146 research outputs found
Geophysical study of the structure and processes of the continental convergence zones: Alpine-Himalayan belt
Studies of the structure of the continental collision zones using seismic and body waves, theoretical modelling of the thermal regime of the convergence processes, and studies of earthquake mechanisms and deformation aspects of the model are covered
MPLS Automatic Bandwidth Allocation via Adaptive Hysteresis
Cataloged from PDF version of article.MPLS automatic bandwidth allocation (or provisioning) refers to the process of dynamically updating the bandwidth allocation of a label switched path on the basis of actual aggregate traffic demand on this path. Since bandwidth updates require signaling, it is common to limit the rate of updates to reduce signaling costs. In this article, we propose a model-free asynchronous adaptive hysteresis algorithm for MPLS automatic bandwidth allocation under bandwidth update rate constraints. We validate the effectiveness of the proposed approach by comparing it against existing schemes in (i) voice and (ii) data traffic scenarios. The proposed method can also be used in more general GMPLS networks. (C) 2010 Elsevier B.V. All rights reserved
Electrokinetics Of A Fluid-Saturated Rock Sample: Laboratory Experiments
The conversion between seismic and electromagnetic energies was discovered in a fluidfilled porous formation. When seismic waves propagate through a fluid-saturated porous formation, relative motion between the pore fluid and the solid matrix is generated and cation motion in the fluid is formed. The streaming electric current induces electromagnetic waves in the formation. There is an opposite phenomenon, i.e., the conversion of electric energy into acoustic energy in the porous formation. The electroseismics in porous sandstone samples are investigated by ultrasonic experiments in our laboratory. A compressional or a shear transducer excites an acoustic wave in the water-saturated sample and the electric signals generated on the surface are measured by an electrode. The relationship between the electric potential and acoustic wave or the conductivity of water-saturated rocks is studied. The electro-seismic conversion in rock samples is also investigated. Electroseismics could provide an effective means for estimating the fluid-saturated porous formation.Massachusetts Institute of Technology. Borehole Acoustics and Logging ConsortiumUnited States. Dept. of Energy (Grant DE-FG02-93ERI4322
Ultrasonic Laboratory Study of Full Waveform Acoustic Logs in Boreholes with Fractures
A set of ultrasonic experiments was carried out to determine the effects of horizontal and vertical fractures on full waveform acoustic logs. Boreholes of 1 cm diameter were drilled in aluminum blocks. Measurements were made with horizontal fractures of 0.05 mm, 1.0 mm, 2.5 mm, and 4.5 mm width and a vertical fracture of 1.0 mm width. The horizontal
fractures of even the smallest thickness significantly attenuate the P, S, and pseudo-Rayleigh waves. The Stoneley waves are the least attenuated, and attenuation increases
with increasing fracture width. The vertical fracture attenuates Stoneley waves most
significantly. Both scattering and fluid flow playa role in attenuation. The results may
qualitatively be extended to inclined open fractures, where we expect strong attenuation
of P and S waves and moderate attenuation of Stoneley waves.Massachusetts Institute of Technology. Full Waveform Acoustic Logging Consortiu
Velocity Anisotropy Of Two Deep Crystalline Samples
Using ultrasonic velocity measurements taken over a multiplicity of directions we show
that samples exhibit weak to moderate anisotropy of seismic velocities. We further
define the anisotropic geometry with high resolution scanning electron microscopy.
Our data indicate that one sample, a granite, is transversely anisotropic, and that the
presence of fine to moderately fine microcracks is the most important factor effecting
the velocities. We model the angular velocity dependence using 5 elastic constants
and show that all 9 observed velocities fit these predictions to within 0.1 km/s. We
are unable to obtain similar fits to a second sample, a mica-schist, in the same fashion.
SEM observations indicate this rock displays orthorhombic symmetry. We made
additional velocity measurements in order to calculate 9 elastic constants, and found
that the predicted angular velocity dependence agreed much better with our velocity
observations.Massachusetts Institute of Technology. Full Waveform Acoustic Logging ConsortiumEnte nazionale per l'energia elettric
Full Waveform Acoustic Logging - From Theory To Applications
This report contains results from the third year of the Full Waveform Acoustic
Logging Consortium and rock physics studies at M.J.T. This year marks the completion
of the first phase of the project which has been directed primarily to the
understanding of the basic theoretical aspects of acoustic waves in a borehole.
With such a background we are ready to emphasize applications as well as to
undertake special problems which require new and different theoretical approaches.
