The Study Of Application To Determine The Shear Wave Velocity Structure Of Shallow Strata By H/V Spectrum Of Surface Wave In Microtremor

Microtremor is a weak vibration caused by the random source of vibration on the ground and underground, which includes short-period caused by artificial vibration and long-period caused by natural forces. Although microtremor shows the map as a continuous noise signal on seismic records, but the observations and studies have shown that is a very important method for classification of site, selection of project location, identification of low-velocity layer and other aspects.At present, the primary methods for surface wave of microtremor are Fourier transform method, the base and higher mode extraction and spectral ratio relative to standard point. H/V spectrum that is a new method is developed rapidly in recent years and concerned because of its simple operation and economics. Based on the theory of layered media, the characteristics for Rayleigh and Love wave in surface wave of microtremor were studied to analyze the relationship between the multi-mode dispersion curves and stratum structure, the character and significance of medium response function, then H/V spectra is achieved and the forward and inversion procedures for writing and testing are completed. By theoretical models and measured data on the test, the main aspects that are obtained for the use of H/V spectral of microtremor to explore the shallow velocity structure are the following:(1) The characteristics of multi-mode dispersion curve and medium response function associated with the stratigraphic interfaces, and its inflection point and the asymptotic lines are an indirect reflection for formation structure. Studied the characteristics of the dispersion curve and medium response function is beneficial to know characteristics of the underground and determine the model and is conducive to inverse;(2) The energy of surface wave is concentrated in the first mode. Rayleigh wave in the H/V spectrum plays a dominant role, controlling the shape and peak value, while Love waves mainly affect the peak value, but the effect for the overall shape of H/V curve is not obvious. Since the existence of higher mode, H/V spectrum considered the multi-mode wave is not always the same with the first mode, but its effect is small and the presence of higher mode waves can provide more useful information to improve the inversion accuracy;(3) H/V spectrum method currently is used in lower frequency, but the application for engineering testing referring to high frequencies have not been reported yet, because Rayleigh wave and Love wave numerical methods are restrictions in higher frequency, in order to broaden the application field, which is to be studied and solved;(4) Generalized least squares method is successfully used for inversion of H/V spectra of theoretical data and measureed data, which shows that codes meet the basic requirements and H/V method can be used for the inversion of velocity structure. Although the inversion process costs less time, fast convergence, but it highly dependents on the initial model. In practice, we need to combine with other inversion method.