104 research outputs found
A Comparative Overview of Geophysical Methods
This report was prepared with support from the Air Force Research Laboratory, under contract FA8718-07-C-0021.The shallow subsurface structure of the Earth is important to understand for many economic and safety reasons. The
problem is usually difficult due to complexity of the earth’s subsurface processes especially near the surface. A
number of geophysical methods are used for this purpose using different physical characteristics of the Earth
materials. A particular geophysical method illuminates part of the problem, but a reliable solution can only be found
by combining results of different methods. In order to synthesize information from different geophysical methods, it
is important to understand their similarities and differences. The aim of this study is to correlate the basic principles
of geophysical methods side-by-side starting from fundamental equations. This study reveals that many analogies
exist among these methods both in their mathematical formulation, and sometimes, in ways they are used in the
geophysical applications
Elastic scattering by unbounded rough surfaces
We consider the two-dimensional time-harmonic elastic wave scattering problem for an unbounded
rough surface, due to an inhomogeneous source term whose support lies within a finite distance
above the surface. The rough surface is supposed to be the graph of a bounded and uniformly
Lipschitz continuous function, on which the elastic displacement vanishes. We propose an upward
propagating radiation condition (angular spectrum representation) for solutions of the Navier equation
in the upper half-space above the rough surface, and establish an equivalent variational formulation.
Existence and uniqueness of solutions at arbitrary frequency is proved by applying a priori estimates
for the Navier equation and perturbation arguments for semi-Fredholm operators
Elastic scattering by unbounded rough surfaces
We consider the two-dimensional time-harmonic elastic wave scattering problem for an unbounded rough surface, due to an inhomogeneous source term whose support lies within a finite distance above the surface. The rough surface is supposed to be the graph of a bounded and uniformly Lipschitz continuous function, on which the elastic displacement vanishes. We propose an upward propagating radiation condition (angular spectrum representation) for solutions of the Navier equation in the upper half-space above the rough surface, and establish an equivalent variational formulation. Existence and uniqueness of solutions at arbitrary frequency is proved by applying a priori estimates for the Navier equation and perturbation arguments for semi-Fredholm operators
Inverse electromagnetic scattering models for sea ice
Journal ArticleInverse scattering algorithms for reconstructing the physical properties of sea ice from scattered electromagnetic field data are presented. The development of these algorithms has advanced the theory of remote sensing, particularly in the microwave region, and has the potential to form the basis for a new generation of techniques for recovering sea ice properties, such as ice thickness, a parameter of geophysical and climatological importance. Moreover, the analysis underlying the algorithms has led to significant advances in the mathematical theory of inverse problems
Uniqueness in inverse scattering of elastic waves by three-dimensional polyhedral diffraction gratings
Literaturverz. S. 35 We consider the inverse elastic scattering problem of determining a
three-dimensional diffraction grating profile from scattered waves measured
above the structure. In general, a grating profile cannot be uniquely
determined by a single incoming plane wave. We completely characterize and
classify the bi-periodic polyhedral structures under the boundary conditions
of the third and fourth kinds that cannot be uniquely recovered by only one
incident plane wave. Thus we have global uniqueness for a polyhedral grating
profile by one incident elastic plane wave if and only if the profile belongs
to neither of the unidentifiable classes, which can be explicitly described
depending on the incident field and the type of boundary conditions. Our
approach is based on the reflection principle for the Navier equation and the
reflectional and rotational invariance of the total field
An inverse method for obtaining the attenuation profile and small variations in the sound speed and density profiles of the ocean bottom
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution May 1985The acoustic properties of marine sediments have a direct effect on the
propagation of sound in the ocean. In the frequency range of interest (50 - 500 Hz)
the sediment can be modelled as a fluid. Assuming horizontal stratification of the
ocean bottom, the acoustic parameters of interest are the compressional wave
speed, the compressional wave attenuation and density as a function of depth.
