Geology Reference
In-Depth Information
• record and display the seismic waveforms on a suitable
seismograph.
The general methodology of examining hidden
structures by studying their effects on artificially gener-
ated acoustic or seismic waves has an enormously wide
range of applications covering a wide range of spatial
scales. Perhaps the smallest scale is ultrasound imaging
in medicine, which can also be applied industrially to
examining engineering structures. Within the more
geophysical applications, the scales range from depths of
a metre or less in engineering, environmental or archae-
ological surveys to tens of kilometres for crustal and
upper mantle studies.
For each application there is a limit to the smallest
structures that can be detected, known as the
resolution
of
the survey.The resolution is basically determined by the
pulse length: for a pulse of any particular length there is
a minimum separation below which the pulses will
overlap in time in the seismic recording. Although the
pulse length may be shortened at the processing stage
by deconvolution (see Section 4.8.2), this is only possible
if the data are of good quality, and is a complement
to, not a substitute for, good survey design. The pulse
width is determined by both the maximum frequency
and the frequency bandwidth of the recorded signal.
Since Earth materials absorb seismic energy in a fre-
quency-selective way (Section 3.5), the optimum wave-
form will be specific to each survey. It is an important
characteristic of all geophysical surveys, and particularly
seismic ones, that they must be designed individually for
each specific case.The general aspects of the equipment
used for seismic surveys are reviewed here; specific
variations for reflection and refraction surveying are
described in Chapters 4 and 5.
wave or S-wave, and generate minimum energy of other
wave types. Such other unwanted energy would degrade
the recorded data and be classed as
coherent noise
.
• The source waveform must be repeatable. Seismic sur-
veys almost always involve comparing the seismograms
generated by a series of sources at different locations.
Variations on the seismograms should be diagnostic of
the ground structure, not due to random variations of
the source.
• The source must be safe, efficient, and environmen-
tally acceptable. Most seismic surveys are commercial
operations which are governed by safety and environ-
mental legislation.They must be as cost-effective as pos-
sible. Sometimes the requirements for efficiency lead to
higher safety and environmental standards than legally
enforced.Whether involving personal injury or not, ac-
cidents are referred to as 'lost-time incidents'. Safety aids
efficiency as well as being desirable from many other
viewpoints.
The complete seismic/acoustic spectrum is shown in
Fig. 3.14.There is a very wide variety of seismic sources,
characterized by differing energy levels and frequency
characteristics. In general, a seismic source contains a
wide range of frequency components within the range
from 1 Hz to a few hundred hertz, though the energy is
often concentrated in a narrower frequency band.
Source characteristics can be modified by the use of
several similar sources in an array designed, for example,
to improve the frequency spectrum of the transmitted
pulse. This matter is taken up in Chapter 4 when dis-
cussing the design parameters of seismic reflection
surveys.
Explosive sources
On land, explosives are normally detonated in shallow
shot holes to improve the coupling of the energy source
with the ground and to minimize surface damage. Ex-
plosives offer a reasonably cheap and highly efficient
seismic source with a wide frequency spectrum, but
their use normally requires special permission and pre-
sents logistical difficulties of storage and transportation.
They are slow to use on land because of the need to drill
shot holes. Their main shortcoming, however, is that
they do not provide the type of precisely repeatable
source signature required by modern processing tech-
niques, nor can the detonation of explosives be repeated
at fixed and precise time intervals as required for efficient
reflection profiling at sea carried out by survey vessels
underway. Since explosive sources thus fail at least two,
3.8.1 Seismic sources and the
seismic/acoustic spectrum
A seismic source is a localized region within which the
sudden release of energy leads to a rapid stressing of the
surrounding medium. The archetypal seismic source is
an explosion. While explosives are still used, there is an
increasing number of more sophisticated and efficient
(and safe!) ways to collect seismic data.
The main requirements of the seismic source are:
• Sufficient energy across the broadest possible fre-
quency range, extending up to the highest recordable
frequencies.
• Energy should be concentrated in the type of wave
energy which is required for a specific survey, either P-
