1,721,802 research outputs found
Crack Models For A Transversely Anisotropic Medium
Full text linkA commonly used model for a transversely anisotropic crack rock is that by Hudson
(1980, 1981). This model is based on a simplified analysis of a thin circular crack, with
displacement and stress conditions specified on the boundary. These papers have a
second order correction in addition to the first order term in porosity/crack density. In
this paper we compare the results of Hudson with those of Anderson et al. (1974) and
Cheng (1978) using the long wavelength static approximation and the ellipsoidal crack
model first proposed by Eshelby (1957). We showed that the Hudson model and those
based on the complete Eshelby theory agree for small aspect ratio cracks and small
crack densities, as expected, provided the weak inclusion version of Hudson's model
(1981) is used. For larger crack densities but small aspect ratios, Hudson's first order
term agrees with the Eshelby solution. The expansion in the second order term in crack
density is an asymptotic series and not a uniformly converging series. Thus there is no
general statement one can make about the accuracy of the second order expansion that
is valid for a variety of situations. A new expansion based on the Pade approximation is
proposed which is identical to Hudson's expansion up to second order in density. This
expansion avoids some of the problems associated with Hudson's second order expansion
such as increasing moduli with crack density at relatively small crack densities.Massachusetts Institute of Technology. Borehole Acoustics and Logging ConsortiumUnited States. Dept. of Energy (Grant DE-FG0286ER13636
Full Waveform Inversion of P Waves for V[subscript s] and Q[subscript p]
No full textWe present an indirect method of determining shear wave velocities from full waveform
acoustic logs based on the inversion of the spectral ratio of the P-wave trains at two
source-receiver separations. This method simultaneously inverts for the formation shear
wave velocity and compressional wave attenuation. The P-wave response is calculated
by means of branch-cut integration. This method is useful in "soft" formations where
the shear wave velocity is lower than the acoustic velocity of the borehole fluid and
thus there are no refracted shear wave or pseudo-Rayleigh wave arrivals. The method
is shown to give good estimates of formation shear wave velocity in both synthetic and
field data. The inversion algorithm is sensitive to local minima; care must be taken to
avoid them.Massachusetts Institute of Technology. Full Waveform Acoustic Logging ConsortiumNational Science Foundation (U.S.) (Grant OCE84-08761
Report Summary
Full text linkThis report contains the results of work completed during the sixth year of the Full
Waveform Acoustic Logging Consortium in the Earth Resources Laboratory at M.LT.
Over the last six years, our work have evolved from simply modelling an open borehole
in an isotropy, elastic formation to the study of logging under more complicated, realistic conditions, especially those that may be encounted in production and development environments.
In the past year, we have modelled elastic wave propagation in a transversely
isotropic formation, in a formation where the borehole is irregular in radius, across an
open fracture, and in a transversely isotropic porous formation, where the horizontal
and vertical permeabilities are different. All of these situations are rather common in
the field. It is important for us to understand how these complications mayor may
not affected our interpretation of the full waveform acoustic logs, and how large an
error we are likely to make if our interpretation is based on a isotropy, elastic model
of the formation. Just as importantly, we need to learn to identify these situations in
the field, so that we can make the appropriate corrections to our interpretation.
In data analysis, we have refined the Extended Prony's Method to calculate the
dispersion and attenuation of the guided waves in the borehole, and have applied
the technique to both laboratory and field data with success. We have also studied
the sensistivity of the guided waves to a transversely anisotropic formation. These
sensistivities are critical in the inversion of the dispersion and attenuation obtained
by the Extended Prony's Method for the actual elastic and anelastic properties of the
formation.
In a continuation of last year's work, we have studied the attenuation of solid particles
in a viscous fluid, applicable to the study of both drilling muds and unconsolidated
sands.
We have studied the diffraction of a cylindrical transducer and its effect on the
measurement of attenuation in laboratory experiments using the spectral ratio technique.
We are also continuing in our effort of scale model experiments to mimic field
situations which are not readily modelled by analytic or numerical methods.
A field study and comparison of velocities from core measurement, borehole compensated
sonic log, full waveform acoustic log, and VSP in the Salton Sea Scientific
Drilling Program is included in this report. Also included is an extended abstract on an
effort to map fractures in the Canadian Shield using borehole geophysical techniques.
The following is a summary of the papers in this report
Vapor-liquid equilibria for the ternary system of carbon dioxide + ethanol + ethyl acetate at elevated pressures
No full textVapor-liquid equilibria of carbon dioxide with isopropyl acetate, diethyl carbonate and ethyl butyrate at elevated pressures
No full textSimulated Annealing Determination Of Shear Wave Travel Time
Full text linkThe method of simulated annealing is introduced to obtain relative moveouts between
different depths from an iso-offset section. This method has been shown to be more
consistent than conventional picks based on peaks, troughs, or zero crossings especially
in situations where the signal-to-noise ratio is low or the wavelet is emergent. This
method also provides a means of quantifying the relative confidence in each pick over
the entire depth of the well. The method has been applied to the data obtained by
the ARCO shear wave logging tool and compared favorably with more conventional
estimates of shear wave slowness and was shown to be robust, even in areas of weak
arrivals.Massachusetts Institute of Technology. Full Waveform Acoustic Logging Consortiu
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