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A robust, low-cost and well-calibrated infrasound sensor for volcano monitoring and other applications
Introduction
Infrasound sensors are of increasing use for volcano monitoring and studies. In particular, volcano acoustic signals are invaluable for detecting or discriminating volcanic explosions, pyroclastic density currents, rock falls or lahars and for locating active vents. It is also useful to detect many natural or artificial phenomena, such as explosions or river floods.
Technical characteristics
We have developed a broadband, robust and low-cost infrasound sensor designed for the detection and analysis of acoustic waves especially on volcanoes. It is based on a microelectromechanical differential pressure transducer (MEMS). The reference pressure is balanced with the atmospheric pressure through an adjustable pneumatic high-pass filter with corner frequency of a few to tens of mHz. Its amplitude range is ± 240 Pa and its sensitivity is 20 mV Pa-1, with a noise level less than 0.05 Pa RMS. The power consumption is 42 mW (3.5 mA with 12 V voltage). A direct output of the MEMS also provides a signal with sensitivity of about 500 µV Pa-1 and range ± 1245 Pa. The outputs can be connected easily with most of electronic recorders. The pressure input is designed for easy connection to any system of wind-noise reduction.
The mechanical elements of the sensor are produced by 3D printer and filled with epoxy resin which guarantees that the sensor can resist hard environmental conditions including corrosive gas, high level of humidity and large temperature variations.
Figure 1. Left panel : block diagram of the infrasound sensor, showing the MEMS, the capillary tube, the reference cavity, the electronic board, and the connector. All these elements are inserted in the packaging displayed on the right side of the picture. Right panel : picture of the sensor.
Table 1. Main features of the IST-2018 infrasound sensor. (*) Adjustable value.
Calibration
The instrumental response of each infrasound sensor is carefully measured using an especially designed calibration system in the frequency range from 1 mHz to more than 100 Hz. The sensor dimensions (26x45x80 mm) and weight (100 g) makes it very easy to handle and install.
Figure 2. Amplitude (top) and phase (bottom) responses of the IST-2018 for its infrasound (IS – blue) and MEMS (red) outputs. Dots are values measured with the calibration system. Amplitude responses of the MB2005 and the GRAS 40AE microphone are also displayed in top panel for comparison.
Availability
The IST-2018 infrasound sensor is built in the instrumentation lab of ISTerre. It is distributed mainly to research institutions in charge of monitoring. Its cost is about 500 €, depending on the type of cable, connector and power supply that are required. Quotation can be sent upon request to lesage univ-smb.fr and the detailed user manual can be downloaded here :
