  Applied Geophysics : Seismics

Seismometer Calibration

Applet ( in sepatate window, ca. 640 x 460 Pixel )

The applet simulates a field test of a short period seismometer, to determine

the natural frequency and damping,
the two parameters, characterizing a seismometer's tranfer function,
the "critical damping resistance" and the open circuit damping,
allowing to adjust a desired damping value,
and
the sensitivity ( motor constant ) of the velocity transducer and the motor constant of the calibration coil.

( See Seismometer Documentation for physical / mathematical background )

Seismometer :

The applet refers to a Geotech Model S-13, configured for vertical operation, equipped with a velocity transducer
with four separate coils ( about 380 [Ohm] / 100 [Vs/m] each )
and
one calibration coil ( about 23 [Ohm] / 0.2 [N/m] ).

Two windings connected in series are used as a signal coil,
driving a high impedance recording device,
the remaining two windings are used for damping only,
to provide a constant sensitivity of the velocity output,
independent of the actual load resistance of the damping circuit.

Colorcode :

 Blue = mechanical system, moving parts, mass displacement, ( "true" seismometer parameters in CALC DISPLAY ), Magenta = coil assembly, ( Amplitude picks in PICK DISPLAY, calibration results in CALC DISPLAY ), Red = signal coil circuit, output signal ( = mass velocity ), Yellow = damping coil circuit, external damping resitance adjustment, symbolic damping assembly ( piston + cylinder, oil / honey filled, select your own preferred interpretaion ), Green = calibration coil circuit, calibration signal ( current / weight ), current adjustment ( frequency and magnitude ).

The actual state of the instrument ( velocity and excursion of the movable mass ) is calculated in constant time steps of 20 [ms] ( = 50 steps/second ) by a recursive filter, applied to the driving signals ( calibration signals, seim. noise ).

- Index -

Program Control

Calibration Procedures
Natural Frequency
Free Motion
Interpretation

Related Applets

HowTo

- Program Control -

Dialogue Panel :
( Screenshot ) The animation is controlled by the buttons RESET, RUN, HALT and STEP :

RESET
erases all data saved for parameter calculation,

RUN / HALT
starts / interrupts the simulation,

STEP
single step mode, one time increment ( 20 [ms] ) per click.

HELP
displays hints to actually possible mouse actions in the graphic area of the applet ( in HALT mode only ).

DISPLAY  Y = ...  /  X ... selects one of three calibration procedures :

Y = U_SIG  /  X  = I_CAL  =>  FREQ

The natural frequency of the seismometer is determined from the phase shift between
a harmonic calibration current
and
the resulting harmonic output voltage,
observed in a XY-display.
( see Natural Frequency and Band Pass B(s) in Seismometer Documentation )

Y = U_SIG  /  X  = TIME  =>  PICK

The damping of the seismometer for different values of the external damping resistance is determined
from the free motion of the movable mass,
resulting from
switching on / off a dc-current in the calibration coil
or
dropping / lifting a small additional mass.
( see Damping and Free Motion of the Seismometer Documentation )

The amplitude values of extrema of the output voltage are
displayed for interpretation by the user,
and can be
saved for interpretation in the procedure CALC.

Y = DAMP  /  X  = 1 / R  =>  CALC

The different values of damping are calculated from the amplitude picks saved and plotted
versus the reciprocal of the total resistance in the damping circuit.
A straight line is fitted to the observations
to determine the open cicuit damping and the "critical damping resistance".

If both, a weight lift test and a dc-current test are performed
for the same damping resistance,
the motor constants of the signal coil and of the calibration coil
are determined from the peak-to-peak amplitudes of both output signals.
( see Damping and Free Motion of the Seismometer Documentation )

In the procedures FREQ and PICK, where the output signal is displayed,

MASS DISPLACEMENT  SHOW
additionally displays the excursion of the mass ( blue ) from its zero-position,

RANGE  I_CAL [mA]U_SIG [mV]  und  X_MAX [mm]  ( F.S. = +/- ... )
determines the scaling of the graphic display in steps 1 - 2 - 5,

and NOISE LEVEL [mm/s**2]  ( R.M.S = ... ) selects
the magnitude of local seismic noise :

NONE
ideal but not realistic,
5.0e-5
corresponds to the ground acceleration at a quiet site and leads to reliable results,
5.0e-3
!don't try to calibrate a seismometer at the third floor of a building !

