Evaluating inversion of microtremor H/V spectrum in extracting shear wave velocity profile with Down-Hole test results

The velocity of shear wave, which is utilized in evaluation of the stiffness and strength of subsurface layers, the geotechnical site effects of earthquakes, and determination of the profile of subsurface layers is one of the most prominent and essential parameters in geotechnical studies. The profile of shear wave velocity of soil layer is usually discerned in the site by means of prevalent direct seismic and geophysical methods such as reflection, refraction like SASW and MASW, and borehole ways or indirectly, by geotechnical methods such as conducting CPT or SPT sounding and employing experimental relations among Vs and CPT or SPT quantities. As far as these methods need to dig boreholes, it is obvious that deploying borehole geophysical methods or geotechnical ones to certify the profile of shear wave velocity is expensive and time-consuming. As an economical and practical alternative, the measurements of microtremors which can be easily implemented on the ground surface at a single station. Firstly, the method registers ambient vibrations; then, the spectral proportion of the average of horizontal components to vertical component of microtremors (H/V) are determined. At the end, shear wave velocity profile of the site are discerned by inversion of the H/V spectrum in a reversal algorithm. The analyses are easily performed by means Dinver program of useful package of Geopsy software.
In this study, single station microtremor measurements are done at six points along Kermanshah Urban Railroad project. In order to determine shear wave velocity profile from inversion of H/V spectrum, four shear wave models with 3,5,7,and 10 layers are initially suggested for all mentioned six points. All initial models were similar in parameters like thickness of layers and range of shear wave velocity in each layer, so that final conclusions and interpretations can be made for future studies. The best of achieved shear wave velocity profiles in terms of minimum misfit error of inversion of horizontal to vertical spectral peak are compared with the result of field Vs measurements that were performed by down-hole experiments. The most outstanding point is that, the procedure is utilized to find shear wave velocity profile without considering the results of down-hole tests for thickness and velocity limits. The comparison showed that there are significant compatibility between shear wave velocity profiles achieved by inversion method and the results of field down-hole tests. Generally, this compatibility increases with increasing the number of layers from three to ten layers of initial models. Furthermore, the depth of exploration grows with increasing the number of layers as well as the accuracy of profiles, so the initial model with 10 layers has more consistency with the results of borehole experiments of all six points in relation to other models. However, it is obvious that the time for analyzing initial model with 10 layers is more than other ones and a tradeoff between accuracy and time of analyses must determine the acceptable results. In conclusion, inversion of H/V spectrum method shows the ability to appropriately estimate shear wave velocity profile even in deeper sediments in relation to down-hole tests.