Showing papers in "Seg Technical Program Expanded Abstracts in 1987"

Cellular-automaton Fluids: A Model For Flow In Porous Media

Abstract: Numerical models of fluid flow through porous media can be developed from either microscopic or macroscopic properties. The large‐scale viewpoint is perhaps the most prevalent. Darcy’s law relates the chief macroscopic parameters of interest—flow rate, permeability, viscosity, and pressure gradient—and may be invoked to solve for any of these parameters when the others are known. In practical situations, however, this solution may not be possible. Attention is then typically focused on the estimation of permeability, and numerous numerical methods based on knowledge of the microscopic pore‐space geometry have been proposed. Because the intrinsic inhomogeneity of porous media makes the application of proper boundary conditions difficult, microscopic flow calculations have typically been achieved with idealized arrays of geometrically simple pores, throats, and cracks. I propose here an attractive alternative which can freely and accurately model fluid flow in grossly irregular geometries. This new method s...

Observations of EDA And PTL Anisotropy In Shear Wave VSPs

Abstract: Detailed analysis of shear-wave splitting recorded in a VSP survey of the Paris Basin (Crampin et al., 1986a. and Crampin & Bush, 1986) interpreted observations as the effective anisotropy of vertically aligned cracks. Synthetic modeling demonstrated that most of the essential features of the observed patterns of particle displacement could be reproduced by propagation through a simple model of aligned cracks. Anomalies where synthetic patterns failed to match observed patterns may now be accounted for by raypaths sampling both sides of the shear-wave polarization transition-zone. The presence of PTL (periodic thin-layer anisotropy) and aligned cracks together moves the position of the transition zone so that wider offset raypaths sample both sides of the transition-zone.

Cooperative Inversion of Geophysical Data

Abstract: Geophysical inversion by iterative modeling involves fitting observations by adjusting model parameters. Both seismic and potential-field model responses can be influenced by the adjustment of the parameters of the rock properties. The objective of this "cooperative in­ version" is to obtain a model which is consistent with all available surface and borehole geophysical data. Al­ though inversion of geophysical data is generally non­ unique and ambiguous, we can lessen the ambiguities by inverting all available surface and borehole data. This paper illustrates this concept with a case history in which surface seismic data, sonic logs, surface gravity data, and borehole gravity meter (BHGM) data are ad­ equately modeled by using least-squares inversion and a series offorward modeling steps. The traditional analysis of various geophysical data sets has been termed "integrated interpretation." Such analysis has traditionally involved much processing, modeling, and subjec­ tive interpretation in order to develop a geologic model which is consistent with various surface and borehole geophysical data. A method which automates some of the above pro­ cedures is least-squares inversion. This procedure perturbs an initial set of model parameters in order to fit the observed data. Least-squares inversion is described in many papers, in­ cluding those of Jackson (1972), Wiggins (1972), Jupp and Vozoff (1975), and Lines and Treitel (1984). This inversion technique could also be termed "automated iterative mod­ eling." In terms of modeling different geophysical data sets, there are many possible approaches which allow inversion of a given data set, and it is doubtful whether one single recipe will prove suitable for all problems. Nevertheless, by jointly invert­ ing various types of geophysical observations, we may lessen the extent of some of the ambiguities inherent in individual data sets. LaFehr (1984) gives a germane discussion of several reasons for this type of coordinated geophysical effort. In the present study, we define "cooperative inversion" as the esti­ mation of a subsurface model which is consistent with various independent geophysical data sets. In the case discussed in this paper, these data sets include surface and borehole observa­ tions of the seismic and gravity responses. In our definition, cooperative inversion may include either "joint inversion" or "sequential inversion."

Full Prestack Versus Shot Record Migration

Abstract: Full prc-stack migration can be replaced by shot record migration plus stacking without loss of accuracy. To prove this, it is most convenient to describe forward seismic modeling in terms of matrix multiplications. Then full pre-stack migration can be described in terms of matrix inversions. By using some simple rules of matrix algebra it can then be shown that the full pre-stack migration result can be wrftte as the sum of shot record migration results. This conclusion is particularly important for the 3-D case because the data management in 3-D shot record migration is dramatically more convenient than in 3-D full pre-stack migration.

Elastic Anisotropy Due to Aligned Cracks

Abstract: All existing theoret ical expressions which provide connections between the density of saturated aligned cracks and elastic anisotropy are restricted in some important way; for example, to the case of soft pore fluids (Kf 0.2), and the equant porosity is often large ($, > 0.2). New expressions are derived which are not limited in these ways. A notable feature of these more general expressions is their insensitivity to the aspect ratio of the cracks; only the crack density is critical. An important conclusion of this more general model is that insights previously achieved, concerning the shear wave splitting due to vertical aligned saturated cracks, are sustained. w 2.4

Decomposition of the Magnetotelluric Impedance Tensor Which is Useful in the Presence of Channeling

Abstract: It is believed that there are many occasions when the magnetotelluric impedance tensor is the result of local galvanic distortion of electric currents which arise from induction in a structure which is approximately two-dimensional on a regional scale. Procedures for rotating the impedance tensor, such as minimizing the mean square modulus of the diagonal elements, cannot in general recover the principal axes of induction nor do they recover the principal impedances but rather linear combinations of them.

