Possible effects of misidentified mode number on Rayleigh wave inversion

Surface wave dispersion data obtained in field surveys are inherently incomplete. Poorly constrained dispersion curves and inclusion of data from higher-mode dispersion curves can be shown to produce erroneous inversion results. The present study evaluates the effects of cross-mode data mixing, and limited data and frequency range on inversion models. Multimodal dispersion curves were generated from synthetic velocity profiles using the forward modeling method. Subsets of the data points obtained by sampling the resulting dispersion curves using different sample intervals, frequency ranges, and by mixing fundamental-mode data with portion of the data from the higher-mode dispersion curves were inverted. The inversion results show that small sample intervals over a wide frequency range and an unambiguous identification of the fundamental-mode dispersion curve are essential factors for the reconstruction of accurate inversion models. The penetration range and accuracy of the inversion models are particularly sensitive to phase velocities at low frequencies of the dispersion curve. The theoretical results were tested with data from a roadbed survey in northern China. Ground model obtained from dispersion data derived by using the F-K method, which contained many cross-mode data points, was found to deviate substantially from the borehole record. A better agreement with the geotechnical record was obtained by inverse modeling of the dispersion data obtained from the fundamental-mode curve of the MASW spectral image.