Automatic system for absolute calibration measuring channels of ocean bottom seismic stations

Absolute amplitude frequency response and phase response are necessary each time when in addition to travel time information it is neccessary to have data about dynamic properties of seismic waves. Unique seismic observations such as sea bottom ones attract special attention. Practically it is impossible to know and control all parameters of all chains in circuit of up-to-date seismic channel so that the total frequency response could be calculated. As far as most channels contain amplifiers frequency filters and (if not digital) some recording and playing back transforming procedure, it is practically reasonable to calibrate the whole channel, already assembled. Besides, calibration of channel with high sensitivity is preferable in its working state, without initial attenuating and further recalculation the result. This paper is devoted to description details and basic principles or calibration system developed for solving these problems. Calibration circuit includes: mechanical exciter with 240 V DC battery, computer, specially designed two boards for controlling high voltage with computer, spectrum analyzer HP 3582A, and calibrated channel itself, consisting of geophones fixed in the frame of exciter and amplifiers, placed in OBSS registrator. Computer and HP 3582A are connected via HP bus with help of AT-GPIB board. The RP 3582A spectrum analyzer working in regime of measuring coherence function provides direct determination of amplitude and phase response of seismic channel Responses are measured for frequencies representing even harmonics of sounding signal For better data representing, computer feeds several periodical sounding signals with different main frequencies. After accumulating, calculating absolute values and reduction data in the memory, computer writes an output file on diskette in ASCII. This file later can be used with any processing, of graphical program. All the procedure is implemented without the presence of operator