540 research outputs found
Rough surface backscatter and statistics via extended parabolic integral equation
This paper extends the parabolic integral equation method, which is very
effective for forward scattering from rough surfaces, to include backscatter.
This is done by applying left-right splitting to a modified two-way governing
integral operator, to express the solution as a series of Volterra operators;
this series describes successively higher-order surface interactions between
forward and backward going components, and allows highly efficient numerical
evaluation. This and equivalent methods such as ordered multiple interactions
have been developed for the full Helmholtz integral equations, but not
previously applied to the parabolic Green's function. In addition, the form of
this Green's function allows the mean field and autocorrelation to be found
analytically to second order in surface height. These may be regarded as
backscatter corrections to the standard parabolic integral equation method
Recommended from our members
Recovery of rough surface in ducting medium from grazing angle scattered wave
A method is developed for rough surface reconstruction using fields scattered at grazing angles in a medium with a linearly varying refractive index and Neumann boundary condition. This regime represents a ducting medium, bounded by a perfectly conducting surface with a TM incident field or an acoustically hard surface. This significantly extends the iterated marching method, based upon the parabolic integral equation for forward-scattered field components [Chen and Spivack, J. Opt. Soc. Am. A 35, 504–513 (2018)]. The approach, which uses a fixed frequency, is able to accurately recover multiscale surfaces and is found to be robust with respect to measurement noise and localized perturbations.</jats:p
Scattering of Ocean Surfaces in Microwave Remote Sensing by Numerical Solutions of Maxwell Equations
Sea-surface scattering has long been studied using various analytical methods. These analytical methods include the two scale method (TSM), the small-slope approximation (SSA), the small-perturbation method (SPM), the Advanced Integral Equation Method (AIEM), and the Geometrical/Physical Optics (GO/PO) method. These analytical methods rely on making approximations and assumptions in the modelling process. Some of these assumptions undermine their applicability in a wide range of situations. The input for analytical methods are usually the ocean spectrum. In real implementations, there are 2 sources of uncertainty in such approaches: (1) the analytical methods have a limited range of applicability to the surface scattering problem; the approximations made in these methods are questionable and (2) the various ocean spectra are another source of uncertainty.
We earlier applied a numerical method in 3-dimensions (NMM3D) to the scattering problem of soil surfaces. Through comparison with measured data, we established the accuracy and applicability of NMM3D. We see a drastic increase of ocean remote sensing applications in recent years. It is thus feasible to extend NMM3D to the sea-surface scattering problem. Compared to soil, sea water has a much higher permittivity, e.g., 75+61i at L-band. The large permittivity dictates the need for using a much denser mesh for the sea surface. In addition, the root mean square (rms) height of the sea surface is large under moderate to high ocean wind speeds, which requires a large simulation area to account for the influence of long scale wave like gravity waves.
Compared to the two-scale model commonly used for the ocean scattering problem, NMM3D does not need an ad-hoc split wavenumber in the ocean spectrum. Combined with a fast computational algorithm, it was shown that NMM3D can produce accurate results compared to measured data like the Aquarius missions. TSM could also match well with Aquarius provided with a pre-selected splitting wavenumber. But it was observed that the result of TSM changes with different splitting wavenumbers. It is seen that TSM is fairly heuristic while NMM3D can serve as an exact method for the scattering problem.
On the other hand, through our study of NMM3D, we found that with a fine grid, the final impedance matrix converges slowly and also it becomes hard to perform simulations for a large surface. This has provoked us to (1) solve low convergence problem for a dense mesh and (2) resolve difficulties in simulations of large surfaces.
Inspired by the existing impedance boundary condition (IBC) method, we proposed a neighborhood impedance boundary condition (NIBC) method to solve the slow convergence problem caused by the dense grid. Different from IBC where the surface electric field and the surface magnetic field are related locally, NIBC relates the surface electric field to the magnetic field within a preselected bandwidth BW. Through numerical simulations, we found that the condition number can be reduced using NIBC. Errors of NIBC are controllable through changing BW. We applied NIBC to various wind speeds and surface types and found NIBC to be quite accurate when surface currents only suffer an error norm of less than 1%.PHDElectrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/145797/1/qiaot_1.pd
Applications on Ultrasonic Wave
This book presents applications on the ultrasonic wave for material characterization and nondestructive evaluations. It could be of interest to the researchers and students who are studying on the fields of ultrasonic waves
A model for the simulation of sidescan sonar
This thesis presents the development of a computer model for the simulation of the
sidescan sonar process. The motivation for the development of this model is the creation
of a unique and powerful visualisation tool to improve understanding and interpretation
of the sidescan sonar process and the images created by it. Existing models tend to generate
graphical or numerical results, but this model produces synthetic sidescan sonar
images as the output. This permits the direct visualisation of the influence of individual
parameters and features of the sonar process on the sidescan images.
