Seismic method - Geophysics for the Mineral Exploration Geoscientist - page 357

Higher-frequency (shorter-wavelength) waves are
increasingly attenuated because these travel through a
greater number of cycles in a given distance than lower-
frequency (longer-wavelength) waves. Consequently, the
different frequency components making up the seismic
wavelet experience different levels of absorption.
A practical demonstration of absorption is the sound of
loud music being played at the other end of the street. The
bass and drums generate lower-frequency sounds whilst
the guitars and vocals tend to produce higher frequencies
which undergo greater attenuation. The result is that at a
distance the sound of the music is dominated by the beat
and the melody is lost. Recall that a seismic wavelet is the
result of interference of waves of different frequencies. The
greater attenuation of higher frequencies changes its fre-
quency content with distance travelled, causing the ampli-
tude and shape of the wavelet to change ( Fig. 6.6b ) . The
greater loss of higher frequencies causes the wavelet to
broaden, i.e. its dominant period increases. This is an
important phenomenon since as the dominant period
increases, Fresnel volumes become larger ( Fig. 6.4c ) and
resolution decreases (see Section 6.7.1 ).
Figure 6.6c shows the combined effects of geometric
spreading and absorption on a seismic wavelet. The effects
of both increase with the distance travelled through the
subsurface by the wavelet. This must be accounted for
when planning a seismic survey, as it is necessary to ensure
that the source produces sufficient energy across a fre-
quency range that can penetrate to the required depth.
Figure 6.7 shows typical frequency ranges for various types
of seismic survey. Note the lower frequencies used for deep
penetrating crustal-scale surveys compared with those of
shallower probing surveys. Tomographic surveys, which
involve waves travelling much shorter distances, can use
much higher frequencies and,

a)

b)

c)

Raypath

Figure 6.6 Distortion of a wavelet propagating through a
homogenous medium. (a) By geometric spreading, (b) by
absorption; (c) the combined effects of geometric spreading
and absorption.

a vertical cylinder ( Fig. 6.4a and b ). The radius of the
wavefront is R surface , so its surface area is 2 R surface Z surface .

The energy of the wave at a point on the wavefront is then
inversely proportional to R surface , i.e. a 1/R relationship,
and the wave

π

' is amplitude is inversely proportional to

1/ R. Clearly then, attenuation due to geometric spreading

is greater for body waves than for surface waves, which is
one reason that surface waves are some of the highest-
amplitude

√

arrivals seen on seismic recordings (for

examples, see Figs. 6.13 and 6.19 ).

6.3.3.2 Absorption
Rocks are not elastic materials, so when a seismic

wavelet travels through them some of its energy is lost
through a phenomenon known as absorption. The princi-
pal cause is friction at grain boundaries and cracks,
although the exact mechanism is not well understood.
Absorption may be described in terms of several param-
eters, but the most common are the quality factor (Q) and
the absorption coef cient (

' perfect '

therefore, have higher

resolution.

6.3.4 Effects of elastic property discontinuities α ) for a particular wavelength

( λ ). These parameters are related via the expression:

It is the interaction of seismic waves with changes in elastic
properties and density of the subsurface that ultimately
provides information about the structure of the subsurface.
For conventional (surface) seismic surveys, it is essential
that the downward travelling waves created by the source
encounter changes in the subsurface that

¼ π
αλ

Q ð 6 : 6 Þ

The absorption coef cient represents the proportion of
energy lost during transmission through a distance equiva-
lent to one wavelength. The amplitude of each cycle of the
wave is reduced so that its amplitude is equal to some
percentage of the preceding cycle; so absorption causes
amplitude to decrease exponentially.

' turn them

'

around by some means so that they return to the surface

where they will be recorded by the detectors. This occurs
through any or all of

three fundamental processes: