Acoustic Daylight Imaging Research Articles

Towards passive non-line-of-sight acoustic localization around corners using uncontrolled random noise sources

Non-line-of-sight (NLoS) imaging is an important challenge in many fields ranging from autonomous vehicles and smart cities to defense applications. Several recent works in optics and acoustics tackle the challenge of imaging targets hidden from view (e.g. placed around a corner) by measuring time-of-flight information using active SONAR/LiDAR techniques, effectively mapping the Green functions (impulse responses) from several controlled sources to an array of detectors. Here, leveraging passive correlations-based imaging techniques (also termed ’acoustic daylight imaging’), we study the possibility of acoustic NLoS target localization around a corner without the use of controlled active sources. We demonstrate localization and tracking of a human subject hidden around a corner in a reverberating room using Green functions retrieved from correlations of broadband uncontrolled noise sources recorded by multiple detectors. Our results demonstrate that for NLoS localization controlled active sources can be replaced by passive detectors as long as a sufficiently broadband noise is present in the scene.

Global acoustic daylight imaging in a stratified Earth-like model

We present an analysis of acoustic daylight imaging in an Earth-like model assuming a random distribution of noise sources spatially supported in an annulus located away from the surface. We assume a situation with scalar wave propagation and that the measurements are of the wave field at the surface. Then, we obtain a relation between the autocorrelation function of the measurements and the trace of the scattered field generated by an impulsive source localized just below the surface. From this relation it is, for example, clear that the eigenfrequencies can be recovered from the autocorrelation. Moreover, the complete scattering operator can be extracted under the additional assumption that the annulus is close to the surface and has a thickness smaller than the typical wavelength.

Passive imaging of scatterers in an acoustic daylight imaging configuration

The arrival-time structure of Green’s functions is extracted by crosscorrelating one-sided aeroacoustic surf noise in an acoustic daylight imaging configuration, where 14 microphones (separation 0.08-3.7 m) are located between the noise and a cylinder. The cylinder is a hollow PVC pipe (radius 20 cm, height 2 m) 2.5 m landside of a microphone array. The frequency band for computing the cross correlation function is 400-2000 Hz, and total averaging time is Tave = 3 min. Results show that arrivals corresponding to the scattered (travel-time for source-scatterer-receiver path) waves and the time-difference (travel-time difference between two scatterer-receiver paths) waves emerge in the crosscorrelation functions for an acoustic daylight imaging configuration. Only one microphone pair is needed to locate the scatterer passively with the scattered and time-difference traveltime. We show imaging of 20 crosscorrelation functions with clear time-difference and scattered waves. Each estimated location is along th...

Passive acoustic localization techniques of Eastern Pacific grey whales

Eastern Pacific grey whales (Eschrichtius robustus) apparently do not actively echolocate, yet still they thrive in shallow water environments where visibility is much reduced. Along their migration route and in their feeding grounds these whales are exposed to high levels of ambient noise, highlyturbid waters and many underwater obstacles. To test possible passive acoustic localization mechanisms (e.g. Acoustic Daylight Imaging and Passive Synthetic Aperture), we made extensive acoustic measurements during comprehensive field studies of these whales on their summer feeding grounds in British Columbia (Canada). In combination with visual observations of the whales and their behaviours, we investigated the acoustical sources available to the whales when navigating within a feeding bay. First, we measured ambient noise levels to construct the acoustic landscape around the whales. Second, we investigated how sound is altered when objects such as kelp beds and rocks are present. We also measured acoustic changes induced by direct, controlled modifications of the near-shore environment. The aim of this research is to understand how grey whales might be finding their way around, and what impact, if any, increased levels of ambient noise might have on the whales' ability to find food and navigate within the feeding grounds.

