Guralp Systems CMG-40T User Manual

CMG-40T

Triaxial Broadband Seismometer

Operator's guide

Part No. MAN-040-0001

Designed and manufactured by
Güralp Systems Limited
3 Midas House, Calleva Park
Aldermaston RG7 8EA
England

Proprietary Notice: The information in this manual is
proprietary to Güralp Systems Limited and may not be copied
or distributed outside the approved recipient's organisation
without the approval of Güralp Systems Limited. Güralp
Systems Limited shall not be liable for technical or editorial
errors or omissions made herein, nor for incidental or
consequential damages resulting from the furnishing,
performance, or usage of this material.

Issue A 2006-01-12

CMG-40T

Table of Contents

1 Introduction............................................................................................................................4

1.1 Response options..............................................................................................................5

2 First encounters......................................................................................................................6

2.1 Handling notes..................................................................................................................6

2.2 Connections......................................................................................................................6

The breakout box...............................................................................................................6

The handheld control unit..................................................................................................7

Signal meter..................................................................................................................8

Calibration.....................................................................................................................9

Control commands........................................................................................................9

Banana plugs.................................................................................................................9

2.3 Zeroing the instrument.....................................................................................................9

Adjusting the mass position offsets manually...................................................................9

Adjusting the mass position offsets with a Handheld Control Unit................................11

Zeroing a 40TD digital instrument..................................................................................11

2.4 Installation notes.............................................................................................................14

3 Installing the 40T.................................................................................................................16

3.1 Installing in vaults..........................................................................................................16

Choosing a location.........................................................................................................16

Temperature stability.......................................................................................................17

Other considerations........................................................................................................18

3.2 Installing in pits..............................................................................................................19

Other installation methods...............................................................................................21

4 Calibrating the 40T..............................................................................................................23

4.1 The calibration pack.......................................................................................................23

Poles and zeroes...............................................................................................................24

Frequency response curves..............................................................................................25

Obtaining copies of the calibration pack.........................................................................25

4.2 Calibration methods.......................................................................................................26

4.3 Calibration with Scream! ...............................................................................................26

Sensor response codes.....................................................................................................30

4.4 Calibration with a handheld control unit........................................................................30

4.5 The coil constant............................................................................................................31

Appendix A Connector pinouts.............................................................................................32

2 Issue A

Operator's guide

Appendix A.1 Output port and breakout box RECORDER connector................................32

Appendix A.2 Output port and breakout box RECORDER connector (high gain option)..33

Appendix A.3 Breakout box power connector.....................................................................34

Appendix B Specifications......................................................................................................35

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CMG-40T

1 Introduction

The CMG-40T is an ultra-lightweight seismometer consisting of three sensors
in a sealed case, which can measure the north/south, east/west and vertical
components of ground motion simultaneously.

The 40T has a rugged, waterproof stainless steel design for ease of installation.
The lightweight sensor elements are designed so that no mechanical clamping is
required. Because of this, the 40T is ready to record ground movements as soon
as you provide it with power. In addition, the sensor does not have to be
levelled or centred as long as the base is within 3 ° of horizontal. For the best
results, however, you should install where possible on a hard, near-horizontal
surface well coupled to the bedrock.

Each seismometer is delivered with a detailed calibration sheet showing its
serial number, measured frequency response in both the long period and the
short period sections of the seismic spectrum, sensor DC calibration levels, and
the transfer function in poles/zeros notation.

4 Issue A

Operator's guide

1.1 Response options

The 40T can be supplied with a response which is flat to velocity from 50 Hz to
any of 0.1 Hz (10 s), 0.050 Hz (20 s) or 0.033 Hz (30 s).

If you do not require high-frequency data, a low-pass filter may be installed at a
frequency (below 50 Hz) that you specify.

Standard 40T instruments output signals representing ground velocity on three
pairs of balanced differential lines. An option is available which provides a
second, parallel set of outputs at higher gain. The high-gain outputs have a
sensitivity nominally 10 times higher than the standard (low-gain) outputs.

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CMG-40T

2 First encounters

2.1 Handling notes

Although the 40T has a rugged design, it is still a sensitive instrument, and can
be damaged if mishandled. If you are at all unsure about the handling or
installation of the device, you should contact Güralp Systems for assistance.

• Avoid bumping or jolting the sensor when handling or unpacking.

• Do not kink or walk on the data cable (especially on rough surfaces such

as gravel), nor allow it to bear the weight of the sensor.

• Do not connect the instrument to power sources except where instructed.

• Do not ground any of the signal lines from the sensor.

All parts of the 40T are waterproof.

2.2 Connections

The instrument has an integrated cable ending in a 26-pin mil-spec socket
which carries both power and output signals. This is suitable for connecting
directly to a Güralp digitizer.

The breakout box, if ordered, provides individual signal and power connectors,
or you can make up your own cable if you prefer.

The breakout box

If you are using a Güralp breakout box, it should be attached to the sensor
through its SENSOR connector. Connectors are also provided at the CONTROL
and RECORDER outputs, for attaching to a handheld control unit or a Güralp
digitizer. If you have ordered a 40T with optional high gain outputs, you will
need to make up a suitable cable to expose these outputs.

