The improvement of the accelerometers’ performance opens the possibility to
compete with standard seismometers. The concept is to install high precision
accelerometers near seismic zones, generally located occurs at the borders of
two tectonic plates.
The interval of time between an earthquake and a local alert can be very small,
from one minute to a few seconds. The waves travel through the ground at 1 to 5
kilometers each second, whereas the signals emitted by the sensors reach the
urban areas almost instantly. This allows the population to execute the security
processes in time, as long as they are not into the blind zone.
Seismometer
Configuration of the System
In order to optimize the chance of detecting an earthquake, theaccelerometersare
generally installed at a constant distance from each other. The use of relays
can be a good solution to reduce radio consumption.
Two vibration sensors installed in the x and y axis are required for P-waves
detection, as they propagate parallel to the ground. On the other hand, three
accelerometers are needed for S-waves detection, as they propagate perpendicular
to the surface .
Conditions needed
The most important point is to read the low amplitude, low frequency vibrations
with net accuracy. The Advanced National Seismic Systems (ANSS) are divided into
3 classes:
• Class A: 134-140 dB
• Class B: 110-120 dB
• Class C: 90 dB
The propagation frequency of a seismic wave varies from 0 to 10 Hz: it mostly
depends whether it is a P-wave or a S-wave. Its amplitude on the ground doesn’t
exceed 1g: the sensors with low ranges are favored for this application.
The MEMS accelerometers
A MEMS accelerometer functions thanks to the displacement of electrostatic
charges on the surface of two fixed electrodes. An acceleration impacts a mobile
mass within the die, creating a voltage difference. MEMS accelerometers read low
frequency vibrations, from continuous voltage (DC 0 Hz) to 1000 Hz.
Piezo ceramic accelerometers, including piezo electric and piezo resistive
sensors, have a very large bandwidth. However, they are not able to measure very
low frequencies, generally those below 0.5-1 Hz.
The servo-accelerometers are high performance, yet expensive sensors. They
function in a closed-loop configuration, which allows them to reach Class A and
Class B requirements.
The VS1000 Series
With a noise density down to 7 µg/√Hz for a recommended range of ±2g, theVS1000is
currently the best vibration sensor that Colibrys can offer. Its dynamic range
reaches 90 dB for a bandwidth of 20 Hz, respecting the precision requirements
imposed by the Class C. The sensor has a non-linearity coefficient of 0.1% FS,
with a bandwidth from DC 0 Hz to 2500 Hz (at ±5% FS)
The company is currently developing a new sensor exclusively designed for
seismic measurements. It will operate in open loop, with a precision approved by
the Class B.