The model considers the main deterministic aspects of the underlying physical
processes which result in the generation of sidescan sonar images. These include the
propagation of the transmitted pulse of acoustic energy through the water column to its
subsequent interaction and scattering from the rough seafloor. The directivity and motion
characteristics of the sonar transducer are also incorporated. The thesis documents the
development of the model to include each of these phenomena and their subsequent effect
on the sidescan sonar images. Finally, techniques are presented for the investigation and
verification of the synthetic sidescan images produced by the model.Defence Research Agenc
Propagation and scattering of electromagnetic waves in low THz band in automotive radar applications
This thesis, firstly, due to the lack of knowledge in influence of harsh outdoor environment on the performance of the low-THz automotive sensors, the investigation has been done to demonstrate the performance of low-THz sensors in the presence of different radome contaminants (mud, oil, grit, etc.) and various weather conditions (rain, snow, fog, etc.) to prove the feasibility of using low-THz frequencies (100 GHz -1 THz) in automotive radar in uncontrolled environmental conditions.
Secondarily, this thesis reports and discuss the important and yet unsolved task on automotive surface recognition and shows the possibility of using Low THz radar for road surface classification by exploring the radar signal backscattering from surfaces with different roughness, and finally this thesis demonstrate the novel approach to surface classification based on the analysis of radar images obtained using the low THz imaging radar and demonstrate the advantage of low THz radar for surface discrimination for automotive sensing. The proposed experimental technique in combination with a convolutional neural network provides high surface classification accuracy
Benthic habitat mapping using multibeam sonar systems
The aim of this study was to develop and examine the use of backscatter data collected with multibeam sonar (MBS) systems for benthic habitat mapping. Backscatter data were collected from six sites around the Australian coastal zone using the Reson SeaBat 8125 MBS system operating at 455 kHz. Benthic habitats surveyed in this study included: seagrass meadows, rhodolith beds, coral reef, rock, gravel, sand, muddy sand, and mixtures of those habitats. Methods for processing MBS backscatter data were developed for the Coastal Water Habitat Mapping (CWHM) project by a team from the Centre for Marine Science and Technology (CMST). The CMST algorithm calculates the seafloor backscatter strength derived from the peak and integral (or average) intensity of backscattered signals for each beam. The seafloor backscatter strength estimated from the mean value of the integral backscatter intensity was shown in this study to provide an accurate measurement of the actual backscatter strength of the seafloor and its angular dependence. However, the seafloor backscatter strength derived from the peak intensity was found to be overestimated when the sonar insonification area is significantly smaller than the footprint of receive beams, which occurs primarily at oblique angles. The angular dependence of the mean backscatter strength showed distinct differences between hard rough substrates (such as rock and coral reef), seagrass, coarse sediments and fine sediments. The highest backscatter strength was observed not only for the hard and rough substrate, but also for marine vegetation, such as rhodolith and seagrass. The main difference in acoustic backscatter from the different habitats was the mean level, or angle-average backscatter strength. However, additional information can also be obtained from the slope of the angular dependence of backscatter strength.It was shown that the distribution of the backscatter. The shape parameter was shown to relate to the ratio of the insonification area (which can be interpreted as an elementary scattering cell) to the footprint size rather than to the angular dependence of backscatter strength. When this ratio is less than 5, the gamma shape parameter is very similar for different habitats and is nearly linearly proportional to the ratio. Above a ratio of 5, the gamma shape parameter is not significantly dependent on the ratio and there is a noticeable difference in this parameter between different seafloor types. A new approach to producing images of backscatter properties, introduced and referred to as the angle cube method, was developed. The angle cube method uses spatial interpolation to construct a three-dimensional array of backscatter data that is a function of X-Y coordinates and the incidence angle. This allows the spatial visualisation of backscatter properties to be free from artefacts of the angular dependence and provides satisfactory estimates of the backscatter characteristics.Using the angle-average backscatter strength and slope of the angular dependence, derived by the angle cube method, in addition to seafloor terrain parameters, habitat probability and classification maps were produced to show distributions of sand, marine vegetation (e.g. seagrass and rhodolith) and hard substrate (e.g. coral and bedrock) for five different survey areas. Ultimately, this study demonstrated that the combination of high-resolution bathymetry and backscatter strength data, as collected by MBS, is an efficient and cost-effective tool for benthic habitat mapping in costal zones
On the use of the finite element method for the modeling of acoustic scattering from one-dimensional rough fluid-poroelastic interfaces
textA poroelastic finite element formulation originally derived for modeling porous absorbing material in air is adapted to the problem of acoustic scattering from a poroelastic seafloor with a one-dimensional randomly rough interface. The developed formulation is verified through calculation of the plane wave reflection coefficient for the case of a flat surface and comparison with the well known analytical solution. The scattering strengths are then obtained for two different sets of material properties and roughness parameters using a Monte Carlo approach. These numerical results are compared with those given by three analytic scattering models---perturbation theory, the Kirchhoff approximation, and the small-slope approximation---and from those calculated using two finite element formulations where the sediment is modeled as an acoustic fluid.Mechanical Engineerin
- …