Guralp Systems Limited
MAN-040-0001 Güralp 40T User's Guide


1. Preliminary Notes 2. Introduction 3. First encounters 4. Installing the 40T 5. Calibrating the 40T 6. Connector pin-outs 7. Specifications 8. Revision history

Section Index: 3.1. Handling notes 3.2. Connections 3.3. Levelling the instrument 3.4. Zeroing the instrument 3.5. Installation notes

Chapter 3. First encounters

3.1 Handling notes

Although the 40T 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 device, you should contact Güralp Systems for assistance.

All parts of the 40T are waterproof.

3.2 Connections

The instrument has an integrated cable ending in a 26-pin mil-spec socket which carries both power and output signals. This is suitable for connecting directly to a Güralp digitizer.

The breakout box, if ordered, provides individual signal and power connectors, or you can make up your own cable if you prefer.

3.2.1 The breakout box

If you are using a Güralp breakout box, it should be attached to the sensor through its SENSOR connector. Connectors are also provided at the CONTROL and RECORDER outputs, for attaching to a hand-held control unit or a Güralp digitizer. If you have ordered a 40T with optional high gain outputs, you will need to make up a suitable cable to expose these outputs.

The breakout box also provides a standard Güralp power connector on a 10-pin mil-spec plug. The 40T draws a nominal current of 48 mA from a 12 V supply when in use; thus, using a 12 V, 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 ACC/VEL mode whilst it is pressed. This mode allows you to monitor the mass positions whilst you adjust the offsets manually. The ENABLE button must also be held in for the CENTRE button to be actioned. If you prefer, you can use the equivalent switch on a Hand-held Control Unit (see below.)

3.2.2 The hand-held control unit

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. Signal meter

The upper section of the HCU contains a simple voltmeter for monitoring various signals from the instrument. Calibration

You can calibrate a 40T sensor through the 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 5 for full details. Control commands

If you have ordered a 40T with the remote null facility, you can zero its mass position offsets from the HCU. Banana plugs

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.

3.3 Levelling the instrument

The instrument must be physicallly level in order to operate correctly.

To level the instrument, first check the bubble level on the lid: the bubble should be in the centre of the printed circle. If it is not, loosen the lock-nuts on the instruument’s feet and then adjust the feet until the bubble is central.

Once levelled, secure the feet by screwing the lock-nuts downwards – not upwards – as shown:

3.4 Zeroing the instrument

Before installing the 40T, you should check that the mass positions are not significantly offset from zero. The 40T must be perfectly level for this to be effective. The mass position offsets can be affected by any tilt to the instrument as well as rough handling during transportation. 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 level and stationary.

The velocity outputs of the 40T are set at the factory to a nominal value below ±3 mV. Once the instrument is installed and has reached thermal equilibrium with its environment, these outputs should be similar to the factory-set value.

The Güralp 40T can be ordered with a option for automated “digital nulling”. See section 3.4.4 for more information and instructions for using this function.

3.4.1 Adjusting the mass position offsets manually

The 40T has three potentiometers (“pots”) accessible within its casing, which should be used to remove any DC offsets electronically:

3.4.2 Adjusting the mass position offsets with an HCU

Some 40T units are equipped with a remote mass centring option, which allows you to adjust the internal potentiometers by applying voltages across control lines to the sensor:

3.4.3 Zeroing a 40TD digital instrument

The offset potentiometers in a 40TD are in the same place as on the 40T. To access them, you will need to remove the digitizer module, which lies on top of the sensor itself. You can monitor the mass position outputs of the sensor using a Hand-held Control Unit and an adapter cable, available from Güralp Systems.

To change the offsets of a 40TD without digital centring:

Alternatively, you can adjust the mass positions and monitor the output digitally.

3.4.4 Digital nulling operation

The Güralp 40T sensor can be ordered with an option for automated “Digital nulling”. This removes the need to manually adjust the internal potentiometers to achieve a near zero mass position output for each component. It comprises a micro-controller and three digital potentiometers that replace the standard electromechanical pots.

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.

3.5 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.


1. Preliminary Notes 2. Introduction 3. First encounters 4. Installing the 40T 5. Calibrating the 40T 6. Connector pin-outs 7. Specifications 8. Revision history