Applied Geophysics : Seismics

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/A] ).

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 ).

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Program Control

Calibration Procedures

Natural Frequency

Free Motion

Interpretation

Related Applets

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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 !

Top of Page    Index

Program Control    Index    Top of Page

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.

Natural Frequency    Program Control    Index    Top of Page

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 relased ( OFF / 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 )

Free Motion    Calibration Procedures    Program Control    Index    Top of Page

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 )

Interpretation    Calibration Procedures    Program Control    Index    Top of Page

Frequency Response of a Seismometer

Free Motion of a Seismometer

Seismometer Demo

Class and html files for a local installation of the seismometer applets ( DEMO + CALIB ) are available as a zip file and as a tar.gz file.

More applets at : Geophysics Department TU Clausthal

Calibration Procedures    Program Control    Index    Top of Page