Seismic Sensors

Overview

Click to EnlargeSeismic sensors are devices used to measure seismic vibrations by converting ground motion into a measurable electronic signal. As the signal is analogue in nature, sensors must be linked to a data acquisition unit to convert its output into a digital format that can be read by computers.

All IMS sensors contain Smart electronics to provide the sensor type, serial number and orientation (dip and roll) in inclined boreholes. In addition, our Smart Sensors have the ability to generate internal vibrations to verify the sensor operation and to detect incorrect cable wiring after installation.

Sensor Types

Seismic sensors are classified according to what form of ground motion it measures, namely ground velocity (geophone) or ground acceleration (accelerometer and FBA); the number of sense axes the sensor has and whether the sensor is deployed in a borehole or against a rock surface. Each sensor type has different advantages in terms of amplitude range, frequency range, reliability and cost.

An IMS seismic network can be based on any combination of geophone, accelerometer and force balance accelerometers, in uni- and tri-axial combinations. The tri-axial configuration affords the most accurate estimates of source parameters.

An IMS systems engineer would be happy to discuss the sensors and configurations best suited to your application – so please contact us at your convenience.

Geophones

Click to EnlargeGeophones are usually the sensor of choice in most mining applications, because of the low cost, large bandwidth and excellent reliability. IMS routinely manufactures two kinds of geophones, with natural frequencies of 4.5 Hz and 14 Hz. The 4.5 Hz geophone has a usable frequency bandwidth of between 3 Hz and 2000 Hz but must be installed to within two degrees of its pre-set orientation with respect to the vertical. The 14 Hz geophone is omni-directional and can be installed at any angle, with a usable frequency bandwidth of between 8 Hz (-3dB point) and 2000 Hz.

For further information about geophones, please refer to the 4.5 Hz and 14 Hz geophone specifications documents.

Standard IMS sensors are pressure tested and rated to withstand 8 MPa. The maximum recommended installation depth is 400 m. For installations into deeper boreholes up to 2000 m, high pressure rated sensors can be manufactured.
See the 14 Hz deep borehole geophone specification.

Accelerometers

Accelerometers are usually used where precision records of smaller (high-frequency) seismic events are required. IMS routinely manufactures two kinds of accelerometers: a 3.7 kHz low-noise version with useful bandwidth between 0.2 and 3,700 Hz, and a 25 kHz high-frequency version with useful bandwidth between 2 and 25,000 Hz.

For further information about accelerometers, please refer to the 3.7 kHz and 25 kHz accelerometer specifications documents.

Non-seismic Sensors

Click to EnlargeAny sensor with a slowly varying output which may be conditioned between -5V and +5V or 4mA and 20mA may be monitored as part of the IMS. Strain meters, creep meters, tilt meters, CSIRO cells and thermometers have all been successfully employed.