MOVING TARGET TRACKING USING TIME REVERSAL METHOD

Time reversal (TR) focusing of ultrasound in granular packings is experimentally investigated. Pulsed elastic waves transmitted from a compressional or shear transducer source are measured by a TR mirror, reversed in time and back-propagated. We find that TR of ballistic coherent waves onto the source position is very robust regardless driving amplitude but provides poor spatial resolution. By contrast, the multiply scattered coda waves offer a finer TR focusing at small amplitude by a lens effect. However, at large amplitude, these TR focusing signals decrease significantly due to the vibration-induced rearrangement of the contact networks, leading to the breakdown of TR invariance. Our observations reveal that granular acoustics is in between particle motion and wave propagation in terms of sensitivity to perturbations. These laboratory experiments are supported by numerical simulations of elastic wave propagation in disordered 2D percolation networks of masses and springs, and should be helpful for source location problems in natural processes.

Corrosion pits on pipelines lead to the formation of small holes, which cause further pipeline damage and even catastrophic consequences. Since many pipelines are located underground, the detection of corrosion pits on pipelines in real time is still an engineering challenge. In this paper, an experimental feasibility study on pipeline corrosion pit detection using the time reversal technique with a piezoceramic transducer as a time reversal mirror was investigated. A specimen of steel pipeline section was fabricated with an artificially drilled hole, which was to mimic a corrosion pit. By gradually increasing the depth of the hole, the evolution of the corrosion pit on the pipeline was simulated and studied. Two piezoceramic transducers were employed to generate a stress wave to propagate along the pipeline and to detect the propagated stress wave. With both the properties of sensing and actuating functions, a piezoceramic transducer was used as a time reversal mirror, which first detected the propagated stress wave signal and then sent ‘back’ the time-reversed signal as a propagating stress wave. With the inherent auto-focusing property of the time reversal technique, the detected time-reversed stress wave had a distinct focused peak. A corrosion pit on a pipeline, as a structural defect, reduces the energy of the focused signal received by the piezoceramic sensor and the attenuation ratio of the focused signal depends strongly on the degree of corrosion depth. Experimental results show that the amplitudes of the focused signal peak decrease with the increase of corrosion pit depth and we can use the peak amplitude of the focused signal to determine the state of pipeline corrosion. The time reversal based method proposed in this paper shows the potential to quantitatively monitor the damage degree of corrosion pits on pipelines in real time.

The concept of time reversal (TR) of a scalar wave is reexamined from basic principles. Five different time-reversal mirrors (TRMs) are introduced and their relations are analyzed. For the boundary behavior, it is shown that for a paraxial wave only the monopole TR scheme satisfies the exact boundary condition while for the spherical wave only the MD-mode TR scheme satisfies the exact boundary condition. The asymptotic analysis of the near-field focusing property is presented for two dimensions and three dimensions. It is shown that to have a subwavelength focal spot, the TRM should consist of dipole transducers. The transverse resolution of the dipole TRM is linearly proportional to the distance between the point source and the TRM. The mixed mode TRM has the similar (linear) behavior in three dimensions, but in two dimensions the transverse resolution behaves as the square root of the distance between the point source and the TRM. The monopole TRM is ineffective in focusing below the wavelength. Contrary to the matched field processing and the phase processor, both of which resemble TR, TR in a weak- or non-scattering medium is usually biased in the longitudinal direction, especially when TR is carried out on a single plane with a finite aperture. This is true for all five TR schemes. On the other hand, the TR focal spot has been shown repeatedly in the literature, both theoretically and experimentally, to be centered at the source point when the medium is multiple scattering. A reconciliation of the two seemingly conflicting results is found in the random fluctuations in the intensity of the Green function for a multiple scattering medium and the notion of scattering-enlarged effective aperture.

Time reversal (TR) is the process of generating a spatio-temporal focus at a probe source (PS) location by backpropagating a time-reversed version of a received signal. While TR focusing requires the PS for a coherent acoustic focus at its origin, the requirement of the PS has been partially relaxed by the concept of a virtual source array (VSA) [J. Acoust. Soc. Am. 125, 3828–3834 (2009)]. A VSA can serve as a remote platform or lens and redirect a focused field to a selected location beyond the VSA for which the field is assumed as a homogeneous medium with constant sound speed. The objective of this study is to extend VSA-based single TR focusing to simultaneous multiple focusing. This is achieved using the optimization theory by employing the multiple constraints method derived from a constraint matrix, which consists of appropriately synchronized transfer functions. It is found that simultaneous multiple focusing can be achieved with distortionless response at selected multiple locations, and its performance degrades in the presence of sound speed mismatch. Possible applications for the VSA-based TR focusing are discussed, and numerical simulation results are presentedTime reversal (TR) is the process of generating a spatio-temporal focus at a probe source (PS) location by backpropagating a time-reversed version of a received signal. While TR focusing requires the PS for a coherent acoustic focus at its origin, the requirement of the PS has been partially relaxed by the concept of a virtual source array (VSA) [J. Acoust. Soc. Am. 125, 3828–3834 (2009)]. A VSA can serve as a remote platform or lens and redirect a focused field to a selected location beyond the VSA for which the field is assumed as a homogeneous medium with constant sound speed. The objective of this study is to extend VSA-based single TR focusing to simultaneous multiple focusing. This is achieved using the optimization theory by employing the multiple constraints method derived from a constraint matrix, which consists of appropriately synchronized transfer functions. It is found that simultaneous multiple focusing can be achieved with distortionless response at selected multiple locations, and its perf...

