Surface Wave Survey Method

When seismic waves are generated, there is a special type of wave propagating 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 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.
Fig. 1.
Fig. 2.  With seismic velocity increasing with depth, longer wavelengths (lower
frequencies) of surface waves penetrating deeper depths travel with faster
velocity than shorter wavelengths (higher frequencies) do.  As a result, different
frequencies arrive at different times on a seismic record, making a dispersive
seismic event.