1,166 research outputs found
Anisotropic Denoising in Functional Deconvolution Model with Dimension-free Convergence Rates
In the present paper we consider the problem of estimating a periodic
-dimensional function based on observations from its noisy
convolution. We construct a wavelet estimator of , derive minimax lower
bounds for the -risk when belongs to a Besov ball of mixed smoothness
and demonstrate that the wavelet estimator is adaptive and asymptotically
near-optimal within a logarithmic factor, in a wide range of Besov balls. We
prove in particular that choosing this type of mixed smoothness leads to rates
of convergence which are free of the "curse of dimensionality" and, hence, are
higher than usual convergence rates when is large. The problem studied in
the paper is motivated by seismic inversion which can be reduced to solution of
noisy two-dimensional convolution equations that allow to draw inference on
underground layer structures along the chosen profiles. The common practice in
seismology is to recover layer structures separately for each profile and then
to combine the derived estimates into a two-dimensional function. By studying
the two-dimensional version of the model, we demonstrate that this strategy
usually leads to estimators which are less accurate than the ones obtained as
two-dimensional functional deconvolutions. Indeed, we show that unless the
function is very smooth in the direction of the profiles, very spatially
inhomogeneous along the other direction and the number of profiles is very
limited, the functional deconvolution solution has a much better precision
compared to a combination of solutions of separate convolution equations. A
limited simulation study in the case of confirms theoretical claims of
the paper.Comment: 29 pages, 1 figure, 1 tabl
Seismic Ray Impedance Inversion
This thesis investigates a prestack seismic inversion scheme implemented in the ray
parameter domain. Conventionally, most prestack seismic inversion methods are
performed in the incidence angle domain. However, inversion using the concept of
ray impedance, as it honours ray path variation following the elastic parameter
variation according to Snell’s law, shows the capacity to discriminate different
lithologies if compared to conventional elastic impedance inversion.
The procedure starts with data transformation into the ray-parameter domain and then
implements the ray impedance inversion along constant ray-parameter profiles. With
different constant-ray-parameter profiles, mixed-phase wavelets are initially estimated
based on the high-order statistics of the data and further refined after a proper well-to-seismic
tie. With the estimated wavelets ready, a Cauchy inversion method is used to
invert for seismic reflectivity sequences, aiming at recovering seismic reflectivity
sequences for blocky impedance inversion. The impedance inversion from reflectivity
sequences adopts a standard generalised linear inversion scheme, whose results are
utilised to identify rock properties and facilitate quantitative interpretation. It has also
been demonstrated that we can further invert elastic parameters from ray impedance
values, without eliminating an extra density term or introducing a Gardner’s relation
to absorb this term.
Ray impedance inversion is extended to P-S converted waves by introducing the
definition of converted-wave ray impedance. This quantity shows some advantages in
connecting prestack converted wave data with well logs, if compared with the shearwave
elastic impedance derived from the Aki and Richards approximation to the
Zoeppritz equations. An analysis of P-P and P-S wave data under the framework of
ray impedance is conducted through a real multicomponent dataset, which can reduce
the uncertainty in lithology identification.Inversion is the key method in generating those examples throughout the entire thesis
as we believe it can render robust solutions to geophysical problems. Apart from the
reflectivity sequence, ray impedance and elastic parameter inversion mentioned above,
inversion methods are also adopted in transforming the prestack data from the offset
domain to the ray-parameter domain, mixed-phase wavelet estimation, as well as the
registration of P-P and P-S waves for the joint analysis.
