The cause and effects of multilayer-generated guided-waves

Abstract

Seismic methods are commonly used by the petroleum industry to obtain reflections from geological boundaries that are prospective for oil and gas production. In the presence of near-surface high-velocity layers (i.e. sea-floor carbonates or basalts), a significant portion of the primary seismic energy is scattered. This results in very poor reflection data as little energy remains to be transmitted further and subsequently reflected back to the surface. Such loss of energy is often characterised by the generation of guided-wave energy in low-velocity layers being sandwiched between two high-velocity layers. This results in the dominant horizontal propagation of seismic energy as guided-waves with leaky modes. Guided-wave data has been physically modelled to allow an in-depth analysis of the problem. This paper shows that there is a relationship between the group velocity, dominant frequency and dominant wavelength of the guided-waves, to the thickness of the low-velocity layer. An empirical formula is presented to estimate the interval velocity of the low-velocity layer, for the case of horizontally layered media. The results of a three-component recording indicate that guided-wave propagation is of pseudo-Rayleigh wave particle motion. This implies that the generation of guided-waves is not solely due to total internal reflections within the waveguide, but also involves surface wave propagation along the surface of the underlying high-velocity layer. It is proposed that this surface wave refracts energy into the low-velocity layer at postcritical angles, thus providing additional energy for total internal reflections within the waveguide. Mode conversion from P- to SV-waves also occur at reflections beyond the critical angle, becoming more prominent as the offset increases (Ogilvie and Purnell, 1996).