Subsurface stiffness imaging using HWAW (Harmonic wavelet analysis of wave) method

Abstract

The surface wave test evaluates the experimental dispersion curve from field measurements and shear wave velocity profile is determined from iterative comparison with theoretical dispersion curve in inversion process. Therefore, to obtain reliable experimental dispersion curve is essential for surface wave test. In the conventional SASW method, the experimental dispersion curve is determined using phase difference between receivers. Although SASW method is effective, it is difficult to determine correct results when there are noise, body waves and lateral variation. Recently, two-dimensional (or three-dimensional) imaging of shear wave velocity profile becomes un-omissible demand because of the lateral non-homogeneity of soil stiffness. Until now, subsurface is assumed to have many horizontal soil layers to bedrock when executing surface wave method. Because subsurface includes several curved/non-horizontal layers, unfortunately, errors can be generated during analysis task of surface wave measurements. Though there are many tomography techniques based on electric-magnetic wave / elastic wave, they cannot give detailed modulus information and bring much cost. Also, the conventional SASW method has weakness to realize the imaging of subsurface stiffness variation for its horizontal constancy assumption. To overcome these weaknesses, new seismic site characterization method using the harmonic wavelet analysis of waves (HWAW) was proposed. HWAW method is based on time-frequency analysis using harmonic wavelet transform. HWAW method mainly uses the signal portion of the maximum local signal/noise ratio to evaluate the phase velocity and it can minimize the effect of noise. The HWAW method can eliminate near field effect and obtain reliable shear wave velocity profile for any soil condition without expertise. Also, the HWAW testing method can explore deep layer properties below about 30m depth with only one short receiver spacing setup. HWAW method not only can obta...