Håvar Gjøystdal

Traveltime and amplitude estimation using wavefront construction

We have developed and implemented a new method for estimation of first and later arrival traveltimes and amplitudes in a general 2D model. The basic idea of this wavefront (WF) construction approach is to use ray tracing to estimate a new WF Erom the old one. The calculation goes along the WFs rather than along the rays.

Inversion of reflection times in three dimensions

When reflection data are available from a grid of crossing seismic lines, it is possible to construct normal incidence time maps from interpreted stacked sections and then apply three‐dimensional (3-D) ray‐tracing techniques following the normal‐incidence raypaths down to the various reflectors. The main disadvantage of this well‐known “time map migration” procedure is that interval velocities must be known a priori, and they must be estimated in advance by some approximate method. A technique is presented here which combines the above procedure with an inversion algorithm, providing direct calculations of interval velocities from the additional use of nonzero offset traveltime observations. A generalized linear inversion scheme is used, making possible a complete calculation of interval velocities and reflection interfaces, the latter represented by bicubic spline functions. To test the method in practice, we have applied it to (1) synthetic data generated from a constructed model, and (2) real data obta...

3-D ray modeling by wavefront construction in open models

A synthesis of two newly developed concepts in 3-D modeling is developed in this paper: (1) The open, (non-complete) seismic model, and (2) the ray tracing based wavefront (WF) construction method. The open model may contain interfaces with holes and other missing parts, which simplifies model building considerably because the input horizon data from standard interpretation and processing packages are often incomplete. A set of volumes is defined in the model. A volume is a logical unit that points to a set of property functions, e.g., P-velocity, S-velocity, and density. The properties are represented either as constants or as B-spline functions of the spatial coordinates (x, y) or (x, y, z). The volumes of the model are assigned to opposing sides of each interface and not to specific spatial areas of the model, which is the case in most (blocky) model representations. The interfaces are given explicitly by triangular grids where the sizes of the triangles are determined locally by the curvature of the interface. We show how modeling by WF construction is both possible and computationally efficient in open models, but only after some modifications to deal with the ambiguity of the model representation. It is not possible to find a unique volume for the spatial positions in an open model. Instead, the volumes (with associated velocities, etc.) are determined from the last interface encountered by each ray in the WF. To find an arrival in a receiver, the volume associated to the receiver has to match the volume of the WF hitting the receiver.

Simulated Prestack Local Imaging: a robust and efficient interpretation tool to control illumination, resolution, and time-lapse properties of reservoirs.

Summary The “Simulated Prestack Local Imaging” (SimPLI 1 ) concept is a new method helping interpreters of migrated seismic data to control and constrain their interpretation of local structures such as oil/gas reservoirs. There is no need in this approach to first generate synthetic data, and then migrate to calculate the migrated seismic response of reservoirs models. Simulated prestack migrated sections are quickly obtained as functions of the survey, emitted pulse, wave-modes, and local reservoir structure. An interpreter can therefore check interactively various reservoir scenarios in terms of illumination (survey-planning, reservoir characterization), resolution, and time-lapse evolution related to rock parameter changes during the production. Input can be interpreted horizons with attributes, property grids from reservoir models and inversion results, or hypothetic models. The results are prestack migrated sections, in depth or time. The SimPLI method is a convolution technique, which goes far beyond the classic 1D convolution as being able to predict 2D/3D effects of illumination, resolution and prestack acquisition, but without requiring experts in modeling and migration.

Improved applicability of ray tracing in seismic acquisition, imaging, and interpretation

Ray-based seismic modeling methods can be applied at various stages of the exploration and production process. The standard ray method has several advantages, e.g., computational efficiency and the possibility of simulating propagation of elementary waves. As a high-frequency approximation, the method also has a number of limitations, particularly with respect to a lack of amplitude reliability in the presence of rapid changes of the model functions representing elastic parameters and interfaces. Given the objective of improving the applicability of the standard ray method, we present a strategy that does not require specific extension to finite frequencies. Instead, we define each ray-based process as an element of a system that, as a composite process, is able to obtain better results than the ray-based process applied alone. Other elements of the composite process can be finitedifference modeling or numerical solutions for surface and volume integrals, which are basic constituents of Kirchhoff modeling and imaging. We also include among the process elements recently developed techniques for simulating the migration amplitude on a target reflector and in a local volume, e.g., a reservoir zone. The model is decomposed according to its complexity into volume elements, surface elements, or a combination. The composite process consists of a specified interaction between process elements and model elements, which fits well with the philosophy of modern software design. Model elements that will be exposed to ray-tracing algorithms may need appropriate preparation, e.g., smoothing and resampling. We demonstrate specifically, in a tutorial example, that simulating the seismic response from a reflector by ray-based composite processes can yield better results than applying standard ray tracing alone.

