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:

(12-19)

2

G = pVs

where

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:

(12-20)

Vp2

G=

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.

12-16

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