Calibrating the 3TB
The
bbnoisecal
script automatically performs appropriate averaging to
reduce the effects of aliasing and cultural noise.
5.3.1 Sensor response codes for 3TB instruments
Sensor
Sensor type code
Units (V/A)
30 s – 50 Hz response
CMG-3B_30S_50HZ
V
100 s – 50 Hz response
CMG-3B_100S_50HZ
V
120 s – 50 Hz response
CMG-3B_120S_50HZ
V
360 s – 50 Hz response
CMG-3B_360S_50HZ
V
360 s – 100 Hz response
CMG-3B_360S_100HZ
V
5.4 Calibration with a hand-held control unit
If you prefer, you can inject your own calibration signals into the system through a
hand-held control unit. The unit includes a switch which activates the calibration
relay in the seismometer, and 4 mm banana sockets for an external signal source.
As above, the equivalent input velocity for a sinusoidal calibration signal is given by
v
=
V /
2 π
f R K
where
V
is the peak-to-peak voltage of the calibration signal,
f
is the signal
frequency,
R
is the magnitude of the calibration resistor and
K
is the feedback coil
constant.
R
and
K
are both given on the calibration sheet supplied with the 3TB.
The calibration resistor is placed in series with the transducer. Depending on the
calibration signal source, and the sensitivity of your recording equipment, you may
need to increase
R
by adding further resistors to the circuit.
5.5 The coil constant
The feedback coil constant
K
is measured at the time of manufacture, and printed on
the calibration sheet. Using this value will give good results at the time of
installation. However, it may change over time.
The coil constant can be determined by tilting the instrument and measuring its
response to gravity. To do this, you will need a mounting harness for the sonde and
apparatus for measuring tilt angles accurately.
1. Measure the acceleration due to gravity,
g
, at your location.
2. Tilt the instrument slightly, and measure its attitude and the gain of the
mass
position
output for the component you wish to calibrate.
57
Issue G - November 2019
