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The Science of Measuring Ground Vibrations

13 Jul 2015

In addition to earthquake, ground vibrations can be induced by human activities such as transportation, explosion, drilling, mining, and construction works. Ground vibrations are low frequencies, typically below 160 Hz. They become harmful to a building’s integrity when the vibration amplitude is large or the frequencies coincide with the building’s natural frequencies. As Singapore is experiencing a construction boom in the recent decades, it is crucial to monitor ground vibrations in existing buildings and at construction sites for safety reasons.

Any vibration can be expressed by three frequency-dependent parameters - amplitude, velocity and acceleration. Typically, ground vibrations are broadband signals with relatively small amplitude but strong velocity and acceleration. Geophones and seismic accelerometers are designed to sense the strong velocity and acceleration respectively. Besides measuring ground vibrations, they can also be used to measure strong seismic vibrations. Geophones are typically designed with a spring-mass system whereby a spring-mounted magnetic mass moving in a wire coil generates electrical signal, corresponding to the vibration velocity. Seismic accelerometers are designed based on piezoelectric effect that converts forces/accelerations exerted on the piezoelectric crystals into electrical charges.

Figure 1: A typical ground vibration sensor and monitoring instrument

The ground vibration sensors and monitoring instrument gathers raw measurement data for further processing to derive information and decisions on building integrity, construction site safety and earthquake magnitude and location. Thus, the accuracy of the measured raw data is of great importance. Metrological standards for ground/seismic vibrations are crucial in ensuring the measurement accuracy through reliable and recognised device performance characterisation.

NMC has established the metrological standards for ground/seismic vibration measurement, covering a frequency range of 0.1 Hz to 160 Hz. With this national infrastructure, ground/seismic vibration sensor performance can be studied through comprehensive characterization including transverse sensitivity, resonance frequencies, linearity and dynamic range. The standards aim to generate well controlled vibration patterns, using air-bearing shaker platforms, characterised by either the primary laser Doppler method or the secondary reference accelerometer method. The electrical outputs from the device of interest can then be correlated accurately with the known vibration levels generated by the shaker platform. The primary standard is able to achieve measurement uncertainty as low as 0.5% in the frequency range of 0.1 Hz to 160 Hz with high operation complexity. In comparison, the secondary standard requires much less effort to operate but gives higher uncertainty. In the following section, the working principles of both the primary and secondary vibration standards will be discussed.

Primary Laser Doppler Based Vibration Standard

As suggested by the name, the primary vibration standard is based on laser Doppler effect. When a moving object has a velocity towards the laser source, the reflected laser beam from the moving object will carry a positive shift in its frequency spectrum. Conversely, when an object has a velocity moving away from the laser source, the reflected laser beam will have a negative shift in its frequency spectrum. By detecting the Doppler frequency shift, the object velocity of interest can be determined as , where is the laser wavelength and is the Doppler frequency shift. Doppler frequency measurement is a phase-based measurement and it is much easier to achieve high accuracy compared with amplitude-based measurement. Thus, it enables measurements to be done with the smallest measurement uncertainty compared to that of other techniques.

Figure 3. Primary Laser Doppler Based Vibration Standard

Secondary Comparison Based Vibration Standard

The secondary vibration standard is realized through comparing the electrical output from the device-under-test with that of a reference accelerometer calibrated using the primary vibration standard. It does not involve laser based measurement. Instead, all measurement is done based on the sensitivity of the reference accelerometer. While the resultant measurement uncertainty is larger, the measurement done using the secondary standard can be much faster as there is no time-consuming laser alignment needed.

Figure 4. Secondary Comparison Based Vibration Standard

Collaboration with industry

NMC is currently working with Geoscan Pte Ltd, a local Small and Medium Enterprise (SME) focusing on seismic monitoring and soil investigation, to understand the performance of their newly designed seismic monitoring device combining construction safety monitoring functions and earthquake.

Mr Chua Wah Ann, Managing Director of Geoscan said, “We have gained much confidence in providing more accurate and reliable seismic monitoring data with a better understanding of our devices.”