When a time reversal mirror (TRM) is laid outside a slotted rectangle metal cavity, the focusing characteristics of time-reversed electromagnetic (EM) wave in the cavity are first studied in this paper. The time reversal (TR) focusing and pseudo focusing occur simultaneously in the initial symmetrical environment. By means of embedding the scatters and adjusting the slot position, two effective ways to eliminate the pseudo focusing are proposed. This research may have great benefits to the source imaging in a quasi-closed environment.

In recent years, time-reversal (TR) mirrors have been developed that create TR waves for ultrasonic transient fields propagating through complex media. A TR wave back propagates and refocuses exactly at its initial source. However, because of diffraction, even if the source is pointlike the wave refocuses on a spot size that cannot be smaller than half a wavelength. Here, by using a TR interpretation of this limit, we show that this latter limitation can be overcome if the source is replaced by its TR image. This new device acts as an acoustic sink that absorbs the TR wave. Here we report the first experimental result obtained with an acoustic sink where a focal spot size of less than 1/14th of one wavelength is recorded.

In order to image a source or a scatterer embedded in a three dimensional solid, acoustic/elastic wave data from an actual experiment are time reversed and backpropagated through a numerical model of the medium. The model makes use of estimates for the elastic constants of the laboratory solid. These estimates may not be very precise, for example, due to experimental uncertainties. Poor characterization of the medium leads to the degradation of the time reversal focus, therefore, to poor medium imaging. In this work, we report on the results of investigating the time reversal focus degradation as the estimates depart from the real values. Very small deviations from the medium’s actual elastic constants degrade the time reversal focus dramatically. However, decreasing the total duration of the signals used for time reversal can attenuate the degradation in some cases. We propose a new method to compensate for the deviations of the model medium’s elastic constants from the actual values. Finally, we explore the effects of scatterers that may exist in the laboratory medium, but are not included in the model medium, and show that their presence does not produce significant effects on the time reversal focus.

Time reversal (TR) is the process of generating a spatio-temporal focus at a probe source (PS) location by transmitting a time-reversed version of a received signal. While TR focusing requires the PS for a coherent acoustic focus at its origin, the requirement of the PS has been partially relaxed by the introduction of the concept of a virtual source array (VSA) (J. Acoust. Soc. Am. 2009, 125, 3828–3834). A VSA can serve as a remote platform or lens and redirect a focused field to a selected location beyond the VSA for which the field is assumed as a homogeneous medium with constant sound speed. The objective of this study is to extend VSA-based single TR focusing to simultaneous multiple focusing. This is achieved using the optimization theory by employing the multiple constraints method derived from a constraint matrix, which consists of appropriately synchronized transfer functions. Through numerical simulations, it is found that simultaneous multiple focusing can be achieved with distortionless response at selected multiple locations, and its performance degrades in the presence of sound speed mismatch. For achieving robust multiple focusing in the mismatch environment, singular value decomposition is applied to obtain the weight vector (i.e., backpropagation vector) that best approximates the column vectors of the constraint matrix. Numerical simulation results show that VSA-based multiple TR focusing using SVD is not a method to simultaneously focus on multiple locations, but a method of constructing a field which robustly passes through multiple locations in sound speed mismatch environment.

