Effects Of Source Motion Research Articles

Modeling the seismic wavefield of moving marine vibrator source

Marine vibrators are an emerging alternative to conventional air guns in ocean-bottom acquisition due to their ability to generate low-frequency waves and limited adverse impact on marine wildlife. However, using marine vibrators introduces challenges not found in conventional air-gun-based acquisition where receivers are placed on the ocean bottom, including handling the phenomena associated with the Doppler effect and time-dependent source-receiver offsets due to the source motion. Standard seismic data processing solutions assume stationary sources and receivers for the duration of any seismic experiment (e.g., a shot); however, accurately accounting for source motion effects in processing is critical for optimal subsurface imaging. To address these challenges, we develop a finite-difference approach for modeling the full acoustic wavefield in a generalized coordinate system that tracks a moving source. Synthetic examples demonstrate that this technique accurately and stably models high-velocity moving sources and accurately accounts for the wavefield distortion predicted by the Doppler effect. This approach is not limited to modeling wavefield propagation for a moving source and can be used to develop advanced imaging and inversion techniques (e.g., reverse time migration and full waveform inversion) for data acquired using marine vibrators. In addition, the developed approach is not only limited to modeling mobile sources but also can model conventional towed-streamer data.

Inversion of range-dependent ocean environmental parameters using radiation noises of an Autonomous Underwater Vehicle

Moving platform is playing an increasingly important role in ocean observatories. We previously reported some field testing results of using the radiation noises of an Autonomous Underwater Vehicle (AUV) as the sound source to invert the range-independent SSP. To incorporate source motion effects, the forward model based on the waveguide Doppler and normal mode theory was applied to compute the replica field, and to resolve the adjacent Doppler shifted frequencies, an analytical solution of the forward model and its simplified version were obtained for arbitrary signal integration intervals with a monochromatic source. In this paper, we reformulate the matched-field inversion problem into a state-space model to track the range-dependent environmental parameters and moving source parameters along the AUV path with the constantly updated measurement equations. Performances of several sequential filters, including extended Kalman, unscented Kalman, and particle filters, are compared. Note that at each point of the track, the estimate is the average from the source to the receiver; by doing the inversion sequentially with AUV moving, one can get the parameter estimate at individual points (sections). This process is equivalent to iteration over space, while iteration over time is considered as a solution to an under-determined inverse problem.Moving platform is playing an increasingly important role in ocean observatories. We previously reported some field testing results of using the radiation noises of an Autonomous Underwater Vehicle (AUV) as the sound source to invert the range-independent SSP. To incorporate source motion effects, the forward model based on the waveguide Doppler and normal mode theory was applied to compute the replica field, and to resolve the adjacent Doppler shifted frequencies, an analytical solution of the forward model and its simplified version were obtained for arbitrary signal integration intervals with a monochromatic source. In this paper, we reformulate the matched-field inversion problem into a state-space model to track the range-dependent environmental parameters and moving source parameters along the AUV path with the constantly updated measurement equations. Performances of several sequential filters, including extended Kalman, unscented Kalman, and particle filters, are compared. Note that at each point ...

Influence of source motion, wind, and temperature profiles on the effective impedance of an absorptive surface

This paper presents a theoretical study of the sound field due to a moving source placed above an absorptive surface in a stratified medium in the presence of wind and sound speed profiles. The current analysis starts with the general equations for an isentropic inviscid flow field. A monopole source is assumed to be traveling parallel to the absorptive surface at a constant speed. The standard Lorentz transform can be applied and the Fourier decomposition can be used to express the sound field in an integral form that is amenable to further mathematical treatments. Either the fast field computation or the method of steepest descent can be used to evaluate the integral. A two-dimensional formulation was considered initially, but it can subsequently be extended to three-dimensional flow fields. The effect of source motion, wind, and temperature profiles on the acoustic properties of the absorptive surfaces have been explored.

