AS-1 Calibration

IRIS Kansas City WorkshopSeptember 19-21, 2007

John Lahr (with Kay Wyatt)

Many of these calibrations and test need only be done once. Results of

the tests should be saved in SAC format files. If you have trouble in the future, you can repeat the tests

and compare them to the tests performed when you first received

your seismograph.

Perhaps the easiest way to monitor the “general health” of your AS1

system is by comparing your records with those from other

schools. See: http://www.iris.edu/hq/ssn/events

for current records.

SIDE NOTE

Keep in mind that the helicorder settings do not effect how the data are recorded on disk.

They just control how the data are displayed on the screen.

1. Setting the zero level2. Check the system’s free period3. Damping adjustment4. Washer calibration5. Adjusting the gain based on the background noise6. Lift test calibration7. Electronic noise calibration8. Inductance calibration

CALIBRATIONS TO BE PERORMED

Setting the zero levelSetting the zero level is probably the most difficult part of setting up an AS1 system. The knob on the AS1 “black

box” is VERY sensitive.

Use the “Show data values” feature under “Settings” to monitor the level while adjusting the knob on the “Black

Box”. Turn it slowly by hand to get within about 100 counts of zero and then make slight adjustments by

tapping lightly with a pencil until the numbers bounce equally above and below zero. It is helpful to put a piece

of tape on the “Black Box” and place a mark by the pointer on the black knob .

Check the System’s Free Period

First, pull the magnet carrier away from the

copper damping vane.

Check the System’s Free Period (cont’d)

Select one of the pulses and click the icon to display a single trace. The oscillations should

continue for at least three minutes

Check the System’s Free Period (cont’d)

First, change the helicorder setting so that it displays one minute per

line.

Undamped oscillations look

like this.

Check the System’s Free Period (cont’d)

Measure the time between two peaks or two troughs. This is the free or natural period of the system.

Here we measured 1.4 seconds.

1.4 sec

We just saw that without damping the system will continue oscillating for a few minutes following a single pulse. For this reason it would be nearly impossible to see any phases after the first P arrival of an earthquake.

On the other hand, if the system is over damped, then the response will be attenuated.

DAMPING ADJUSTMENT

Prepare to Mark the Location of Magnet Block

Tape a piece of paper to the area under the magnet block so that when the block has been positioned for

proper damping, its location can be saved. If the instrument needs to be moved later, the block can be

replaced in exactly the right spot.

DAMPING ADJUSTMENT (cont’d)

Place a small ruler vertically next to the red magnet, perhaps holding it in place with some clay.

Note magnet’s equilibrium position.

Lift the magnet 10 mm, release it, and then note how much below the equilibrium position the magnet goes.

Adjust the position of the damping magnet block so that the overshoot is 0.5 mm, which gives the desired ratio of 20.

DAMPING ADJUSTMENT (cont’d)

The more overlap between the magnet damping block and the copper vane, the

greater the damping.

DAMPING ADJUSTMENT (cont’d)

If the damping is too little when the copper

vane is completely within the damping

block, then adjust nuts on the block so that

the magnets are closer together

DAMPING ADJUSTMENT (cont’d)

Mark the final position of the magnet block.

DAMPING ADJUSTMENT (cont’d)

Tie a thread to a washer and place it near the boom magnet and coil.

WASHER CALIBRATION

Pulling the washer away suddenly creates this

calibration pulse: down, up, and then return to zero.

WASHER CALIBRATION (cont’d)

The system is under damped on the left. The system on the right is correctly damped.

Too little damping Proper damping

WASHER CALIBRATION (cont’d)

Note that the overshoot during a washer calibration is primarily

due to the AS-1 filters.

This overshoot will be much more than 1/20th of the first pulse,

as it should be.

WASHER CALIBRATION (cont’d)

When the washer was pulled away, which way did the magnet move?

Which way did the seismogram trace move?

Why?

Save your calibration as a SAC file.

WASHER CALIBRATION (cont’d)

When the ground moves down,the magnet will tend to remain fixed, which means it moves up with respect to the coil.

WASHER CALIBRATION (cont’d)

As soon as your AS-1 is setup, perform a washer calibration.

Save this initial calibration for future reference in a SAC file in

case there is some question about the system’s operation.

WASHER CALIBRATION (cont’d)

In the “Settings/Helicorder/Gain”set the helicorder display gain so that the

background noise is quite clear.

Adjusting the gain based on the background noise

The gain on this record is too low. The gain may need to be reduced

during storms and increased during quiet times.

Adjusting the gain based on the background noise (cont’d)

To do a lift-test calibration of your system, make a

5/16”-diameter hole in the top of the cover, near the

hinge end.

14.5 cm from the end and

7.7 cm from the side.

Lift Test Calibration

Mark the location for the lift-test weight. Make a white patch on the boom with whiteout and then

draw a line across it 10 cm from the mast.

Lift Test Calibration (cont’d)

A 0.034-gram weight is made

from a 2 cm x 1 cm piece of a manila

folder.

The weight is lowered onto the boom, ready for a lift-test calibration.

Lift Test Calibration (cont’d)

Quickly lift the weight. Here is an example lift-test calibration pulse.

Be sure to save your test in a SAC file.

Lift Test Calibration (cont’d)

To check for electronic noise, place a weight on the boom.

Electronic Noise Test

The electronic noise when the boom is held fixed is just 3 counts peak-to-peak.

Electronic Noise Test (cont’d)

For an “inductance” calibration, lift boom and place one loop of 32

gauge wire around the coil.

Inductance Calibration

Here is a closer view

of the loop of wire.

Inductance Calibration (cont’d)

A pulse is generated when a AAA battery (1.4 V measured) is attached and another pulse when the

battery is removed from the single loop of wire.

Inductance Calibration (cont’d)

This calibration is independent of the AS1 spring and damping

system.

Again, save this calibration for comparison with future calibrations.

Inductance Calibration (cont’d)

For more information or help regarding these calibration tests, contact:

Kay Wyattkwyatt@geocouple.com

CALIBRATION TESTS