An inverse method based on a perturbation technique is presented in this
thesis for the determination of these parameters. A monochromatic source
experiment is proposed because of the desirability of such an experiment for
determining the acoustic properties of an anelastic medium. The input information
is the plane wave reflection coerricent as a function of the angle of incidence at a
fixed frequency. A nonlinear integral equation relating the variations of these
acoustic properties from a known reference value to the plane wave reflection
coefficient is derived. This is then linearised using the Born approximation. The
region of validity of the Born approximation is derived and based on this the
optimum angular aperture for the input data is obtained.
The linearised integral equation is a Fredholm integral equation of the first
kind. An acceptable stable solution of the integral equation is obtained by imposing
a priori constraints on the solution. The inversion method is tested using synthetic
data and inversions are carried out for various examples of the attenuation
coefficient profile and the sound speed profile. The results obtained with noise free
data show good agreement between the true profiles and the reconstructed profiles.
The resolution obtainable with the data set is studied using the resolving power
theory of Backus and Gilbert and the inversion method is shown to provide
adequate resolution. The effect of additive noise in data is examined and inversions
performed with noisy data yielded stable acceptable results.I acknowledge the financial support provided by the education office
in the Woods Hole Oceanographic Institution and the Office of Naval Research
Exact reconstruction of ocean bottom velocity profiles from monochromatic scattering data
Submitted in partial fulfillment of the requirements for the degree of Doctor of Science at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution January 1987This thesis presents the theoretical and computational
underpinnings of a novel approach to the determination of the
acoustic parameters of the ocean bottom using a monochromatic
source. The problem is shown to be equivalent to that of the
reconstruction of the potential in a Schrodinger equation
from the knowledge of the plane-wave reflection coefficient
as a function of vertical wavenumber, r(kz) for all real
positive k z. First, the reflection coefficient is shown to
decay asymptotically at least as fast as (1/kz2) for large kz
and is therefore inteqrable. The Gelfand-Levitan inversion
procedure is extended to include the case of basement
velocity higher than the velocity of sound in water. The
neglect of bound states is shown to be justified in both
clayey silt and silty clay at the 220 Hz frequency of
operation.
Three methods for the numerical solution of the integral
equation are investigated. The first one is an "Improved
Born approximation" wherein the solution is given as a series
expansion the first term of which is the Born approximation
while the second term represents a substantial and yet easy
to implement improvement over Born.
The two other methods are based on a discretization of
the Gelfand-Levitan integral equation, and both avoid a
matrix inversion: one by employing a recursive procedure,
and the other by coupling the Gelfand-Levitan equation with a
partial differential equation. Bounds are obtained on errors
in the solution due either to discretization or to data inaccuracy.
These methods are tested on synthetic data obtained
from known geoacoustic models of the ocean bottom. Results
are found to be very accurate particularly at the top of the
sediment layer with resolution of less than the wavelength of
the acoustic source in the water. Several effects are investigated,
such as sampling, attenuation, and noise. Also
examined is the gradual restriction of the reflection coefficient
to a finite range of vertical wave numbers and the consequent
progressive deterioration of the reconstruction.
The analysis shows how to reconstruct velocity profiles
in the presence of density variation when the experiment is
conducted at two frequencies.
Our results provide a good understanding of the issues
involved in conducting a monochromatic deep ocean bottom
experiment and constitute a promising technique for processing
the experimental data when it becomes available.Support for this thesis was provided in part by the
education office at WHOI and by the Office of Naval Research
Uniqueness in inverse scattering of elastic waves by three-dimensional polyhedral diffraction gratings
We consider the inverse elastic scattering problem of determining a three-dimensional diffraction grating profile from scattered waves measured above the structure. In general, a grating profile cannot be uniquely determined by a single incoming plane wave. We completely characterize and classify the bi-periodic polyhedral structures under the boundary conditions of the third and fourth kinds that cannot be uniquely recovered by only one incident plane wave. Thus we have global uniqueness for a polyhedral grating profile by one incident elastic plane wave if and only if the profile belongs to neither of the unidentifiable classes, which can be explicitly described depending on the incident field and the type of boundary conditions. Our approach is based on the reflection principle for the Navier equation and the reflectional and rotational invariance of the total field
- …