Seismometer Transfer Function Is Accurate Determination

The accurate measurement of seismometer transfer function is important to interpret and use ofseismographic observation data. Some fundamental problems in accurate measurement of seismometertransfer funtion are studied in dhail. The main contents and results are concluded as fellows.The Mode FBS-3A seismometer principle of operation s-plane pole-zero plot and frequencycharasteristics are presentod. Results show that tha FBS-3A frequency characterishc in the lowerfrequency part is main1y related to mechanical pendulun and feedback circuit the frequencycharacteristic in the higher frequency pat is mainly related to low-pass filter circults. The measurmentof seismometer transfer function is implemented mainly in the low-frequency prt, and transferfunchion in the high-frequency part is obtained with analyis and edculation.The measurement signa1s, such as sine-wave signal, step signai, impules signal, high-orderimpulse signal and white noise, are compared and have different characterishcs. According to theanalysis, the signal-noise-ratios of seismometer frequency response are obtained.Step signai cdlibration is a very simple and often used in seismic system. The damping andfrequency of seismometer is obtained by using the step calibration function. A theory is developed toobtain the damping of seismometer from the step response and nonlinear least square method. Themethod is confirmed by JC-Vl00-3D and FBS-3A feedbaCk broad band seismometer applications. Themethod Of the damping and frequency of seismometer is obtained in time domain is introduced. withthe method, some application examples of FBS-3A seismometer are given.White noise is ideal signal, its power spectrai density is constant It is ideal choice use of whitenoise in seismometer measurment. But there are two primary problems in using white noise signal, thefirst is white noise signai is difficult to generate, the second is it is needed infinite 1ong time to measurethe seismometer In order to use white noise excellent properties, the pseudo random binary signal(PRBS) is used to seismometer measurement. The power spectrum of pseudo random binary signal issimilar with white noise signal power spectral density, and random binary signals are easily generatedby periodically clocking a long shift reglster at fixed interval and have finite periods. The theory ofrandom binary signal is given in detail. M sequence and L sequence of order l0 and ll are generaredby computer, and are stored in Model EDAS-C24 data acquisition system.The system characteristics of seismometer can be described by many forms, one form can changeto another easily, but the transfer function is important and convenient to describe the systemcharacteristics. Generally, frequeucy characteristic of seismometer is obtained by measuring data, butthe measuring data is disturbed by noise. For obtaining the seismometer transfer function, it is neededto determinatc the seismometer transfer function from frequency response data, the LEVY method is incommon use to determinate the transfer function. because the transfer function of FBS-3A is dds tonormal used form. LEVY method can not be used directly, a new method is developed to determinatethe transfer function of particular form from frequency response, the method is conformed by digitallimulation.Ther are many noise soures in measuring the seismometer transfer function, the main noisesources are investigated. The method of cross-spectral analysis is presented, flow process diagram ofcomputing is glven, the algorithms are discussed in detail, such as low-pass digital filter, computing~ofseismometer transfer function inihai values, time domain convolution,bilinear thansformation,estimation of powre spectrum and cross-spectrum, window funchons aud preprocessing of measuringdata ets.The experiments had been implemented in 1aboratory and field seismic stahon, measuring signalwas pseudo random binary signal, FBS-3A seismometer was measured,...