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.
Avoid bumping or jolting 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.
All parts of the 40T are waterproof.
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.
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 handheld 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. If you prefer, you can use the equivalent switch on a Handheld Control Unit (see below.)
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 low-gain outputs, switch the dial to V, N/S or E/W LOW VEL according to the component you want to monitor.
To monitor the high-gain outputs (on a 40T with that option), switch the dial to V, N/S or E/W HIGH VEL.
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 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 Chapter 4, “Calibrating the 40T” for full details.
If you have ordered a 40T with the remote null facility, you can zero its mass position offsets from the HCU.
Select the component you want to centre from the CENTRING SELECT dial.
Switch the signal meter dial to one of the MASS POS settings.
Switch the rightmost switch to 1 SEC VEL to enable the centring lines.
Press the +/– switch towards – to centre a mass from a positive value, or towards + to centre it from a negative value.
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.
Before installing the 40T, you should check that the mass positions are not significantly offset from zero. The mass position offsets can be affected by any tilt to the instrument, as well as 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 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 40T has three potentiometers (“pots”) accessible within its casing, which should be used to remove any DC offsets electronically:
Bring the instrument into 1 second response mode by applying a voltage across the Acc/Vel and Signal Ground pins of the input. If you are using a DM24, you can do this by sending a CENTRE command. If you are using a Handheld Control Unit, you should select 1 SEC VEL from the Velocity Select switch.
Measure the vertical mass position output with a 10 V voltmeter (see Appendix A, “Connector pinouts”) or by selecting MASS POS, V from a Handheld Control Unit's Display Select knob. If using a HCU, also check that the Centring Select knob is set to OFF.
If the vertical component needs adjusting, remove the cap on the lid which protects the Verticalpot with a flat-bladed screwdriver (provided).
Insert the screwdriver through the opening, and engage the pot. An LED flashlight may be useful for locating the head.
Turn the pot either way until the offset readout is as close to 0 V as possible.
Repeat steps 2 – 5 for the north/south and east/west components.
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:
Bring the instrument into 1 second response mode by selecting 1 SEC VEL from the Velocity Select switch.
Measure the vertical mass position output by selecting MASS POS, V from the Handheld Control Unit's Display Select knob.
Set the Centring Select knob to V.
Press the spring-loaded switch towards + or – to bring the mass position offset from negative or positive values towards zero.
Repeat steps 3 and 4 for the N/S and E/W components.
Return the instrument to broadband mode by selecting BB VEL from the Velocity Select switch.
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 Handheld Control Unit and an adapter cable, available from Güralp Systems.
To change the offsets of a 40TD without digital centring:
Check the bubble level on the lid of the instrument, to ensure it is not tilted. If necessary, re-level the instrument by adjusting its feet.
Unscrew the vent cap on the lid to allow the air pressure to equalise.
Using an Allen key, remove the screws holding the digitizer module onto the sensor.
Place a flat-head screwdriver in the notches provided, and twist to lever off the digitizer module.
Carefully lift off the digitizer module, and unplug the ribbon cable from the sensor electronics.
Attach a Handheld Control Unit and adapter cable to the ribbon connector, and power up the sensor through the control unit.
Set the CENTRING SELECT switch on the Handheld Control Unit to 1 SEC VEL, and the monitoring dial to V MASS POS.
There are three holes in the topmost electronics board, which provide access to the offset potentiometers. Insert a screwdriver through the appropriate hole, and engage the potentiometer for the vertical component.
Adjust the potentiometer until the mass position output reads close to zero.
Repeat steps 4 – 6 for the north/south and east/west components.
Alternatively, you can adjust the mass postions and monitor the output digitally.
Remove the vent cap, and use an Allen key to remove the screws holding the digitizer module onto the sensor as above.
Carefully lift off the digitizer module, and support it nearby, leaving the ribbon cable connected. Extension cables can be obtained from Güralp Systems; otherwise, you can use the casing of the sensor itself as a support.
The steel sensor housing can be removed with an Allen key.
Whilst monitoring the mass position outputs (channels M8, M9 and MA), adjust the potentiometers as above.
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,
instruments may need to be deployed rapidly, perhaps to monitor the activity of a volcano showing signs of rejuvenation, or to study the aftershocks of a major earthquake;
installations may be required in remote locations, or otherwise in circumstances where it is unfeasible to build a vault.
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
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.
In temporary installations, environmental factors are also important. The sensor needs to be well protected against
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 heavy machinery (even at a distance), or by overhead power lines.
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.