A noise shot has been recorded by using explosive as the source, and a line of 47 vertical geophones. The figure on the bottom shows the seismic data. Data are sampled every 16 ms and the recording's duration is limited to 4 seconds. On this recording, we can see a refracted wave, a reflected arrival of strong amplitude associated to a deep reflector, and two dispersed Pseudo-Rayleigh waves characterized by low apparent velocity and by low frequency contents.

The figure shows on the top, the 2D amplitude spectrum of the seismic recording. The spectrum is presented with and without normalisation in a wavenumber k. Without a normalisation in k, the two branches associated to two dispersed waves are clearly visible. After a normalisation in k, the refracted wave appears at frequencies superior to 17 Hz (k between 0.01 and 0.02 m^-1), and the reflected waves concentrated near the null wave number (k=0 m^-1). The first branch of the Pseudo-Rayleigh mode, situated in the frequencies period 3-10 Hz, corresponds to the slow Pseudo-Rayleigh. The second branch in the period 10-20 Hz corresponds to the fast Pseudo-Rayleigh. These two branches are associated to the first two modes.