Fresnel Area Research Articles

A Novel Low-Complexity Method for Near-Field Sources Based on an S-IMISC Array Model

Array optimization has recently received significant attention owing to its several advantages, such as larger array aperture and greater degrees of freedom (DOFs). However, current works focus on far-field sources, while array optimization for near-field sources has not been adequately investigated. Therefore, this work develops a new symmetry sparse array model for near-field sources based on the improved maximum inter-element spacing constraint (IMISC). The proposed symmetry IMISC (S-IMISC) array model has all the advantages of traditional sparse array models. Compared with traditional sparse array models, the S-IMISC array model affords more uniform DOFs and is less affected by mutual coupling. Additionally, in order to improve the real-time performance of near-field sources localization, the characteristic equation-based method (CEM) is used to obtain the azimuth information of near-field sources which can avoid eigenvalue decomposition (EVD), and a spectrum peak search and compression scheme is used to obtain the distance information by searching the partial area instead of the whole Fresnel area, thereby significantly reducing computation complexity. Extensive simulations verify the advantages of the proposed algorithm and the S-IMISC array model.

A Novel Method Based on Oblique Projection Technology for Mixed Sources Estimation

Reducing the computational complexity of the near-field sources and far-field sources localization algorithms has been considered as a serious problem in the field of array signal processing. A novel algorithm caring for mixed sources location estimation based on oblique projection is proposed in this paper. The sources are estimated at two different stages and the sensor noise power is estimated and eliminated from the covariance which improve the accuracy of the estimation of mixed sources. Using the idea of compress, the range information of near-field sources is obtained by searching the partial area instead of the whole Fresnel area which can reduce the processing time. Compared with the traditional algorithms, the proposed algorithm has the lower computation complexity and has the ability to solve the two closed-spaced sources with high resolution and accuracy. The duplication of range estimation is also avoided. Finally, simulation results are provided to demonstrate the performance of the proposed method.

The Passive Evaluation Method of Electromagnetic Environment in ILS Construction of Airport

In the process of new and expand airport, electromagnetic environment of navigation equipments in air traffic management are affected by more and more factors, especially ILS. For voiding to influence fly safety, and ensuring a nice electromagnetic environment, the electromagnetic environment of airport should be assessed by passive evaluation through clearance analysis, Fresnel area analysis and simulation analysis.

A Lens Assisted Phase Microscope Based on Ptychography

Based on the recently developed ptychographical iterative engine (PIE), we suggest a lens assisted microscopy to realize quantitative phase imaging without using interferometry. The sample is imaged with a lens system; a pinhole on the image plane scans the image at a proper step interval; the diffraction pattern is recorded simultaneously by a CCD at Fresnel area. With a slightly changed PIE algorithm, the phase image of the sample can be accurately reconstructed from the recorded diffraction pattern. The main advantage of this suggested method lies in its capability to retrieve the phase information from the recorded intensity directly, and thus it has more flexibility over conventional interferometric techniques. The feasibility of the suggested method is verified by reconstructing the modulus and phase image of a biological sample from a set of 10 by 10 diffraction patterns, and the result matches the analysis well.

Determination of the influence zone for surface wave paths

SUMMARY An approximate description of the zone of influence around the propagation path for a surface wave is provided by investigating the Fresnel zones for the frequency range of interest. The influence zone about surface wave paths, over which surface waves are coherent in phase, is identified as approximately one-third of the width of the first Fresnel zone. A technique called Fresnel-area ray tracing (FRT) for surface waves has been used to estimate this region around the ray path for each frequency. The FRT technique is developed by combining two standard ray tracing methods, i.e. kinematic ray tracing (KRT) and dynamic ray tracing (DRT). To obtain the exact Fresnel area in a laterally heterogeneous structure would require the solution of a large number of KRT equations. In contrast, the FRT approach requires just a few ray tracing calculations. In the first step, the trajectory of the surface wave is computed by solving the KRT system for the phase-velocity distribution at the required frequency. In the next step, the behaviour of rays in the zone surrounding the KRT path is calculated by solving the DRT system twice; once from the source to the receiver and once more from the receiver to the source along the same trajectory. Finally, combining the solutions of these ray tracing systems using paraxial ray theory, the Fresnel area around a central ray can be estimated. Using FRT, stationary-phase fields can be constructed around a central ray path in a laterally heterogeneous structure. The influence zone around the ray path is then estimated from the stationary-phase function with simple assumptions concerning the perturbed wavefield. The estimate of the influence zone can be efficiently calculated in laterally heterogeneous structure by using the FRT technique, and allows an extension of current methods of surface wave analysis, which have commonly been based on geometrical ray theory and on the approximation of great-circle propagation. This approach allows the treatment of finite-width rays as well as deviations in propagation from the great circle induced by moderate lateral heterogeneity as revealed by recent tomography models. Such finite-width rays should be of major benefit in enhancing ray-based surface wave tomography.

Validity of surface-wave ray theory on a laterally heterogeneous earth

SUMMARY A necessary and sufficient condition for the validity of surface-wave ray theory is S/m << 1, where S is the root-mean-square degree of the phase-velocity perturbation 6c, and 1 is the degree of the equivalent mode ,,Sl or ,,T. This condition, which is independent of the number of orbits, is obtained by consideration of the Fresnel area along the surface-wave ray path between the source and receiver. We assess the accuracy of surface-wave ray theory by comparing the phase, arrival angle and amplitude anomalies obtained using the JWKB approximation with the corresponding quantities measured using 'ground-truth' synthetic coupled-mode seismograms on models of varying roughness. The JWKB results agree well with the coupled-mode results for model S12-WM13; on a contrived model with slightly rougher variations, however, the agreement deteriorates as the condition S/@ << 1 is less well satisfied. The misfit of the ray approximation, which is dependent upon the quantity S/@, can be attributed to diffraction and other finite-frequency effects within the Fresnel area along the ray path; these effects are ignored by the JWKB theory but are fully accounted for by the coupled-mode summation. The wavefront smoothing produced by this Fresnel-area averaging limits the resolution of surface-wave inversion studies based upon the JWKB approximation.

Elastic finite difference modelling of lower crustal reflections

It is a common feature to observe on deep reflection profiles, short (less than the Fresnel area diameter) and strongly discontinuous lower crustal reflections. Such wave patterns can be attributed to variations in the crustal properties on the scale of a few wavelengths. In order to explain these characteristics, we have computed finite-difference synthetic seismograms for various types of deep heterogeneities. Focusing, defocusing and interference effects were especially studied. The results can only be interpreted in a qualitative manner because our models exhibit departures from the real cases (lower frequency content, and only two-dimensional structures). It is found that simple undulations of the deep interfaces can hardly produce the discontinuous pattern observed, but high velocity intrusions generate complex interferences matching the reflection character of real data. Furthermore, the reflection patterns strongly depend both on length and shape of the inclusions.