Guralp Systems Limited
MAN-T60-0002 - 6TD Operator's Guide

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1. Preliminary Notes 2. Introduction 3. First encounters 4. Installing the 6TD 5. Configuration with Scream! 6. Calibrating the 6TD 7. Command-line interface 8. Updating the 6TD 9. Connector pin-outs 10. Specifications 11. Revision history

Section Index: 6.1. The calibration pack 6.2. Poles and zeroes 6.3. Frequency response curves 6.4. Obtaining copies of the calibration pack 6.5. Calibration methods 6.6. The coil constant

Chapter 6. Calibrating the 6TD

6.1 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:

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

6.2 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. Transfer functions for the vertical and horizontal sensors may be provided separately.

6.3 Frequency response curves

The frequency response of each component of the 6TD 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.

6.4 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.

6.5 Calibration methods

Velocity sensors such as the 6TD 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 calibrate a 6TD sensor using any of these methods, using its built-in signal generator.

6.5.1 Noise calibration with Scream!

The most convenient way to calibrate a 6TD instrument is to use its pseudo-random broadband noise generator with Scream!'s noise calibration extension. The extension is part of the standard distribution of Scream!, and contains all the algorithms needed to determine the complete sensor response in a single experiment.

Information on other calibration methods is available on the Güralp Systems Web site.

6.5.2 Sensor response codes

The correct response code for use with your instrument is shown in the table below. If the response of your instrument is not listed, please contact support@guralp.com for advice.

Sensor

Sensor type code

Units (V/A)

CMG-40T-1 or 6T-1,
1 second – 50 Hertz response

CMG-40_1HZ_50HZ

V

CMG-40T-1 or 6T-1,
1 second – 100 Hertz response

CMG-40_1S_100HZ

V

CMG-40T or 6T,
2 second – 100 Hertz response

CMG-40_2S_100HZ

V

CMG-40T or 6T,
10 second – 100 Hertz response

CMG-40_10S_100HZ

V

CMG-40T or 6T,
20 second – 50 Hertz response

CMG-40_20S_50HZ

V

CMG-40T or 6T,
30 second – 50 Hertz response

CMG-40_30S_50HZ

V

6.6 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. First encounters 4. Installing the 6TD 5. Configuration with Scream! 6. Calibrating the 6TD 7. Command-line interface 8. Updating the 6TD 9. Connector pin-outs 10. Specifications 11. Revision history