Authors have developed the elastic wave exploration method as a nondestructive investigation to examine the current situation of embankment of irrigation tank. The method applied the concept of impact-echo used for concrete structure. However, the method had a difficulty to identify the resonance frequencies from the Fourier transform of the elastic wave. There might be, therefore, a personal error of investigator. To improve the difficulty, the new procedure for Fourier transform was proposed in this paper. The spectrum at a frequency was added to the one at double the frequency. This concept used the tendency that the resonance vibration had the peak at double the resonance frequency. The spectrum at resonance frequency was, thus, emphasized. The personal error was not included in this procedure. By converting the frequency to the distance, the location having the reflected wave was identified. The proposed method was verified at the laboratory and then applied to the in-situ investigation. In comparison with the previous method and electric exploration, the efficiency of the newly proposed method was examined.