Multiazimuthal modeling and inversion ofqPreflection coefficients in fractured media

Author:

Simões‐Filho Ivan A.1,Neves Fernando A.2,Tinen Júlio S.3,Protázio João S.4,Costa Jessé C.4

Affiliation:

1. UNICAMP/IG/AGP, Rua Senador Dantas, 105-12 Andar, Centro‐Rio de Janeiro, RJ 20031-201, Brazil.

2. PGS Reservoir Services, 1001 South Dairy Ashford, Houston, Texas 77077.

3. PETROBRAS/E&P and UNICAMP/IG/AGP, Av. Elias Agostinho 665, Macaé, RJ 27913-350, Brazil.

4. UFPA/CCEN. Dep. Geofísica, GG, Federal University of Pará, Belem 66017-900, Brazil. Emails:

Abstract

We present a method for the exact modeling and inversion of multiazimuthal qP-wave reflection coefficients at an interface separating two anisotropic media. This procedure can be used for media with at least one of its planes of symmetry parallel to the interface (i.e., monoclinic or higher symmetries). To illustrate the method, we compute qP-wave reflection coefficients at an interface separating an isotropic medium (representing a seal rock) from a transversely isotropic medium (representing a reservoir rock with vertical aligned fractures). Forward modeling shows that the difference in the offset of the critical angles for different azimuths is proportional to the fracture density: the higher the fracture density, the larger the difference. In the second part of the paper, we use a global optimization technique (genetic algorithm) to invert wide‐angle amplitude variation with offset (AVO) synthetic data. The model space consists of mass density and five elastic parameters of a transversely isotropic medium with a horizontal symmetry axis (HTI medium), which, to the first order, represents the fractured reservoir rock. For this model, we find that the configuration of three azimuths of data acquisition is the minimum number of acquisition planes needed to invert amplitude variation with offset/amplitude variation with azimuth (AVO/AVA) data. Further, there is a need for incidence angles up to 40°; a more narrow range of angles can lead to models that fit the data perfectly only up to the “maximum” incidence angle. We assume that the velocities and density of the isotropic rock are known, but use no prior information on the values of the model space parameters of the fractured rock except for reasonable velocity values in crustal rocks and constraints of elastic stability of solid media. After inversion for the model space parameters, we compute statistics of the 30 best models and likelihood functions, which provide information on the nonuniqueness and quality of the AVO/AVA inverse problem.

Publisher

Society of Exploration Geophysicists

Subject

Geochemistry and Petrology,Geophysics

Reference39 articles.

1. Abreu, C. E. B. S., and Simões‐Filho, I. A., 1995, Modelagem sísmica para caracterização de fraturamento: I Conf. Latin‐American Union of Geophys., Expanded Abstracts, 823–826.

2. Rapid Determination of the Critical Temperature in Simulated Annealing Inversion

3. Waveform Inversion of Expanding Spread Profile 5 From the North Atlantic Transect

4. Evaluation of anisotropy by shear‐wave splitting

5. Evaluation of anisotropy by shear‐wave splitting

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