Guralp Systems CMG-3T User Manual
CMG-3T
Triaxial Broadband Seismometer
Ope rat or's g uide
Part No. MAN-030-0001
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 N 2009-07-06
CMG-3T
Table of Contents
1 Introduction...............................................................................................................3
1.1 Options................................................................................................................4
2 Installing the 3T........................................................................................................7
2.1 First encounters..................................................................................................7
2.2 Installation notes..............................................................................................10
2.3 Installing in vaults............................................................................................11
2.4 Installing in pits................................................................................................14
2.5 Installing in post-holes....................................................................................18
3 Calibrating the 3T...................................................................................................20
3.1 The calibration pack.........................................................................................20
3.2 Calibration methods.........................................................................................23
3.3 Calibration with Scream! ................................................................................23
3.4 Calibration with a hand-held control unit.......................................................28
3.5 The coil constant..............................................................................................28
4 Accessories...............................................................................................................30
4.1 The breakout box..............................................................................................30
4.2 The hand-held control unit..............................................................................31
4.3 Integrated State-of-Health Controller...............................................................33
5 Inside the 3T............................................................................................................35
5.1 The sensors.......................................................................................................35
5.2 The control system...........................................................................................36
5.3 RS232 control interface....................................................................................41
5.4 The feedback system........................................................................................42
6 Connector pin-outs..................................................................................................47
6.1 Sensor and control unit pin output.................................................................47
6.2 Sensor output (“D”-type connector option).....................................................48
6.3 Sensor output (waterproof connector option).................................................49
6.4 Breakout box POWER connector......................................................................50
7 Specifications..........................................................................................................51
8 Revision history.......................................................................................................53
2 Issue N
Operator's guide
1 Introduction
The CMG-3T is a three-axis seismometer consisting of three sensors in
a sealed case, which can measure the north/south, east/west and
vertical components of ground motion simultaneously. Each sensor is
sensitive to ground vibrations in the frequency range 0.003 – 50 Hz, a
broadband frequency response made possible by advanced forcebalance feedback electronics. Because of this wide response range, the
3T can replace many of the instruments conventionally used in a
seismic observatory; it also produces true pulse-shape records suitable
for modern earthquake mechanism analysis.
The 3T is designed for mounting on a hard, near-horizontal surface
well coupled to the bedrock. After levelling and orienting the case, you
can perform accurate adjustments internally by sending the
instrument control signals. These electronics allow it to compensate
for a tilt of up to 3 ° from horizontal.
Once levelled and centred, the 3T will begin operating automatically.
It outputs analogue voltages representing ground velocity on balanced
differential lines. These voltages can be recorded using a separate
logging device or digitizer. For testing and installation purposes, a
hand-held control unit is supplied which can monitor the instrument's
output.
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The seismometer unit is self-contained apart from its 12 Volt power
supply. Centring and mass locking can be carried out by sending
control signals to the instrument, either through the hand-held control
unit or through an attached Güralp digitizer.
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 Options
Form factors
The CMG-3T can be supplied in several forms, besides its standard
configuration:
• The CMG-3T Compact is internally identical to the standard 3T,
but has a different arrangement of components allowing it to fit
in a smaller casing.
• The 3T can also be supplied in a slimline form factor, with
vertically-stacked sensors, suitable for installation in post-holes
(see Section 2.5, page 18.).
• The 3T is also available as a fully-fledged borehole instrument,
the CMG-3TB.
Any of these can be supplied with integral digitizers and data
modules, allowing the 3T to form a complete, integrated seismic
installation. For example, the 3TD is a full-height 3T with an
integrated CMG-DM24 digitiser. As an additional option, the 3TD can
be ordered with an Integrated State-of-Health Controller, which
provides the mass-control functions (normally accessed via the breakout box) directly from controls mounted on the instrument. See
section 4.3 on page 33 for more details.
Output types
The standard 3T has a single 26-pin mil-spec waterproof connector for
signals, control and power:
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Operator's guide
As an option, the 3T can be supplied with a 26-way “D”-type
connector with standard pin assignments (see section 6.2, page 48.)
Other output connector types may also be supplied, to your own pinout specification.
Sensor control interface
3T instruments can be fitted with an optional RS232 control interface.
The control interface can be used to lock, unlock, and centre the
sensor masses, and query mass positions. See section 5.3, page 41, for
more details.
