Guralp Systems Limited
MAN-030-0001 - Güralp 3T Operator's guide

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1. Preliminary Notes 2. Introduction 3. Installing the 3T 4. Calibrating the 3T 5. Accessories 6. Inside the 3T 7. Connector pin-outs 8. Specifications 9. Revision history

Section Index: 3.1. First encounters 3.2. Installation notes 3.3. Installing in vaults 3.4. Installing in pits 3.5. Installing in post-holes

Chapter 3. Installing the 3T

3.1 First encounters

3.1.1 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. (The softer, low-density foam should be on the outside and the stiffer, high-density foam should be placed next to the instrument when re-packing, in order to prevent the instrument from damaging the soft foam while still providing the necessary shock-absorption.) 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 a digitiser or breakout box; and

  • a calibration and installation sheet.

If you have ordered the optional break-out box, you will also have received:

  • the breakout box (which provides separate connections for the signal, control and power lines); and

  • a 10-pin connector for your power lead (see below).

You will also have received, if ordered, a Hand-held Control Unit (HCU) for monitoring sensor outputs and calibration

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

    Warning: Instruments are assembled at sea level. If working at altitude, there may be a considerable pressure difference between inside and outside the casing. Never remove this screw unless instructed to do so by GSL support staff.

  • three adjustable feet

  • two accurate orientation pins, one brass and one steel. The steel stud is opposite the brass stud, pictured here.

3.1.1.1 Serial number

The sensor’s serial number is stamped onto the side of the sensor base, next to the N/S indicator (seen to the right of the photograph above). It can also be found on the label on the top lid of the sensor. You should quote this serial number if you need assistance from Güralp Systems.

3.1.1.2 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.

3.1.2 Connections

The instrument has a single connector, which can be joined using the cable provided to a digitiser 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.

3.1.2.1 Using a digitiser

The 3T can be connected directly to any Güralp Systems digitiser using the signal cable provided. This is the simplest way to use a 3T instrument. All the instrument's functions are available through the digitiser - including centring, locking and unlocking - and the instrument takes its power from the digitiser, along the single connection cable.

We recommend that you keep the digitiser 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 digitiser in the quiet, stable conditions of a seismic installation also provides it with an optimum environment for the on-board ADCs.

3.1.2.2 Breakout box and hand-held control unit

The 3T can be supplied with an optional breakout box, which provides mass control functions in installations which do not use compatible digitisers.

“Pig-tail” sensor cables, which terminate bare ends, are available, to aid connection to third-party digitisers.

A hand-held control unit is also available which can control calibration lines and monitor sensor outputs in addition to mass control. See chapter 5, for more details.

The digitiser can be connected either to the break-out box, as in the previous illustration, or to the hand-held control unit, as shown above.

3.1.2.3 Power supply

or

The sensor requires a DC power supply of between 10 and 36 Volts, connected via the socket on the breakout box, hand-held control unit or digitiser. If you are powering the instrument via the break-out box or hand-held controller, you will need to make up a suitable cable to connect your power source to the 10-pin connector (spare 10-pin bayonet connectors are provided for this purpose). Using a 12 Volt, 25 Ampere-hour, sealed, heavy-duty, lead-acid 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 7.)

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 a battery-powered 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.

3.1.2.4 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 digitiser. You can use any multi-channel recording system, provided that it has high-impedance floating differential inputs that can accept ±10 Volt signals on each leg of each input.

If you are using a Güralp Systems digitiser, you can connect the instrument directly to the digitiser without using the breakout box; power will be supplied through the digitiser. The digitiser can also activate the sensor control lines under software control.

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 7.

3.2 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:

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.

Temporary installations can be protected against spurious vibrations by:

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-centre the instrument after this time.

3.3 Installing in vaults

You can install a 3T in an existing seismic vault with the following procedure:

3.4 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.

3.4.1 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 introduced 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

Again, 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.

installation-completeFinally, 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.

By following these guidelines, you will ensure that your seismic installation is ready to produce the highest quality data.

3.5 Installing in post-holes

The 3T is suitable for installation in post-holes. In soft subsoil, a hole two to four metres deep and 20cm wide can be conveniently excavated using a tractor-mounted or hand-operated post-hole auger. To minimize surface effects, you should ensure that the hole is one metre deeper than the length of the instrument or, preferably, somewhat more.

Since the hole has no lining, it may occasionally flood. However, most soil types are sufficiently permeable to allow water to soak away, leaving the packing material moist.

A slim-profile 3T instrument is available with vertically stacked sensors and built-in inclinometer, specifically designed for post-hole installations. Digitisers or other recording equipment can be placed either within the hole or in a separate enclosure; alternatively, the Güralp DM24 is available in modular form, allowing you to attach the digitiser directly to the instrument.

To install a 3T in a post-hole:

PreviousNext

1. Preliminary Notes 2. Introduction 3. Installing the 3T 4. Calibrating the 3T 5. Accessories 6. Inside the 3T 7. Connector pin-outs 8. Specifications 9. Revision history