Geology Reference
In-Depth Information
The energy that land field crews expend in setting out geophones individu-
ally by hand is regarded with amazement by marine crews, who simply drag
their hydrophone streamers through the water. It is not easy to develop simi-
lar systems for land use, because of poorer coupling to the vibrating ground,
higher background noise, electrical and mechanical cross-talk in the cables,
and mechanical wear, but this is a rapidly developing field of design. At
present, streamers are being used mainly in surface wave studies (see Chap-
ter 14), where amplitudes may be an order of magnitude greater than for
body waves and short spreads and closely spaced detectors are the norm.
11.3.2 Detection of S-waves
Although S-waves are regarded as noise in most seismic work, there are
occasions when S-wave information is specifically sought. For example, both
S- and P-wave velocities are required to determine elastic properties (see
Section 11.1.2), and S-waves are sometimes preferred for high-resolution
reflection surveys in noisy environments (see Section 12.2.7).
'S-wave' geophones have coils that move horizontally rather than ver-
tically, the assumption being that any wavefronts of interest will be rising
more or less vertically and that the S-wave vibrations will therefore be in
the plane of the ground surface. Knowing this does not, of course, define
the vibration direction, which may be determined by the way the source is
operated but can be completely unknown. It is sometimes necessary to use
two S-wave phones, at right angles to each other, in each location.
Because direct waves travel parallel to the ground surface, S-wave geo-
phones are more sensitive to direct P-waves than to direct S-waves, just as
P-wave geophones are sensitive to vertically polarised direct S-waves.
11.3.3 Detection in swamps and water
Normal geophones are rainproof rather than waterproof, and are connected
to cables by open crocodile clips. Geophones are also available that are
completely enclosed and sealed into waterproof cases, for use in swamps.
These do not have external spikes but are shaped so that they can be easily
pushed into mud.
Motion-sensitive instruments cannot be used in water. Piezo-electric
hydrophones
respond to variations in pressure rather than motion and are
equally sensitive in all directions. Discrimination between P- and S-waves
is not required since S-waves cannot travel through fluids.
11.3.4 Noise
Any vibration that is not part of the signal is
noise
. Noise is inevitable, and
coherent
noise is generated by the shot itself. S-waves, Love and Rayleigh
waves and reflections from surface irregularities are all forms of coherent
noise in P-wave surveys. In shallow refraction work these late-arriving, but