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CMG-3T
Sensor response
The 3T can be supplied with a response which is flat to velocity from
100 Hz to any of 0.1 Hz (10 seconds), 0.033 Hz (30 seconds), 0.016 Hz
(60 seconds), 0.01 Hz (100 seconds), 0.0083 Hz (120 seconds) or 0.0027
Hz (360 seconds). Alternatively, a hybrid response function may be
provided. See Chapter 5 for more details.
If you do not require high-frequency data, a low-pass filter may be
installed at any frequency (below 100 Hz) that you specify.
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Operator's guide
2 Installing the 3T
2.1 First encounters
Unpacking
The 3T seismometer is delivered in a single transportation case. The
packaging is specifically designed for the 3T 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. The package should contain:
• the seismometer;
• a cable to join the sensor to the breakout box;
• the breakout box (which provides separate connections for the
signal, control and power lines);
• a Hand-held Control Unit (HCU) for monitoring sensor outputs
and calibration, if ordered;
• a 10-pin connector for your power lead (see below); and
• 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,
• a multi-pole socket for input and output,
• other optional connectors as ordered;
• a bubble level,
• an air vent port,
• three adjustable feet, and
• two accurate orientation pins (one brass and one steel).
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CMG-3T
Serial number
The sensor's serial number can be found on the label stuck to the top
lid of the sensor. It is also stamped onto the side of the sensor base,
next to the N/S indicator, and into the lid itself. You should quote this
serial number if you need assistance from Güralp Systems.
Handling notes
The 3T is a sensitive instrument, and is easily damaged if mishandled.
If you are at all unsure about the handling or installation of the device,
you should contact Güralp Systems for assistance.
• Do not bump or jolt any part of 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.
• Do not move the instrument whilst the masses are unlocked.
You should report any sign of loose components, or any sound
of parts moving inside the instrument, to Güralp Systems.
Connections
The instrument has a single connector, which can be joined using the
cable provided to a digitizer or breakout box. Individually shielded
twisted-pair cabling must be used for the sensor outputs, control lines
and power supply. If you need to make up a suitable cable, you should
confirm the cable type with Güralp Systems.
Using a digitizer
The 3T can be connected directly to any Güralp Systems digitizer
using the signal cable provided. This is the simplest way to use a 3T
instrument. All the instrument's functions are available through the
digitizer, including centring, locking and unlocking.
We recommend that you keep the digitizer near the instrument if at all
possible, to minimize the length of analogue cable required. Once
digitized, the signal is robust to degradation by noise or attenuation.
Keeping the digitizer in the quiet, stable conditions of a seismic
installation also provides it with an optimum environment for the on-
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Operator's guide
board ADCs.
Breakout box and hand-held control unit
The 3T can be supplied with an optional robust breakout box, which
provides mass control functions in installations which do not use
compatible digitizers. A hand-held control unit is also available which
can control calibration lines and monitor sensor outputs in addition to
mass control. See chapter 4, page 30, for more details.
Power supply
The sensor requires a 12 Volt power supply, connected via the socket
and breakout box or digitizer. You will need to make up a suitable
cable to connect a 12 V power source to the 10-pin connector on the
breakout box (spare 10-pin mil-spec connectors are provided for this
purpose.) Using a 12 Volt, 25 Ampere-hour, sealed, heavy-duty, leadacid battery, you should expect the instrument to operate for around a
week without recharging.
If you prefer, you can power the 3T directly from the connector on the
top panel (see Chapter 6, page 47.)
A power management module can be installed as an option, which
allows the 3T to operate from a 10 – 15 V supply range. This module
also cuts the input power to the sensor electronics if it drops below
10.5 V, to minimize discharge from battery-operated installations.
Units with serial numbers beginning T33 are provided with a wide
input range DC-DC converter, and can be powered from a 10 – 36 V
supply.
The 3T draws a nominal current of 75 milliamps from a 12 Volt supply
when in use. During locking and unlocking of the sensor masses, this
current rises briefly to 600 milliamps. It is recommended that you
carry a spare 12 Volt battery when visiting an installation for
maintenance, in case the sensor needs to be moved and the on-site
batteries no longer have sufficient charge to perform the locking
procedure.
Signal output
The sensors output voltages representing ground velocity on floating
differential lines. The breakout box provides a RECORDER connector
for attaching to a recording system or digitizer. You can use any multichannel recording system, provided that it has high-impedance
floating differential inputs.
If you are using a Güralp Systems digitizer, you can connect the
instrument directly to the digitizer without using the breakout box;
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CMG-3T
power will be supplied through the digitizer, which can also activate
the sensor control lines.
