Geophones
A geophone senses the vibrations created by an energy source which could be explosives, a shot gun blast or the pounding of a hammer. The geophones transfer the captured vibrations information to a seismograph which records the data for future processing and analysis.
While there are is a notable range of capacity and uses, essentially geophones are employed to monitor velocity. They consist of a magnetic core, surrounded by an electric coil. The magnetic core is attached to the housing and moves with it. The coil is attached to the housing using delicate leaf springs. As the ground vibrates, the housing moves but the coil tends to stay stationary. Movement of the magnet within the coil induces an electrical current that is calibrated to the velocity of vibration.
An important feature of geophones is that they can only monitor frequencies above their natural frequency, up to a specified spurious frequency. A natural frequency on the order of 10 Hz, and a spurious frequency on the order of 250 Hz are common. This frequency range precludes using geophones for certain applications, such as monitoring of pile driving. Nevertheless, geophones are one of the most common sensors employed for monitoring blasting and construction activities.
The different types of geophones are as follows:
- 4.5 Hz geophones - To capture refraction seismic waves and micro-tremors.
- 10 – 24 Hz geophones - To capture reflection seismic waves.
- 100 Hz geophones – High detail refraction seismic waves.
- Down-hole geophones – To capture down-hole and cross-hole compressional and shear waves.
