Key Questions to Ask When Ordering Geophones for Sale

Accelerometers, Geophones and Seismometers – Which to ...

Report by John Shelton, AcSoft Ltd, Svantek UK Ltd

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Vibration measurements have surged in recent years for various applications, compelling acoustic consultants to engage actively in vibration surveys, in addition to noise measurements. These applications range from health and safety, such as hand-arm and whole-body vibration, to structural concerns, such as building or blasting vibrations. Understanding sound level meters is crucial for practitioners, but vibration instrumentation often presents confusion. This article addresses these key questions.
Geophones for Sale are a valuable tool among many others in your toolkit. The discussion below will help you identify which instruments best suit your needs.

Transducers

Vibration transducers come in two primary types: accelerometers and geophones (or seismometers). Accelerometers measure acceleration, while geophones measure velocity. Both can measure vibration based on the relationship between acceleration and velocity. Conversion between the two is feasible using integration techniques.
Integrating an acceleration signal to get a velocity signal is often done in the time domain with a filter (integrator) but can also be achieved in the frequency domain by dividing an acceleration spectrum by 2πf (where f is the frequency). This conversion helps eliminate high-frequency components in a velocity spectrum.

Accelerometers

Most accelerometers for contemporary applications are piezoelectric devices, where a piezoceramic crystal sandwiched between a base and seismic mass generates a charge proportional to the acceleration. This simple mass/spring system exhibits a fundamental high-frequency resonance. Below this resonance, the response remains flat and linear.
Accelerometers generate a charge output that necessitates a specialized charge amplifier with high input impedance to interface with measuring systems. Today, these amplifiers are integrated into the accelerometer, using IEPE (Integrated Electronic Piezo-Electric) systems. Ensure compatibility with your equipment before use. Some accelerometers do not respond at DC frequency and can roll off at low frequencies, so choose an appropriate device for your needs.
Note that accelerometers can measure across all axes but require care to avoid damage, especially sensitive designs with larger masses.

Geophones

Geophones typically feature a moving coil design where a magnet within a coil generates an output proportional to velocity. Resembling reversed loudspeakers, these resonant devices operate best above their low-frequency resonant point (around 3-4Hz). High-quality damping and design make modern geophones very effective.
Geophones are usually oriented either vertically or horizontally and must be installed correctly. Triaxial units include multiple directional coils. These instruments are often rugged, economical, and preferred in rigorous environments like mining.
Modern geophones also support IEPE, offering compatibility with accelerometers, thus facilitating easier use and integration across systems.

Choose your weapons

Your choice of transducer should align with what you measure: use accelerometers for acceleration (VDV, MTVV) and geophones for velocity (PPV). Integrated accelerometers can substitute geophones but consider low-frequency noise that could distort data.
If phase performance and noise sensitivity are critical but manageable with high pass filters, accelerometers might suffice. Geophones remain ideal for PPV measurements due to their inherent design benefits. Whichever you choose, understand its performance and limitations.
Consider this: blowing on an accelerometer introduces low-frequency noise; careful design mitigates but does not remove all errors. Mastering Seismic App: Your Ultimate Crash Course

Future Technologies

Emerging technologies like MEMS (MicroElectroMechanical Systems) show great promise in both sound and vibration transducers. MEMS devices are prevalent in consumer electronics, and their application in sensitive ground vibration measurement is imminent.
MEMS accelerometers, notable for their DC response and stability, are expected to become mainstream. They facilitate easy calibration and are durable, making them optimal for various deployments.

Calibration

Field calibrators, crucial for sound level meters, have not become standard for vibration measurement due to complex, costly equipment. Yet, the BS EN ISO 8041:2005 standard enforces calibration requirements.
For accurate calibration, consider devices operating at key frequencies (15.91Hz and 79.6Hz) and employing complete measurement chains, including accelerometers and geophones. Vertical geophones can be field-calibrated like accelerometers while horizontal ones might need lab testing, introducing time and cost challenges. Standards for broader adoption and efficient field calibration are evolving.Related links:
A brief spectrum analyzer comparison guide for beginners
Mastering Seismic App: Your Ultimate Crash Course
For more information, please visit Short-Period Nodal Seismometers.

Conclusion

This article aims to guide practitioners on essential considerations when acquiring vibration instrumentation. Many more nuances exist in the standards. Future articles will delve deeper into these topics. John Shelton is a member of the IOA Measurement & Instrumentation Committee, and AcSoft Ltd sponsors the Institute.

Questions from a Potential New Customer

I've begun exploring seismic instruments and discovered the Raspberry Pi website. I'm seeking product recommendations.

Living in southern Alberta, Canada where nearby seismic activities stem from Washington, British Columbia, and the Intermontane Seismic Belt, I wonder about detectable magnitudes from those regions and globally from places like Japan or Chile.

I'm uncertain about 'boards' and Raspberry Pi. Do I need a Raspberry Pi computer or install a board in my PC laptop to use devices like Raspberry Shake 3D or 4D?

Thanks for your insights and knowledge.

Gary

Contact us to discuss your requirements of High-Density Seismic Data Acquisition. Our experienced sales team can help you identify the options that best suit your needs.