Guralp Systems Limited
MAN-T60-0001 - CMG-6T Operator's Guide

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1. Introduction 2. First Encounters 3. Calibrating the CMG-6T 4. Connector pin-outs 5. Specifications 6. Revision history

Section Index: 3.1. The calibration pack 3.2. Poles and zeroes 3.3. Frequency response curves 3.4. Obtaining copies of the calibration pack 3.5. Calibration methods 3.6. Noise calibration with Scream! 3.7. The coil constant

Chapter 3. Calibrating the CMG-6T

3.1 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/ms-1. Because the 6T 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/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/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 terms of the current (in amperes) drawn from 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.)

3.2 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. 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.

3.3 Frequency response curves

The frequency response of each component of the 6T 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).

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 digitizer or your own signal generator, and record the instrument's response.

3.4 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.

From: your@email.net

To: caldoc@guralp.com

Subject: T6315

The server will reply with the calibration documentation in Microsoft Word format (note that, since these documents contain active content, they cannot be viewed in, for example, openoffice.org's writer software.) The body of your e-mail will be ignored.

3.5 Calibration methods

Velocity sensors such as the 6T 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 6T sensor using any of these methods, using the signal generator built in to all Güralp Systems' digitisers.

3.6 Noise calibration with Scream!

The most convenient way to calibrate a 6TD instrument is to use a CMG-DM24's pseudo-random broadband noise generator along 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.

Waveview-noise-calibration-select

3.6.1 Sensor response codes

Sensor

Sensor type code

Units (V/A)

CMG-40T-1 or 6T-1,
1 s – 50 Hz response

CMG-40_1HZ_50HZ

V

CMG-40T-1 or 6T-1,
1 s – 100 Hz response

CMG-40_1S_100HZ

V

CMG-40T or 6T,
2 s – 100 Hz response

CMG-40_2S_100HZ

V

CMG-40T or 6T,
10 s – 100 Hz response

CMG-40_10S_100HZ

V

CMG-40T or 6T,
20 s – 50 Hz response

CMG-40_20S_50HZ

V

CMG-40T or 6T,
30 s – 50 Hz response

CMG-40_30S_50HZ

V

CMG-40T or 6T,
60 s – 50 Hz response

CMG-40_60S_50HZ

V

3.7 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. Introduction 2. First Encounters 3. Calibrating the CMG-6T 4. Connector pin-outs 5. Specifications 6. Revision history