13-12-2012, 04:13 PM
Using Micro-ElectroMechanical Systems (MEMS) accelerometers for earthquake monitoring
Using Micro-ElectroMechanical Systems.pdf (Size: 989.57 KB / Downloads: 82)
The first recording (above) is of an ML 4.7 earthquake where the seismic recorder was at a
hypocentral distance of about 8km. The top three channels (from top: east-west, northsouth
and vertical) are velocity recordings from a modern short period sensor, which has
obviously clipped due to the high level of ground motion. The lower three channels (again
E-W, N-S and vertical) are from a MEMS accelerometer that has a full scale range of ±2g.
The velocity sensor saturated at about 12mm/s, but the accelerometer still has a great deal
of recording range left, with the ground motion peaking at 0.36g in the vertical axis.
By converting the vertical acceleration channel into velocity (red trace below) and
overlaying the original vertical velocity channel (white trace below) we can see that the
peak vertical velocity was actually around 24 mm/s. The accelerometer was sampled at 200
samples per second (sps), which reveals a number of velocity peaks not visible on the
100sps seismometer channels.
The diagram below illustrates the practical result of combining a seismometer and
accelerometer. The black bar represents the full range of ground motion generated by
earthquakes (logarithmically scaled to from zero to 10g for illustrative purposes).
Seismometers can record down to the smallest of levels of vibrations (the red bar) but have
a limited range at the top end of ground motion. Accelerometers (yellow, green and blue
bars) can easily handle strong motion but have limited sensitivity at the bottom end.
What you want to achieve is the maximum recording range possible (the rightmost bar) by
combining a seismometer and accelerometer. All of the common earthquake-oriented
accelerometers, including the ±2g 100dB MEMS accelerometer, have a sufficient overlap
with the seismometer to achieve the desired result.
Using Low-Noise Accelerometers
So, when are low-noise accelerometers useful? If you are primarily interested in recording
strong motion (in structural monitoring, for example), but also wish to be able to see the
smallest events possible without going to the expense of adding a seismometer, then a lownoise
accelerometer may be of advantage, but only if your site noise is below the sensitivity
level of the accelerometer.
It is important to remember to not get caught up in the dynamic range specifications of an
accelerometer when the application cannot justify the extra cost for no practical advantage.
For example, when using an accelerograph to monitor a structure such as a dam for
earthquake response, is a ±2g 140dB accelerometer that can detect ground movements of
less than 1μg required, or would a ±2g 100dB accelerometer that can measure down to
around 60μg (and costs significantly less to purchase) suffice? Would 60μg, or even 200μg,
be an issue for the structure? Is the background noise level at the structure above the
lower threshold of the sensor? Appropriate technology for the application should always be
considered.