CMG-3T
Triaxial Broadband Seismometer
Operator's guide
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.
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CMG-3T
Table of Contents
1 |
Introduction............................................................................................................... |
3 |
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1.1 |
Options................................................................................................................ |
4 |
2 |
Installing the 3T........................................................................................................ |
7 |
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2.1 |
First encounters.................................................................................................. |
7 |
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2.2 |
Installation notes.............................................................................................. |
10 |
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2.3 |
Installing in vaults............................................................................................ |
11 |
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2.4 |
Installing in pits................................................................................................ |
14 |
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2.5 |
Installing in post-holes.................................................................................... |
18 |
3 |
Calibrating the 3T................................................................................................... |
20 |
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3.1 |
The calibration pack......................................................................................... |
20 |
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3.2 |
Calibration methods......................................................................................... |
23 |
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3.3 |
Calibration with Scream! ................................................................................ |
23 |
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3.4 |
Calibration with a hand-held control unit....................................................... |
28 |
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3.5 |
The coil constant.............................................................................................. |
28 |
4 |
Accessories............................................................................................................... |
30 |
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4.1 |
The breakout box.............................................................................................. |
30 |
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4.2 |
The hand-held control unit.............................................................................. |
31 |
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4.3 |
Integrated State-of-Health Controller............................................................... |
33 |
5 |
Inside the 3T............................................................................................................ |
35 |
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5.1 |
The sensors....................................................................................................... |
35 |
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5.2 |
The control system........................................................................................... |
36 |
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5.3 |
RS232 control interface.................................................................................... |
41 |
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5.4 |
The feedback system........................................................................................ |
42 |
6 |
Connector pin-outs.................................................................................................. |
47 |
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6.1 |
Sensor and control unit pin output................................................................. |
47 |
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6.2 |
Sensor output (“D”-type connector option)..................................................... |
48 |
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6.3 |
Sensor output (waterproof connector option)................................................. |
49 |
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6.4 Breakout box POWER connector...................................................................... |
50 |
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7 |
Specifications.......................................................................................................... |
51 |
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8 |
Revision history....................................................................................................... |
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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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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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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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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:
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a handle with North indication,
amulti-pole socket for input and output, other optional connectors as ordered;
abubble level,
an air vent port,
three adjustable feet, and
two accurate orientation pins (one brass and one steel).
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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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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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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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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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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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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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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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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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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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