Accurate fast field formulation for a uniformly moving source traveling above an impedance plane

The Lorentz transform, which can be applied to a uniformly moving source, effectively converts the time-domain wave equation to an equivalent problem in frequency-domain due to a stationary source. Standard fast field program (FFP) implementations apply the property of conjugate symmetry in the wavenumber domain for improved efficiency. Recent literature [D. Dragna et al., AIAA 52, 1928–1939 (2014)] suggests the use of a Dopplerized frequency-dependent impedance model to account for the effects of source motion. Consequently, the FFP kernel function is no longer identical for the positive and negative wavenumbers. Additional complications are introduced by the necessity to compute the positive and negative horizontal separation distances in the Lorentz frame to obtain a complete time history of sound pressures for the source approaching and receding from the receiver. Further development of the FFP algorithm in the Lorentz frame is explored such that a frequency-dependent impedance model is developed. Both moving line and point monopole sources are considered in the current investigation. Results are validated against direct numerical integration schemes and with the published time-domain solutions. Applications for an aircraft operating in cruise condition is also examined in the present study. [Work Sponsored by the Federal Aviation Administration.]

The effects of source motion and bottom bathymetry on temporal coherence in shallow water propagation

Previous studies on coherence have been focused on the effects of water column fluctuations on temporal coherence with a stationary source/receiver setup. However, with focus being turned to moving source setups there has been documented a significant drop in temporal coherence. With a moving platform, the propagation path will change based on relative source/ receiver position, and hence the bathymetry along the path will vary. Here, we will examine the effects that this bathymetric variation and related ship speed contribute to coherence loss. A range dependent parabolic equation model will be used to predict the temporal coherence for individual mode arrivals. A slowly varying random bottom will be introduced to the model and the coherence calculated for different ship speeds and for both radial and tangential tracks. Results will be compared with stationary source/receiver setups in order to determine at what ship speed/ bottom bathymetry does source motion become the driving factor in loss of coherence versus water column fluctuations from a stationary setup. Preliminary results indicate that at a speed of 2 knots, there is remarked loss of coherence at all modes except the first with even small variations in bottom bathymetry, which is in agreement with experimental results.

Performance of active noise barrier with a moving sound source

Active noise control is an effective technology to improve noise reduction performance of traditional passive noise barriers at low-frequencies. Previous researches on active noise barriers are mainly on the assumption that the primary sound source is stationary. Effects of source motion on the performance of an active noise barrier are investigated in this paper. First, an analytical model of a passive barrier with a moving sound source is introduced, which can be used to calculate the primary sound field distribution in time domain. Then, the performance of applying active noise control on such a barrier is investigated numerically. Finally, the experimental results with a practical prototype of active noise barrier will be reported and be compared with the numerical results.

Effect of source-motion and self-motion on the resetting of auditory scene analysis

Auditory scene analysis needs to parse the incoming flow of acoustic information into perceptual streams, such as distinct musical melodies or sentences from a single talker. Previous studies have demonstrated that the formation of auditory streams is not instantaneous: rather, streaming builds up over time and can be reset by sudden changes in the acoustics of the scene. The present study examined the effect of changes induced by voluntary head motion on streaming. A telepresence robot in a virtual-reality setup was used to disentangle all potential consequences of head motion: changes in acoustic cues at the ears, changes in apparent sound location, and changes in motor processes. The results showed that self-motion induced resetting of auditory streaming. An additive model analysis further revealed that resetting was largely influenced by acoustic cues and apparent sound location rather than by non-auditory factors related to head motion. Thus, low-level changes in sensory cues can affect perceptual organization, even though those changes are fully accounted for by self-motion of the listener. It is suggested that our results reflect a widely distributed neural architecture for the formation of auditory streams.

The effect of source motion on contaminant distribution in the cleanrooms

In the recent decades, cleanrooms have found growing applications in broad range of industries such as pharmacy and microelectronics. Concerns about negative effects of the contaminant exposure on the human health and product quality motivate many researchers towards understanding of the airflow and contaminant distribution though these environments. With an improvement in computational capacity of the computers, computational fluid dynamics (CFD) technique has become a powerful tool to study the engineering problems including indoor air quality (IAQ). In this research, indoor airflow in a full-scale cleanroom is investigated numerically using Eulerian–Eulerian approach. To evaluate the ventilation system effectiveness, a new index, called final efficiency, is introduced which takes all aspects of the problem into account. The results show that the contaminant source motion and its path have a great influence on the contaminant dispersion through the room. Based on the results, the contaminant distribution indexes, e.g. final efficiency and spreading radius, are improved when the source motion path is in the dominant direction of the ventilation airflow. Consequently, the efficiency of an air distribution system which provides a directional airflow pattern shows the least source path dependency. This study and its results may be useful to gain better understanding of the source motion effects on the indoor air quality (IAQ) and to design more effective ventilation systems.