The breakout box also provides a CONTROL output, which can be
connected to the Hand-held Control Unit. This device lets you monitor
output signals from the instrument, and perform on-site calibration.
For more information, see Section 6, page 47.
2.2 Installation notes
The goal of any seismic installation is to ensure that wave-trains
arriving at the instrument accurately reflect the internal motion of
subsurface rock formations. To achieve this, the seismometer and its
emplacement need to be considered as a mechanical system, which
will have its own vibrational modes and resonances. These
frequencies should be raised as high as possible so that they do not
interfere with true ground motion: ideally, beyond the range of the
instrument.
In particular, the sensor needs to be protected against environmental
factors such as
• fluctuations in temperature,
• turbulent air flow around walls or trees, or around sharp
corners or edges in the immediate vicinity of the sensor;
• vibration caused by equipment in or near the installation,
particularly computer equipment; and
• vibration caused by heavy machinery (even at a distance), or by
overhead power lines.
In seismic vaults, instruments are often installed on piers. It is
important to ensure that the interface between the pier and the floor
does not introduce noise, and that the pier itself does not have
resonant frequencies within the passband. Ideally, a seismic pier will
be significantly wider than it is high (to minimize flexing) and will
form a single piece with the floor, e.g. by moulding a poured concrete
floor within a wooden frame.
Many situations do not allow for the construction of a seismic vault.
For example, you may need to deploy quickly to monitor the activity of
a volcano showing signs of rejuvenation, or to study the aftershocks of
a major earthquake. In other cases the site itself may be too remote to
ship in construction equipment.
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Operator's guide
Temporary installations can be protected against spurious vibrations
by
• selecting a suitable site,
• placing the instrument in a protective enclosure (an open-sided
box of 5 cm expanded polystyrene slabs, placed over the
instrument and taped down to exclude draughts, makes an
excellent thermal shield),
• standing the sensor on bedrock where possible, or at least deep
in well-compacted subsoil;
• clearing the floor of the hole of all loose material; and
• using as little extra mass as possible in preparing the chamber.
After installation, the instrument case and mounting surface will
slowly return to the local temperature and settle in their positions.
This will take around four hours from the time installation is
completed. If you require long-period recording, you should re-zero
the instrument after this time.
2.3 Installing in vaults
You can install a 3T in an existing seismic vault with the following
procedure:
1. Unpack the sensors from their container, saving the shipping
boxes for later transportation.
2. Prepare the mounting surface, which should be smooth and free
of cracks. Remove any loose particles or dust, and any pieces of
loose surfacing. This ensures good contact between the
instrument's feet and the surface.
3. If it is not already present, inscribe an accurate North-South line
on the mounting surface.
4. Place the sensor over the scribed line, so that the brass and steel
pointers are aligned with the marked directions, with the brass
pointer facing North. This can be done by rotating the base of
the sensor whilst observing it from above. The brass pointer can
be found next to one of the feet.
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CMG-3T
If you cannot easily see the pointers, you should align the
sensor using the north arrow on the handle. However, the
alignment of the handle with the sensors inside is less accurate
than the metal pointers, so they should be used wherever
possible.
5. The top panel of the 3T includes a spirit level.
Level the sensor using each of the adjustable feet of the
instrument in turn, until the bubble in the spirit level lies
entirely within the inner circle. (The instrument can operate
with up to 2° of tilt, but with reduced performance.)
The feet are mounted on screw threads. To adjust the height of a
foot, turn the brass locking nut anticlockwise to loosen it, and
rotate the foot so that it screws either in or out. When you are
happy with the height, tighten the brass locking nut clockwise
to secure the foot. When locked, the nut should be at the bottom
of its travel for optimal noise performance.
6. Connect the sensor to a breakout box, or a Güralp digitizer if
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Operator's guide
you are using one.
7. Connect a 12 Volt power supply, either directly or through the
breakout box or digitizer.
8. Unlock the sensor. If you have a breakout box or hand-held
control unit, you can do this by holding the ENABLE and
UNLOCK buttons on the unit down together for 7 seconds. The
BUSY LED will start flashing, and then go out.
Alternatively, if you are using a DM24 digitizer and Scream!,
right-click on the digitizer's entry in Scream! and select
Control.... Click on the Mass control tab, followed by Unlock. (If
the Mass control tab is unavailable, check the sensor type in the
Sensor type tab, apply, and open a new Control window.)
