Geoscience Reference
In-Depth Information
300 km or so, the distance necessary to determine a structure for the continental
crust and uppermost mantle. It is necessary to have a seismometer at least every
5km along each line, and preferable to have them very much closer together.
Since the instruments are expensive, this can sometimes entail detonating one
shot at the shotpoint, moving the seismometers along the line to new positions,
and then detonating another shot at the shotpoint, and so on. Thus, large refraction
experiments on land are usually carried out cooperatively by several universities
or institutions so that enough people and recording instruments are available.
The situation at sea is different. In marine work, the source and receiver loca-
tions (Fig. 4.33) are usually exchanged so that a small number of seismometers
or hydrophones (pressure recorders) is deployed and then a large number of shots
is fired at ever increasing distances. Seismometers for use at sea are very expen-
sive because they are necessarily enclosed in waterproof pressure vessels, have
a homing detection device to ensure their subsequent recovery and, if laid on the
seabed, must be equipped with a release device. Fewer people are needed for a
marine experiment because the research ship can steam along, firing charges as
it goes (no drilling needed here). At shorter ranges, an air gun (an underwater
source that discharges a volume of very-high-pressure air) can be used as an
energy source. Because the oceanic crust is much thinner than the continental
crust, marine refraction lines need to be only 50 km or so in length to allow one
to determine the crustal structure and an uppermost mantle velocity.
Most textbooks on exploration geophysics (e.g., Telford
et al
. 1990; Dobrin
and Savit 1988)give the details of the field procedures and the corrections to be
applied to seismic-refraction data. These are not discussed further here.
The coding used for crustal and uppermost-mantle phases is as follows:
P-wave through the upper continental crust
7
P
g
S
g
S-wave through the upper continental crust
P
m
P
P-wave reflection from the Moho
S
m
S
S-wave reflection from the Moho
P
n
upper mantle P-head wave
S
n
upper mantle S-head wave.
4.3.2 A two-layered model
Let us assume that the crust beneath a refraction line consists of two horizontal
layers, with distinct and constant P-wave velocities
α
2
>α
1
(Fig. 4.34). Energy from the source can then reach the seismometer by a variety
of paths: directly through the top layer, by reflection from the interface between
the two layers, by multiple reflections within the top layer or by travelling along
the interface as a critically refracted wave or 'head' wave. The head wave, which
is often called a refraction or refracted wave, has a travel time corresponding to
α
1
and
α
2
such that
7
The subscript g was first used to denote granite (Jeffreys 1926).
