Sensor SMT-200 Operation And Technical Manual page 107

SENSOR
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TEST PRINCIPLE.......
A DC voltage (5-10V) is applied between the geophone negative terminal and an external probe
connected to the leakage terminal. The current flowing between the geophone terminals and the
leakage probe is amplified and compared with the current flowing in an internal reference resistor. The
ratio of the two readings gives a value for the leakage of the geophone string.
B.5 Resistance Test
This is a DC resistance test of the coil, shunt and leader resistance. It quickly determines if the string
has open coils or shunt resistors. It also confirms the correct series / parallel combination of
geophones are connected. Most recording systems use this as the basic geophone QC test.
TEST PRINCIPLE.......
The same procedure for the leakage test is used except the readings are taken between the two
geophone terminals.
B.6 Pulse Test
This single test provides a wealth of information about a geophone. From this test we can determine
three parameters: Natural Frequency (Fn), Damping (Bo or Bt) and Sensitivity (Go).
Fig.4 shows the geophone response (in the time domain) to an impulse. The oscillations will reduce to
zero. The rate at which this will happen is determined by the damping. The time to the first zero
crossing is dependant on the natural frequency of the phone.
A1 = Amplitude of first peak
A2 = Amplitude of second peak
T = Time of first zero crossing in
seconds
Natural Frequency
SMT-200 Operations and Technical Manual 2.00/R1
This is the frequency that the geophone coil will naturally oscillate at when
freely suspended. The actual frequency depends primarily on the stiffness of
the spring and the suspended mass. A geophone having a natural frequency
of say 10 Hz means that any signal at that frequency will tend to be boosted.
An undamped phone will not to have a flat response over the seismic
frequency band and sometimes high amplitude low frequency energy such
as ground roll can swamp higher frequency data unless it is controlled. The
normal way of doing this is a process know as damping.
A1
A2
Geophone Pulse response
Appendix B Test Overview
T
Time in seconds
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