Geology Reference
In-Depth Information
11
SEISMIC METHODS: GENERAL CONSIDERATIONS
Seismic methods are the most effective, and the most expensive, of all
the geophysical techniques used in investigating layered media. Features
common to reflection and refraction surveys are discussed in this chapter.
Chapter 12 is concerned with the special features of small-scale reflection
work, and Chapter 13 with shallow refraction. Chapter 14 deals with the
developing use of surface waves. Deep reflection surveys, which involve
large field crews, bulky equipment and complex data processing, are beyond
the scope of this topic.
11.1 Seismic Waves
A seismic wave is acoustic energy (a sound wave) transmitted by vibration
of rock particles. Low-energy waves are approximately elastic, leaving the
rock mass unchanged by their passage, but close to a seismic source the rock
may be shattered and permanently distorted.
11.1.1 Types of elastic wave
When a sound wave travels in air, the molecules oscillate backwards and
forwards in the direction of energy transport. This
pressure
or 'push' wave
thus travels as a series of compressions and rarefactions. In a solid medium it
has the highest velocity of any of the possible wave motions and is therefore
also known as the
primary
wave, or simply the
P-wave.
Particles vibrating at right angles to the direction of energy flow (which
can only happen in a solid) create an
S
(
shear
, 'shake' or, because of its re-
latively slow velocity,
secondary
) wave. In many consolidated rocks, the S-
wave velocity is roughly half the P-wave velocity. It depends slightly on the
plane in which the particles vibrate but these differences are not significant
in small-scale surveys.
P- and S-waves are
body-waves
and expand within the main rock mass.
Other waves, known as
Love waves,
are generated at interfaces, while par-
ticles at the Earth's surface can follow elliptical paths to create
Rayleigh
waves
. Love and Rayleigh waves may carry a considerable proportion of the
source energy but travel very slowly. Traditionally they were simply lumped
