A robust experimental determination of Thomsen’s δ parameter

A novel inversion method for the laboratory determination of Thomsen’s [Formula: see text] anisotropy parameter on cylindrical rock specimens from ultrasonic data has been recently reported in the literature. We further assessed this method through a direct comparison of the results of the traditional method (involving a single off-axis P-wave velocity measurement at 45°) and the new method (involving 65 P-wave velocity measurements at several angles to the symmetry axis). We prepared and characterized two vertical shale specimens from the same preserved vertical core to assess their similarity in terms of structure, mineralogy, porosity, and density. The shale was assumed to be transversely isotropic in view of the observed (horizontal) bedding. We subjected both specimens to the same brine saturation and effective stress state. Using the two methods, we obtained similar results for Thomsen’s [Formula: see text] (vertical P-wave) and [Formula: see text] (P-wave anisotropy) parameters. However, a significant discrepancy was observed for Thomsen’s [Formula: see text] parameter: We obtained results of 0.13 using the new method and 0.39 using the traditional method. As a result of the overdetermined nature of the P-wave velocity measurements used in the new method, we believe that the corresponding [Formula: see text] value is more reliable. Also, the value derived with the new testing method seems to match more closely the reported field data.