146 research outputs found
Borehole effects on downhole seismic measurements
No full textThesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1994.Includes bibliographical references (p. 270-281).by Chengbin Peng.Ph.D
Acoustic Wave Propagation In And Around A Fluid-Filled Borehole Of Irregular Cross-Section
No full textBoreholes with 10% or more ellipticity are not uncommon. In this paper, we consider
the coupling of an incident elastic wave into a borehole of irregular cross-section and
investigate the cross-mode coupling phenomenon in sonic well logging in the presence
of borehole irregularity. The mode-matching method is used. Different from its original
formulation, we employ the Reichel et al. algorithm to obtain the discrete least square
approximation by trigonometric polynomials, a technique closely related to the fast
Fourier transform (FFT). Our method not only yields great accuracy but also gains
computational speed. Our study shows that the pressure in the borehole fluid is sensitive
to the irregularity of the borehole cross-section, it is larger if the incident wave is along the effective minor axis and smaller if the incident wave is along the effective major axis. In the frequency range of a typical borehole experiment, the solid displacement in the formation is much less affected by the borehole irregularity. In an elliptical borehole, a monopole source excites dipole wave trains that are characteristic of the tube waves, and a centered dipole source excites monopole wave trains that are characteristic of the flexural waves.Massachusetts Institute of Technology. Borehole Acoustics and Logging Consortiu
Optimal Absorbing Boundary Conditions For Finite Difference Modeling Of Acoustic And Elastic Wave Propagation
No full textAn optimal absorbing boundary condition is designed to model acoustic and elastic wave
propagation in 2D and 3D media using the finite difference method. In our method,
extrapolation on the artificial boundaries of a finite difference domain is expressed as
a linear combination of wave fields at previous time steps and/or interior grids. The
acoustic and elastic reflection coefficients from the artificial boundaries are derived.
They are found to be identical with the transfer functions of two cascaded systems: one
is the inverse of a causal system and the other is an anticausal system. This method
makes use of the zeros and poles of reflection coefficients in a complex plane. The
optimal absorbing boundary condition designed in this paper yields about 10 dB smaller
in magnitude of reflection coefficients than Higdon's absorbing boundary condition, and
around 20 dB smaller than Reynolds' absorbing boundary condition. This conclusion is
supported by a simulation of elastic wave propagation in a 3D medium on an nCUBE
parallel computer.ERL/nCUBE Geophysical Center for Parallel Processin
Tube Wave Generation At A Layer Boundary For An Incident Compressional Plane Wave
No full textAn approximate theory for the scattering of an incident plane P wave into tube waves
in a fluid-filled borehole drilled through two homogeneous half-spaces is proposed in
this paper. This theory is in excellent agreement with the zero frequency formulation
(White, 1983) for frequencies below hundreds of Hertz (in the range of conventional
crosshole or VSP experiments) and finite difference simulation at high frequencies. At
low frequency the excited tube wave is found to be independent of the borehole radius
and shows stronger sensitivity to the formation shear velocity contrast across the layer
boundary. The sensitivity towards the compressional velocity perturbation is opposite
to that of the shear wave and density such that little tube wave can be generated if the
compressional and shear velocities are both increased or decreased accordingly. Unlike
the tube wave excited in the borehole when an incident plane wave hits a fracture,
the reflected and transmitted tube waves generated at a layer boundary show opposite
polarities.Massachusetts Institute of Technology. Borehole Acoustics and Logging ConsortiumnCUBE (Graduate Student Fellowship
Borehole Effects On Downhole Seismic Measurements
No full textAn exact formulation for borehole coupling, which is valid for all frequencies and all
azimuthally symmetric and nonsymrnetric components, is given in this paper. The borehole
effects on downhole measurements are studied in detail as functions of frequency,
incidence angle and polarization of an incident wave as well as geophone orientation.
We found that correction of the borehole effect for downhole measurements should be
made for frequencies above 500 Hz in a hard formation. In a soft formation, if the
incidence angle is well away from the resonance angle for a SV incidence, no borehole
correction is needed for frequencies below 300 Hz; while for frequencies above 300 Hz,
the borehole can cause severe problems on downhole measurements. The borehole can
also significantly alter the particle motion direction such that horizontal components
rotation from data itself is unreliable for experiments with frequencies above 1 kHz in
the hard formation and around 500 Hz in the soft formation.ERL/nCUBE Geophysical Center for Parallel PrecessingMassachusetts Institute of Technology. Borehole Acoustics and Logging Consortiu
Cased Borehole Effect On Downhole Seismic Measurements
No full textApproximate and exact formulations are presented for the interaction of an incident
wave with a cased borehole. In the approximate method, the borehole coupling theory
is used to compute pressure in the fluid at a low frequency. The results are simple
and explicit. They are useful in the study of cased borehole coupling and as well
as borehole radiation. In the exact method, elastic potentials in each annulus are
represented as a superposition of fundamental solutions to the Helmholtz equations.
