Métodos de inversão de dados acústicos em poços preenchidos com fluído por integração da semelhança espectral ao longo da curva de dispersão

Abstract

This work presents a comparative analysis of different acoustic data inversion methods in fluid-filled boreholes for determining Stoneley mode propagation velocity. Initially, an acoustic model was defined based on the wave equation solution for linear elastic media. Subsequently, two iterative methods for inverting the borehole parameters were defined, using the Stoneley mode velocity dispersion curve. In the first method, employed as a reference, the quality of fit was based on conventional techniques using the maximum spectral semblance curve, while in the second, this measure was obtained through a defined energy, result of integrating the dispersion curve over the spectral semblance. For the statistical combination of different shots, three different techniques were used and compared: weighted arithmetic mean, weighted geometric mean and weighted conflation. The methods were applied to experimental data generated by a monopole source at low frequencies, ideal conditions for Stoneley mode excitation. In the tested data, the method of integrating the dispersion curve over the spectral semblance yielded results closer to the known accurate ones. Among the different combination methods, the weighted geometric mean showed an RMS difference of 278.1 m/s compared to reference values, significantly lower than the 579.1 m/s obtained by the maximum curve. The reference values were experimental propagation velocity values of the S wave collected by a dipole tool.