Chapter 13. Calibration Values

In the channel subframe description field, the calibration of the sensor and digitiser can be given. Current Güralp hardware is not capable of automatically filling out these fields, so they must be entered manually. If left empty, default values of 1.0 will be used for both the calibration gain and period.

In order to fill out the calibration value, a period within the pass band of the sensor must be chosen. 1 s is often a good choice and simplifies the calculations. In any case, the period T is computed from the frequency at which the gain is considered, which will be referred to as f in the calculations below.

— relationship between period T (seconds) and frequency f (Hz).

It is the value T which goes into the calibration period or calper field of the gdi2cd11 configuration and is stored in the channel subframe description.

The digitiser sensitivity of the corresponding channel must also be known. This is given in units of µV/count and is nominally 3.2 µV/count for a DM24mk3 seismic channel (Z, N, E or X) or (again, nominally) 300 µV/count for a multiplexed or mass-position channel (M8, M9, MA, MB, MC, MD, ME, MF).

The text below is from a typical DM24 calibration sheet (relevant excerpt only):

The important details here are:

• the sensitivities for the three “seismic” channels Z2, N2 and E2, which should be around 3.2 µV/count;

• the sensitivity for the “calibration”/auxiliary channel X2, which should be around 3.2 µV/count;

• the calibrations for the mux (mass position) channels at 4sps, which should be around 300 µV/count.

The “seismic” and “calibration” labels are only labels; it is possible to digitise any analogue signal on any channel (although note that the X channel is temporarily replaced by a calibration signal when a calibration is initiated).

13.1 Seismic sensors (velocity)

For velocity sensors, such as the 3T, 40T and 6T, the sensor's calibration value will be given as a value in V/ms-1.

The text below is from a typical 3V (3 sensor, vertical component only) calibration sheet (relevant excerpt only):

The important fields are the velocity output, used in this section, and the mass position output, used in section 13.2.

The CD1.1 calibration value for seismic sensors should be in units of nm/count, which is the displacement response. To convert from a velocity gain to a displacement gain, it is necessary to divide the velocity gain by 2πf.

Taking into account conversion from µV to V (×106) and from m to nm (×10-9), we introduce a scale factor of 1000, leading to:

— CD1.1 calibration value for seismic sensor with velocity response, where C is the calibration value in nm/count, S is the digitiser sensitivity in µV/count, G is the gain of the instrument in V/ms-1, and f is the frequency of the calibration point in Hz.

13.2 Seismic sensors (acceleration)

For sensors such as the Fortis or 5T, with an acceleration response, or for mass position channels (which are also measuring an acceleration), the following method should be used to compute the CD1.1 calibration value.

As Güralp sensors and digitisers use differential signalling, the instrument gain value (in V/ms-²) on the calibration sheet is written as e.g. 2×0.512. Use the doubled value (in this case 1.024). This does not apply to mass positions/mux channels, which are single-ended.

The text below is from a typical 5T calibration sheet (relevant excerpt only):

The CD1.1 calibration value for seismic sensors (including mass position channels) should be in units of nm/count. To convert from an acceleration gain to a displacement gain, it is necessary to divide by (2πf)², or 4π²f². We also need to introduce a scale factor of 1000, leading to:

- where C is the calibration value in nm/count, S is the digitiser sensitivity in µV/count, G is the gain of the instrument in V/ms-1, and f is the frequency of the calibration point in Hz.

13.3 Wind speed

For a wind speed sensor with an output that is given as V/ms-1, use:

- where C is the calibration value in ms-1/count, g is the gain of an optional auxiliary signal conditioner (use 1 if no conditioner is present), S is the digitiser sensitivity in µV/count and G is the gain of the instrument in V/ms-1.

13.4 Wind direction

For a wind direction sensor with an output that is given as V/° (volts per degree), use:

- where C is the calibration value in °/count, g is the gain of an optional auxiliary signal conditioner (use 1 if no conditioner is present), S is the digitiser sensitivity in µV/count and G is the gain of the instrument in V/°.

13.5 Acoustic or infrasound sensors

For sensors which measure acoustic/infrasound responses, the sensitivity (in units of V/Pa or VPa-1) should be known. The CD1.1 calibration value is expected to be in Pa/count so, to convert:

- where C is the calibration value in Pa/count, g is the gain of an optional auxiliary signal conditioner (use 1 if no conditioner is present), S is the digitiser sensitivity in µV/count and G is the gain of the instrument in V/Pa.

13.6 Temperature

For a temperature sensor with an output that is given either as V/°C or
V/K (VK-1), use:

- where C is the calibration value in K/count, g is the gain of an optional auxiliary signal conditioner (use 1 if no conditioner is present), S is the digitiser sensitivity in µV/count and G is the gain of the instrument in V/°C or V/K.