As examples of the latter, we can mention uncentered tools, vertical fractures around
boreholes, thinly bedded formations and anisotropy. The third year studies fall into four general areas: theoretical aspects of wave propagation in the borehole, applications to the characterization of formations, integrated log analysis and physical properties of sedimentary rocks relevant to logging. There are fifteen papers in this report which discuss individual topics in detail. In this introduction we summarize the major points and also list the potential applications of full waveform acoustic logs and future directions of our research
Modeling Of Seismoelectric Effects In A Borehole
We present a method to simulate the propagation of seismic and converted electromagnetic
waves generated by a mechanical borehole source embedded in a layered poroelastic
medium. The electroseismic conversions occur at both the borehole wall and the layer
boundaries. Most studies in electroseismic effects have been modelled and tested with
seismic sources and detectors (geophones and antennas) at the surface. In this paper,
we investigate the case of a seismic source in a borehole and receivers either at the
surface or embedded in the medium. The method is formulated as a boundary element
technique (where the poroelastic displacement and relative flow Green's functions are
calculated by the discrete wavenumber method. The singular properties of the Green's
functions are determined analytically using static Green's functions to regularize the
integrals. This is necessary to calculate the element's self interaction. The borehole is
cylindrical and its axis rulls normal to the interfaces. The coupled electroseismic effects in the layered media are included by using the global matrix technique.
The developed method is an extension of the model of Biot-Rosenbaum, who applied
the wavenumber integration technique to investigate the effect of formation permeability
on Stoneley waves, using Biot's theory to model the wave propagation effects of a homogeneous permeable formation surrounding a borehole. We extend the Biot-Rosenbaum
model by including the effect of a heterogeneous permeable formation surrounding the
borehole. The effect of formation permeable zones (or fractured zones) on Stoneley
waves can now be investigated. The other modification is the inclusion of conversions of
mechanical into electromagnetic waves at mechanical and/or electrical contrasts in the
poroelastic formation. The converted electromagnetic fields are sensitive to large permeability contrasts and fluid chemistry contrasts inside a reservoir. Using the electroseismic method downhole will provide more information about permeability/permeability contrasts in the formation, as well as additional lithological information (salinity of the fluids)
Electroseismic Waves From Point Sources In Layered Media
The macroscopic governing equations controlling the coupled electromagnetics and acoustics of porous media are numerically solved for the case of a layered poro-elastic medium.It is shown that these coupled equations decouple into two equation sets describing two uncoupled wavefield pictures. That is, the PSVTM picture where the compressional and vertical polarized mechanical waves drive currents in the PSV particle motion plane that couples to the electromagnetic wavefield components of the TM mode. And the SHTE picture where the horizontal polarized rotational mechanical waves drive currents in the SH particle motion plane that couples to the electromagnetic wavefield
components of the T E mode. The global matrix method is employed in computing
electroseismograms in layered poro-elastic media in the PSVTM picture.
The principal features of the converted electromagnetic signals are the following:
(1) contacts all antennas at approximately the same time; (2) arrives at the antennas
at half of the seismic traveltime at normal incidence reflected P waves; and (3) changes
sign on opposite sides of the shot.
The seismic pulse is shown to induce electric fields that travel with the compressional
wavespeed and magnetic fields that travel with the rotational wavefield. The frequency
content of the converted electromagnetic field has the same frequency content of the
driving incident seismic pulse, as long as the propagation distances are much less than
the electromagnetic skin depth.
Snapshots in time and converted electromagnetic amplitudes versus seismic point
source-antenna offset-are calculated for contrasts in mechanical and/or electrical medium
property. Conversion happens there where the seismic wavefront passes a contrast in
medium properties due to generated imbalances in current across the contrast. The TM
component amplitude radiation pattern away from the interface shows similarities with an effective electric dipole radiation pattern, or its dual, an effective magnetic current
loop radiation pattern centered right beneath the source at the contrast's depth. The
TM mode amplitudes decay rapidly with traveled distance and suggest the importance
of a Vertical Electroseismic Profiling geometry to enhance recording of the converted
electromagnetic signal by positioning the antennas closer to the target (contrast) of
interest.United States. Dept. of Energy. Office of Energy Research (Grant DE-FG02-93ERI4322
Inversion For Permeability From Stoneley Wave Velocity And Attenuation
The in situ permeability of a formation is obtained by the inversion of Stoneley wave
phase velocity and attenuation, which are evaluated by applying the Extended Prony's
method to the array sonic logging data. The Maximum Likelihood inversion is used
together with logarithmic parameterization of the permeabilities. Formation shear
wave velocity is also inverted for. This process is tested on both synthetic and field
data. Logarithmic parameterization contributes to rapid convergence of the algorithm.
Permeabilities estimated from field data are in good agreement with core measurements.Massachusetts Institute of Technology. Full Waveform Acoustic Logging Consortiu
Dispersive Wave Analysis – Method and Applications
A technique for estimating the dispersion characteristics of propagating waves as measured by an array is detailed. The technique consists of bandpass filtering the data through a filterbank and then processing the filtered waveforms non-dispersively. The results can show the dispersion of the entire time series or be parsed in time to analyze the dispersion characteristics of any section of the time series. Processing LWD field data shows that this method can extract dispersion characteristics over a broadband of frequencies and with low amplitude signals. Both the field data and laboratory scale data show that multiple modes present over the same frequency band can be identified.Halliburton CompanyMassachusetts Institute of Technology. Earth Resources LaboratoryMassachusetts Institute of Technology. Borehole Acoustics and Logging Consortiu
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