Performance analysis of subaperture processing using a large aperture planar towed array

Thesis (Ph. D.)--Joint Program in Applied Ocean Science and Engineering (Massachusetts Institute of Technology, Dept. of Ocean Engineering; and, the Woods Hole Oceanographic Institution), 2004.Includes bibliographical references (v. 2, leaves 211-215).In recent years the focus of passive detection and localization of submarines has moved from the deep ocean into the littoral regions. the problem of passive detection in these regions is complicated by strong multipath propagation with high transmission loss. Large aperture planar arrays have the potential to improve detection performance due to their high resolution and high gain, but are suceptible to two main performance degradation mechanisms: limited spatial coherence of signals and nonstationarity of high bearing rate interference sources common in littoral regions of strategic importance. This thesis presents subarray processing as a method of improving passive detection performance using such large arrays. This thesis develops statistical models for the detection of performance of three adaptive, sample-covariance-based subarray processing algorithms which incorporate the effects of limited spatial coherence as well as finite snapshot support. The performance of the optimum processor conditioned on known data coveriances is derived as well for comparison. These models are then used to compare subarray algorithms and partitioning schemes in a variety of interference environments using plane wave and matched-field propagation models.(cont.) The analysis shows a tradeoff between the required adaptive degrees of freedom, snapshot support, and adaptive resolution. This thesis shows that for both plane-wave and matched-field processing, the Conventional-Then-Adaptive (CTA) algorithm optimizes this tradeoff most efficiently. Finally, a comparison of the CTA algorithm to beam-space adaptive processing shows that for moderate beam coverage, the subarray algorithm performs as well as or superior to the adaptive beamspace algorighm.by Jennifer Anne Watson.Ph.D

Resilience and Adaptive Capacity of Aquatic Environmental Law in the EU : An evaluation and comparison of the WFD, MSFD, and MSPD

Peer reviewe

First-arrival Travel-Time Tomography using Second Generation Wavelets

International audienceWavelet decomposition of the slowness model has been proposed as a multiscale strategy for seismic first-arrival time tomography. We propose the introduction of so-called second generation wavelets which could be used for any mesh structure and does not need a number of samples as the power of two in each direction. Moreover, one can handle easily boundary effects. A linearized procedure for inverting delayed travel-times considering either slowness coefficients or wavelet coefficients. The ray tracing is solved at each iteration through an eikonal solver while the linear system to be solved at each iteration goes through an iterative solver as LSQR algorithm. We develop wavelet decomposition over constant patches (Haar wavelet) or over linear patches (Battle-Lemarie wavelet) of coefficients at different scales. This decomposition is introduced in the linear system to be solved and wavelet coefficients are considered as unknowns to be inverted. Synthetic examples show that the inversion behaves in a better way as wavelet decomposition seems to act as a preconditioner of the linear system. Local discretisation is possible but requires additional implementation as artefacs once built inside the model description never disappear because of the linearized approach. A binary mask operator is designed for each scale grid and could be applied locally leading to quite different spatial resolution depending on the analysis we could perform of the expected resolution at a given position of the medium. We show that indeed it is possible to design this binary operator and we apply it to synthetic examples as a crosswell experiment inside the Marmousi model. An application to a surface-surface experiment has been performed and the waveled decomposition shows that indeed we may recover detailed features nearby the free surface while preventing imprints of ray coverage at greater depths giving us smooth features at that depths. In spite of the increase demand of computer resources, the wavelet decomposition seems to be a rather promising alternative for controlling the resolution variation of seismic first-arrival tomography

Projection operators in correlated noise fields

Includes bibliographical references (p. 7-8).Supported by the ARO. DAAL03-92-G-0115 Supported by AFOSR. F49620-92-J-0002 Supported by the NSF. MIP-9015281H. Krim, P. Forster and A.S. Willsky

A Tutorial on Speckle Reduction in Synthetic Aperture Radar Images

Speckle is a granular disturbance, usually modeled as a multiplicative noise, that affects synthetic aperture radar (SAR) images, as well as all coherent images. Over the last three decades, several methods have been proposed for the reduction of speckle, or despeckling, in SAR images. Goal of this paper is making a comprehensive review of despeckling methods since their birth, over thirty years ago, highlighting trends and changing approaches over years. The concept of fully developed speckle is explained. Drawbacks of homomorphic filtering are pointed out. Assets of multiresolution despeckling, as opposite to spatial-domain despeckling, are highlighted. Also advantages of undecimated, or stationary, wavelet transforms over decimated ones are discussed. Bayesian estimators and probability density function (pdf) models in both spatial and multiresolution domains are reviewed. Scale-space varying pdf models, as opposite to scale varying models, are promoted. Promising methods following non-Bayesian approaches, like nonlocal (NL) filtering and total variation (TV) regularization, are reviewed and compared to spatial- and wavelet-domain Bayesian filters. Both established and new trends for assessment of despeckling are presented. A few experiments on simulated data and real COSMO-SkyMed SAR images highlight, on one side the costperformance tradeoff of the different methods, on the other side the effectiveness of solutions purposely designed for SAR heterogeneity and not fully developed speckle. Eventually, upcoming methods based on new concepts of signal processing, like compressive sensing, are foreseen as a new generation of despeckling, after spatial-domain and multiresolution-domain method