The ray impedance inversion methods are successfully applied to different types of
datasets. In each individual step to achieving the ray impedance inversion, advantages,
disadvantages as well as limitations of the algorithms adopted are detailed. As a
conclusion, the ray impedance related analyses demonstrated in this thesis are highly
competent compared with the classical elastic impedance methods and the author
would like to recommend it for a wider application
A Primal-Dual Proximal Algorithm for Sparse Template-Based Adaptive Filtering: Application to Seismic Multiple Removal
Unveiling meaningful geophysical information from seismic data requires to
deal with both random and structured "noises". As their amplitude may be
greater than signals of interest (primaries), additional prior information is
especially important in performing efficient signal separation. We address here
the problem of multiple reflections, caused by wave-field bouncing between
layers. Since only approximate models of these phenomena are available, we
propose a flexible framework for time-varying adaptive filtering of seismic
signals, using sparse representations, based on inaccurate templates. We recast
the joint estimation of adaptive filters and primaries in a new convex
variational formulation. This approach allows us to incorporate plausible
knowledge about noise statistics, data sparsity and slow filter variation in
parsimony-promoting wavelet frames. The designed primal-dual algorithm solves a
constrained minimization problem that alleviates standard regularization issues
in finding hyperparameters. The approach demonstrates significantly good
performance in low signal-to-noise ratio conditions, both for simulated and
real field seismic data
INVERSE ATTENUATION-FILTERING
When seismic waves propagate through the Earth, they are affected by numerous inelastic effects of the medium. These effects are usually characterized by the concept of the Q-factor and lead to variations of spectra of the signal and shapes of the waveforms, which further affect the results of reflection seismic imaging. Attenuation compensation, also often called the inverse Q filtering is a signal-processing procedure broadly used to compensate both of these effects of attenuation in reflection sections or volumes. The objective of this thesis is to present and investigate a new attenuation-compensation approach that is much more general than the conventional inverse Q filtering
Sources of uncertainties and artefacts in back-projection results
Back-projecting high-frequency (HF) waves is a common procedure for imaging rupture processes of large earthquakes (i.e. M_w > 7.0). However, obtained back-projection (BP) results could suffer from large uncertainties since high-frequency seismic waveforms are strongly affected by factors like source depth, focal mechanisms, and the Earth's 3-D velocity structures. So far, these uncertainties have not been thoroughly investigated. Here, we use synthetic tests to investigate the influencing factors for which scenarios with various source and/or velocity set-ups are designed, using either Tohoku-Oki (Japan), Kaikoura (New Zealand), Java/Wharton Basin (Indonesia) as test areas. For the scenarios, we generate either 1-D or 3-D teleseismic synthetic data, which are then back-projected using a representative BP method, MUltiple SIgnal Classification (MUSIC). We also analyse corresponding real cases to verify the synthetic test results. The Tohoku-Oki scenario shows that depth phases of a point source can be back-projected as artefacts at their bounce points on the earth's surface, with these artefacts located far away from the epicentre if earthquakes occur at large depths, which could significantly contaminate BP images of large intermediate-depth earthquakes. The Kaikoura scenario shows that for complicated earthquakes, composed of multiple subevents with varying focal mechanisms, BP tends to image subevents emanating large amplitude coherent waveforms, while missing subevents whose P nodal directions point to the arrays, leading to discrepancies either between BP images from different arrays, or between BP images and other source models. Using the Java event, we investigate the impact of 3-D source-side velocity structures. The 3-D bathymetry together with a water layer can generate strong and long-lasting coda waves, which are mirrored as artefacts far from the true source location. Finally, we use a Wharton Basin outer-rise event to show that the wavefields generated by 3-D near trench structures contain frequency-dependent coda waves, leading to frequency-dependent BP results. In summary, our analyses indicate that depth phases, focal mechanism variations and 3-D source-side structures can affect various aspects of BP results. Thus, we suggest that target-oriented synthetic tests, for example, synthetic tests for subduction earthquakes using more realistic 3-D source-side velocity structures, should be conducted to understand the uncertainties and artefacts before we interpret detailed BP images to infer earthquake rupture kinematics and dynamics
Joint inversion of seismic PP- and PS-waves in the ray parameter domain
Seismic inversion is a quantitative analysis technique in reservoir geophysics to
reveal subsurface physical properties from surface-recorded seismic data. But the
most widely used inversion in oil and gas exploration for decades is PP-wave based.
P-to-S converted wave, which has shown great success in the imaging of gas clouds,
has a different response to rocks and pore-fluids from the PP-wave. A joint use of the
PS-wave and PP-wave in the inversion can reduce the ill-posedness of the inverse
problem and in particular enables simultaneous inversion for three independent elastic
parameters.
Conventionally, prestack seismic inversion is based on the incidence
angle-dependent reflection coefficients. In my research, I define the seismic
reflections and impedances along the ray paths of wave propagation, and these ray
paths obey Snell’s law. I adopt the ray-impedance concept, which is a
frequency-dependent parameter and is sensitive to fluid contents. Joined interpretation
of PP- and PS-wave ray impedances can identify reservoirs, and also has potential in
fluid discrimination.
Joint inversion of PP- and PS-waves is performed on the constant ray parameter
(CRP) profiles. For a constant ray parameter, a pair of PP- and PS-wave traces has
exactly the same ray path between the source and the reflection point, which means
the PP- and PS-wave reflection events represent exactly the same reflection point, in
the horizontal direction. Therefore, PP and PS-wave calibration transforms PS-wave
reflection events from PS-wave time to the corresponding PP-wave time, and
reflections events in a pair of PP- and calibrated PS-wave traces with a constant ray
parameter should correspond to each other, sample by sample, both horizontally and
vertically. I also present a procedure which preserves the original wavelets in the
transformed PS-wave trace.
I use a bending ray-tracing method to construct the common image point (CIP)
gathers in the ray-parameter domain. I estimate mixed-phase wavelets for each
constant ray-parameter (CRP) profile through a frequency domain high-order
statistical method, and then invert for the reflectivity series using weighted constraints.
From the reflectivity sections, I estimate PP- and PS-wave ray impedances separately
and also estimate three elastic parameters simultaneously in a joint inversion.
I have applied the entire procedure to a couple of field data sets, to verify the
robustness and effectiveness of the method, and to demonstrate the great potential of joint inversion in ray-parameter domain
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