Guralp Systems CMG-6T User Manual

CMG-6T

Triaxial Broadband Seismometer

Operator's Guide

Part No. MAN-T60-0001

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 B 2011-07-19

CMG-6T

Table of Contents

1 Introduction.............................................................................................................. 3

1.1 Response options............................................................................................... 4

2 First Encounters........................................................................................................5

2.1 Handling notes...................................................................................................5

2.2 Unpacking..........................................................................................................5

2.2.1 Serial number............................................................................................. 6

2.3 Connection.........................................................................................................6

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

2.3.2 The hand-held control unit (HCU)............................................................7

2.4 Centring (zeroing) the instrument.....................................................................8

2.4.1 Remote centring option..............................................................................9

2.5 Test installation.................................................................................................9

2.6 Installation notes.............................................................................................11

2.7 Installing in vaults...........................................................................................12

2.8 Installing in pits...............................................................................................15

2.9 Other installation methods..............................................................................18

3 Calibrating the CMG-6T.........................................................................................20

3.1 The calibration pack........................................................................................20

3.2 Poles and zeroes...............................................................................................21

3.3 Frequency response curves.............................................................................22

3.4 Obtaining copies of the calibration pack........................................................22

3.5 Calibration methods........................................................................................23

3.6 Noise calibration with Scream! ......................................................................23

3.6.1 Sensor response codes............................................................................. 26

3.7 The coil constant.............................................................................................27

4 Connector pin-outs................................................................................................. 28

4.1 Instrument, HCU RECORDER and SENSOR connectors...............................28

4.2 Breakout box and HCU power connector.......................................................29

5 Specifications......................................................................................................... 30

6 Revision history......................................................................................................31

2 Issue B

Operator's Guide

1 Introduction

The CMG-6T is an ultra-lightweight seismometer consisting of three
sensors in a sealed case, which can measure simultaneously the
North/South, East/West and vertical components of ground motion
over a wide frequency range. This frequency response is made
possible by advanced force-balance feedback electronics.. It is ideally
suited for rapid, temporary installations in locations with medium
noise.

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CMG-6T

The 6T has a rugged, waterproof, lightweight aluminium design for
ease of installation. The robust sensor elements are designed so that
no mechanical clamping is required. Because of this, the 6T 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 the 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.

1.1 Response options

The 6T can be supplied with a response which is flat to velocity from
100 Hz to any of 0.1 Hz (10 s), 0.033 Hz (30 s) or 0.01 Hz (100 s). The
6T is also available as the high-frequency CMG-6T-1 variant, which
has a 100Hz to 1s response.

Two variants, the CMG-6V (vertical) and CMG-6H (horizontal), are
available for single-component applications.

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Operator's Guide

2 First Encounters

2.1 Handling notes

Although the 6T 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 sensor, you should contact Güralp
Systems for assistance.

• Avoid bumping or jolting the seismometer 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 6T are waterproof.

2.2 Unpacking

The 6T seismometer is delivered in a single transportation case. The
packaging is specifically designed for the 6T and should be reused
whenever you need to transport the sensor. Please note any damage to
the packaging when you receive the equipment, and unpack on a safe,
clean surface. For each instrument in the packaging, you should have
received:

• the seismometer;

• the breakout box (which provides separate connections for the

signal, control and power lines);

• a data cable, for connecting the instrument to a Güralp digitiser,

such as the CMG_DM24;

• a calibration and installation sheet.

Assuming all the parts are present, stand the seismometer in the centre
of a bench and identify its external features:

• a handle with North indication,

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CMG-6T

• a 26-way bayonet plug for data and power;

• a pressure-release cap;

• a cover-plate for the output offset adjustment potentiometers;

• a spirit level,

• three feet (two adjustable, and one fixed), and

• two accurate orientation pins (one brass and one steel).

2.2.1 Serial number

The sensor's serial number can be found on the label affixed to the
side of the instrument. You should quote this serial number if you
ever need assistance from Güralp Systems.

2.3 Connection

The instrument is supplied with a 26-way cable with mil-spec bayonet
connectors on each end; this carries both power and output signals
and is suitable for connecting directly to a Güralp digitizer.

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

2.3.1 The breakout box

If you are using the optional Güralp breakout
box, the data cable from the instrument should
be attached to its SENSOR connector.
Connectors are also provided at the CONTROL
and RECORDER outputs, for attaching to an
optional hand-held control unit or a Güralp
digitizer.

The breakout box also provides a standard
Güralp power connector on a 10-pin mil-spec
plug. The 6T draws a nominal current of 38 mA
from a 12 V supply when in use; thus, using a
12V, 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 one-second mode
whilst it is pressed. This mode allows you to monitor the mass

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Operator's Guide

positions whilst you adjust the offsets manually. If you prefer, you can
use the equivalent switch on a Hand-held Control Unit (see below.)

