Spatial Coherence Estimates Research Articles

Coherence of the frequency-difference autoproduct deduced from high-frequency acoustic fields scattered from a rough sea surfacea).

The prevalence of random scattering from a rough ocean surface increases with increasing χ=kh cos θ, where k is the acoustic wavenumber, h is the root-mean-square surface height, and θ is the incidence angle. Generally, when χ≫1, coherence between incident and surface-scattered fields is lost. However, such coherence may be recovered when χ≫1 by considering the frequency-difference autoproduct of the surface-scattered field, a quadratic product of complex fields at nearby frequencies. Herein, the autoproduct's coherent reflection coefficient for χ> 20 is determined from surface-scattered sound fields obtained from 50 independent realizations of the rough ocean surface measured in pelagic waters off the coast of California in January 1992. The recordings were made with a source at a depth of 147 m that broadcasted 30 and 40 kHz signals to a single receiver 576 m away at depth of 66 m. An analytic formula for the coherent reflection coefficient of the frequency-difference autoproduct, based on the Kirchhoff approximation and a Gaussian surface autocorrelation function, compares favorably with measurements. Improved agreement with the single-receiver measurements is possible via a minor adjustment to the surface autocorrelation length. The adjustment identified here matches that determined previously from horizontal spatial coherence estimates utilizing the experiment's eight-element receiving array.

Regional-Lag Signed Delay Multiply and Sum Beamforming in Ultrafast Ultrasound Imaging.

Ultrafast ultrasound imaging provides very high frame rates but provides poor imaging quality due to unfocused beams. The delay multiply and sum (DMAS) beamformer has been used to improve ultrafast ultrasound imaging contrast but is always accompanied by oversuppression, which produces low-quality speckle images and degrades the contrast performance. A smaller maximum lag in the signed DMAS (sDMAS) contributes better speckle preservation but lower resolution for hyperechoic scatters. To overcome this tradeoff, a regional-lag signed delay multiply and sum (rsDMAS) beamformer is proposed in this article. Innovatively, a region discrimination tool realized by the generalized coherence factor (GCF) is used to limit the maximum lag for spatial coherence estimation. Subaperture coherence smoothing estimates the short-lag coherence instead of multiplication in pairs, thereby reducing calculation complexity and smoothing the speckle texture. Normalization and sign correction are also introduced to achieve better beamforming output. The simulated, phantom, and in vivo data are adopted to evaluate the effectiveness of the proposed beamformer. Numerical results show that the proposed method achieves improvements of the contrast ratio (CR) by 9%, contrast-to-noise ratio (CNR) by 41%, speckle signal-to-noise ratio (sSNR) by 41%, and generalized contrast-to-noise ratio (gCNR) by 0.0004 compared with DMAS (in simulation). Resolution experiments show that the proposed method obtains a loss of 0.07 mm in the full width at half maximum (FWHM) and the same separability of close point scatters as DMAS. These findings indicate that the proposed method achieves higher contrast performance at less obvious sacrifice of the lateral resolution than DMAS.

Short-lag spatial coherence imaging using minimum variance beamforming on dual apertures

BackgroundShort-lag spatial coherence (SLSC) imaging, a newly proposed ultrasound imaging scheme, can offer a higher lesion detectability than conventional B-mode imaging. It requires a high focusing quality which can be satisfied by the synthetic aperture imaging mode. However, traditional nonadaptive synthesis for the SLSC still offers an unsatisfactory resolution. The spatial coherence estimation on the receive aperture cannot fully utilize the coherence information in two-dimensional (2D) echo data.MethodsTo overcome these drawbacks, an improved SLSC scheme with adaptive synthesis on dual apertures is proposed in this paper. The minimum variance (MV) beamformer is applied in synthesizing both the receiving and transmitting apertures, while the SLSC function is estimated on both apertures as well. In this way, the resolution is enhanced by the MV implementation, while the coherence in dual apertures is fully utilized.ResultsSimulations, phantom experiments, and in vivo studies are conducted to evaluate the performance of the proposed method. Results demonstrate that the proposed method achieves the best performance in terms of the contrast ratio (CR), contrast-to-noise ratio (CNR), and the speckle signal-to-noise ratio (SNR). Specifically, compared with the delay-and-sum (DAS) method, the proposed method achieves 42.5% higher CR, 412.7% higher CNR, and 402.9% higher speckle SNR in simulations. The resolution is also better than the DAS and conventional SLSC beamformers.ConclusionsThe proposed method is a promising technique for improving the SLSC imaging quality and can provide better visualization for medical diagnosis.

