Guralp Systems Limited
MAN-C3E-0002 - Güralp 3ESPCD Digital Broadband Seismometer (DM24 Versions) - Users' Guide

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1. Preliminary Notes 2. Introduction 3. Installing the 3ESPCD 4. Calibrating the 3ESPCD 5. Using Scream! 6. Command line interface 7. Inside the 3ESPCD 8. Appendix A - Connector pin-outs 9. Appendix B - Sensor Specifications 10. Appendix C - Digitiser specifications 11. Revision History

Section Index: 4.1. The calibration pack 4.2. Calibration methods 4.3. Calibration with Scream! 4.4. The coil constant

Chapter 4. Calibrating the 3ESPCD

4.1 The calibration pack

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

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

( V / x ) (s) = G × A × H (s)

In this equation

In the calibration pack, G is the sensitivity given for each component on the first page, whilst the roots zn and pm, 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 3ESPCD 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 cutoff values (also known as –3 dB or half-power points).

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 caldoc@guralp.com 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.

4.2 Calibration methods

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

Calibration is performed using the built-in DM24 digitiser, which can generate step and sinusoidal calibration signals as well as broadband noise, and integrated scripts within Güralp Systems' Scream! software.

Initiating a calibration closes a relay which normally isolates the feedback electronics from the calibration signal generator and causes signals of the specified nature to be generated and routed through the feedback system. You can then measure the signal's equivalent velocity using Scream!

4.3 Calibration with Scream!

Sine-wave, step and broadband noise calibrations are available. In this section, broadband noise calibration will be used to determine the complete sensor response in one action.

4.3.1 Sensor response codes

Sensor

Sensor type code

Units (V/A)

3T or 3ESP, 30 s – 50 Hz response

CMG-3_30S_50HZ

V

3T or 3ESP, 60 s – 50 Hz response

CMG-3_60S_50HZ

V

3T or 3ESP, 100 s – 50 Hz response

CMG-3_100S_50HZ

V

3T or 3ESP, 120 s – 50 Hz response

CMG-3_120S_50HZ

V

3T or 3ESP, 360 s – 50 Hz response

CMG-3_360S_50HZ

V

3TB/3V/3ESP borehole, 30 s – 50 Hz

CMG-3B_30S_50HZ

V

3TB/3V/3ESP borehole, 100 s – 50 Hz

CMG3B_100S_50HZ

V

3TB/3V/3ESP borehole, 120 s – 50 Hz

CMG3B_120S_50HZ

V

4.4 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.

PreviousNext

1. Preliminary Notes 2. Introduction 3. Installing the 3ESPCD 4. Calibrating the 3ESPCD 5. Using Scream! 6. Command line interface 7. Inside the 3ESPCD 8. Appendix A - Connector pin-outs 9. Appendix B - Sensor Specifications 10. Appendix C - Digitiser specifications 11. Revision History