a. One of the broad, more or less well-defined, elongate zones in which
most earthquakes originate. Stokes
b. An elongate earthquake zone, esp. a zone of subduction or sea-floor
spreading. AGI
See:seismometer
See:diffraction
Applied to any definite signal change or amplitude difference on a seismic
record. It may be a reflection, a refraction, a diffraction, or a random
signal. AGI
Special forms of blasting gelatin, or gelatin and ammonia gelatin
dynamites, used in geophysical prospecting by the seismic method;
developed to shoot consistently at their characteristic rate of detonation
under unusually heavy water pressure. Lewis
The place of origin within the Earth of an earthquake; usually some more
or less restricted area of a fault surface. If the focus is to be some
particular point, it is the central point of the area over which fault
movement occurred and caused the earthquake. Challinor
a. Measure of frequency and magnitudes of earthquakes in a given area;
e.g., the average number of earthquakes per year and per 100 km2
Schieferdecker
b. The phenomenon of earth movements. Hy
A geophysical prospecting method based on the fact that the speeds of
transmission of shock waves through the Earth vary with the elastic
constants and the densities of the rocks through which the waves pass. A
seismic wave is initiated by firing an explosive charge (or by equivalent
artificial sources) at a known point (the shot point); records are made of
the travel times taken for selected seismic waves to arrive at sensitive
recorders (geophones). There are two main subdivisions of seismic
operations: the reflection method and the refraction method. The seismic
method has been applied to a lesser extent to elucidate mining problems,
partly due to its high cost. It has been used to investigate the base of
drift deposits, and drift-filled channels have been successfully outlined.
Nelson
See:microseism.
A method of geophysical prospecting in which vibrations are set up by
firing small explosive charges in the ground or by other artificial
sources. Precise measurements of the resulting waves are taken, from which
the nature and extent of underlying strata are revealed. Hammond
In this geophysical prospecting technique, the structure of subsurface
formations is mapped by making use of the times required for a seismic
wave (or pulse), generated in the Earth by a near-surface explosion of
dynamite or by other artificial sources, to return to the surface after
reflection from the formations themselves. The reflections are recorded by
detecting instruments responsive to ground motion, which are laid along
the ground near the site of generation of the seismic pulse. Variations in
the reflection times from place to place on the surface usually indicate
structural features in the rock below. Syn:reflection method
Dobrin
In refraction shooting, the detecting instruments are laid down at a
distance from the shothole that is large compared with the depth of the
horizon to be mapped. The seismic waves travel large horizontal distances
along distinct interfaces in the Earth, and the time required for travel
gives information on the velocity and depth of certain subsurface
formations. Dobrin
a. The initiation of seismic waves in the rocks by the firing of an
explosive charge at a known point. The disturbance must be capable of
accurate timing and must be such that, after traveling considerable
distances through varying strata, it produces a sharply defined effect on
the seismograph. These requirements may be supplied by the shock produced
by detonating a charge of high explosive. The intensity of the shock and
its effective range can be controlled by varying the quantity of explosive
charge. See also:reflection shooting; refraction shooting.
Nelson
b. A method of geophysical prospecting in which elastic waves are produced
in the Earth by the firing of explosives or by other means.
See also:reflection shooting; refraction shooting. AGI
A hole drilled for a seismic shot. It is usually a slim hole, although it
has also been termed core hole. See also:slim hole;
structure test hole. Williams
An exploration technique utilizing the variation in the rate of
propagation of shock waves in layered media. It is used primarily to
delineate subsurface geologic structures of possible economic importance.
Long
The Earth motion produced by a natural (earthquake) or synthetic
disturbance on the surface or underground; utilized in the seismic method
of geophysical exploration and for investigating the Earth's interior.
Three types of waves are produced: (1) longitudinal or P waves; (2)
traverse or S waves; and (3) surface or Raleigh and L waves. The speed of
propagation is characteristic for each type of rock, depending largely on
its compactness. In sandy clay, the speed of the P wave is about 4,000
ft/s (1.22 km/s); in sandstone, 10,000 ft/s (3.05 km/s); and in igneous
rock up to 22,000 ft/s (6.71 km/s). Nelson
An instrument designed to check ground stability. It amplifies 2.5 million
times, and can detect a rock movement as small as 0.000001 in (2.54 mu m).
Receiving phones are placed in holes in the area being tested. Either
earphones or automatic recording apparatus may be used for listening. A
rate of 3 or more microseisms per second indicates probable collapse, and
any rate over 25 or 30 per minute is considered dangerous. This instrument
is also finding use above ground in checking highway cut slopes.
Nichols, 1
The record of Earth motion made by a seismograph. AGI
a. This process produces an artificial reflection record from a
continuous-velocity log or an electric log. With this system the log is
converted from a depth scale to a time scale and is run through a scanning
device that transforms the fluctuations on the log into electrical
impulses that vary with time so as to simulate reflections. These impulses
are passed through appropriate filters and are then recorded on an
oscillograph in the same way as signals from a geophone. Dobrin
b. The theoretically calculated ground motion that would be recorded for a
given Earth structure and seismic source.