Sonar Technology and Shifts in Environmental Ethics

For a philosopher, the history of sonar technology is fascinating. During the first and second World Wars, sonar technology was primarily associated with activity on the part of the sonar technicians and researchers. Usually this activity is concerned with creation of sound waves under water, as in the classic “ping and echo”. The last fifteen years have seen a shift toward passive, ambient noise “acoustic daylight imaging” sonar. Along with this shift a new relationship has begun between sonar technicians and environmental ethics.

周囲雑音を用いた無音物体探知における高分解能アレイ信号処理手法の比較

Much attention has been paid to the new idea of detecting objects by using ocean ambient noise affirmatively. This concept is called acoustic daylight imaging. The authors have tried to detect a silent target object using ambient noise by employing a wide-band beam former consisting of an array of receivers. In experimental results obtained in air, we have applied the delay-sum array (DSA) method and the multiple signal classification (MUSIC) method to successfully detect a silent target object in an acoustic noise field generated by a large number of transducers by using a wide-band beam former. In this paper, we applied the minimum variance (MV) method to detect silent targets as one of the promising high resolution technique of the array signal processing method. The relative power levels near the target obtained with MV and MUSIC were compared with those obtained by DSA. Then, the effectiveness of the MV and MUSIC method was evaluated according to the rate of increase and the direction in the maximum relative power levels.

Applying the Multiple Signal Classification Method to Silent Object Detection Using Ambient Noise

The revolutionary concept of using ocean ambient noise positively to detect objects, called acoustic daylight imaging, has attracted much attention. The authors attempted the detection of a silent target object using ambient noise and a wide-band beam former consisting of an array of receivers. In experimental results obtained in air, using the wide-band beam former, we successfully applied the delay-sum array (DSA) method to detect a silent target object in an acoustic noise field generated by a large number of transducers. This paper reports some experimental results obtained by applying the multiple signal classification (MUSIC) method to a wide-band beam former to detect silent targets. The ocean ambient noise was simulated by transducers decentralized to many points in air. Both MUSIC and DSA detected a spherical target object in the noise field. The relative power levels near the target obtained with MUSIC were compared with those obtained by DSA. Then the effectiveness of the MUSIC method was evaluated according to the rate of increase in the maximum and minimum relative power levels.

Relations between reflection and transmission responses of three-dimensional inhomogeneous media

Relations between reflection and transmission responses of horizontally layered media were formulated by Claerbout in 1968 and by many others. In this paper we derive similar relations for 3-D inhomogeneous media. As the starting point for these derivations, we make use of two types of propagation invariants, based on one-way reciprocity theorems of the convolution type and of the correlation type. We obtain relations between reflection and transmission responses, including their codas, due to internal multiple scattering. These relations can be used for deriving the reflection response from transmission measurements (which is useful for seismic imaging of the subsurface, using passive recordings of noise sources in the subsurface, also known as acoustic daylight imaging) as well as for deriving the transmission coda from the reflection measurements (which is useful for seismic imaging schemes that take internal multiple scattering into account). Furthermore, following the same approach, we obtain mutual relations between reflection responses with and without free-surface multiples. The convolution-type relations are similar to those used by Berkhout and others for surface-related multiple elimination, whereas the correlation-type relations resemble Schuster's relations for seismic interferometry. Last, but not least, we obtain expressions for the reflection response at a boundary below an inhomogeneous medium, which may be useful for imaging the medium ‘from below’. The main text of this paper deals with the acoustic situation; the Appendices provide extensions to the elastodynamic situation.

Effect of Received Beam Width on Passive Detection of Silent Target Using Ambient Noise

Much attention has been paid to the new idea of detecting objects by using ocean ambient noise affirmatively. This concept is called acoustic daylight imaging. The authors have attempted to detect a silent target object using ambient noise by employing a wide-band beamformer consisting with an array of receivers. In the experimental results obtained in air, we applied the delay-sum array method to successfully detect the silent target object in the acoustic noise field generated by many transducers with a line array of receivers. In this paper, some experimental results of the effect of the beam width formed by the wide-band beamformer on target detection are reported. The ocean ambient noise is simulated by transducers decentralized to many points in air. The spherical target object is detected by changing the number of receivers. The beam width is compared with the angle subtended by the spherical target at the center of the receiver array.

