Page 35 - Acquisition and Processing of Marine Seismic Data

26 1. INTRODUCTION

respectively. P waves express compression side-to-side or horizontal transverse vibration
(therefore, they are also known as compres- (which is the horizontal shear or SH wave).
sional waves), while S waves create shear The movements of the particles affected by SV
(hence, they are also known as shear waves) and SH waves are schematically shown in
through the particles during the wave passage. Fig. 1.17. The velocity of the S waves can be
P waves can travel in solids such as rocks and expressed by elastic constants of the propagat-
sediments of the subsurface, liquids such as ing medium as
the water column of the oceans, and gasses. Par- r μ
ffiffiffi
ticles affected by P waves move in the same V S ¼ (1.3)
direction, parallel to the ray path that the wave ρ
propagates on (Fig. 1.17). The velocity of the P where μ is rigidity or shear modulus and ρ is
waves can be expressed by elastic constants of density. Velocity of S waves is always slower
the propagating medium, as than P waves.
v ffiffiffiffiffiffiffiffiffiffiffiffiffi The second type of seismic wave is the surface
u 4
k + μ
u wave, which propagates at the free surface of
3
t
V P ¼ (1.2) a medium such as the earth’s surface. They
ρ
can be divided into two types, called Rayleigh
where k is bulk modulus, μ is rigidity or shear (R) waves and Love (L) waves, which are
modulus, and ρ is density of the medium. distinguished by their different types of particle
S waves propagate only in solids, and motions. Particle movement of Love waves is
particles affected by S waves move in a perpen- similar to SH waves, which show an entirely
dicular direction to the ray path that the wave horizontal polarization, whereas Rayleigh
propagates along. This may be in two directions: waves exhibit a retrograde elliptical particle
up and down or vertical transverse vibration motion near the surface, which moves the parti-
(which is the vertical shear or SV wave), or cle up and down and side-to-side in the same

FIG. 1.17 Particle movements affected by P waves, S waves, Love waves, and Rayleigh wave in vertical and horizontal
planes.