In this study, time reversal (TR) signal processing methods are used to create a focus of airborne ultrasound (between approximately 30 kHz and 50 kHz) in a room. There are unique challenges presented by ultrasonic sources, such as the highly directional nature of the sources, and attenuation of ultrasound in air. The aim of this study is to create a TR focus whose amplitude is as high as possible using the clipping processing method. Because ultrasonic sources tend to be very directional, this study explores the impact of using methods to make the sources more omnidirectional, which increases the number of reflective paths that contribute to the TR focus. This study explores various configurations of source and microphone in the room to determine which configuration yields the highest amplitude. Because thermoviscous loses for ultrasound are higher than for audible frequencies, a smaller room is used in the experiments to increase focal amplitudes.In this study, time reversal (TR) signal processing methods are used to create a focus of airborne ultrasound (between approximately 30 kHz and 50 kHz) in a room. There are unique challenges presented by ultrasonic sources, such as the highly directional nature of the sources, and attenuation of ultrasound in air. The aim of this study is to create a TR focus whose amplitude is as high as possible using the clipping processing method. Because ultrasonic sources tend to be very directional, this study explores the impact of using methods to make the sources more omnidirectional, which increases the number of reflective paths that contribute to the TR focus. This study explores various configurations of source and microphone in the room to determine which configuration yields the highest amplitude. Because thermoviscous loses for ultrasound are higher than for audible frequencies, a smaller room is used in the experiments to increase focal amplitudes.

The focusing resolution effects of time reversal mirror (TRM) are studied based on time-reversal focusing theory. Using the coherence of multi-path the effective aperture of TRM is enlarged and focusing gain can be obtained more than 10dB. Focusing resolution with a TRM through random medium is much better than that obtained in homogeneous medium.

Time reversal acoustics (TRA) techniques provide a means to focus energy at a localized time and space, or at an unknown location (i.e., source or scatterer not known a priori) without the need to calculate time delays, as is necessary for phased arrays. The experiments demonstrate that time reversal focusing provides localized surface vibration with amplitude sufficient to induce measurable nonlinear effects and does so over a wide frequency band. Because the amplitude is spatially focused, time reversal enables local probing of this nonlinearity that can be used for nonlinear imaging. The nonlinear effects for wide band frequency signals were measured by phase-inversion method combining two signals of opposite sign in order to cancel the linear response and preserve quadratic nonlinearities. The method works effectively for nonlinear NDE of thin plates where laser Doppler Vibrometer can be used for measurements of the surface vibration and TRA focusing of acoustic waves. Presented results include crack and delamination diagnostic is epoxy graphite composites, diffusion bonded metallic disks and impact surface damage in glass. Scanning electron microscopy and optical images are also shown to demonstrate the types of damage features detected using this method.

Self-focusing and time recompression of Lamb waves using a time reversal mirror

In this paper, we present a novel and completely different approach to focusing on defects beneath plane or curved surfaces: the time reversal mirror method. The time reversal technique is based on the concept of time reversal of ultrasonic fields and takes into account both the phase and modulus information coming from the defect. This technique is self-adaptative and requires only the presence of a target in the solid sample. In highly scattering media, it is shown that the time reversal process allows a new approach to speckle noise reduction. Experimental results obtained with a 121-channel time reversal mirror on titanium and duralumin samples are presented. They demonstrate the ability of time reversal to compensate for the distortions induced by liquid-solid interfaces of different geometries and to detect small defects in a noisy background.

Time-reversal mirrors (TRMs) refocus an incident wavefield to the position of the original source regardless of the complexity of the propagation medium. TRMs have now been implemented in a variety of physical scenarios from GHz microwaves to MHz ultrasonics and to hundreds of Hz in ocean acoustics. Common to this broad range of scales is a remarkable robustness exemplified by observations at all scales that the more complex the medium (random or chaotic), the sharper the focus. A TRM acts as an antenna that uses complex environments to appear wider than it is, resulting for a broadband pulse, in a refocusing quality that does not depend on the TRM aperture. Moreover, when the complex environment is located in the near field of the source, time-reversal focusing opens completely new approaches to super-resolution. We will show that, for a broadband source located inside a random metamaterial, a TRM located in the far field radiated a time-reversed wave that interacts with the random medium to regenerate not only the propagating but also the evanescent waves required to refocus below the diffraction limit. This focusing process is very different from that developed with superlenses made of negative index material only valid for narrowband signals. We will emphasize the role of the frequency diversity in time-reversal focusing.

Minimum variance distortionless response (MVDR) algorithm has broken through the Rayleigh limit restrictions and makes higher resolutions possible. However, they are easily affected by complex underwater acoustic environments and are not sufficiently robust, so it is difficult to use them in real underwater systems. Time reversal mirror technology is a new method of localization, which may be effectively applied to target detection and localization. To improve its robustness, the MVDR algorithm is combined with time reversal mirroring for passive detection and location. This method makes full use of underwater multipath channel information and reduces the effects of complex underwater environments. The performance of time-reversal MVDR that is based on fixed diagonal loading and robust capon beamforming (RCB) was also studied. Simulations, water tank experiment and sea trial proved it is feasible and practical Keywords: MVDR; time reversal mirror; robust capon beamforming; passive location