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JOURNAL OF SEISMIC EXPLORATION

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CONTENTS Volume 16, Number 1, July 2007
R.R. Stewart, C. Xu and N.L. Soubotcheva
Interpreting multicomponent (3C) seismic data: Examples of sand channel
identification
R. Gunther and M. Reshef
Dip corrections for velocity analysis in super-gathers
P.F. Daley and E.S. Krebes
Quasi-compressional group velocity approximation in a general 21-parameter
weakly anisotropic medium
U.K. Tiwari and G.A. McMechan
Estimation of effective pressure and water saturation by viscoelastic inversion of
synthetic time-lapse seismic data for a gas sandstone reservoir
V. Grosfeld and L.T. Santos
An AVO indicator based on the impedance concept
C.P. Ursenbach, A.B. Haase and J.E. Downton
An efficient method for AVO modeling of reflected spherical waves

ABSTRACTS Vol. 16, No. 1, July 2007
Stewart, R.R., Xu, C. and Soubotcheva, N.L., 2007. Interpreting
multicomponent (3C) seismic data: Examples of sand channel identification.

We analyse two multicomponent (3C) seismic data sets in the quest to delineate sand reservoirs. The first case is a 3C-3D seismic dataset over the Ross Lake oilfield in southwestern Saskatchewan, Canada. We find a reasonable correlation among associated logs, synthetic seismograms, VSP, and surface seismic volumes. The PS synthetic seismogram increases the confidence of PS seismic event identification and provides an essential guide to pick PS horizons. Combining the PP and PS time-thickness maps yields a VP/VS map between horizons bounding the reservoir. Relatively low VP/VS values are interpreted as a sand indicator. A break in the mapped low VP/VS values suggests that there is a shale-cut or shaly part within this target sand body. This interpretation is supported by a subsequent gamma ray log from a horizontal well drilled through the sand and shaly region. PP and PS impedance inversions are performed on the post-stack PP and PS data. A VP/VS map, resulting from the ratio of the P and S impedances, also shows anomalies roughly correlated with the time-thickness results. The second example is from the Pikes Peak heavy oilfield, also in Saskatchewan. In this 2D-3C case, we follow a similar procedure as above to outline the productive sand intervals. In addition, we use geostatistical methods, which combine multicomponent seismic data and well logs, to estimate density and porosity along the line. Again, known sand areas are indicated on the PP/PS results as well as several new, yet untested, anomalies.
Gunther, R. and Reshef, M., 2007. Dip corrections for velocity analysis in
super-gathers.

This paper shows that structural dip information can be used for practical enhancements to velocity analysis workflows. During the analysis of stacking velocities, a dip field is computed from a preliminary stack section. By using the travel-time formulas of the Common Reflection Surface (CRS) method, the dip information alleviates the need for dip move-out (DMO) correction and improves the resolution of semblance panels by correcting for lateral changes in reflection times. Optimal construction of larger super-gathers may be used to increase the
quantity of data used for the calculation. A similar improvement can be achieved for pre-stack migration velocity analysis. Dips are measured on a migrated stack section and then applied during the residual move-out (RMO) process as corrections to semblance computations on the migrated super-gathers.
Alternatively, a dip field measured on a zero-offset stack can be used for a pre-migration mix, allowing for velocity updates with less computation cost than a full pre-stack migration. Examples of both stacking and migration velocity analysis are shown for a 3D land dataset.
Daley, P.F. and Krebes, E.S., 2007. Quasi-compressional group velocity
approximation in a general 21-parameter weakly anisotropic medium.

Using a linearized approximation of the phase velocity, related to quasi-compressional (qP) wave propagation in a general 21-parameter weakly anisotropic medium, an approximate eikonal equation is constructed. The corresponding expression for the related group velocity is then derived. The degenerate (ellipsoidal) case of (qP) wave propagation in an anisotropic medium is explored and an exact group velocity expression obtained, together with the exact expressions for the slowness vector components, for this reduced case. This ellipsoidal group velocity is taken as the reference or background velocity surface.
Slowness vector components are determined in terms of the group velocity vector angles. The reference group velocity is employed as a trial solution in the approximate eikonal equation to obtain an expression for group velocity in a general 21-parameter weakly anisotropic medium. Group velocity expressions, both approximate and exact, are numerically compared for anisotropic models that may be classified as weakly anisotropic or possibly more accurately, weakly anellipsoidal, as the background group velocity surface used is an ellipsoid.
Tiwari, U.K. and McMechan, G.A., 2007. Estimation of effective pressure and
water saturation by viscoelastic inversion of synthetic time-lapse seismic data for
a gas sandstone reservoir.

Inversion of synthetic seismic data for a viscoelastic reservoir model provides estimates of changes in effective pressure (Pe) and water saturation (Sw) over time.
Application is to analysis of simulated time-lapse seismic data for a gas sandstone reservoir. Estimation of unique values of Pe and Sw from two independent seismic data [such as P-wave velocity (VP) and S-wave quality factor (QS)] is theoretically feasible, if the other reservoir properties are known. In the Sw-Pe plane, the solution corresponds to a point for noise-free data and a region for noisy data. For the reservoir properties [porosity (í), clay content, permeability (k)], and the dominant seismic frequency (40 Hz) used in the synthetic examples, density, QS and QS/QP are primarily dependent on Sw, VP is primarily dependent on Pe, and VS, QP and VP/VS are dependent on both Sw and Pe.
Grosfeld, V. and Santos, L.T., 2007. An AVO indicator based on the impedance
concept.

During the last two decades, many approximations for the PP reflection coefficient have been proposed in the literature. Basically, all of them are derived from the classical approximation of Aki and Richards, using additional assumptions on the medium parameters, and using some inversion procedure. The aim of constructing such approximations is to establish reliable attributes that can be capable to indicate the presence of oil or gas. In this work we review some well-known approximations and their respective attributes, and introduce a new
indicator based on a impedance-type approximation for the reflection coefficient. Such indicator can be computed without any additional inversion scheme. Numerical examples are also provided and illustrates the potential of the new approach.
Ursenbach, C.P., Haase, A.B. and Downton, J.E., 2007. An efficient method for
AVO modeling of reflected spherical waves.

Spherical-wave reflection coefficients, which are vital in modeling supercritical reflections, are considerably more difficult to calculate than their plane-wave analogues. We seek to narrow this gap by performing one of the requisite integrations analytically. The key is to assume a wavelet of the Rayleigh form. The resulting reflection coefficients are accurate, even if the true wavelet is not a Rayleigh wavelet, as long as one follows a simple prescription for choosing the wavelet parameters. The method is also efficient enough to implement in an
interactive program, and is thus promising for use in high-volume, supercritical modeling.





 

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