Abstract: Using the existing exact analytical and Rayleigh approximate solutions of vertical displacement at surface of a homogenous ground with the Poisson’s ratio 0.25 under a vertical harmonic excitation, the variation of its phase angle as well as the computed R wave velocity with spacing between the exciting and measuring points is investigated in this paper. It was shown that,when the tolerable error of computed R wave velocity was limited, the minimum interval between the exciting and nearer measuring point would decrease with increasing spacing of two measuring points, which is quite different from the conventional establishment of a surface wave measurement.