Airborne Interferometry for Underwater Sound Source Triangulation
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Solution Overview
Problem
Existing interferometry techniques struggle to distinguish between acoustically-induced motions and gravity-capillary waves on water surfaces, as conventional assumptions treat the water surface as rigid to acoustic excitation, failing to account for microscopic movements caused by sound waves.
Innovation Solution
The system employs wide-field imaging interferometry to exploit the dispersion relationships between acoustic and gravity-capillary waves, using laser interferometry to detect microscopic surface movements induced by sound waves, separating them from the larger amplitude gravity-capillary waves by filtering out slow-moving components and leveraging the differences in frequency-velocity dispersion.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional interferometry techniques are used to detect water surface movements, then the detection capability is limited by the assumption of rigid water surface, but the system cannot distinguish between acoustically-induced motions and gravity-capillary waves
Solution Approach 1:
The system changes the detection parameters by measuring both the amplitude and phase of surface movements, and by analyzing the frequency spectrum. This allows differentiation between acoustic waves (higher frequency, specific amplitude-phase relationship) and gravity-capillary waves (lower frequency, different dispersion characteristics), resolving the inability to distinguish wave types while maintaining detection precision
Solution Approach 2:
The system transitions from static surface assumption to dynamic surface measurement by capturing time-varying surface movements. By measuring the temporal evolution of surface displacement and using the known dispersion relationships of different wave types, the system can dynamically separate acoustic signals from background wave noise
2Device complexity
If the water surface is treated as rigid to acoustic excitation, then the measurement system is simplified, but the system fails to detect microscopic surface movements caused by sound waves
Solution Approach 1:
The system replaces direct mechanical contact measurement with optical interferometry. This non-contact method uses light wave interference to detect microscopic surface displacements (on the order of nanometers) caused by acoustic waves, achieving high measurement precision without the complexity of mechanical contact sensors that would be required to detect such small movements
3Measurement precision
If laser interferometry is used to detect microscopic surface movements, then acoustically-induced motions can be detected, but the detection is overwhelmed by larger amplitude gravity-capillary waves
Solution Approach 1:
The system extracts the acoustic signal from the composite surface movement by utilizing the different dispersion characteristics of acoustic waves and gravity-capillary waves. By analyzing the frequency spectrum and applying the known dispersion relationships, the system separates and extracts the acoustic component from the overwhelming gravity-capillary wave background, enabling detection of microscopic acoustic-induced movements
Solution Approach 2:
The system uses the dispersion relationship as an intermediary filter. By measuring both amplitude and phase and comparing against the theoretical dispersion characteristics of different wave types, the system acts as a selective mediator that passes acoustic signals while rejecting gravity-capillary wave interference, resolving the harmful effect of background waves
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
Enables the detection and location of underwater sound sources by creating a 'movie' of sound waves on the water surface, effectively disregarding gravity-capillary waves and isolating acoustically-driven surface motions, allowing for precise triangulation of the sound source using back propagation techniques.
Implementation Method 1
the two images are coherently superposed. The lateral displacement is called the shear of the images. The superposition of the two images is called a shearogram, which is an interferogram of an object wave with the sheared surface wave as a reference wave
Implementation Method 2
Bodies of water such as lakes and oceans respond to physical disturbances with a variety of motions, including surface waves, currents, and acoustic waves
Implementation Method 3
the dominant assumption is that the surface is rigid with respect to acoustic excitation—the approximate 'pressure release' condition
Implementation Method 4
The physics of acoustic waves, and of water-surface gravity-capillary waves, are widely exploited in underwater communications and marine engineering
Data Source
AI summary
An interferometry system and method thereof detects movements of the surface of a body of water in response to acoustic waves generated from a sub-surface source interacting with the surface. Movements of the surface of the body of water are viewed over multiple interferometric images that can be pieced together to generate an interferometric movie or video. The interferometric movie or video depicts the movement of the acoustic wave propagating through the viewing area. Once the movement of the acoustic wave propagating through the viewing area is known, then back propagation techniques are employed to determine or triangulate the location of the sub-surface source that generated the acoustic wave.