When seismic waves are generated, a special type of wave propagates along the free surface called surface waves, whose penetration depth is wavelength-dependent; the longer wavelength influences the deeper portion of the earth (Fig. 1). Because of this property, surface waves are usually dispersive (Fig. 2), meaning different frequencies have different propagation velocities, whereas body waves (refraction, reflection, head, etc., waves) rarely take such property to a noticeable extent. Two types of surface waves are generally known: Rayleigh and Love waves. The disturbance (vibration) direction of the former is mainly perpendicular to the surface, whereas it is parallel for the latter. Theoretically, the dispersion property of surface waves is determined by several elastic properties, including density (rho) and depth-variation of S- and P-wave velocities (Vs. and Vp). Among these parameters, the depth-variation of Vs. is the most influencing factor. Because of this, surface waves are often used to deduce Vs. properties of near-surface earth materials.
In comparison to using conventional body-wave methods to achieve similar Vs. information (for example, S-wave refraction, reflection, down-hole, cross-hole surveys), the surface-wave method has several advantages:
- Field data acquisition is very simple and tolerant because surface waves always take the strongest energy
- The data processing procedure is relatively simple and easy even for the non-experienced.
- A large area can be covered within a relatively short time period.
- Because of all the above reasons, it is highly cost-effective and time-efficient.
Utilization of surface waves for geotechnical engineering purposes has a history dating back to the early 1950s. Since the early 2000s a multichannel approach called the MASW (multichannel analysis of surface waves) method has been widely used.