6 Issue A

Operator's guide

The breakout box also provides a standard Güralp power connector on a 10-pin
mil-spec plug. The 40T draws a nominal current of 48 mA from a 12 V supply
when in use; thus, using a 12 V, 25 Ah sealed heavy-duty lead-acid battery, you
should expect the instrument to operate for around a week without recharging.

The CENTRE button switches the instrument into ACC/VEL mode whilst it is
pressed. This mode allows you to monitor the mass positions whilst you adjust
the offsets manually. If you prefer, you can use the equivalent switch on a
Handheld Control Unit (see below.)

The handheld control unit

This portable control unit provides easy access to the seismometer's control
commands, as well as displaying the output velocity and mass position (i.e.
acceleration) on an analogue meter.

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CMG-40T

Signal meter

The upper section of the HCU contains a simple voltmeter for monitoring
various signals from the instrument.

• To monitor the low-gain outputs, switch the dial to V, N/S or E/W LOW

VEL according to the component you want to monitor.

• To monitor the high-gain outputs (on a 40T with that option), switch the

dial to V, N/S or E/W HIGH VEL.

• To monitor the mass position outputs, switch the dial to V, N/S or E/W

MASS POS. Whilst you are adjusting mass position offsets, you should

also switch the instrument out of broadband mode by switching the
rightmost CENTRING SELECT switch to 1 SEC VEL, or by holding
down the CENTRE button on a breakout box.

• You can set the range of the meter with the RANGE switch. When

switched to 10 V, the meter ranges from –10 to + 10 V (as marked.)
When switched to 1 V, the range is –1 to +1 V.

8 Issue A

Operator's guide

Calibration

You can calibrate a 40T sensor through the HCU by connecting a signal
generator across the yellow and green CALIBRATION SIGNAL inputs and
setting the adjacent switch to ON. The sensor's response can now be monitored
or recorded, and calibration calculations carried out. See Chapter 4, “Calibrating

the 40T” for full details.

Control commands

If you have ordered a 40T with the remote null facility, you can zero its mass
position offsets from the HCU.

1. Select the component you want to centre from the CENTRING SELECT
dial.

2. Switch the signal meter dial to one of the MASS POS settings.

3. Switch the rightmost switch to 1 SEC VEL to enable the centring lines.

4. Press the +/– switch towards – to centre a mass from a positive value, or
towards + to centre it from a negative value.

Banana plugs

The remainder of the HCU provides useful connections for each of the signal
lines from the instrument, for attaching to your own equipment as necessary.

2.3 Zeroing the instrument

Before installing the 40T, you should check that the mass positions are not
significantly offset from zero. The mass position offsets can be affected by any
tilt to the instrument, as well as handling during transportation. The normal
range of the mass positions is ±10 V; you should zero the instrument if any
mass reads more than around ±3.5 V when the sensor is stationary.

The velocity outputs of the 40T are set at the factory to a nominal value below
±3 mV. Once the instrument is installed and has reached thermal equilibrium
with its environment, these outputs should be similar to the factory-set value.

Adjusting the mass position offsets manually

The 40T has three potentiometers (“pots”) accessible within its casing, which
should be used to remove any DC offsets electronically:

1. Bring the instrument into 1 second response mode by applying a voltage
across the Acc/Vel and Signal Ground pins of the input. If you are using

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CMG-40T

a DM24, you can do this by sending a CENTRE command. If you are
using a Handheld Control Unit, you should select 1 SEC VEL from the
Velocity Select switch.

2. Measure the vertical mass position output with a 10 V voltmeter (see

Appendix A, “Connector pinouts”) or by selecting MASS POS, V from a

Handheld Control Unit's Display Select knob. If using a HCU, also
check that the Centring Select knob is set to OFF.

3. If the vertical component needs adjusting, remove the cap on the lid
which protects the Verticalpot with a flat-bladed screwdriver (provided).

4. Insert the screwdriver through the opening, and engage the pot. An LED
flashlight may be useful for locating the head.

5. Turn the pot either way until the offset readout is as close to 0 V as
possible.

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Operator's guide

6. Repeat steps 2 – 5 for the north/south and east/west components.

Adjusting the mass position offsets with a Handheld Control Unit

Some 40T units are equipped with a remote mass centring option, which allows
you to adjust the internal potentiometers by applying voltages across control
lines to the sensor:

1. Bring the instrument into 1 second response mode by selecting 1 SEC
VEL from the Velocity Select switch.

2. Measure the vertical mass position output by selecting MASS POS, V
from the Handheld Control Unit's Display Select knob.

3. Set the Centring Select knob to V.

4. Press the spring-loaded switch towards + or – to bring the mass position
offset from negative or positive values towards zero.

5. Repeat steps 3 and 4 for the N/S and E/W components.

6. Return the instrument to broadband mode by selecting BB VEL from the
Velocity Select switch.

Zeroing a 40TD digital instrument

The offset potentiometers in a 40TD are in the same place as on the 40T. To
access them, you will need to remove the digitizer module, which lies on top of
the sensor itself. You can monitor the mass position outputs of the sensor using
a Handheld Control Unit and an adapter cable, available from Güralp Systems.

To change the offsets of a 40TD without digital centring:

1. Check the bubble level on the lid of the instrument, to ensure it is not
tilted. If necessary, re-level the instrument by adjusting its feet.

2. Unscrew the vent cap on the lid to allow the air pressure to equalise.

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