Arcangelo G. Sena

Seismic traveltime equations for azimuthally anisotropic and isotropic media; estimation of interval elastic properties

I derive analytical expressions giving traveltime‐offset curves for multilayered weakly azimuthally isotropic and anisotropic media on terms of the elastic properties of each layer. This method is based upon an approximate skewed hyperbolic moveout formula involving three measured bulk velocities for each reflector. The primary benefits of this technique are: (1) it allows for fast traveltime computation; (2) it makes possible an extremely rapid estimation of the interval elastic parameters; and (3) it provides physical insight into the wave propagation in anisotropic media. I establish a traveltime inversion algorithm which allows the estimation of the five elastic constants together with the orientation of the axis of symmetry for each layer. In the isotropic limit, this algorithm reduces to the conventional one used for determining the interval velocities from stacking velocity measurements in isotropic media. The inversion technique is applied to surface seismic measurements and VSP field surveys. In ...

Simultaneous reconstruction of permittivity and conductivity for crosshole geometries

We develop the theory and algorithm for the simultaneous inversion of permittivity and conductivity maps in the region between two boreholes in the earth. The three‐dimensional region is modeled as an inhomogeneous annulus of known thickness and height with radial and vertical variations of the electrical properties. The medium of interest is probed by a vertical magnetic dipole located on the axis of the cylindrical geometry and the receivers can be placed anywhere outside the inhomogeneous annulus. The inversion procedure is formulated in terms of a source‐type integral equation using monochromatic data. The integral equation is solved using an iterative approach where a Born approximation is applied at each iteration step. The nonuniqueness of the problem is overcome by imposing additional constraints on the solution using the method of regularization of Tikhonov. The distribution of electrical properties is obtained directly by this method. Numerical simulations, including multisource inversions, show...

Kirchhoff migration and velocity analysis for converted and nonconverted waves in anisotropic media

We develop asymptotic expressions and outline a procedure to perform Kirchhoff migration in anisotropic media. This technique is based on a new Green’s tensor representation for azimuthally isotropic media obtained by using analytical forms for the ray amplitudes and traveltimes. Since in real applications the usage of general anisotropy in a migration scheme will be limited by the availability and reliability of the velocity model considered, we also develop a new anisotropic velocity analysis scheme to generate realistic anisotropic models for migration in azimuthally isotropic media for nonconverted and converted qP-qSV waves. This velocity analysis technique is based on nonhyperbolic traveltime‐offset formulas explicitly given in terms of the five elastic constants of azimuthal isotropy. The imaging technique is applied to nonconverted‐as well as converted‐wave surface seismic data. In both cases the method provides accurate images of the subsurface. Even with a weak to moderate percentage of anisotro...

3D AVO And Seismic Inversion In María Inés Oeste Field, Santa Cruz, Argentina, a Case Study

The successful exploration for new reservoirs in mature areas, as well as the optimal development of existing fields, requires the integration of unconventional geological and geophysical techniques. In particular, the calibration of 3D seismic data to well log information is crucial to obtain a quantitative understanding of reservoir properties. The advent of new technology for prestack seismic data analysis and 3D visualization has resulted in improved fluid and lithology predictions prior to expensive drilling. Increased reservoir resolution has been achieved by combining seismic inversion with AVO analysis to minimize exploration risk. In this paper we present an integrated and systematic approach to prospect evaluation in the Maria Ines Oeste field. We will show how petrophysical analysis of well log data can be used as a feasibility tool to determine the fluid and lithology discrimination capabilities of AVO and inversion techniques. Then, a description of effective AVO and prestack inversion tools for reservoir property quantification will be discussed. Finally, the incorporation of the geological interpretation and the use of 3D visualization will be presented as a key integration tool for the discovery of new plays. Geological framework The Maria Ines sandstones represent the base of a Second Order Transgressive System Tract (TST), deposited in a coastal to open shelf environment. These sands represent a transgressive clastic depositional system in the Austral Foreland Basin. The discontinuity over which these sandstones were deposited represents one of the orogenic pulses of the Andean Orogeny. In a few wells located to the west and northwest of the area of study it is possible to define a lower sand body, which represents the Lowstand System Tract (LST), limited by incised valleys. To the east of the study area, the sandstones onlap the regional forebulge represented by an antiform with a north-south axis that plunges to the south. The Maastrichtian – Paleocene sandstones are seismically well defined, due to their thickness (50 meter average) and hydrocarbon content. In the study area the traps are mainly structural, associated with an east-west normal fault system. As they are charged to the spillpoint, the faults themselves determine the amount of the hydrocarbon trapped. Seismically, the response to this combination is in the form of bright spots, limited at their northern edge by faults. Petrophysical Analysis The development of the Maria Ines field was essentially based on the detection of these bright spots and their structural evaluation using 3D seismic data. After drilling almost ten wells, a further investigation of these amplitude anomalies became necessary. A petrophysical analysis was carried out to evaluate the various relationships between lithology and fluid type via trend analysis and cross-plotting techniques. This firststage determined which petrophysical attribute, or combination of attributes, shows characteristics that can be used as an identifier of a specific lithology and pore fluid. This understanding provides a preliminary assessment on which AVO attribute products can be diagnostic. Prior to AVO modeling, all wells were processed using standard log editing and interpretation techniques. Two of the wells used in this study had measured shear velocities. For those wells without shear velocities, a local estimator was developed and applied; care was taken to ensure that shear velocities across pay were properly calculated. Gassman’s equation was used to replace the insitu fluid in each well with three different fluids. Thus, brine, gas and two oil cases were generated for each well evaluated. Figure 1 shows the interval of interest from one of the wells. Figure 1.Log display of one of the gas wells from the study area. Note the sharp contrast in acoustic impedance between the overlying shale and the sand. VSH V/V 0 1

Simultaneous reconstruction of permittivity and conductivity maps by iterative born inversion

In the present paper, we develop the theory and the algorithm for the simultaneous inversion of permittivity and conductivity maps in the region between two boreholes in the earth. The three-dimensional region is modelled as an inhomogeneous annulus of known thickness and height with radial and vertical variations of the electrical properties. The medium of interest is probed by a vertical magnetic dipole located on the axis of the cylindrical geometry and the receivers can be placed anywhere outside the inhomogeneous annulus. This allows for a crosshole set up, The inversion procedure is formulated in terms of a sourcetype integral equation using monochromatic data. The integral equation obtained is then solved using an iterative approach where a Born approximation is applied at each iteration step. The nonuniqueness of the problem is overcome by imposing additional constraints on the solution using the method of regularization of Tikhonov. Numerical simulations, including multi-source inversions, show that good results can be obtained for smoothly varying electrical properties, even for large contrast cases. In the case of rapid local variations of such properties, convergence can also be reached but at a slower rate and the reconstructions are smoothed versions of the original properties.