Abstract:Because of the effect of higher modes of surface waves and the distance between two receivers,it is very difficult to calculate absolute phase differences from the wrapped phase curve given in frequency domain by the conventional unwrapped technique. The difference between the absolute and the wrapped phases is 2p by an integer for each frequency point. The phase velocities calculated from a wrapped phase are varied with the integer, which do not correspond to the phase velocities of modes respectively. When the influences of different mode surface waves are separated in frequency domain,the dispersion curves of modes can be obtained in the corresponding frequency zones by the non-determinacy analysis of phase. Of these curves,one always corresponds to the so-called‘measured phase velocity vs frequency’ curve. When all curves of phase velocity for each distance are overlapped and the overlapped probabilities are figured by brightness,the dispersion curves of the fundamental mode and higher mode waves can be distinguished from brightness variation.