- Calibration Procedures -

The natural frequency of the seismometer, the parameters necessary for the adjustment of the damping and the sensitiviies of signal and calibration coil are determined in three procedures.

Y = U_SIG  /  X  = I_CAL  =>  FREQ :
( Screenshot ) XY-display ( red ) of harmonic calibration current ( =X ) and output voltage ( =Y ).

The external damping resitance ( i.g. approx. 0.2 - 0.5 [kOhm] ) is adjusted
via R_EXT [kOhm] ( orange ),
the amplitude of the harmonic calibration current
via I_CAL [mA] ( green ).
( Activate HELP for details. )

CAL. SIG.  ON ( dialogue panel )
applies the current to the calibration coil and starts the simulation.

The phase difference between output signal and calibration current depends
on the difference between F_CAL and the natural frequency F_0 of the seismometer.

The momentary XY-position ( green point ) draws an ellipse,
clockwise for F_CAL > F_0,
ccw for F_CAL > F_0, and
follows a straight line for F_CAL = F_0.

The frequency of the calibration current is varied via F_CAL [Hz] ( green ) until
a straight line is drawn, i.e.
the value of F_CAL corresponds to the natural frequency of the seismometer.

For frequency differences |ΔF| < 0.04∗F_0 a symbolic path, circle / straight line ( grey, lower left ), is displayed additionally.

Y = U_SIG  /  X  = TIME  =>  PICK :
( Screenshot ) Graphic display of calibration signal ( green ) and output voltage ( red, = velocity of free motion of the mass ) versus time.

CAL. SIG. ... selects the calibration signal :

+I [mA]  / -I [mA]
dc-current of selectable polarity and adjustable amplitude ( I_CAL [mA],  green ), controlled by ON / OFF,

1.0 [g]  / 0.2 [g]
additional mass, contolled by DROP / LIFT.

R_EXT [kOhm] ( orange )
determins the external damping resistance.

At a relatively high damping value ( R_EXT approx. 0.2 - 0.5 [kOhm] )
the simulation is startet by ON resp. DROP, and
the mass moves to a new postion ( X_MASS ≠ 0 ).

After the new position is reached ( output voltage = velocity = 0 ),
a new value R_EXT is selected, and
the mass is relasedOFF / LIFT ), and the simulation is restarted ( if in HALT mode ).

The mass returns to its zero position, and
the output voltage ( red ) of the free motion is recorded.

After HALT ( manually or autom., if the start of the free motion signal has reached the left edge of the display )
up to 10 amplitudes of consecutive extrema are "picked",
displayed ( AMPLITUDE PICKS SHOW )
and
stored ( AMPLITODE PICKS SAVE ) for an interpretation in the procdure CALC.

Graphic display ( and saving ) of amplitude picks is inhibited, if the values or their distance in time is "obscure" ( especially at high noise level ).

SAVED nn ( magenta, upper left ) displays
the number of saved data sets ( max. 20 ), and
a mouse click into the surrounding frame
lists the data to the screen / to the JAVA-Console. ( see Example )

Y = DAMP  /  X  = 1 / R  =>  CALC :
( Screenshot ) Graphic display of damping versus 1 / resistance
with different symbols for different calibration signals ( table upper left, magenta ) and
straight line fitted to the observations :
y-intercept = open circuit damping ( α_0 ),
slope = "critical damping resistance" ( CDR ).

In two tables
the results of the calibration ( CALIB, magenta )
can be compared to
the parameters of the seismometer ( SEIS, blue ).

The weight of an observation for its use in the iterpretation can be set
to 1 by the left mouse button,
to 0 by the right button.
( USED nn, magenta, upper left, displays the number of observations used,
a mouse click into the surrounding frame lists the obsevations to the screen / to the JAVA-Console. )

List of Observations :
( Screenshot ) ( See Seismometer Documentation for physical / mathematical background )

- Related Applets -

Frequency Response of a Seismometer

Free Motion of a Seismometer