2.3.2 The hand-held control unit (HCU)

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.

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

• To monitor the velocity outputs, switch

the dial to V, N/S or E/W LOW VEL
according to the component you want to
monitor.

• 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.

You can calibrate a 6T sensor using an 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
section 3 on page 20 for full details.

You can zero the mass position offsets using the HCU. 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. See section 2.4 on page 8 for full details.

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.

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CMG-6T

2.4 Centring (zeroing) the instrument

For optimal operation, the instrument's
masses should be at their centre
positions in the absence of a seismic
signal. The dynamic range of the
instrument is reduced if the masses
stray too far from the centres. The
mass positions are affected by the angle
at which the instrument is installed as
well as by the ambient temperature.
The instrument should be allowed to
acclimatise itself before final centring
is carried out.

The mass positions are directly output by the instrument and are
available from the bayonet connector where they can be monitored by
a Güralp Systems digitiser (where they appear as streams M8 (vertical),
M9 (N/S) and MA (E/W)) or by using the meter on the hand-held
control unit. The offsets can be nulled using three potentiometers,
located under a protective cover on the instrument's lid and adjustable
using a trim-pot tool or a very small flat-bladed screwdriver.

The cover can be removed without
tools: three potentiometers are then
exposed. The one nearest the
“North” of the instrument controls
the vertical mass offset and the
other two, proceeding clockwise,
control the North/South and East
West offsets.

1. Select the component you want to centre using 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. Switch the RANGE selector to 10V (coarse adjustment);

5. Adjust the relevant potentiometer until the signal meter reads
zero.

6. Switch the RANGE selector to 1V (fine adjustment);

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Operator's Guide

7. Adjust the relevant potentiometer until the signal meter reads
zero.

2.4.1 Remote centring option

For instruments fitted with the remote centring option, the mass
position offsets are adjusted using a micro-controller and three digital
potentiometers that replace the standard electromechanical
potentiometers.

When installing the instrument, ensure that it is levelled accurately by
checking that the bubble in the level lies within the central circle.

On power-up, the micro-controller will automatically zero the mass
positions of all three axes simultaneously. Zeroing can be further
triggered via the “centre” control line on pin U. (On instruments
without the digital nulling option, pin U is the 'Acc/Vel' line).

Automatic zeroing takes approximately forty-five seconds to complete,
after which the sensor reverts to long period operation and the nulling
module enters a low power “sleep” mode. During nulling, the sensor's
outputs will fluctuate as the pots are adjusted in a binary search before
settling with a mass position of ±0.5V. If the sensor is poorly levelled,
the micro-controller will make three attempts to zero the mass before
giving up and using the closest match.

A test mode is available to check the operation of the digital centring
pots. This mode is entered by holding the centre line low during
power up. The unit will then set the pots to maximum for thirty
seconds; then minimum for thirty seconds; then to the centre position
for six minutes. The centre line must be held low continuously,
otherwise the unit will abort the test mode and null the sensors as
normal.

2.5 Test installation

This section gives an overview of how to set up a CMG-6T and begin
recording data. We recommend that you set up a test instrument in
your office or laboratory as a “dry run” to gain a basic understanding of
the system and to check that it is functioning as expected.

This test installation will use the instrument's default settings. Data
will be digitised using a CMG-DM24 and received using Güralp
Systems' Scream! software, available from our website

http://www.guralp.com/

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CMG-6T

You will need access to a PC with a 9-pin RS232 port, and a 12 V
power source.

1. Install Scream! on your PC and run it.

2. Connect the GPS receiver to the CMG-DM24 using the brown
cable supplied with the digitiser. Position the GPS receiver so
that it has a good view of the sky. If you do not have a view of
the sky, you can operate the sensor without a GPS unit, but
timing information may be inaccurate.

3. Connect the grey data cable between the CMG-6T and the
SENSOR A .connector on the digitiser.

4. Using the blue/grey combined data/power cable that is included
with the CMG-DM24, connect the digitiser to the PC's serial port
and then to a 12V power supply.

The instrument is now fully operational, and will already be
producing data.

5. After a few seconds you should see the digitizer appear under
Network – Local – Com1 in the left-hand panel of Scream!'s
main window. (If your PC has multiple serial ports, it may
appear under some other Com port name.) Soon after, data
streams will begin appearing in the right-hand panel. Streams
with higher sample rates will appear sooner than those with
lower sample rates.

If this does not happen, check all connections, and ensure the
power supply is providing the correct voltage and current.

10 Issue B