Instrumental Methods for Extending the Amplitude-Frequency Responses of a Geophone

To solve various engineering problems, it is often necessary to record signals in the 0.1–2000 Hz range. The lower limit of this range is beyond the operating band of geophones. This article considers extending of geophone frequency responses in two ways: multiplication of the transfer functions and introduction of negative resistance. The applicability limits of these methods are estimated. The upper limit of the velocity recorded by the geophone is determined by the gap in which the coil is shifted relative to the sensor frame. The lower limit depends on the total instrument noise of the measurement channel, the main contribution to which is the Brownian noise of the mechanical oscillatory system and the noise of the measuring circuit. A prototype was constructed using multiplication of the transfer functions. Laboratory measurements on a shaking table and microseismic noise recordings demonstrated that the modified geophone operates as a velocity sensor with an eigenfrequency of 2 Hz. This value depends on the microseismic conditions of where of the measurement system is placed. Recording seismicity with the modified geophone made it possible to record massive blasts in mines and quarries in the frequency range up to 2 Hz. Thus, the research demonstrates that the modified geophone can be used to monitor local and regional seismicity as a counterpart to short-period seismometers.