15 AUGUST 2005
identify (being slower, it arrives last; traveling near the surface, it contains more relative
energy). Because R- and S-wave velocities are relatively close, the velocity of the R-
wave is frequently used in computations for elastic properties.
Because amplitudes in seismic survey are very small, the computed shear and
Young's moduli are considerably larger than those obtained from conventional
laboratory compression tests.
The shear modulus, G, may be calculated from the S- (approximately the R-
wave) wave velocity as follows:
G = pVs
p = y/32.2 = mass density of soil using wet or total unit weight
Vs = S-wave velocity (or R-wave), feet per second
This equation is independent of Poisson's ratio. The Vs value is taken as
representative to a depth of approximately one-half wavelength. Alternatively, the shear
modulus can be computed from the P-wave velocity and Poisson's ratio from:
p(1 - 2)
2(1 - )
The use of this equation is somewhat limited because the velocity of a P-wave is
approximately 5000 feet per second (approximately the velocity in many soils) and
Poisson's ratio must be estimated. For saturated or near saturated soils, -> 0.5. The
theoretical variation of the ratio Vs/Vp with u is shown in figure 12-8.