Realistic modeling of adaptive beamformer performance in nonstationary noise

Most adaptive beamformers (ABFs) operate under the assumption that the noise field is quasistationary. They estimate the present noise field by averaging, assuming stationarity over the estimation time. The adaptive beamformer responds to slow changes in the noise field across multiple estimation intervals. Unfortunately, in many low-frequency underwater sound applications, the shipping noise may change rapidly, due to nearby ship motion. This motion can be significant during the estimation interval and degrade ABF performance. A realistic model has been developed, including two effects of source motion on horizontal towed arrays. Bearing rate produces a differential Doppler shift across the array. Range rate produces an amplitude modulation as the multipath interference pattern shifts along the array. The ABF model begins with a realization of ship locations and motion based on historical shipping density. Each ship generates realistic random noise composed of tonals in a broadband background. The noise is propagated from each ship to each hydrophone by a normal mode model. For each time sample the position of each ship is updated and propagation recalculated. The ability of a variety of ABF algorithms to reduce shipping noise clutter is simulated and compared.

Motion compensation for adaptive horizontal line array processing.

Large aperture horizontal line arrays have small resolution cells and can be used to separate a target signal from an interference signal by array beamforming. High-resolution adaptive array processing can be used to place a null at the interference signal so that the array gain can be much higher than that of conventional beamforming. But these nice features are significantly degraded by the source motion, which reduces the time period under which the environment can be considered stationary from the array processing point of view. For adaptive array processing, a large number of data samples are generally required to minimize the variance of the cross-spectral density, or the covariance matrix, between the array elements. For a moving source and interference, the penalty of integrating over a large number of samples is the spread of signal and interference energy to more than one or two eigenvalues. The signal and interference are no longer clearly identified by the eigenvectors and, consequently, the ability to suppress the interference suffers. We show in this paper that the effect of source motion can be compensated for the (signal) beam covariance matrix, thus allowing integration over a large number of data samples without loss in the signal beam power. We employ an equivalent of a rotating coordinate frame to track the signal bearing change and use the waveguide invariant theory to compensate the signal range change by frequency shifting.

Temporal resolutions of time-reversal and passive-phase conjugation for underwater acoustic communications. II. The effect of source motion

Multipath arrivals cause interference between transmitted symbols and hence, symbol errors in underwater acoustic communications. A time-reversal mirror uses the ocean to combine the multipath arrivals and can be used for underwater acoustic communications. Passive-phase conjugation uses the received data of a probe signal to deconvolve the channel transfer function and thereby removes the multipath effects. In this paper, we study the following temporal resolutions of a time-reversal or passive-phase conjugation process as applied to underwater acoustic communications: (1) the time resolution or the pulse width of a backpropagated, time-compressed pulse as compared with the original transmitted pulse; (2) the effectiveness of temporal focusing as measured by the peak-to-sidelobe ratio of the backpropagated or phase conjugated pulse; both pulse elongation and sidelobe leakages are causes of intersymbol interference and bit errors for communications; (3) the duration of temporal focusing or the temporal coherence time of the underwater acoustic channel; and (4) the stability of temporal focusing as measured by the phase fluctuations of successive pulses (symbols). We analyze BPSK signals collected at sea from a moving source, extract the above four parameters, and evaluate the bit error rate. [Work supported by ONR.]

Source motion effects on matched field processing

Past matched field processing (MFP) analyses have typically overlooked the effects of source motion. However, in real-world applications of MFP, especially those involving the use of large arrays or in the highly dynamic shipping environments common to littoral waters, source motion can exacerbate the conflicting requirements of sufficient samples for adequate data covariance estimation and short-term stationarity assumptions. In the context of MFP, short-term stationarity can be assumed provided that the source does not traverse more than one beamwidth of the matched field processor. Thus, theoretically derived estimates of MFP resolution for a vertical line array are compared to those determined empirically from a shallow water acoustics experiment off the coast of San Diego. Predictions of the eigenvalue spreading that arises from source motion, based upon the work of Baggeroer and Cox [Baggeroer and Cox, Proc. of 1999 Asilomar Conference 1, 103–108 (1999)], are then compared to experimental eigenspectra. Finally, the impact of interferer motion on adaptive MFP processor performance is presented.