Elastic Kirchhoff Migration For Vertical Seismic Profiles

Abstract: Elastic Kirchhoff migration is implemented for the VSP recording geometry. The resulting migration formula requires measurement of the stress as well as the displacement. Since stress is not measured in a VSP, and in many cases the horizontal component of displacement is not measured, approximate migration formulas are given for these cases. The elastic migration formula for the case where only the vertical components are available, is the same as the acoustic migration formula, where the pressure data are replaced by the magnitudes of the elastic data as reconstructed from the vertical components, and the acoustic Green’s functions are replaced with either the P or S wave elastic Green’s functions. Two expressions for migration of two component displacement data are presented. In the first, the terms involving traction data are simply ignored. In the second, an improved backpropagation operator for the displacement field is obtained by replacing the traction data in the Kirchhoff integral by displacement data using Hooke’s law. The migration expressions for the cases where two component data are available produce images which are less contaminated by artifacts than the migration images of one component data.

Separation of Seismic Waves

Abstract: This paper presents a multichannel method for enhancing the signal-to-noise ratio, when the noise is purely random. We show that under some assumptions the contribution of each wave may be individualized. The quality of the results is sensitive to the estimation of the interspectral matrix from the initial traces. The proposed method is general: according to the type of document (shot point or CDP gather, stacked seismic section), we indicate how the interspectral matrix is best estimated.

Effect of Normal Moveout Errors On Amplitude Versus Offset-derived Shear Reflectivity

Abstract: P-wave amplitude-versus-offset information contains the shear-wave reflectivity. An algorithm for estimating the shear-wave reflectivity from P-wave CDP gathers, based on a small angle expansion of Zoeppritz equation, is shown to be extremely sensitive to move-out velocity. A velocity error equivalent to a 2 milliseconds misalignment from the inner to the outer trace can produce an additional spurious shear component that is larger than the correct result. The problem is more fundamental. If the true shear-wave reflectivity is ‘spurious”, i.e. does not look exactly like the P-wave reflectivity over an interval, then the effect on the P-wave gather can look like a slight change in move-out velocity. Implicit in velocity estimation is the assumption that short offset traces (mostly P-wave reflectivity) “look like” long offset traces (which contain more shear-wave information) over time intervals sufficiently long to estimate a velocity. Only that part of the shear-wave reflectivity that correlates with the P-wave reflectivity can be extracted from P-wave data. As a result the new information derived from amplitude variations with offset is inherently low frequency.

3-D Seismic Imaging of Heat Zones At an Athabasca Tar Sands Thermal Pilot

Abstract: Vast resources of relatively untapped tar sands petroleum reserves are waiting to be developed when the technology becomes available to efficiently mobilize this highly viscous hydrocarbon For example, at this pilot site, there are approximately 2000 bbls/acre ft of oil in place In recent years multi-millions of dollars have been invested in pilot projects throughout Western Canada in order to evaluate in-situ techniques, such as fire-floods and steam injection, as a means of producing this oil

New Directions For Marine 3-D Surveys

Abstract: In the late 1960’ s and early 1970’ s there were severe constraints on how a marine three-dimensional (3-D) seismic reflection survey could be designed. Streamer location techniques were almost nonexistent, and seismic data processing procedures were developed only for 2-D surveys. For these reasons and others, 3-D surveys were designed as a collection of parallel 2-D seismic lines. Geologic considerations had a minimal impact because 3-D surveys were designed with the concept of dip direction and strike direction. As survey sizes grew, this simple geologic picture broke down.

Velocities In Hydrocarbons And Hydrocarbon-saturated Rocks And Sands

Abstract: Compressional ultrasonic velocities were measured in pure alkanee, alkenes, and naphthenes (cycloparaffins) as a function of temperature. It was found that the velocities in all the hydrocarbons decreased with increasing temperature approximately linearly. Furthermore, the velocities show also a linear relationship with the inverse of the carbon number (or equivalently the molecular weight). Measured wave velocities in mixtures of the above hydrocarbons were found to obey very closely values calculated by a simple mixing model.

Factors Limiting Potential of Evaporites As Hydrocarbon Source Rocks

Abstract: and suspended organic matter retards the evaporites would provide excellent source rocks. settling rate of the organic matter and In practice, however, this is not the case. An examination of effective prolongs its exposure to oxidative source rocks within evaporite sequences in processes; (2) high concentrations of sulfates, and possibly nitrates, provide a the major producing provinces reveals that

Stratigraphic Modeling of Sedimentary Basins

Abstract: A new deterministic computer program has been developed to simulate the stratigraphic development of two-dimensional transects across sedimentary basins. The program operates within the framework of generally accepted stratigraphic, sedimentologic, and geophysical principles to define overall basin geometry, depositional sequence geometry, and details of stratigraphic relations between intervals. Clastic, carbonate, and mixed carbonate-elastic regimes can be modeled. A movie will be shown illustrating the stratigraphic evolution of a typical passive margin transect in response to changes in the rates of subsidence, sea-level rise, ard sedimentation.