Physical modelling of multi-layered transversely isotropic media

Abstract

Physical modelling facilities and anisotropic inversion techniques are applied in this study to recover the average and interval anisotropy parameters, and average P-wave velocity field information, in layered transversely isotropic media. The elastic parameters recovered are the vertical P-wave velocity (a0), the vertical S-wave velocity (b0), the P-wave anisotropy (e) and the near-vertical P-wave anisotropy (d). The experimental models consist of isotropic and anisotropic simulating materials with vertical axes of symmetry. Inversion codes developed by Li et al. (1998) are used to invert traveltime data to recover the average anisotropy parameters e and d. The physical modelling transmission experiments simulate walkaway VSP field surveys. The first arrival traveltime data, recorded at different ray angles, are required to determine the average vertical P and S wave velocities a0 and b0 and are also used as input to the inversion program that recovers the average elastic parameters. The interval parameters for each particular layer are recovered by inversion codes that utilise the average elastic parameters of the multilayered models. Physical modelling transmission data obtained from experiments indicate that the effect of the transducer size must be considered. Variations in the interval anisotropy parameters e and d recovered from the two-layer and three-layer models indicate that corrections must be applied to compensate for the large size of the piezoelectric transducers. The accuracy of the inversion results was observed to decrease as the ratio of the transducer width to the model thickness increased. When the ratio of the size of the transducers to the thickness of the model is too large, the effects of transducer size must be taken into account. Point-sized transducers were successfully used to overcome the transducer size effects, and hence obtain results that are more accurate. The accurate determination of these parameters in multilayered media enables the computation of the correct velocity field. Successful recovery of elastic parameters in the physical modelling laboratory may lead to direct application of this analysis to walkaway VSP surveys in the field. The use of correct velocity functions will result in improved seismic data processing and interpretation.