The effects of source motion on the performance of matched field processors

To obtain a reliable and accurate estimate of the cross-spectral matrix, it is necessary to average individual matrices over a number of snapshots. However, an upper limit in the number of snapshots that can be used to average the cross-spectral matrix is imposed by source and interferer motion. Averaging while the target moves through resolution cells introduces signal gain degradation, spreading the beam-former peak response through the resolutions cells occupied by the source. Averaging while interferences move smears sidelobes, increasing the number of eigenvalues in the cross-spectral matrix and reducing the effectiveness of adaptive noise cancellation processes. During the May 1996 Shallow Water evaluation cell Experiment (SwellEx-96), which was performed in shallow water over a relatively flat bottom off the coast of San Diego, data was obtained with towed sources and interfering surface ships moving at many different rates. Data were recorded on several arrays including two 118-m 64-phone vertical array, one tilted by 45 degrees. This paper presents an analysis of source and interferer motion effects using SwellEx-96 data. Signal gain degradation, target and side-lobe smearing, and adaptive noise cancellation effectiveness are examined as a function of integration time, and number of elements used in the processing.

Acoustic field generated by moving sources in stratified waveguides

We investigate sound propagation from moving sources in stratified waveguides, both completely composed of fluids and with a solid top layer. Asymptotic formulas are derived for the acoustic field generated by a narrow-band point source moving non-uniformly either in a waveguide or on its surface. The asymptotic formulas, expressed in terms of retarded times and Doppler-shifted frequencies, are very simple and suitable for numerical computations. They take into account normal modes and lateral wave contributions and they have clear physical interpretations. Numerical results are presented to illustrate the effects of source motion, in particular, mode cutoff and Doppler effects.

Performance analysis of wavefront curvature methods for range estimation of a moving source

In numerous practical situations a large array of sensors is required, especially for improving the system ability to detect and track moving sources. However, the array performance is strongly affected by the lack of stationarity of the impinging signals. The use of a large array of sensors allow us to perform the source localization by using the wavefront curvature. The analysis of the range estimates based on the wavefront curvature have been thoroughly performed by various authors but, to our knowledge, is restricted to the case of fixed sources. The study of the effects of source motion for range estimation (Section III) and source detection (Section V) constitutes an important part of this work. This study reveals that the array performance can be far from the expected values, especially for large array. Therefore, the robustness of a direct processing of the whole sensor array is very questionable. The computational burden constitutes another serious drawback of this (direct) approach. The use of partitioned processings (Sections IV and VI) seems thus quite promising even if (and perhaps because) it is not optimal so as to fulfil the computation cost and robustness requirements. The performance of such processings for estimating the range of moving sources is presented. >

Performance bounds on the passive localization of a moving source for ocean acoustics

Matched-field processing (MFP) for locating a point acoustic source in the ocean using a vertical array is extended to treat a moving source problem. The extension involves both temporally nonstationary and spatially inhomogeneous structure of the sound field generated by a time-harmonic point source moving uniformly in a stratified oceanic waveguide. Using normal mode description of the sound field, the focus was on the effect of source motion on MFP. An optimum receiver based on maximum likelihood method is developed in the presence of spatially and temporally white noise. The generalized ambiguity function (GAF) was used to analyze problems of accuracy, ambiguity, and resolution. The principal result is the demonstration that a moving source problem can be treated as a stationary source problem if the source travel distance (uncompensated speed × time window) is less than half the wavelength of trapped modes. Also, a closed-form expression for the optimum potential resolution is derived based on the Cramer-Rao bound. The lower bound provides physical insight of how each mode contributes to the localization process, and can be easily evaluated for a wide range of source positions in any sound channel using sound channel eigenfunctions, eigenvalues, and the number of modes involved. Simulations of GAF and the bounds for Arctic environment illustrate the coupling of ocean environment to the localization performance.

