Geology Reference
In-Depth Information
Vibroseis sweep signal
Reflection from base of layer 1
Reflection from base of layer 2
Reflection from base of layer 3
(phase inverted)
Field recording (superposition
of above reflections)
Output trace resulting from
correlation of field recording
with sweep signal
t
= 0
Time
Fig. 3.15
Cross-correlation of aVibroseis
®
seismogram with the input sweep signal to locate the positions of occurrence of reflected
arrivals.
Firing
pin
pulse-encoded signal from buried interfaces. Peaks in
the cross-correlation function reveal the positions of
reflected signals in the recordings.
Weight drops and hammers.
Perhaps the simplest
land seismic source is a large mass dropped on to the
ground surface. Weight drops have been manufactured
in a wide variety of forms from eight-wheel trucks
dropping a weight of several tonnes, to a single person
with a sledgehammer. If the source energy required
is relatively low, these types of sources can be fast and
efficient. The horizontal impact of a weight or hammer
on to one side of a vertical plate partially embedded
in the ground can be used as a source for shear wave
seismology.
Shotguns, buffalo guns and rifles
. One solution to gaining
additional energy for small-scale surveys is to use the
compact chemical energy in small-arms ammunition.
Rifles have been used as seismic sources by firing the
bullet into the ground. While effective as a very high-
frequency source, this is banned by legislation in many
countries. An alternative is to fire a blank shotgun car-
tridge in a hole using a suitable device, commonly
termed a buffalo gun (Fig 3.16).The blank shotgun car-
tridge offers an impulsive source giving considerably
more energy than a sledgehammer, with few of the
safety problems of explosives.
Ground
surface
Auger
flight
Firing
chamber
Fig. 3.16
Schematic cross-section of a typical buffalo gun.The
cartridge is fired by dropping a simple firing pin on to the
cartridge.
