سید رضا منصوری

Seismometer is a divice that recordes the ground oscillations caused by the arrival of seismic waves as time-dependent signals. It operates as an electric seismic converter and records the displacement, velocity or acceleration of ground motion.Geophone electromagnetic sensor is a receiver of seismic wave in land, which is used to measure the ground motion. Due to its high sensitivity, it is mostly used in recording ground vibrations and ultimately in exploring underground resources of oil. In a geophone that is affected by ground motion, there would be a relative movement between magnet and coil, this motion cuts the magnet's lines through the coil and generates a voltage proportional to the speed of the earth's movement in the coil. Eventually this procedure will produce electrical oscillating signals of a certain frequency. The seismic signals are recorded by several hundred to several thousand geophones and finally the data is analyzed by computer. Transfer function of geophone is described using a cubic equation, and provided that magnetic induction coefficient of the sensor is considered insignificant, it can be estimated using a quadratic equation. If a capacitor series circuit and a resistor (RC) are used, transfer function would be a quartic function, which is able to be changed into a cubic function (according to poles-zeros analyses). If appropriate relative amounts are considered for R and C, corner frequency can be reduced and thus, frequency range can be changed. In this study, for different amounts of resistor and capacitors, by considering effective damping coefficient in a range from 0.7 to 0.707, suitable amounts were achieved for corner frequency which is far smaller in comparison to 4.5 Hertz corner frequency of geophone (SS-4.5N); so, by employing a simple and cheap method (comparing with wide band seismographs), a wider range of frequencies were achieved by short period seismographs and as a result, analysis of ground motion would be simpler an