GNGTS 2015 - Atti del 34° Convegno Nazionale

14 GNGTS 2015 S essione 3.1 A first assessment of the estimated RPMs can be done by comparing the true elastic properties, derived from well-log recordings, with those predicted by including the actual petrophysical data in the RPMs. The Fig. 1 shows this comparison together with the resulting correlation coefficients. First, we note that independently from the method applied (SR, NN, GA or TRPM) the quality of the prediction decreases passing from density (Fig. 1c) to Vp (Fig. 1a) and to Vs (Fig. 1b). Indeed, as it is well known, the relation linking the density to the petrophysical properties is simpler than the relations linking these properties to Vp and Vs (Aveseth et al. 2005). The higher correlation coefficient obtained for Vp (Fig. 1d) with respect to Vs (Fig. 1e) can be explained with the lower performance of the logging tools in measuring the S-velocity, and then the lower reliability of the Vs measurements with respect to the Vp and the density ones. For the density, the four methods return very similar results. Differently, for Vp and, particularly, for Vs the NN method yields final estimates with the highest correlation coefficients, whereas slightly lower correlation coefficient are obtained by the SR, GA and the TRPM methods. The slightly higher correlation coefficients obtained by the empirical methods with respect to the TRPM can be easily explained taking into account that the empirical approaches are data-driven procedures, and thus derive the final f RPM on the basis of the actual petrophysical and elastic properties. Conversely, the TRPM is based on theoretical equations with general validity. Among the empirical approaches, NN produces a slightly better match than SR and GA, whereas the non-linear GA method and the multilinear SR algorithm return very similar predictions. The main advantage of the SR and GAmethods over the NN approach Fig. 1 – Comparison between the true and the predicted elastic properties. Vp, Vs and density are represented in (a), (b) and (c), respectively. The correlation coefficients computed for each approach are shown in (d), (e) and (f) for Vp, Vs and density, respectively.