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MB3: a new infrasound sensor with remote calibration capability |
General description
The MB3 is an infrasound sensor developed by the CEA, following the MB2000 and MB2005 series [1], which allows the recording of very-low-frequency acoustic signals over a broad frequency band, with an excellent resolution (i.e. self-noise lower than Infrasound Low Noise Model) and a large dynamic range. Moreover, it has remote electrical calibration capability.
This sensor is available in two versions:
- MB3a, with analog output.
- The MB3d, which is a digital sensor encapsulating a low-noise 24 bit digitizer (ADC), with a calibration board, a time card and a storage capacity of 1 Go.
Both the MB3a and the MB3d provide a pressure output or a pressure derivative output. The sensitive element of the sensor is made up of a metallic bellows sealed under vacuum (to minimize heating effects) and a high-performance magnet-coil transducer based on technologies developed for seismic applications. Variations of environmental pressure lead to a deformation of the bellows. The movements produced by this deformation are converted by the transducer into an electrical signal, which is generated by electromagnetic induction. This signal is then processed in an electronic circuit consisting of a pre-amplifier and an analog integrating filter with differential output. The integrator/high-pass filter is set at 100 s to obtain an output signal proportional to the pressure. Auxiliary sensors of atmospheric pressure and temperature are integrated into the MB3; their information is available in analog form for the MB3a and digital form for the MB3d.
Figure 1: Transducer block: 1) vacuum bellows; 2) magnet 3) coil 4) calibration coil wound around measurement coil.
Figure 2: CAD view (transverse section) of the bellows and the transducer.
Figure 3: The analogue sensor (on the left) and the digital sensor (on the right, with its GPS antenna).
Performances
Like its predecessors MB2000 and MB2005, the MB3 is an observatory sensor perfectly adapted for the round-the-clock operational infrasound monitoring, in the context of the Nuclear Test Ban Treaty, as well as for use in scientific studies of volcanic activity, gravity waves or acquiring data on the atmosphere. The smal size of the MB3d is an advantage for temporary measurements, which only require deployment of a single MB3d with its GPS antenna and battery, since the device stores the digital data.
Instrumental noise
The MB3 is characterized by an extremely low level of instrumental noise (< -80 dB Pa/√Hz @ 1 Hz). This allows the sensor to resolve by more than 10 dB the Infrasound Low Noise Model [2] (cf 4). This sensor was particularly designed to meet the requirements of the network of infrasound stations of the IMS (International Monitoring System) set up in the framework of the CTBT (Comprehensive Nuclear Test-Ban Treaty).
Figure 4: Power spectral density of instrumental noise for MB3 sensor (blue) compared with Infrasound Low Noise Model of Bowman over the effective band-width of the IMS used for CTBT (above 10 Hz, the measurement is influenced by vibratory disturbances of the laboratory).
Figure 5: Power spectral density of very-low frequency infrasound measured by a MB3a sensor at an observatory (Lormes station in the department of Nièvre, France) compared with a co-localized MB2005 sensor (after correction of the high-pass filters) showing 24h periodic variation attributed to atmospheric tides.
Transfer function and sensitivity
The transfer function of the sensor is an important parameter to ensure the good quality of results obtained from processing the acquired signals. It describes how the sensor deforms the initial signal in terms of amplitude and phase as a function of the frequency.
Figure 6 shows the measurement of the transfer function of a MB3a sensor (presure output) with an infrasound calibrator (Patent no. 0102063) [3] developed by the CEA/DASE. The sensor is adjusted in the factory to a sensitivity of 20 mV/Pa at 1 Hz.
Figure 6: Measurement of the transfer function of a MB3a in amplitude and phase (in blue) compared with the theoretical response.
Technology transfer
The MB3 sensor was developed with the support of PROLANN-SRMP, a company based in Lannion (Brittany, France) which has been involved in the manufacture of seismic and infrasound sensors for the CEA for more than 25 years. The MB3 sensor is manufactured by PROLANN-SRMP and marketed by its subsidiary company SEISMO-WAVE under licence with the CEA.
Specifications
Technical
characteristics |
Outputs |
Pressure (Pa)
Pressure derivative (Pa/s) |
Bandwidth (f -3 dB) |
Pressure output: 0.01 - 28 Hz
Pressure derivative output: DC - 28 Hz |
BLDR [0.02 ; 4] Hz
(Bandwidth Limited Dynamic Range) |
117 dB @ f< 1.6 Hz
109 dB @ f= 4 Hz |
Instrumental noise |
< 10 dB under LNM [2] |
Resolution |
1,75 mPaRMS over [0.02 ; 4] Hz |
Sensitivity |
Pressure output: 20 mV/Pa
Pressure derivative output: 2 mV/Pa.s-1 |
MB3a |
Output range |
48 V peak-to-peak |
| Type of output |
Differential (Symmetrical) |
Output impedance |
2 × 50 Ω |
Saturation level |
Output P (Pa) : ±min (12000 [Pa/s]/2.π.f[Hz] ; 1200[Pa])
Output ΔP : ±12000 (Pa/s) |
MB3d |
Saturation level
and output range |
Pressure : ±min (12000 [Pa/s]/2.π.f[Hz] ; 1000[Pa])
Pressure derivative: ±10000 (Pa/s) |
Sampling rate |
20, 50,100 Hz |
Sensitivity |
1.178 10-4 Pa/lsb ou 1.178 10-3 Pa/s/lsb @ gain = 1 |
Parametrable gain |
1, 2, 4, 8 |
Storage |
1 Go in miniSEED format |
Integrated calibration |
Yes (internal adjusted calibration coil) |
Auxiliary
outputs |
Temperature sensor |
[-40 ; + 110]°C, 10 mV/°C, ±0.2°C |
Atmospheric pressure sensor |
[150; 1150] hPa, 1 mV/hPa
Offset stability of offset: 0.25% full scale
Uncertainty: 1.5% full scale |
Physical
characteristics |
MB3a |
Dimensions |
Diameter: 110 mm, height: 139 mm |
Weight |
3.0 kg |
MB3d |
Dimensions |
Diameter: 110 mm, height: 205 mm |
Weight |
3.4 kg |
Power supply |
Power supply |
7-20 V, DC |
MB3a |
Consumption |
7.5 mA @ 12 V |
MB3d |
70 mA @ 12 V |
Environment |
Temperature range |
-20 à + 50°C |
Storage temperature |
-30 à + 70°C |
Seismic sensitivity |
< 30 Pa/m.s-2 |
Sealing |
CEI 60529-IP67 (air inlets plugged) |
Shocks/falls |
NF EN 60721-3-1, 2M1 (free fall, impact, shocks) |
Transport |
NF EN 60721-3-2, 2M3 (vibration) |
EMC |
NF EN 55024 classes A and B (immunity)
NF EN 55022 class B (emission) |
MB3 spécifications table.
[1] J. Marty et al, 2013 : ‘’Surface pressure fluctuations produced by the total solar eclipse of 1st August 2008’’, J. Atmos. Sci 70, 809-823. doi:10.1175/JAS-D-12-091.1.
[2] Bowman et al, ‘’Infrasound station ambient noise estimates and models: 2003-2006’’, Infrasound technology Workshop, Tokyo, Japan, November 13-16, 2007.
[3] Brevet n° 0102063. Titre : Dispositif de calibrage d’un capteur de pression, notamment d’un capteur de pression infrasonore.
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