Guralp Systems Limited
MAN-T60-0001 - CMG-6T Operator's Guide

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1. Introduction 2. First Encounters 3. Calibrating the CMG-6T 4. Connector pin-outs 5. Specifications 6. Revision history

Section Index: 2.1. Handling notes 2.2. Unpacking 2.3. Connection 2.4. Centring (zeroing) the instrument 2.5. Test installation 2.6. Installation notes 2.7. Installing in vaults 2.8. Installing in pits 2.9. Other installation methods

Chapter 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.

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:

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

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 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.

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 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 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.

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.

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/

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

2.6 Installation notes

For the best possible results, a seismometer should be installed on a seismic pier in a specially-built vault, where conditions are near perfect. Here, wave-trains arriving at the instrument reflect very well the internal motion of subsurface rock formations. However, this is not always feasible. For example,

In these situations, 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. This is done by

In temporary installations, environmental factors are also important. The sensor needs to be well protected against

This can be done by selecting a suitable site, and 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.

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.

2.7 Installing in vaults

You can install a CMG-6T in an existing seismic vault with the following procedure:

2.8 Installing in pits

For outdoor installations, high-quality results can be obtained by constructing a seismic pit.

pit-cutaway-bedrock

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.

2.9 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.

Some installations introduce a layer of ceramic tiles between a rock or concrete plinth and the seismometer (left):

pit-installation-with-tiles pit-installation-with-tiles-removed

However, noise tests show that this method of installation is significantly inferior to the same concrete plinth with the tiles removed (right). Horizontal sensors show shifting due to moisture trapped between the concrete and tiling, whilst the vertical sensors show “pings” as the tile settles.

Other installations have been attempted with the instrument encased in plaster of Paris, or some other hard-setting compound (left):

installation-in-plaster-of-paris installation-in-sand

installation-completeAgain, this method produces inferior bonding to the instrument, and moisture becomes trapped between the hard surfaces. We recommend the use of fine dry sand (right) contained in a box if necessary, which can also insulate the instrument against convection currents and temperature changes. Sand has the further advantage of being very easy to install, requiring no preparation.

Finally, many pit installations have a large space around the seismometer, covered with a wooden roof. Large air-filled cavities are susceptible to currents which produce lower-frequency vibrations, and sharp edges and corners can give rise to turbulence. We recommend that a wooden box is placed around the sensor to protect it from these currents. Once in the box, the emplacement may be backfilled with fresh turf to insulate it from vibrations at the surface, or simply roofed as before.

PreviousNext

1. Introduction 2. First Encounters 3. Calibrating the CMG-6T 4. Connector pin-outs 5. Specifications 6. Revision history