Seismic sensors are classified by the following characteristics:
The form of ground motion it measures, namely ground velocity or ground acceleration
The number of sense axes the sensor has
Whether the sensor is deployed in a borehole or against a rock surface
Each type of sensor 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 a uni- or tri-axial configuration. The tri-axial configuration affords the most accurate estimates of source parameters.
which digitally report sensor orientation (dip and roll) to resolve waveform directionality
for automated system configuration with IMS software
which create repeatable mechanical vibrations to verify sensor component health and wiring after installation
overcomes the limitations of clipping and saturation of conventional sensors and ADCs
measures ground velocity
Geophones 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, respectively.
The 4.5 Hz geophone has a usable frequency bandwidth from 3 Hz but must be installed to within 2° of its pre-set orientation. The 14 Hz geophone is omni-directional and can be installed at any angle. It has a usable frequency bandwidth from 11 Hz. For further information about geophones, please refer to the 4.5 Hz and 14 Hz geophone specifications documents.
Other Geophone types, for example geophones for monitoring regional seismicity (1H z) or for use in reflection seismology (28 Hz) are also available.
Intrinsically Safe Geophones
IMS carries a range of geophones, which have IECEx ia Ma certification for use in coal mine applications.
measures ground acceleration
Integrated electronic piezoelectric (IEPE) accelerometers are usually used where precision measurements of smaller (high-frequency) seismic events are required. IMS routinely manufactures two kinds of accelerometers: a 3.7 kHz low-noise version with a usable frequency bandwidth of between 0.2 and 3.700 Hz, and a 25 kHz high-frequency version with usable bandwidth between 2 and 25 000 Hz.
For further information on accelerometers, please refer to the 3.7 kHz and 25 kHz accelerometer specifications documents.
MEMS accelerometers are used where large ground motions, greater than the upper-measurement range of geophones and IEPE accelerometers, are expected to occur.
complimentary sensors to improve quality of the seismic event catalogue
For decades, miniature geophones and integrated electronic piezoelectric (IEPE) accelerometers have been the sensors of choice for microseismic monitoring in mines. Which of these sensors to use is not always an obvious decision – there are clear advantages and disadvantages to both. IEPE accelerometers are more sensitive to higher frequencies, and so are better suited to denser arrays monitoring smaller seismic events occurring closer to the sensors. They are also less sensitive to the low frequencies generated by larger events, but saturate more easily on the higher frequencies of even moderately sized events. Geophones have a lower noise floor (and higher dynamic range) over most of their usable bandwidth, but clip more easily due to the large displacements of nearby large events. IMS now offers hybrid sensors where both types of sensor can be included in a single sensor package. Combining the complementary characteristics of geophones and IEPE accelerometers means that better coverage of a wider variety of seismic events is achievable, which ultimately results in higher quality of the seismic event catalogue.
A common problem with traditional seismic sensors is their limited upper measurement range. Geophones typically clip at ground displacements above a few millimetres, and IEPE accelerometers saturate at ground accelerations above a few tens of g’s. Adding to this, when the signal from these sensors is outside the recorder’s analogue-to-digital converter (ADC) voltage range, an additional type of non-linear signal distortion is introduced. All this means that the ground motions of large and/or nearby seismic events cannot be accurately recorded. This is a serious problem because large (often damaging) events cannot be properly analysed, which makes back-analysis, seismic hazard assessments and other seismological work more difficult.
To overcome the limitations of clipping and saturation of conventional sensors and ADCs, IMS has introduced a new range of composite sensors called XGM sensors, which rely on digital MEMS accelerometer technology. MEMS accelerometers are miniature sensors that have higher noise than conventional seismic sensors, but are able to record (with high fidelity) seismic signals with amplitudes that would overwhelm conventional sensors or ADCs.
Every seismic sensor manufactured by IMS, whether geophone, accelerometer or a hybrid combination, now includes digital XGM sensors. The XGM sensor is able to record the strong ground motions that conventional sensors and ADCs cannot. Furthermore, because they are sensitive to frequencies down to DC, they will also be capable of recording lower frequency signals.
Surface Mount Sensors
IMS has a range of surface-mount sensors available for use in applications where it is important to monitor structural integrity or the site effects of the excavation face.
Sensors for Deep Boreholes
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 2 000 m, high-pressure rated sensors can be manufactured upon request.
Removable Borehole Sensors
For applications subject to rapid excavation progression, or where temporary monitoring of microseismic activity is required, IMS has developed a reusable borehole sensor that can be recovered and redeployed as soon as a more suitable location is available. The removable sensor allows the seismic system layout to adapt to mine progression, within the limitations of open-borehole microseismic monitoring.
For any questions or custom orders, please feel free to contact us.