Continuity of displacements and stresses across layer boundaries are used to determine
unknown coefficients. The global matrix method is employed to simultaneously compute
these coefficients in individual layers. This method is advantageous over the Thomson Haskell propagator matrix method in handling evanescent waves. Our results show that,
in a cased borehole, the borehole effects on downhole seismic measurements are more
significant than those in an open borehole, especially when the formation is soft and
the casing is steel. For hard formations and frequency below 1 kHz, cased borehole
influence on downhole geophone measurement is minimal, while at high frequencies,
large discrepancies occur, especially at grazing incidence. For soft formations, both
the pressure in the fluid and the solid displacement on the borehole wall show strong
dependence on frequency and incidence angle, even at very low frequencies. Strong
resonance occurs in the fluid for an SV incidence at angle δ = cos[superscript -1]β/C[subscript T] where CT is the tube wave velocity in a cased borehole. This resonance is prominent even at a very high frequency and large incidence angle because the tube wave velocity is raised well above the formation shear velocity by the steel pipe. This behavior is very different from that in an open borehole. At a particular angle of incidence of a plane P wave, the pressure in the fluid is near zero at low frequencies. This angle is dependent on the casing thickness and can be computed exactly. In general the casing behaves like a shield in such a way that the amplitude of both pressure in the fluid and solid motion on the borehole wall are reduced compared to those in an open borehole.ERL/nCUBE Geophysical Center for Parallel PrecessingMassachusetts Institute of Technology. Borehole Acoustics and Logging Consortiu
Pressure in a fluid‐filled borehole caused by a seismic source in stratified media
No full textA method for numerically simulating hydrophone vertical seismic profiles (VSP) and crosswell data measured in a fluid-filled borehole (either open or cased) embedded in stratified media is presented. The method makes use of both the borehole coupling theory and the global matrix formulation for computing synthetic seismograms in a stratified medium. The global matrix formulation is used to calculate the stress field at the borehole location. Borehole coupling theory is then employed to obtain the pressure in the borehole fluid. Comparisons with exact solutions for an open borehole
in a homogeneous and unbounded formation show that this method is accurate for frequencies below 2 kHz. This method is used to model the Kent Cliffs hydrophone VSP data, where good agreement between the numerical simulations and the field measurements has been found, in both traveltimes and rms amplitudes of the direct P-wave. Examples show that this method is efficient and accurate, and can be applied to model VSP and crosswell experiments using an array of hydrophones.United States. Department of Energy (Grant No. DE-FG02-86ER1363)Reservoir Delineation ConsortiumMassachusetts Institute of Technology. Earth Resources Laboratory (nCUBE/ERL Geophysical Center for Parallel Processing
Finding Community Structures In Social Activity Data
No full textSocial activity data sets are increasing in number and volume. Finding community structure in such data is valuable in many applications. For example, understand- ing the community structure of social networks may reduce the spread of epidemics or boost advertising revenue; discovering partitions in tra c networks can help to optimize routing and to reduce congestion; finding a group of users with common interests can allow a system to recommend useful items. Among many aspects, qual- ity of inference and e ciency in finding community structures in such data sets are of paramount concern. In this thesis, we propose several approaches to improve com- munity detection in these aspects.
The first approach utilizes the concept of K-cores to reduce the size of the problem. The K-core of a graph is the largest subgraph within which each node has at least K connections. We propose a framework that accelerates community detection. It first applies a traditional algorithm that is relatively slow to the K-core, and then uses a fast heuristic to infer community labels for the remaining nodes.
The second approach is to scale the algorithm to multi-processor systems. We de- vise a scalable community detection algorithm for large networks based on stochastic block models. It is an alternating iterative algorithm using a maximum likelihood ap- proach. Compared with traditional inference algorithms for stochastic block models, our algorithm can scale to large networks and run on multi-processor systems. The
time complexity is linear in the number of edges of the input network.
The third approach is to improve the quality. We propose a framework for non- negative matrix factorization that allows the imposition of linear or approximately linear constraints on each factor. An example of the applications is to find community
structures in bipartite networks, which is useful in recommender systems.
Our algorithms are compared with the results in recent papers and their quality
and e ciency are verified by experiments
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