Nonlinear brain dynamics as macroscopic manifestation of underlying many-body field dynamics

Neural activity patterns related to behavior occur at many scales in time and space from the atomic and molecular to the whole brain. Here we explore the feasibility of interpreting neurophysiological data in the context of many-body physics by using tools that physicists have devised to analyze comparable hierarchies in other fields of science. We focus on a mesoscopic level that offers a multi-step pathway between the microscopic functions of neurons and the macroscopic functions of brain systems revealed by hemodynamic imaging. We use electroencephalographic (EEG) records collected from high-density electrode arrays fixed on the epidural surfaces of primary sensory and limbic areas in rabbits and cats trained to discriminate conditioned stimuli (CS) in the various modalities. High temporal resolution of EEG signals with the Hilbert transform gives evidence for diverse intermittent spatial patterns of amplitude (AM) and phase modulations (PM) of carrier waves that repeatedly re-synchronize in the beta and gamma ranges at near zero time lags over long distances. The dominant mechanism for neural interactions by axodendritic synaptic transmission should impose distance-dependent delays on the EEG oscillations owing to finite propagation velocities. It does not. EEGs instead show evidence for anomalous dispersion: the existence in neural populations of a low velocity range of information and energy transfers, and a high velocity range of the spread of phase transitions. This distinction labels the phenomenon but does not explain it. In this report we explore the analysis of these phenomena using concepts of energy dissipation, the maintenance by cortex of multiple ground states corresponding to AM patterns, and the exclusive selection by spontaneous breakdown of symmetry (SBS) of single states in sequences.Comment: 31 page

Scattering Characteristics In Heterogeneous Fractured Reservoirs From Waveform Estimation

Offset-dependent characteristics of seismic scattering are useful in the interpretation of fractured reservoirs. Synthetic seismograms generated by a 3-D finite difference modeling are used to study elastic wave propagation and scattering in heterogeneous fractured reservoirs. We use two models having different background medium properties and different azimuthal AVO responses and build heterogeneous fracture density realizations through stochastic modeling. Gas-saturated fractured reservoirs and waveforms ill fracture normal and strike directions are considered in this paper. The multiple signal classification (MUSIC) frequency estimator is used in waveform estimation to provide frequency domain attributes related to seismic wave scattering by fractures. Our results indicate that the strength of the scattered field increases with increasing fracture scatter density and decreasing correlation length of spatial variations of fracture density. It is also a function of the background medium. The strength of the scattering field is stronger in model 1 which has smaller property contrasts in the background medium than model 2. The scattering characteristics for both models are different at the top and the base of the fractured reservoir. Our results show that the scattered field is weak at the top of a fractured reservoir. The first order results are dominated by velocity anisotropy of a mean crack density field. However, the base of the fractured reservoir corresponds to a strong scattered field on which fracture heterogeneity has a larger effect.Massachusetts Institute of Technology. Borehole Acoustics and Logging ConsortiumMassachusetts Institute of Technology. Earth Resources Laboratory. Reservoir Delineation ConsortiumUnited States. Dept. of Energy (Grant DE-82066-223II-1

Hilbert–Huang spectral analysis of cavity flows incorporating fluidic spoilers

Numerical aeroacoustic analysis was conducted on an M219 cavity geometry, incorporating signature suppression features and leading-edge fluidic spoilers. The numerical model was validated against existing experimental data. The palliative properties of fluidic spoilers were investigated at Mach numbers of 0.85, 1.20, and 1.80 with blowing coefficients of 0.03 and 0.06. The results are presented for the acoustic spectrum, and further analysis was conducted using the Hilbert–Huang methodology. The fluidic spoilers were able to considerably reduce the overall level of acoustic noise and to reduce and/or suppress the resonant modes typical of cavity flows. The effectiveness of the spoilers was a direct consequence of their effect on the detached shear layer, of which the trajectory and coherence were altered. The Hilbert–Huang spectral analysis provided an enhanced understanding of the complex nature of the aeroacoustic behavior of the cavity. Acoustic modes were identified that, together with the Rossiter–Heller tones, governed the behavior of the spectrum. This demonstrated how the generated tones, appearing inside the cavity, were a result of complex nonlinear interactions between shear-layer acoustic instabilities and centrifugal instabilities originating in the flow recirculating in the internal part of the cavity. This also demonstrated that the fundamental frequencies had frequency and amplitude modulation characteristics that spread the energy in a wide bandwidth. This is not captured by classical Fourier analysis

Sensor array signal processing : two decades later

Caption title.Includes bibliographical references (p. 55-65).Supported by Army Research Office. DAAL03-92-G-115 Supported by the Air Force Office of Scientific Research. F49620-92-J-2002 Supported by the National Science Foundation. MIP-9015281 Supported by the ONR. N00014-91-J-1967 Supported by the AFOSR. F49620-93-1-0102Hamid Krim, Mats Viberg