Imaging in the ocean with ambient noise: the ORB experiments

Acoustic daylight imaging is a new technique that has been proposed for creating pictorial images of objects in the ocean from the ensonification provided by the incident ambient noise field. To investigate the feasibility of the technique, a series of experiments was performed from the research platform ORB, moored in San Diego Bay, Southern California. Central to these experiments was an acoustic receiver known as ADONIS (acoustic daylight ocean noise imaging system), which consists of a spherical reflector, 3 m in diameter, with an elliptical array of 130 hydrophones at the focal surface. This system, which is broadband, operating between 8 and 80 kHz, forms a total of 126 receive-only beams spanning the vertical and horizontal. The ambient noise power in each beam is mapped into a pixel on a VDU. Various types of targets were used in the experiments, including planar panels and cylindrical, polyethylene drums containing wet sand, seawater or syntactic foam (essentially air), and most of the experiments were conducted with the targets at ranges between 20 and 40 m. At the time of the experiments the noise field in the area was created primarily by snapping shrimp. Moving, color images of the object space were successfully created with ADONIS. Some representative static images from the moving sequences are presented and discussed in the paper.

Acoustic daylight imaging via spectral factorization: Helioseismology and reservoir monitoring

The acoustic time history of the sun’ s surface is a stochastic t x y -cube of information. Helioseismologists cross-correlate these noise traces to produce impulse response seismograms, providing the proof of concept for a long-standing geophysical conjecture. We pack the x y -mesh of time series into a single super-long one-dimensional time series. We apply Kolmogoroff spectral factorization to the super-trace, unpack, and findthe multidimensional acoustic impulse response of the sun. State-of-the-art seismic exploration recording equipment offers tens of thousands of channels, and permanent recording installations are becoming economically realistic. Helioseismology, therefore, provides a conceptual prototype for using natural noises for continuous reservoir monitoring.

Imaging the acoustical sources beneath individual breaking waves

Breaking surface waves produce oscillating bubbles which generate high-frequency acoustic energy in the ocean. An experiment was conducted in May 1996 to examine the spatial and temporal evolution of the acoustically active regions beneath breaking surface waves within the frequency band 8–80 kHz. A multibeam, high-resolution, passive sonar, originally designed for acoustic daylight imaging, was aimed at the surface with a slant range of 45 m. The surface footprint was 9 m in diameter and contained 126 beams, each 0.75 m wide between the −3-dB points at the highest operating frequency. With this resolution, pictorial images have been obtained which show the spatial distribution and the spectral structure of the acoustic sources within individual breaking events. The spectral structure of the sources evolves on a time scale that is at least as fast as 40 ms, which is the temporal resolution of the system. The experimental site was 25 miles from the coast of southern California, with wind speeds rarely exceeding 15 kn. Examples of breaking-wave images will be presented and interpreted in terms of wind speed, wind direction, and horizontal anisotropy in the ambient noise field. [Work supported by ONR.]

THEORY OF ACOUSTIC IMAGING IN THE OCEAN WITH AMBIENT NOISE

Following a recent experiment in which ambient noise (acoustic daylight) was used as the sole source of acoustic illumination for detecting objects in the ocean, a full wave-theoretic analysis is presented in this article of the visibility of a pressure-release spherical target when illuminated by ambient noise showing various degrees of anisotropy. The basis of the analysis is a new, accurate approximation for the Green's function representing the field around the sphere. From this expression, an analysis of the acoustic contrast is developed, as observed at the output of an endfire line array, which constitutes the acoustic lens. Our essential conclusion is that, even in isotropic noise, which presents one of the least favourable conditions for incoherent imaging, the visibility of the sphere (i.e. the ratio of intensities with the sphere present and absent) is approximately 4 dB. This is consistent with our early experimental observations, and is adequate to form the basis of an acoustic daylight imaging system.