Guralp Systems Limited
MAN-030-0001 - Güralp 3T Operator's guide
Section Index: 4.1 The calibration pack 4.2 Calibration methods 4.3 Calibration with Scream! 4.4 Calibration with a hand‑held control unit 4.5 The coil constant

Chapter 4. Calibrating the 3T

4.1 The calibration pack

All Güralp sensors are fully calibrated before they leave the factory. Both absolute and relative calibration calculations are carried out. The results are given in the calibration pack supplied with each instrument:

Works Order : The Güralp factory order number including the instrument, used internally to file details of the sensor's manufacture.

Serial Number : The serial number of the instrument

Date : The date the instrument was tested at the factory.

Tested By : The name of the testing engineer.

There follows a table showing important calibration information for each component of the instrument, VERTICAL, NORTH/SOUTH, and EAST/WEST. Each row details:

Velocity Output (Differential) : The sensitivity of each component to velocity at 1 Hz, in volts per m/s. Because the 3T uses balanced differential outputs, the signal strength as measured between the +ve and –ve lines will be twice the true sensitivity of the instrument. To remind you of this, the sensitivities are given as 2 × (single-ended sensitivity) in each case.

Mass Position Output : The sensitivity of the mass position outputs to acceleration, in volts per ms-². These outputs are single-ended and referenced to signal ground.

Feedback Coil Constant : A constant describing the characteristics of the feedback system. You will need this constant, given in amperes per ms-², if you want to perform your own calibration calculations (see below.)

Power Consumption : The average power consumption of the sensor during testing, given in amperes and assuming a 12 V supply.

Calibration Resistor : The value of the resistor in the calibration circuit. You will need this value if you want to perform your own calibration calculations (see below.)

4.1.1  Poles and zeroes

Most users of seismometers find it convenient to consider the sensor as a “black box”, which produces an output signal V from a measured input x. So long as the relationship between V and x is known, the details of the internal mechanics and electronics can be disregarded.

This relationship, given in terms of the Laplace variable s, takes the form

In this equation

In the calibration pack, G is the sensitivity given for each component on the first page, whilst the roots Zᵢ and Pⱼ, together with the normalising factor A, are given in the Poles and Zeros table. The poles and zeros given are measured directly at Güralp Systems' factory using a spectrum analyser. Transfer functions for the vertical and horizontal sensors may be provided separately.

4.1.2  Frequency response curves

The frequency response of each component of the 3T is described in the normalised amplitude and phase plots provided. The response is measured at low and high frequencies in two separate experiments. Each plot marks the low-frequency and high-frequency cut-off values (also known as –3 dB or half-power points).

3T-calibration-graph   3T-calibration-graph-2

If you want to repeat the calibration to obtain more precise values at a frequency of interest, or to check that a sensor is still functioning correctly, you can inject calibration signals into the system using a Güralp digitiser or your own signal generator, and record the instrument's response.

4.1.3  Obtaining copies of the calibration pack

Our servers keep copies of all calibration data that we send out. In the event that the calibration information becomes separated from the instrument, you can obtain all the information using our free e-mail service.

Simply e-mail with the serial number of the instrument in the subject line, e.g.

The server will reply with the calibration documentation in Word format. The body of your e-mail will be ignored. If you want data for several sensors, put their serial numbers together on the subject line, separated by commas.

4.2 Calibration methods

Velocity sensors such as the 3T are not sensitive to constant DC levels, either as a result of their design or because of an interposed high-pass filter. Instead, three common calibration techniques are used.

You can perform calibration either using a Güralp DM24 digitiser, which can generate step and sinusoidal calibration signals, or by feeding your own signals into the instrument through a hand-held control unit.

Before you can calibrate the instrument, its calibration relays need to be activated by pulling low the CAL ENABLE line on the instrument's connector for the component you wish to calibrate. Once enabled, a calibration signal provided across the CAL SIGNAL and SIGNAL GROUND lines will be routed through the feedback system. You can then measure the signal's equivalent velocity on the sensor's output lines. Güralp Hand-held Control Units provide a switch for activating the CAL ENABLE line.

4.3 Calibration with Scream!

Güralp digitisers provide calibration signal generators to help you set up your sensors. Calibration is most easily done through a PC running Güralp's Scream! software.

Depending on the digitiser type, sine-wave, step and broadband noise signal generators may be available. In this section, broadband noise calibration will be used to determine the complete sensor response in one action. Please refer to the digitiser's manual for information on other calibration methods.

4.3.1  Sensor response codes


Sensor type code

Units (V/A)

3T or 3ESP, 30s – 50 Hz response



3T or 3ESP, 60s – 50 Hz response



3T or 3ESP, 100s – 50 Hz response



3T or 3ESP, 120s – 50 Hz response



3T, 360s – 50 Hz response



3TB or 3V / 3ESP borehole, 30s – 50 Hz response



3TB or 3V / 3ESP borehole, 100s – 50 Hz response



3TB or 3V / 3ESP borehole, 120s – 50 Hz response



4.4 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 the equation

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 value of the calibration resistor and K is the feedback coil constant. R and K are both given on the calibration sheet supplied with the instrument.

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.

4.5 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 apparatus for measuring tilt angles accurately.