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Russian Geology and Geophysics

2006 year, number 5


L.T. Santos and M. Tygel
Department of Applied Mathematics, IMECC, State University of Campinas (UNICAMP),
CP 6065, 13081-970, Campinas (SP), Brazil
Keywords: Reflection wave, impedance, inversion, AVO, AVA, AVP
Pages: 574-581


Amplitude versus offset (AVO) or amplitude versus angle (AVA) curves are nowadays regularly extracted from seismic data for various purposes of reservoir studies and characterization. After adequate processing, these curves represent the variation of the reflection coefficients with respect of offset or angle at points of key target reflectors, such as, for example, the top of a reservoir. Besides the information that can be obtained merely from their shape, AVO/AVA curves can also be used to invert more quantitative attributes, such as the intercept and the gradient of the reflection coefficient or, even better, the elastic-parameter contrasts. For a common-midpoint gather, the AVA curve is generally derived from its AVO counterpart by means of a well-known expression that relates the reflection angle to offset. It is to be noted that a successful inversion of the sought-for attributes is strongly dependent on the approximations of the reflection coefficient that are considered.
The recently introduced reflection impedance concept provides an attractive approximation of the elastic PP-reflection coefficient as a function of the ray parameter. In this sense, that approximation can be of value when amplitude versus ray parameter (AVP) curves are available from seismic data. It is to be noted that an AVP curve tend to be more reliable that its AVA counterpart. This is because the ray parameter, as a function of offset, depends on the RMS-velocity only, as opposed to the incidence angle, which also depends on the (more unstable) interval velocity.
In this paper, we propose an algorithm to invert elastic-parameter contrasts from AVP curves using the reflection impedance approximation of the PP-reflection coefficient. First results shown on synthetic data indicate that the procedure may offer a promising alternative to existing methods of inverting reservoir attributes from AVO/AVA curves.