Review of Ray Theory Applications in Modelling and Imaging of Seismic Data

Throughout the last twenty years, 3D seismic ray modelling has developed from a research tool to a more operational tool that has gained growing interest in the petroleum industry. Various areas of application have been established and new ones are under development. Many of these applications require a modelling system with flexible, robust and efficient modelling algorithms in the core. The present paper reviews the basic elements of such a system, based on the ‘open model’ concept and the ‘wavefront construction’ technique. In the latter, Červenýs dynamic ray tracing is an intrinsic part. The modelling system can be used for generating ray attributes and synthetic seismograms for realistic 3D surveys with tens of thousands of shots and receivers. Moreover, some other types of application areas are illustrated: Production of Greens functions for prestack depth migration and hybrid modelling (combined ray and finite-difference modelling), attribute mapping and illumination analysis, both for survey planning and interpretation. Finally, the concepts of ‘isochron rays’ and ‘velocity rays’ related to seismic isochrons have been introduced recently, with very interesting future applications.

Prestack map migration as an engine for parameter estimation in TI media

Summary An approach to parameter estimation in TI media is presented, using prestack map migration as a primary process. The input is interpreted maps of prestack travel time corresponding to various wave modes, e.g., PP and PS. As output, RMS depth deviation is obtained as a function of the Thomsen parameters epsilon and delta, for selected horizontal locations. It is required that the isotropic component of the velocity model has been obtained beforehand. By manual inspection of minima in the RMS maps, a lateral variation of epsilon and delta can be established. An example using sea-bed seismic data shows that the prestack map migration technique is well suited for estimation of epsilon and delta.

Fast repeated seismic modelling of local complex targets.

Summary A new modelling scheme is described, for very fast repeated calculation of the seismic response of a local complex target below a structurally simpler overburden. The method combines a petrophysical modelling system with a hybrid ray tracing/finite difference scheme. Both acoustic and elastic waves can be modelled. For the example shown, the new modelling scheme required about 2.5% of CPU resources and 1.7% of the grid size when compared to the global finite difference calculation.

Combining Saturation Changes and 4D Seismic for Updating Reservoir Characterizations

This paper was selected for presentation by an IPTC Programme Committee following review of information contained in an proposal submitted by the author(s). Contents of the paper, as presented, have not been reviewed by the International Petroleum Technology Conference and are subject to correction by the author(s). The material, as presented, does not necessarily reflect any position of the International Petroleum Technology Conference, its officers, or members. Papers presented at IPTC are subject to publication review by Sponsor Society Committees of IPTC. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of the International Petroleum Technology Conference is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of where and by whom the paper was presented. Write Librarian, IPTC, P.O. Box 833836, Richardson, TX 75083-3836, U.S.A., fax 01-972-952-9435.

Information Content in Forward 4D-Seismic Modelling and Elastic Inversion

This paper was selected for presentation by an IPTC Programme Committee following review of information contained in an proposal submitted by the author(s). Contents of the paper, as presented, have not been reviewed by the International Petroleum Technology Conference and are subject to correction by the author(s). The material, as presented, does not necessarily reflect any position of the International Petroleum Technology Conference, its officers, or members. Papers presented at IPTC are subject to publication review by Sponsor Society Committees of IPTC. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of the International Petroleum Technology Conference is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of where and by whom the paper was presented. Abstract Elastic seismic inversion is a tool frequently used in analysis of seismic data. Elastic inversion relies on a simplified seismic model, and generally produces 3D cubes for Vp, Vs and density. By applying rock physics theory such volumes may be interpreted in terms of lithology and fluid properties. Understanding the robustness of forward and inverse techniques is important when deciding how much information seismic data really carry. This paper discusses the observed deviation between a reference and simulated reservoir, and its dependency on the seismic parameters and the reservoir characterization parameters. The ability to utilize the results from a 4D seismic survey in reservoir characterization will depend on several aspects. To investigate this, a loop that performs independent forward seismic modeling and elastic inversion at two time stages has been established. The multidisciplinary workflow has several independent steps: 1. Generation of a synthetic reference reservoir, by realistic geostatistical modeling. 2. Flow simulation of the reference reservoir to predict reservoir conditions at survey acquisition times. 3. Establishing a relationship between petrophysical and fluid properties and seismic parameters by a rock physics model. 4. Generation of seismic AVA responses corresponding to reservoir conditions at base and monitor survey times. 5. Elastic seismic inversion of both AVA response sets. 6. Simulation of lithology and fluid parameters conditioned on seismic inversion. 7. Comparison of static reservoir parameters of reference and simulated realization. 8. Comparison of seismic responses at initial and monitor survey times. By working on a realistic synthetic reservoir, full knowledge of the reservoir characteristics is achieved. This …