Sensor SMT-200 Operation And Technical Manual page 108

Appendix B Test Overview
Damping
Sensitivity
100
10
1
1
TEST PRINCIPLE......
As its name implies the pulse test applies a short pulse to the coil of the geophone. The current used
to do this is calculated (using the stored geophone parameters) to normally give a 70% excursion from
its rest position. When the current is turned off the coil will drop and the result is a sinusoid that decays
exponentially.
98
Energy hitting the phone perpendicular to its normal direction of travel can cause
unwanted oscillations in the springs, which in turn causes a spurious frequency. This
is specific to each geophone type but tends to be at relatively high frequencies and
normally above 150 Hz.
Damping tends to reduce oscillation at the phones natural frequency. This is
achieved by adding an external resistor across the terminals of the phone. In this
case large excursions that occur at the geophone natural frequency are now
opposed by the current flowing in the resistor giving rise to a "motor action" . This
gives a more uniform response curve. Open circuit damping Bo (intrinsic damping) is
the damping with no external damping resistor.
Damping resistor electrical configuration
Measured in V/m/s (i.e. Volts per meter/second) this gives an indication of the
magnitude of the output voltage in response to a certain velocity applied to a phone.
Sometimes known as transduction. Sensitivity depends on the load the geophone is
driving. Shunt resistors have an effect on the sensitivity of the geophone and so
does the recording systems input stages. The seismic detector response curves
shown in the Sensor geophone specification sheets show the open circuit (OC)
damping sensitivity curve at the top and the damping curve with various values of
shunt resistors underneath. See figure below.
GEOPHONE RESPONSE CURVE
SM-4 model B 10 Hz
10
Frequency (Hz)
Damping Resistor
Coil
Shunt: 0 . Bs = 0.25
Shunt: 1330 Bs = 0.60
Shunt: 1000 Bs = 0.69
100 1000
SMT-200 Operations and Technical Manual 2.00/R1
Io
Geophone response curves
present information in the frequency
domain. The x axis shows the
frequency on a logarithmic scale.
The y axis shows the geophone
sensitivity on a logarithmic scale.
This gives a clear indication of the
geophone response over the full
range of seismic frequencies.
SENSOR