Geology Reference
In-Depth Information
Echo
sounders
Pingers
Boomers
Sparkers
Air guns
Vibroseis
Quarry blasts
Earthquake body waves
Earthquake surface waves
10
-2
10
-1
10
1
10
2
10
3
10
4
10
5
1
Frequency (Hz) (log scale)
Fig. 3.14
The seismic/acoustic spectrum.
and usually three, of the basic requirements for modern
surveys, their use is steadily declining and limited to lo-
cations where alternative sources cannot be used.
and, to increase the transmitted energy for deep penetra-
tion surveys, vibrators are typically employed in groups
with a phase-locked response. Multiple sweeps are com-
monly employed, the recordings from individual sweeps
being added together (stacked) to increase the SNR.
A particular advantage of vibrators is that they can
be used in towns since they cause no damage or sig-
nificant disturbance to the environment. The cross-
correlation method of extracting the signal is also capa-
ble of coping with the inherently high noise levels of
urban areas. SomeVibroseis
®
trucks are adapted so that
the vibration direction can be horizontal rather than ver-
tical. In this case the truck can also be used as an S-wave
source. A principal disadvantage of the Vibroseis
®
method is that each fully configured truck costs of
the order of half a million dollars. While the method is
effective for major hydrocarbon surveys, the costs are
prohibitive for small surveys. Small electro-mechanical
vibrators have been developed for shallow geophysical
surveys, and these are gaining increasing acceptance as
seismographs capable of receiving and correlating the
signals are developed.
Mini-Sosie
adapts the principle of using a precisely
known source signature of long duration to cheaper,
lower energy applications. A pneumatic hammer deliv-
ers a random sequence of impacts to a base plate, thus
transmitting a pulse-encoded signal of low amplitude
into the ground. The source signal is recorded by a
detector on the base plate and used to cross-correlate
with the field recordings of reflected arrivals of the
Non-explosive land sources
Vibroseis
®
is the most common non-explosive source
used for reflection surveying. It uses truck-mounted
vibrators to pass into the ground an extended vibration
of low amplitude and continuously varying frequency,
known as a
sweep signal
. A typical sweep signal lasts from
several seconds up to a few tens of seconds and varies
progressively in frequency between limits of about 10
and 80 Hz. The field recordings consist of overlapping
reflected wave trains of very low amplitude concealed
in the ambient seismic noise. In order both to increase
the signal-to-noise ratio (SNR) and to shorten the pulse
length, each recorded seismogram is cross-correlated
(see Section 2.4.3) with the known sweep signal to
produce a correlated seismogram or
correlogram
. The
correlogram has a similar appearance to the type of
seismogram that would be obtained with a high-energy
impulsive source such as an explosion, but the seismic
arrivals appear as symmetrical (zero phase) wavelets
known as
Klauder wavelets
(Fig. 3.15).
The Vibroseis
®
source is quick and convenient to use
and produces a precisely known and repeatable signal.
The vibrator unit needs a firm base on which to operate,
such as a tarmac road, and it will not work well on soft
ground.The peak force of a vibrator is only about 10
5
N
