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.
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CMG-6T
Table of Contents
1 |
Introduction.............................................................................................................. |
3 |
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1.1 |
Response options............................................................................................... |
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2 |
First Encounters........................................................................................................ |
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2.1 |
Handling notes................................................................................................... |
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2.2 |
Unpacking.......................................................................................................... |
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2.2.1 Serial number............................................................................................. |
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2.3 |
Connection......................................................................................................... |
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2.3.1 The breakout box........................................................................................ |
6 |
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2.3.2 The hand-held control unit (HCU)............................................................ |
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2.4 |
Centring (zeroing) the instrument..................................................................... |
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2.4.1 Remote centring option.............................................................................. |
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2.5 |
Test installation................................................................................................. |
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2.6 |
Installation notes............................................................................................. |
11 |
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2.7 |
Installing in vaults........................................................................................... |
12 |
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2.8 |
Installing in pits............................................................................................... |
15 |
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2.9 |
Other installation methods.............................................................................. |
18 |
3 Calibrating the CMG-6T......................................................................................... |
20 |
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3.1 |
The calibration pack........................................................................................ |
20 |
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3.2 |
Poles and zeroes............................................................................................... |
21 |
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3.3 |
Frequency response curves............................................................................. |
22 |
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3.4 |
Obtaining copies of the calibration pack........................................................ |
22 |
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3.5 |
Calibration methods........................................................................................ |
23 |
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3.6 |
Noise calibration with Scream! ...................................................................... |
23 |
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3.6.1 Sensor response codes............................................................................. |
26 |
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3.7 |
The coil constant............................................................................................. |
27 |
4 Connector pin-outs................................................................................................. |
28 |
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4.1 |
Instrument, HCU RECORDER and SENSOR connectors............................... |
28 |
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4.2 |
Breakout box and HCU power connector....................................................... |
29 |
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Specifications......................................................................................................... |
30 |
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6 |
Revision history...................................................................................................... |
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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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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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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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•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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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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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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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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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.
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