GB/T 20485.42-2018Methods for the calibration of vibration and shock transducers -- Part 42: Calibration of seismometers with high accuracy using acceleration of gravity (English PDF)

4.1 Method of using absolute gravimeter

Determine the local absolute gravitational acceleration using a free fall absolute gravimeter (FG5 or other device). The local weight obtained thereby

The extended uncertainty of the force acceleration is approximately 5x10-8 m/s2 (5 µGal).

4.2 Method of using gravity acceleration normalization net and relative gravimeter

At the local gravity acceleration reference point established by IGSN-71, a relative gravimeter can be used to determine the local absolute gravity acceleration.

degree. In this case, there is no need to correct the latitude and altitude. The extended uncertainty value of the relative gravimeter is given by the manufacturer.

National Geological Exploration Institute, Meteorological Institute, Geodesy or Geophysics Institute can provide uncertainty than IGSN-71

Smaller gravitational acceleration measurements. This value can be used if possible.

4.3 Method of standardizing the net using gravity acceleration

Based on the latitude and altitude of the measurement point, the local gravity acceleration can pass the nearest relative geographic point given in the IGSN-71 database.

The value is calculated.

The uncertainty of the local gravity acceleration thus obtained is approximately 1 x 10-5 m/s2 (1 mGal). This method is only applicable to no

He is a situation with irregular terrain.

National Geological Exploration Institute, Meteorological Institute, Geodesy or Geophysics Institute can provide uncertainty than IGSN-71

Smaller gravitational acceleration measurements. This value can be used if possible.

Since the 1m height difference is equivalent to a gravitational acceleration change of about 3x10-6m/s2 (0.3mGal), the uncertainty of the height difference measurement is uncertain.

The degree should be less than 2m.

Note 1. The effect of the 1° deviation at a latitude of approximately 45° corresponds to a change in gravitational acceleration of approximately 1 x 10 -6 m/s 2 (0.1 mGal).

Note 2. The local gravity map includes the geoid and altitude values.

5.1 Calibration environment

Standard reference atmospheric conditions. temperature (23 +/- 3) ° C; relative humidity is not more than 75%. Temperature, humidity and atmospheric pressure should be measured and reported

The report is given.

5.2 Basic and vibration environment (isolation block of calibration equipment)

The calibration equipment should be placed on a sufficiently heavy basis to effectively isolate the vibrations of the building.

5.3 voltmeter

The voltmeter that measures the voltage output of the seismometer should contribute no more than 0.1% to the measured relative extended uncertainty (see Table A.1).

5.4 Adjustable low pass filter

a) cutoff frequency

The cutoff frequency should be 10 Hz, 30 Hz or 60 Hz. A typical cutoff frequency is 30 Hz.

b) attenuation rate (filter steepness)

The attenuation or insertion loss in the filter stopband should be no less than 24dB/octave.

5.5 Power supply

When measuring the sensitivity of a seismometer under certain gain conditions, the stability and signal-to-noise ratio of the power supply should meet the nominal measurement uncertainty.

The rate of contribution.

5.6 tilting table

The angular resolution of the tilting table should be no more than 0.05°, and the contribution to the extended uncertainty should be less than 0.03°. Tilting table should have support

Seismic meter quality with sufficient stiffness, small enough clearance and hysteresis, and sufficient linearity.

6.1 Principle of calibration

Figures 1 and 2 show schematic and operational examples of the calibration device.

Description.

1---based;

2---platform;

3---seismometer;

4---filter;

5---voltmeter;

6---Environmental instrument (temperature and atmospheric pressure);

7---tilt table;

8---ground.

Figure 1 Calibration device

Description.

1---based;

2---platform;

3---seismometer;

4---ground.

Figure 2 Example of applying arbitrary acceleration from the initial setting

Place the tilting table on the platform, which should be rigidly connected to the base described in 5.2 to form the vibration isolating block of the calibration equipment. Vibration isolation block top surface

The horizontal plane should be perpendicular to the local gravity field and the angular deviation from the horizontal plane should be less than 0.03°. Due to the deviation of the horizontal angle

The amount of influence of the force component is about 10-7. The angular deviation of the horizontal plane should be measured with an inclinometer with sufficient resolution.

On the tilting table, the vertical component of the acceleration athetav or the horizontal component athetah sensed by the seismometer is given by.

Athetav=costheta.g

Athetah=sintheta.g

(1)

In the formula.

theta---the inclination angle with the horizontal plane obtained when placing the inclined table, the unit is degree (°);

g---Local gravitational acceleration in meters per square second (m/s2).

The vertical and horizontal components in the seismometer output signal are given by equation (2).

The measured seismometer output signal E is.

E=Sathetav B or E=Sathetah B (2)

In the formula.

S --- Seismometer sensitivity calculation value, in volts per meter negative quadratic seconds [V/(m . s-2)];

Athetav or athetah---the accelerometer acceleration component of the accelerometer sensitive axis, in meters per square second (m/s2);

B --- The calculated value of the output signal offset component in volts (V);

E --- Seismometer output voltage measurement in volts (V).

When thetav or thetah changes from theta1 to thetan, the n measurements of the seismometer output signal are.

E1=Satheta1 B

E2=Satheta2 B (3)

En=Sathetan B

The sensitivity S of the seismometer and the offset value B of the output are given by the following linear regression equations (4) and (5).

S=

Nsum

i=1

athetaiEi-sum

i=1

Athetaisum

i=1

Ei

Nsum

i=1

(athetai)2-(sum

i=1

Athetai)2

(4)

B=

i=1

(athetai)2sum

i=1

Ei-sum

i=1

athetaiEisum

i=1

Athetai

Nsum

i=1

(athetai)2-(sum

i=1

Athetai)2

(5)

6.2 Calibration procedure

Place the seismometer on the tilting table so that its sensitive axis is aligned with the vertical axis. The angular deviation between the sensitive axis and the vertical axis should be less than

0.03°, after adjusting the setting accurately, it should be adapted to the ambient temperature for a period of time.

Seismometers typically have an angular misalignment error relative to the sensitive axis due to the package housing, also referred to as housing alignment error. School

The quasi-seismic timing, as described above, should adjust the direction axis of the measured acceleration input to coincide with the direction of the sensitive axis of the seismometer. Really here

The sensitive axis is determined by obtaining the inclination of the seismometer's maximum output signal, otherwise the uncertainty of the housing alignment error should include the tilt

The inclination of the inclined table is not determined. On the tilting table, the vertical component of the acceleration induced by the seismometer at the initial tilt angle athetav0 or horizontal

Quantity athetah0.

Athetav0=costheta0.g or athetah0=sintheta0.g

The necessary multiple measurements of the output are made at a certain sampling time interval. Output record at acceleration athetav0 or athetah0 will be applied

Is V0. Then, the tilt angle is adjusted to theta1, and the vertical component athetav1 or the horizontal component athetah1 of the acceleration is.

Athetav1=costheta1.g or athetah1=sintheta1.g

After setting the inclination of the tilting table, take measurements. Perform the necessary multiple output measurements at a certain sampling interval. See

figure 2.