A Framework for Indeterministic Model-Based Microtremor Inversion to Estimate Mechanical Properties of Rock Mass in Tectonic Mélange Area

Tectonic mélanges have complex lithological assemblage characteristics with block matrix structure that makes the mechanical properties of its rock mass vary widely. Since enormous engineering constructions are carried out in tectonic mélange regions, estimation of mechanical properties on tectonic mélange rock mass is of significance. This study proposed a novel framework for indeterministic model-based microtremor inversion with uncertainty analysis to independently resolve phase velocity into the shear modulus and basic quality (BQ) index of mélange rock mass. To verify the effectiveness of this framework, microtremor surveys were arranged in a linear array and conducted over a mélange postlandslide deposit in Southwestern China. First, through calculating wave-shape coherence of microtremor recordings, the dispersion spectra within the frequency range of 2–30 Hz were extracted. Initial models were established according to the laboratory experiments data and the measured dispersion curves. Then, uncertainty analysis is used to evaluate the reliability of the demonstration inversion results because of the absence of fully deterministic models. By adjusting the initial model and the extracted dispersion curves, the final phase velocity structures with uncertainty coefficient (UC) of less than 10% were determined. The dynamic elastic constants were calculated based on the phase velocities. Finally, the BQ classification index related to the obtained dynamic shear modulus was introduced and estimated. Three grades of mélange rock mass were classified and the interfaces between different mechanical properties are distinguished. This proposed computational framework could be useful for the estimation of mechanical properties and the engineering classification of such a complex rock mass.