Io
SENSOR
Appendix B Test Overview
SMT-200 Operations and Technical Manual 2.00/R1
99
Damping
(B
t
) is calculated by comparing
the first and second peak. High damping
means the magnitude of the second peak
will be much less than the first. No other
variables are needed to determine
damping.
Natural frequency
(Fn
)
is primarily
determined by the time to the first zero
crossing. There is a small correction for
damping as the time period will decrease
slightly with the increase in damping.
Sensitivity
(G) is the most complex
calculation. There are a number of
variables that will have a direct effect on
the sensitivity. The SMT-200 will fed a
current into the coil which then move.
The amount it moves will be dependent
on the magnitude of the current and the
moving mass.
B.7 Distortion
Test
The ideal geophone would convert movement into an exact electrical equivalent that can be fed to a
recording system. This is not always possible. For instance the magnetic field within the element is
more or less linear around the rest position in the centre of the magnet. As the moving mass
approaches the limit of its travel it will start to encounter non linearities in the magnetic field around the
poles of the magnet, hence the output will appear distorted.
TEST PRINCIPLE……
The accepted way to measure distortion is to feed a low distortion signal into the unit under test and to
measure the output voltage. Distortion will appear as harmonics of the signal fed into the unit under
test (referred to as the fundamental). By analysing the output in the frequency domain using a Fast
Fourier Transform (FFT.) the amplitude of each harmonic can be calculated. This can be expressed as
a percentage of the fundamental.
In the case of the SMT-200 the test frequency is determined by the user but for each period during the
recording interval, 32 samples are taken and averaged. The result is then converted to the frequency
domain so a 32 point FFT can be applied to work out the fundamental, second and third harmonics.
From this the final percentage distortion is calculated.
Bt = Total damping
A1 = Amplitude of first peak
A2 = Amplitude of second peak
G = Sensitivity
I = Current through geophone
Bt = Total damping
Fn = Frequency
m = Mass
Bt
Bt
Bt
Bt
e
I
FnmA
G
2
2
1
1
1
arctan
2
−
−
=
π
=
−
=
π
2
1
2
arctan
2
1
1
2
1
A
A
In
TCos
Bt
T
Fn
(
)
2
2
1
2
2
1
2
1
arctan
A
A
A
A
A
A
In
In
In
Sin
Bt
+
=
=
π
π
