SUMMARY

Interval attenuation measurements provide valuable information for reservoir characterization and lithology discrimination. Here, we present a methodology for estimating the interval S-wave attenuation coefficient from mode-converted (PS) data. By identifying PP and PS events with shared ray segments and applying the PP+PS=SS method, we first perform kinematic construction of pure shear (SS) events in the target layer and overburden. Then, the modified spectral-ratio method is used to compute the effective shear-wave attenuation coefficient for the target reflection. Finally, application of the dynamic version of velocity-independent layer stripping to the constructed SS reflections yields the interval S-wave attenuation coefficient in the target layer. The algorithm does not require knowledge of velocity and attenuation in the overburden, as long as it is composed of laterally homogeneous layers with a horizontal symmetry plane. The attenuation coefficient estimated for a range of source-receiver offsets can be inverted for the interval attenuation-anisotropy parameters. The method is tested on multicomponent synthetic data from layered VTI (transversely isotropic with a vertical symmetry axis) media generated with the anisotropic reflectivity method.

INTRODUCTION

Attenuation analysis provides seismic attributes sensitive to the physical properties of the subsurface. Reliable attenuation measurements have become feasible with acquisition of high-quality reflection and borehole data. Attenuation is often found to be anisotropic (directionally dependent) due to a variety of factors such as the intrinsic anisotropy of the material, the presence of aligned fluid-filled fractures (Batzle et al., 2005), or interbedding of thin layers with different properties (Zhu et al., 2007). Dvorkin and Mavko (2006) observe that the ratio of the P- and S-wave quality factors (QP/QS ) can serve as an indicator of hydrocarbons because QP ! QS in fluid-saturated rocks, while in dry or gas-saturated rocks QP " QS. De et al. (1994) report measurements of QS from vertical seismic profiling (VSP) surveys and sonic logs. For reservoir-characterization purposes, it is important to be able to obtain S-wave attenuation from reflection data. Behura and Tsvankin (2009) combine the velocity independent layer stripping (VILS) method of Dewangan and Tsvankin (2006) with the spectral-ratio method to estimate the interval attenuation of pure modes. Here, we extend their technique to the combination of PP and PS data with the goal of evaluating interval S-wave attenuation.

METHODOLOGY

For simplicity, the method is described for 2D models, but it can be extended to 3D wide-azimuth data. We operate with pure-mode (PP) and mode-converted (PS) reflections for a medium with an arbitrarily anisotropic, heterogeneous target layer overlaid by a laterally homogeneous overburden with a horizontal symmetry plane in each layer. In the 2D version of the method, the vertical incidence plane is supposed to be a plane of mirror symmetry for the whole model. Therefore, both rays and the corresponding phase-velocity vectors are confined to the incidence plane, and converted waves represent in-plane polarized PSV modes. The P-to-S conversion is assumed to occur only at the reflector. Suppose P-wave sources and receivers of both P- and S-waves are continuously distributed along the acquisition line.

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