Surface wave, internal wave, and source motion effects on matched field processing in a shallow water waveguide

Given well-known environmental conditions, matched field processing has been shown to be a promising signal processing technique for the localization of acoustic sources. However, when environmental data are incomplete or inaccurate, a ‘‘mismatch’’ occurs between the measured and model fields that can lead to a severe degradation of the localization estimator. The possible mismatch effects of surface and internal waves on matched field processing in a shallow water waveguide were investigated. A modified ray theory was utilized, based on the work of Tindle [J. Acoust. Soc. Am. 70, 813–819 (1981); 73, 1581–1586 (1983)], to calculate the acoustic pressure field. Thus it was possible to simply incorporate range-dependent environmental conditions as well as to generalize this work to deeper waveguides. In general, the conventional (Bartlett) matched field beamformer does not provide sufficient resolution to unambiguously locate a source, even in a perfectly matched environment. The maximum likelihood method matched field beamformer has much better resolution, but is extremely susceptible to mismatch. The mismatch due to surface roughness can result in a large reduction of the estimator peak. Part, but not all, of the peak can be regained by (1) using a model that includes incomplete reflection at the surface based on actual sea surface statistics and (2) short-time averaging of the measured signal, with times on the order of the period of the surface waves. Mismatch due to internal waves can also result in a large degradation of the estimator. Averaging over the same time period as surface waves provides little improvement and leads one to surmise that internal waves may be a limiting constraint on matched field processing. Finally, the surface and internal wave fields were combined with a slowly moving source. This example highlights the necessity for the development of a beamformer that has a broader mainlobe while maintaining adequate sidelobe suppression. This issue is addressed by looking at two such beamformers.

Sound propagation phenomena and source motion effects in the measurement of motor vehicle noise

At short ranges the phenomena relevant to the propagation of motor vehicle noise include inverse square law, interference between direct and ground reflected waves, the finite impedance of the ground, refraction, and turbulence. The interference between direct and reflected waves is a rapidly varying function of receiver distance and height from the source. If the ground has a detectable finite acoustic impedance a shadow region is produced which is penetrated at low frequencies by a ground wave. For ground having impedance discontinuities a diffracted wave originates at the interface. Strong wind and temperature gradients close to the ground produce a refractive shadow that is penetrated by a creeping wave. Wind and temperature fluctuations (turbulence) cause sound energy to be redirected by scattering and enhance sound pressure levels in the vicinity of acoustic shadow regions. In addition to the sound propagation phenomena, mechanisms associated with the moving vehicle cause variability in motor vehicl...

A ray theory simulator of a moving broadband source

A computer model is described which simulates and processes the time series received from a moving broadband source. Ray theory is used to compute the set of eigenrays instantaneously valid at each source emission time in a range invariant environment characterized by an arbitrarily depth‐dependent water‐column sound‐speed profile atop a single sediment layer with linear sound speed and attenuation profiles overlying a semi‐infinite homogeneous substrate. The received time series is obtained by a discrete time domain convolution of an arbitrary source waveform with a set of impulse responses which depend on emission time and propagation path. The received time series can be autocorrelated or crosscorrelated to examine the impact on broadband signal processing due to environmental (particularly bottom interaction) effects, source motion effects (Doppler and multipath structure evolution), and the choice of signal processing parameters. Example correlation functions are shown to illustrate the effects of some of these mechanisms. [Work supported by Naval Ocean Research and Development Activity, Code 530.]

Degradation of coherence and correlation when sound from a moving source is reflected from solid surfaces

The propagation of sound in ice‐covered waters, as in Canadian Arctic archipelago, is complicated by the sound's repeated encounters with the under surface of the ice. The essential elements of such a propagation environment were modeled, in air, in an anechoic chamber. The study concentrated on the effects of source motion, in the presence of various reflecting surfaces, on the coherence of sound received at a pair of sensors. The electrical signals from the sensors were processed in real time by a typical dual‐channel spectrum analyzer. The study shows that when the sound reaches the sensors by reflection from an irregular solid surface, the degradation may be attributed to changes in the Doppler shifted components of the sounds at the sensors and to changes in acoustic interference patterns.