SEP-98 (1998)

3-D Imaging

3-D Seismic Imaging

(ps.gz 255K) (pdf 0K)
Biondi B.
This document was generated using the LaTeX2HTML translator Version 97.1 (release) (July 13th, 1997) Copyright © 1993, 1994, 1995, 1996, 1997, Nikos Drakos, Computer Based Learning Unit, University of Leeds. The command line arguments were:
latex2html -dir /public/docs/sep98/biondo/paper_html index.ltx. ...

25th SEP Reunion

The History of the Mitchell Building Skylight

(ps.gz 3904K) (pdf 0K) (src 33635K)
Levin S. A.
During recent earthquake reinforcement of the Ruth Wattis Mitchell Earth Sciences Building, funds were successfully raised to install a skylight in the th}}$"> floor. The skylight was completed in January of 1998 and dedicated during the Stanford Exploration Project's th}}$"> year reunion on July th}}$">, 1998. Here is the story.

Rock Physics

Double porosity modeling in elastic wave propagation for reservoir characterization

(ps.gz 618K) (pdf 0K) (src 171K)
Berryman J. G. and Wang H. F.
To account for large-volume low-permeability storage porosity and low-volume high-permeability fracture/crack porosity in oil and gas reservoirs, phenomenological equations for elastic wave propagation in a double porosity medium have been formulated and the coefficients in these linear equations identified. The generalization from single porosity to double porosity modeling increases the number of independent inertial coefficients from three to six, the number of independent drag coefficients from three to six, and the number of independent stress-strain coefficients from three to six for an isotropic applied stress and assumed isotropy of the medium. The analysis leading to physical interpretations of the inertial and drag coefficients is relatively straightforward, whereas that for the stress-strain coefficients is more tedious. In a quasistatic analysis of stress-strain, the physical interpretations are based upon considerations of extremes in both spatial and temporal scales. The limit of very short times is the one most relevant for wave propagation, and in this case both matrix porosity and fractures are expected to behave in an undrained fashion, although our analysis makes no assumptions in this regard. For the very long times more relevant for reservoir drawdown, the double porosity medium behaves as an equivalent single porosity medium. At the macroscopic spatial level, the pertinent parameters (such as the total compressibility) may be determined by appropriate field tests. At the mesoscopic scale pertinent parameters of the rock matrix can be determined directly through laboratory measurements on core, and the compressiblity can be measured for a single fracture. We show explicitly how to generalize the quasistatic results to incorporate wave propagation effects and how effects that are usually attributed to squirt flow under partially saturated conditions can be explained alternatively in terms of the double-porosity model. The result is therefore a theory that generalizes, but is completely consistent with, Biot's theory of poroelasticity and is valid for analysis of elastic wave data from highly fractured reservoirs.

Anisotropy

The azimuth moveout operator for anisotropic media

(ps.gz 671K) (pdf 0K) (src 26K)
Alkhalifah T.
The azimuth moveout (AMO) operator in homogeneous, moderate to strong, anisotropic media, like in isotropic media, has an overall skew saddle shape. Yet, the AMO operator in anisotropic media is complicated; it includes, among other things, triplications at low angles. Even, in weaker anisotropies, with anisotropy parameter =0.1 (  anisotropy), the AMO operator differs from the isotropic one, though does not include triplications. The structure of the operator in VTI media (positive ) is stretched compared to operators in the isotropic media, with the amount of stretch dependent on the strength of anisotropy. If the medium is both v(z) and anisotropic, a likely combination in practical problems, the shape of the operator again differs from that for isotropic media. Yet, the difference in the AMO operator for the homogeneous case and the v(z) one is small even for anisotropic media. Simply stated, anisotropy impacts AMO more than typical smooth vertical velocity variations.

An acoustic wave equation for orthorhombic anisotropy

(ps.gz 527K) (pdf 0K) (src 54K)
Alkhalifah T.
Using a dispersion relation derived under the acoustic medium assumption for P-waves in orthorhombic anisotropic media, I obtain an acoustic wave equation valid under the same assumption. Although this assumption is physically impossible for anisotropic media, it results in wave equations that are kinematically and dynamically accurate for elastic media. The orthorhombic acoustic wave equation, unlike the transversely isotropic (TI) one, is a six-order equation with three sets of complex conjugate solutions. Only one set of these solutions are perturbations of the familiar acoustic wavefield solution in isotropic media for in-coming and out-going P-waves, and thus, are of interest here. The other two sets of solutions are simplify the result of this artificially derived sixth order equation, and thus, represent unwanted artifacts. Like in the TI case, these artifacts can be eliminated by placing the source in an isotropic layer, where such artifacts do not exist.