Alternatively, if you are using a DM24 digitizer and a DCM,
navigate to the Actions Digitizer Setup Port → → x page for the
digitizer and click on the Unlock instrument button at the
bottom of the page.
After unlocking the masses, you should be careful not to tilt the
instrument or you may damage it.
9. Check the mass position outputs using a digital multimeter,
digitizer or hand-held control unit.
Re-centre the masses if required. If you have a breakout box or
hand-held control unit, you can do this by holding the ENABLE
and CENTRE buttons on the unit down together for 7 seconds.
The BUSY LED will start flashing, and then go out
Alternatively, if you are using a DM24 digitizer and Scream!,
right-click on the digitizer's entry in Scream! and select
Control.... Click on the Mass control tab, followed by Centre.
Alternatively, if you are using a DM24 digitizer and a DCM,
navigate to the Actions Digitizer Setup Port → → x page for the
digitizer and click on the Centre instrument button at the
bottom of the page.
10.Cover the instrument with thermal insulation, for example, a 5
cm expanded polystyrene box. This will shield it from thermal
fluctuations and convection currents in the vault. It also helps
to stratify the air in the seismometer package. Position the
thermal insulation carefully so that it does not touch the sensor
package.
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CMG-3T
11.Ensure that the sensor cable is loose and that it exits the
seismometer enclosure at the base of the instrument. This will
prevent vibrations from being inadvertently transmitted along
the cable.
2.4 Installing in pits
For outdoor installations, high-quality results can be obtained by
constructing a seismic pit.
Depending on the time and resources available, this type of
installation can suit all kinds of deployment, from rapid temporary
installations to medium-term telemetered stations.
Ideally, the sensor should rest directly on the bedrock for maximum
coupling to surface movements. However, if bedrock cannot be
reached, good results can be obtained by placing the sensor on a
granite pier on a bed of dry sand.
1. Prepare a hole of 60 – 90 cm depth to compacted subsoil, or
down to the bedrock if possible.
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Operator's guide
2. On granite or other hard bedrock, use an angle grinder to plane
off the bedrock at the pit bottom so that it is flat and level.
Stand the instrument directly on the bedrock, and go to step 7.
3. On soft bedrock or subsoil, you should install a pier as depicted
below.
4. Pour a layer of loose, fine sand into the pit to cover the base.
The type of sand used for children's sand-pits is ideal, since the
grains are clean, dry and within a small size range. On top of
the sand, place a smooth, flat granite plinth around 20 cm
across, and shift it to compact the sand and provide a nearlevel surface.
Placing a granite plinth on a sand layer increases the contact
between the ground and the plinth and improves the
performance of the instrument. There is also no need to mix
concrete or to wait for it to set, as in step 5.
5. Alternatively, if time allows and granite is not available,
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CMG-3T
prepare a concrete mix with sand and fine grit, and pour it into
the hole. Agitate (“puddle”) it whilst still liquid, to allow it to
flow out and form a level surface, then leave to set. Follow on
from step 7.
Puddled concrete produces a fine-textured, level floor for
situating the seismometer. However, once set hard, the
concrete does not have the best possible coupling to the subsoil
or bedrock, which has some leeway to shift or settle beneath it.
6. Alternatively, for the most rapid installation, place loose soil
over the bottom of the pit, and compact it with a flat stone.
Place the seismometer on top of this stone. This method
emulates that in step 3, but can be performed on-site with no
additional equipment.
7. Set up the instrument as for a vault installation (Section 2.3,
page 11, steps 4 to 9.)
8. The instrument must now be shielded from air currents and
temperature fluctuations. This is best done by covering it with
a thermal shield.
An open-sided box of 5 cm expanded polystyrene slabs is
recommended. If using a seismic plinth on sand (from steps 3–
4 or 5), ensure that the box is firmly placed in the sand,
without touching the plinth at any point. In other installations,
tape the box down to the surface to exclude draughts.
9. Alternatively, if a box is not available, cover the instrument
with fine sand up to the top.
The sand insulates the instrument and protects it from thermal
fluctuations, as well as minimizing unwanted vibration.
10. Ensure that the sensor cable is loose and that it exits the
seismometer enclosure at the base of the instrument. This will
prevent vibrations from being inadvertently transmitted along
the cable.
11. Cover the pit with a wooden lid, and back-fill with fresh turf.
Other installation methods
The recommended installation methods have been extensively tested
in a wide range of situations. However, past practice in seismometer
installation has varied widely.
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