Geology Reference
In-Depth Information
Resonant
frequency
Resonant
frequency
(a)
(b)
180
0.80
150
0.40
h = 0.2
120
h = 0.5
90
0.20
10
20
30
40
h = 0.7
Frequency (Hz)
60
0.10
30
0
0
10
20
30
40
Frequency (Hz)
Fig. 3.20 Amplitude and phase responses of a geophone with a resonant frequency of 7 Hz, for different damping factors h . Output phase
is expressed relative to input phase. (After Telford et al . 1976.)
sympathy with the ground surface during the passage of
a seismic wave, causing relative motion between the sus-
pended coil and the fixed magnet. Movement of the coil
in the magnetic field generates a voltage across the ter-
minals of the coil. The oscillatory motion of the coil is
inherently damped because the current flowing in the
coil induces a magnetic field that interacts with the field
of the magnet to oppose the motion of the coil. The
amount of this damping can be altered by connecting a
shunt resistance across the coil terminals to control the
amount of current flowing in the coil.
Ideally, the output waveform of a geophone closely
mirrors the ground motion and this is arranged by
careful selection of the amount of damping. Too little
damping results in an oscillatory output at the resonant
frequency, whilst overdamping leads to a reduction of
sensitivity. Damping is typically arranged to be about 0.7
of the critical value at which oscillation would just fail
to occur for an impulsive mechanical input such as a
sharp tap. With this amount of damping the frequency
response of the geophone is effectively flat above the
resonant frequency. The effect of differing amounts of
damping on the frequency and phase response of a geo-
phone is shown in Fig. 3.20.
To preserve the shape of the seismic waveform, geo-
phones should have a flat frequency response and mini-
mal phase distortion within the frequency range of
interest. Consequently, geophones should be arranged
to have a resonant frequency well below the main
frequency band of the seismic signal to be recorded.
Most commercial seismic reflection surveys employ
geophones with a resonant frequency between 4 and
15 Hz.
Above the resonant frequency, the output of a
moving-coil geophone is proportional to the velocity of
the coil. Note that the coil velocity is related to the very
low particle velocity associated with a seismic ground
motion and not to the much higher propagation veloci-
ty of the seismic energy (see Section 3.4).The sensitivity
of a geophone, measured in output volts per unit of ve-
locity, is determined by the number of windings in the
coil and the strength of the magnetic field; hence, instru-
ments of larger and heavier construction are required for
higher sensitivity. The miniature geophones used in
commercial reflection surveying typically have a sensi-
tivity of about 10 V per m s -1 .
Moving-coil geophones are sensitive only to the
component of ground motion along the axis of the coil.
Vertically travelling compressional waves from subsur-
face reflectors cause vertical ground motions and are
therefore best detected by geophones with an upright
coil as illustrated in Fig. 3.19. The optimal recording of
seismic phases that involve mainly horizontal ground
motions, such as horizontally-polarized shear waves, re-
quires geophones in which the coil is mounted and
constrained to move horizontally.
Hydrophones are composed of ceramic piezoelectric
elements which produce an output voltage proportional
to the pressure variations associated with the passage of
a compressional seismic wave through water. The sensi-
tivity is typically 0.1 mV Pa -1 . For multichannel seismic
surveying at sea,
large numbers of individual
 
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