Efficient Strategies for Estimating the Spatial Coherence of Backscatter.

The spatial coherence of ultrasound backscatter has been proposed to reduce clutter in medical imaging, to measure the anisotropy of the scattering source, and to improve the detection of blood flow. These techniques rely on correlation estimates that are obtained using computationally expensive strategies. In this paper, we assess the existing spatial coherence estimation methods and propose three computationally efficient modifications: a reduced kernel, a downsampled receive aperture, and the use of an ensemble correlation coefficient. The proposed methods are implemented in simulation and in vivo studies. Reducing the kernel to a single sample improved computational throughput and improved axial resolution. Downsampling the receive aperture was found to have negligible effect on estimator variance, and improved computational throughput by an order of magnitude for a downsample factor of 4. The ensemble correlation estimator demonstrated lower variance than the currently used average correlation. Combining the three methods, the throughput was improved 105-fold in simulation with a downsample factor of 4- and 20-fold in vivo with a downsample factor of 2.

Coherent acoustic communication in a tidal estuary with busy shipping traffic

High-rate acoustic communication experiments were conducted in a dynamic estuarine environment. Two current profilers deployed in a shipping lane were interfaced with acoustic modems, which modulated and transmitted the sensor readings every 200 s over a period of four days. QPSK modulation was employed at a raw data rate of 8 kbits on a 12-kHz carrier. Two 16-element hydrophone arrays, one horizontal and one vertical, were deployed near the shore. A multichannel decision-feedback equalizer was used to demodulate the modem signals received on both arrays. Long-term statistical analysis reveals the effects of the tidal cycle, subsea unit location, attenuation by the wake of passing vessels, and high levels of ship-generated noise on the fidelity of the communication links. The use of receiver arrays enables vast improvement in the overall reliability of data delivery compared with a single-receiver system, with performance depending strongly on array orientation. The vertical array offers the best performance overall, although the horizontal array proves more robust against shipping noise. Spatial coherence estimates, variation of array aperture, and inspection of array angular responses point to adaptive beamforming and coherent combining as the chief mechanisms of array gain.

Satellite radar interferometry for deformation monitoring: a priori assessment of feasibility and accuracy

Abstracts Conventional satellite repeat-pass radar interferometric measurements can be used for monitoring subsidence phenomena with high accuracies. This methodology was developed for mostly contiguous phase observations, enabling spatial coherence estimation and 2D phase unwrapping. Unfortunately, in many areas in the world, complete temporal decorrelation of the scattering characteristics occurs in a period reaching from days to months. In these circumstances, only urban areas and isolated stable scatterers maintain coherent. The problem is to detect these scatterers amidst their decorrelated neighbors, as spatial coherence estimation is not possible anymore. Methodology for the detection and analysis of such points, labeled permanent scatterers (PS), has been developed by Ferretti et al. [Ferretti, A., Prati, C., Rocca, F., 2000. Nonlinear subsidence rate estimation using permanent scatterers in differential SAR interferometry. IEEE Trans. Geosci. Remote Sens. 38(5), 2202–2212], using many (30 or more) SAR images of a particular site. This approach enables coherence estimation using temporal, pixel-based evaluation. Together, these points form a geodetic network of opportunity with different characteristics when compared to traditional geodetic network design. For practical purposes, it is necessary to define a set of guidelines to assess the feasibility of contiguous or PS radar interferometry for a specific deformation problem. This feasibility is dependent on the number of SAR data acquisitions available, their spatial and temporal baselines and observation statistics, and the expected spatial and temporal behavior of the deformation process. Here, we will discuss the evaluation procedure to assess a priori whether the techniques of contiguous and PS–InSAR are feasible for specific deformation studies in terms of precision and reliability.

