Geoscience Reference
In-Depth Information
Figure 4.33.
Examples of
some layouts of energy
sources (usually
explosives) and
seismometers used in
refraction experiments:
(a) a reversed refraction
line; (b) a split profile; (c)
fan shooting; and (d) a
refraction experiment
designed to determine
the three-dimensional
structure, including
velocity anistropy. For
refraction experiments on
land, * is the explosive
source and squares are
the seismometer
locations. For refraction
experiments at sea, the
source and receiver
locations are exchanged:
*isthe seismometer/
hydrophone location and
squares are the locations
of explosive sources.
(a)
*
*
(b)
*
(c)
*
(d)
*
*
*
along the northern boundary of the Pacific plate. They found that one set changed
slowly and systematically in direction and the others showed no consistency. The
correct horizontal components of slip were the consistent set, and so the fault
and auxiliary planes were determined. An example of this method is shown in
Fig. 4.29.
4.3 Refraction seismology
4.3.1 Refraction experiments
Earthquake seismology is able to reveal the broad details of the velocity and
density structure of the Earth (Section 8.1), but to look in greater detail at the
structure of the crust and uppermost mantle, especially in those regions well
away from active seismic zones, it becomes impracticable to use earthquakes as
the energy source. In refraction seismology, portable seismometers are deployed
in the regions to be studied, and explosives are used as the energy source -
careful logistical and environmental planning is necessary. Figure 4.33 shows
some examples of the positioning of sources and receivers for such refraction
experiments.
Refraction seismology on land is expensive and messy because tons of explo-
sive are needed in order to make it possible to record energy at distances of
