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FP5

OPTSDET Report Summary

Project ID: EVG1-CT-2002-00062
Funded under: FP5-EESD
Country: Romania

Report containing the procedures for calibration and field testing of new optical seismic sensors

For the new optical sensors developed in our project, the problem of the calibration is special because, so far, there are no systematic methods dedicated for this. In order to minimize the errors of the parameters determined by calibration, we propose a particular yet direct method. The principle is to perform a calibration against a reference sensor.

The reference sensor is a typical electromagnetic sensor (having known parameters) and the “unknown” sensor is the new optical sensor.

Using a ground noise or other seismic signals, an unknown sensor can be calibrated against a known one by operating the two sensors side by side [Pavlis & Vernon 1994]. As a method of relative (frequency-response) calibration, this is limited to a frequency bandwidth where suitable seismic signals are present well above the instrumental noise level, and are spatially coherent between the instruments.

However, when the frequency response of the sensor has been measured electrically, then its absolute gain may be determined quite accurately using this method. The two responses should be digitally equalized in terms of electric signal before the amplitudes are compared.

This method can be used to get the information concerning the behaviour of the optical sensor in a quite large seismic range events. Thus, it become necessary to design two complementary approaching ways:
- The behaviour at the background noise and the small seismic events can be determined by calibration of the sensors on a special vault having a good acoustic coupling with the competent rock. For properly obtained results, the vault has to be placed in quiet locations, like tunnels, caves or basements, situated in granite or crystalline schist areas. It is important to obtain a good signal to noise ratio.

- The behaviour at the large seismic events can be determined by calibration using a shaker-table, which is designed to move with a prescribed motion. Both seismometers will be fixed on the shake table, and the outputs for the programmed input will be measured. The inconvenience of the shaker-table consists in that is difficult to calibrate the sensors at low level seismic signals. Thus, we remove this problem using the complementary calibration method on the vault.

Usually the input has a prescribed frequency and the outputs will be compared in order to calibrate the unknown seismometer relative to the seismometer with known response.

For calibration of the optical sensors, we used several steps:
-Selecting the reference sensor;

-Calibration and verifying the parameters of the reference sensor;

-Comparative measurements in the vaults using the reference sensor and the optical sensor;

-Calibration of both sensors using the shaker-table;

-Computing of the recorded data and applying the results for determination of the optical sensor characteristics.

Following the existing information and the work experience we propose to use as the reference sensor a short-period seismometer, model S-13 (the vertical version), manufactured by Teledyne Geotech. Our experience in maintenance of the Romanian Telemetered Seismic Network showed this instrument is quite stable and easy to operate. The calibration and the adjustment of the seismometer parameters should be done without special mechanical devices or sophisticated acquisition systems.

Related information

Contact

Mircea RADULIAN, (Deputy Director)
Tel.: +40-214-930118
Fax: +40-214-930052
E-mail
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