layer. The fluctuation of ray-path effects to the change of scattering strength and number of eigen ray at boundaries. In consequence, these changes cause the fluctuation of reverberation signals.
In order to validate the model, we have simulated bistatic reverberation with various environmental parameters (bottom type, wind speed, sound speed and bathymetry) and system parameters (pulse length, geometry of source and receiver) for range independent and dependent environments.
Measured and predicted reverberation signals are compared to verify the reverberation model. The measured reverberation signals (6 kHz) were obtained using hull-mounted sonar in range-dependent environments. As a result, the proposed hybrid MID-FREQ scattering strength model for low grazing angle is turned to be a suitable means to generate reverberation signals in mid-high-frequency band.
This dissertation includes (1) overall description of the algorithm for bistatic reverberation modeling, (2) theoretical review on bistatic scattering strength models, (3) the validation of reverberation modeling and (4) model and data comparison.

고주파용 양상태 잔향음 모델을 개발하였다.
민수용, 군수용 양상태 소나 활용 및 개발에 활용이 가능하다.

고주파용 양상태 잔향음 모델을 개발하였다.
민수용, 군수용 양상태 소나 활용 및 개발에 활용이 가능하다.