Seismic method - Geophysics for the Mineral Exploration Geoscientist - page 364

re ection surveys), the greater is the energy required to
counter the effects of attenuation ( Section 6.3.3 ) and energy
partitioning (see Energy partitioning in Section 6.3.4.2 ) .
A seismic source

logistics and survey economics require that these are
cheap, small, light and easy to deploy. Normally only the
vertical component of ground velocity is measured since
this corresponds with the direction of motion of the
ground for P-waves travelling steeply upwards (see Section
6.2.2 ) , but also means that Rayleigh waves produce strong
responses (see Section 6.2.3 ). Waterborne surveys use
hydrophones in which the sensor is a pressure-sensitive
piezoelectric element that produces a voltage in response to
pressure variations associated with the propagation of P-
waves through the water.
A seismic detector rarely consists of only a single geo-
phone or hydrophone. The recording is the combined
response of a number of geophones/hydrophones known
as an array or group. The position of the detector is taken
as the centre of the group and their spacing is known as the
group interval. For the case of hydrophones, they are
mounted within a flexible waterproof tube, usually lled
with oil of a speci c density to create neutral buoyancy,
and towed in the water behind a moving vessel. This is
known as a streamer.
Appropriate arranging of the individual detectors allows
the group to have a direction dependent response, designed
to favour seismic waves travelling upwards from below
rather than those which have travelled laterally and, there-
fore, have not sampled the subsurface. The combining of
the outputs from multiple detectors is a form of stacking
which suppresses random noise (see Section 2.7.4.1 ).

is ultimate function is to deform the
Earth and generate elastic waves. The most common seis-
mic sources for land surveys are explosives, impact devices
and a vibrating impact technology known as Vibroseis.
Explosives and impact devices produce a short duration
pulse of energy. The widespread use of explosives has led to
the seismic source often being referred to as the shot, and
the term fired meaning activation of the source. Impact
devices range from a sledge hammer to mobile machinery
incorporating a heavy mass which is dropped to the ground.
Vibroseis introduces seismic energy into the ground via a
vehicle-mounted vibrating base plate. The base plate is
placed in contact with the ground and vibrated at a range
of frequencies during the course of a sweep, which lasts
several seconds. This necessitates that the recorded data be
mathematically transformed into the equivalent response
that would have been recorded from an impulse (short
duration) source.
For waterborne surveys, an airgun releasing a pulse of
compressed air into the water is a common source. An
array of airguns of different size can be

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in combin-
ation, but at slightly different times, to control the fre-
quency and energy of the wavelet and to counteract the
effects of gas (air) bubbles, whose periodic expansion and
collapse create the equivalent of a series of gradually
diminishing pulses.
The choice of seismic source depends on financial, logis-
tical and technical considerations such as source energy
and frequency requirements. Explosives are good sources
of P-waves but require expensive drilling of shotholes, and
the source signature is not always consistent, owing to
different geological conditions local to each shothole. On
the other hand, Vibroseis produces a more consistent
source waveform and also can easily operate in urban
areas. However, only a fraction of the seismic energy
created is in the form of body waves, with the majority
occurring as surface waves.

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6.4.3 Displaying seismic data

As noted previously, the deformation associated with the
passage of seismic waves at a detector location is displayed
in the form of a seismic trace. This is a graph representing
the velocity of the movement of the ground (or pressure
variation in water) at the location of the detector as a
function of time. The resulting time series begins at the
instant (T 0) that the seismic waves were generated by

the source ( Fig. 6.1 ). The detector output is digitised using
a sampling interval chosen to avoid aliasing ( Section 2.6.1 ).

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6.4.3.1 Traces
A simple line graph of ground velocity (or pressure vari-
ations) is referred to as a wiggle trace ( Fig. 6.12a , i ) . An
alternative display is the variable-area display, where the
wave is shaded when it exceeds a speci ed threshold value
( Fig. 6.12a , ii ). Sometimes the trace itself is omitted leaving
just the shaded areas ( Fig. 6.12a , iii ). Also commonly used

6.4.2 Seismic detectors

Land-based seismic detectors are known as geophones.
These are a form of microphone which converts the vel-
ocity of the ground motion associated with the passage of a
seismic wave into a voltage response. Large-scale land
surveys require hundreds to thousands of detectors, so