414 research outputs found
Estimating anisotropy parameters and traveltimes in the tau-p domain
The presence of anisotropy influences many aspects of
seismic wave propagation and has therefore implications
for conventional processing schemes. To estimate the
anisotropy, we need both forward modelling and inversion
tools. Exact forward modelling in anisotropic media
is generally done by raytracing. However, we present a
new and fast method, using the tau-p transform, to calculate
exact P and SV reflection moveout curves in stratified,
laterally homogeneous, anisotropic media which
requires no ray tracing. Results are exact even if the
SV-waves display cusps. In addition, we show how the
same method can be used for parameter estimation.
Since inversion for anisotropic parameters is very
nonunique, we develop expressions requiring only a reduced
number of parameters. Nevertheless, predictions
using these expressions are more accurate than Taylor
series expansions and are also able to handle cusps in
the SV traveltime curves. In addition, layer stripping is
a linear process. Therefore, both effective (average) and
local (interval) estimates can be obtained
Traveltime and conversion-point computations and parameter estimation in layered, anisotropic media by tau-p transform
Anisotropy influences many aspects of seismic wave
propagation and, therefore, has implications for conventional
processing schemes. It also holds information
about the nature of the medium. To estimate anisotropy,
we need both forward modeling and inversion tools. Forward
modeling in anisotropic media is generally done
by ray tracing. We present a new and fast method using
the tau-p transform to calculate exact reflection-moveout
curves in stratified, laterally homogeneous, anisotropic
media for all pure-mode and converted phases which requires
no conventional ray tracing. Moreover, we obtain
the common conversion points for both P-SV and P-SH
converted waves. Results are exact for arbitrary strength
of anisotropy in both HTI and VTI media (transverse
isotropy with a horizontal or vertical symmetry axis,
respectively).
Since inversion for anisotropic parameters is a highly
nonunique problem, we also develop expressions describing
the phase velocities that require only a reduced
number of parameters for both types of anisotropy. Nevertheless,
resulting predictions for traveltimes and conversion
points are generally more accurate than those
obtained using the conventional Taylor-series expansions.
In addition, the reduced-parameter expressions
are also able to handle kinks or cusps in the SV traveltime
curves for either VTI or HTI symmetry
Reconstruction of Lame moduli and density at the boundary enabling directional elastic wavefield decomposition
We consider the inverse boundary value problem for the system of equations
describing elastic waves in isotropic media on a bounded domain in
via a finite-time Laplace transform. The data is the dynamical
Dirichlet-to-Neumann map. More precisely, using the full symbol of the
transformed Dirichlet-to-Neumann map viewed as a semiclassical
pseudodifferential operator, we give an explicit reconstruction of both
Lam\'{e} parameters and the density, as well as their derivatives, at the
boundary. We also show how this boundary reconstruction leads to a
decomposition of incoming and outgoing waves
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Seismic characterization of naturally fractured reservoirs
Many hydrocarbon reservoirs have sufficient porosity but low permeability (for example, tight gas sands and coal beds). However, such reservoirs are often naturally fractured. The fracture patterns in these reservoirs can control flow and transport properties, and therefore, play an important role in drilling production wells. On the scale of seismic wavelengths, closely spaced parallel fractures behave like an anisotropic media, which precludes the response of individual fractures in the seismic data. There are a number of fracture parameters which are needed to fully characterize a fractured reservoir. However, seismic data may reveal only certain fracture parameters and those are fracture orientation, crack density and fracture infill. Most of the widely used fracture characterization methods such as Swave splitting analysis or amplitude vs. offset and azimuth (AVOA) analysis fail to render desired results in laterally varying media. I have conducted a systematic study of the response of fractured reservoirs with laterally varying elastic and fracture properties, and I have developed a scheme to invert for the fracture parameters. I have implemented a 3D finite-difference method to generate multicomponent synthetic seismic data in general anisotropic media. I applied the finite-difference algorithm in both Standard and Rotated Staggered grids. Standard Staggered grid is used for media having symmetry up to orthorhombic (isotropic, transversely isotropic, and orthorhombic), whereas Rotated Staggered grid is implemented for monoclinic and triclinic media. I have also developed an efficient and accurate ray-bending algorithm to compute seismic traveltimes in 3D anisotropic media. AVOA analysis is equivalent to the first-order Born approximation. However, AVOA analysis can be applied only in a laterally uniform medium, whereas the Born-approximation does not pose any restriction on the subsurface structure. I have developed an inversion scheme based on a ray-Born approximation to invert for the fracture parameters. Best results are achieved when both vertical and horizontal components of the seismic data are inverted simultaneously. I have also developed an efficient positivity constraint which forbids the inverted fracture parameters to be negative in value. I have implemented the inversion scheme in the frequency domain and I show, using various numerical examples, that all frequency samples up to the Nyquist are not required to achieve desired inversion results.Geological Science
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