Effects of tidally driven temperature fluctuations on shallow-water acoustic communications at 18 kHz

A communications experiment was conducted to investigate the effects of oceanographic fluctuations on high-frequency acoustic transmissions. Source transmissions containing broadband channel probes and phase-shift-keyed (PSK) communication sequences were carried out in 100-m water depth using a stationary 18-kHz source and a stationary 64-hydrophone receive array. Thermistor string data indicate the presence of high-frequency temperature fluctuations generated by the internal tide as it progresses along the continental shelf. Data are analyzed from three periods when the thermal activity is: (1) absent; (2) confined to the lower water column; and (3) confined to the mid-water column. Because of a downward refracting profile, ray paths are most dense near the bottom, which maximizes wavefront interaction with the fluctuating temperature field during period 2. Statistics of the individual ray paths indicate a strong dependence of the temporal correlation on the presence and location of the thermal activity. Ray paths interacting strongly with the portion of the water column exhibiting rapid temperature fluctuations have coherence times of a few tens of seconds, whereas the correlation remain high over the 60-s observation window when the temperature structure is stable. Because of calm sea surface conditions, surface interacting rays are also highly correlated over this time. Spatial coherence estimates and eigenvalue analysis of the array cross-spectral density matrix further indicate that the thermal activity decorrelates the signal in space. Binary PSK (BPSK) data are processed for each of the three periods using two receiver structures: (1) a multichannel decision feedback equalizer and (2) a single-channel decision feedback equalizer preceded by an eigenvector beamformer which projects the data onto the first empirical orthogonal function. Performance of the two receivers is comparable except during period 2. During this period, the eigenvector beamformer performance suffers due to the spatial decorrelation of the signal imposed by the temperature fluctuations.

Estimates of coherence of low-frequency low-grazing angle sound reflected from the sea surface

Sea surface scattering experiments were conducted in which sound from underwater explosives was reflected from the sea surface at grazing angles 5° to 40° and was received by vertically separated sensors of a hydrophone array. The sensor separation varied from l0 to 250 m. The surface-reflected arrivals were isolated and processed to obtain estimates of spatial coherence and reflection-loss spectra for frequencies 20 to 250 Hz. The coherence estimates were obtained as a function of frequency, grazing angle, sensor separation, and bearing for different sea surfaces of low sea state. Simultaneous with the acoustic experiment, ocean wave height time series were measured by three oceanographic data buoys operating in the general region of the experiments. Processing of these data yielded estimates of the ocean wave roughness spectra. Sea surface roughness estimates from the acoustic data were consistent with the measured sea surface roughness. [Work supported by NORDA and Naval Electronic System Command.]

Estimates of spatial coherence in multipath channels

Pure‐tone response in a multipath channel, wherein by virtue of reflection or refraction many paths connect the source to any receiver point, is highly variable due to interference between the components transmitted along different paths. A statistical measure of these fluctuations is the spatial coherence—the averaged product of the complex response field evaluated at two spatially lagged points. Estimates of the coherence are readily formed by combining (i) a field description in which the average response “energy” is expressed as a density in wavenumber or propagation direction, and (ii) the hypothesis that the local properties of each component are those of a free plane wave. This procedure models the field as being locally homogeneous, but with statistical parameters that vary slowly as a consequence of refraction and attenuation. Analytical estimates of coherence have been made for several canonical problems of ocean acoustics: the isospeed channel with flat bottom; the bigradient duct with a limiting ray.