Dispersive seismic wave tools in geophysics

Guide mode waves are important tools for geophysical prospecting. These waves are dispersive which means that their group and phase velocities are different and they are functions of frequency. The use of such waves for a geophysical investigation requires a 3 steps process. Firstly, enhancement of signal to noise ratio, secondly an analysis of the dispersion and finally the interpretation of the wave characteristics in terms of physical parameters. The waves are guided either due to the presence of a velocity gradient or by various reflectors. Thus the waves are confined within a specific wave guide. In seismic prospecting such waves are: Rayleigh waves (3 to 50 Hz), and Love waves (2 to 20 Hz) in seismic reflection propection, or Stoneley waves (1 to 5 kHz) in acoustic logging. For example when the impedance of the substrata has a velocity profile without large discontinuities, dispersive waves are present in a shallow water seismic section [Nardin et al 1998]. In presence of sediments. the impedance has a regular variation and dispersion occurs because of the interference of different travel paths for the same wave. Field data comprising Rayleigh, Love, Stoneley waves and shallow water refractien's (SWR) are presented in figures 1 to 4. The Rayleigh wave seismic data have been recorded on an array of 47 two component geophones. (vertical component is presented on figure 1). The Love wave section (figure 2) has been generated using a hammer with the strike action applied horizontally. The Stoneley section (figure 3) shows a 3.25 m offset section (acoustic imaging) recorded in the depth interval of 400-450 m. The acoustic tooI used was a slim-hole SEMM tooI with a monopole source and two pairs of receivers. Shallow water refractien's are presenred in figure 4 after a velocity correction of 1520 m/s over a distance of 5.5 km. First arrival of water guide is plotted as time zero. The signal was recorded by a hydrophone Iying on the sea floor at a depth of 70 m. The seismic source was a 16 liters airgun.