Work packages

Detailed project plans by work package 

Development of primary low-frequency calibration methods for sound in air, underwater acoustics and vibration metrology

The aim of this work package is to develop traceable, laboratory-based primary calibration methods for seismic and acoustic (air and underwater) sensors for the low frequency range needed for environmental measurements, but which are not yet covered by national metrology services worldwide.  Existing primary standards for air and underwater sound pressure are based on the three-transducer reciprocity method while the existing primary standard for vibration is achieved using laser interferometry. These methods provide the absolute sensitivity of the devices under calibration with metrological traceability to appropriate standards. These methods are routinely applied by the NMIs and can be extended to low frequencies with specific adaptations of the existing calibration devices or by the development of new calibration devices and methods especially designed for low frequency purposes. However, for infrasound applications (frequencies below ~20 Hz), the extension of the reciprocity method to the lowest frequencies of interest is not straightforward and results in implementation problems particularly because of low signal-to-noise ratios and acoustic leakages, requiring the evaluation of alternative methods as potential candidates for primary calibrations. In the work package, primary calibration techniques for infrasound will be evaluated over a frequency range of 40 mHz to 20 Hz in Task 1.1. Existing primary calibration devices will be further developed in Task 1.2 in order to extend their capabilities for seismometer calibration in a frequency range of 10 mHz to 20 Hz. Primary calibration techniques in underwater acoustics will be evaluated over a frequency range of 0.5 Hz to 100 Hz in Task 1.3. Finally, in Task 1.4 the developed primary and secondary calibration methods and devices will be validated by comparison measurements in sound in air, underwater acoustics and seismometry. 

Dissemination of primary standards: Secondary calibration and test methods for environmental measurement infrastructure

The aim of this work package is to develop secondary calibration methods for laboratory calibrations. Unlike the primary calibration techniques established in WP1, which often require sensors with special properties or high-level quality, secondary calibration techniques can be applied to a wide range of sensors. Thus, they represent a basic element of dissemination of units and form the foundation of traceability for many applications of sensors. The calibrations are realised in laboratories where, in general, transfer standards are compared with a primary standard. In most cases the equipment is less sophisticated than for primary techniques and the processes are fast and reliable.  In this work package methods will be developed, adapted or transferred which cover all application areas of this project and suitable sensors and transfer standards, respectively. They focus on the exploitation of low frequencies down to 40 mHz as a minimum. Microphones and micro-barometers will be applied in Task 2.1 for infrasound measurement in sensor networks or for environmental noise measurement. Hydrophones for underwater measurement are also multi-functional devices and the focus of Task 2.3 will be on the development of low-frequency methods which are still not yet available. Task 2.4 will assess and select one or more suitable transfer standards for sound in air calibrations for on-site calibrations. The assessment of seismic detection sensors, which are mainly exploited for the detection of ground vibrations e.g. from explosions or other geological events, is included in Task 2.5. Although several measurement techniques have been realised using a variety of sensors for the different modalities of low-frequency measurement, the choice and development of transfer standards primarily used to calibrate the sensors at international sensor networks stations has hardly advanced to date. Thus, this work package will evaluate the potential technologies and will select methods and devices suitable for use as reliable transfer standards. The chosen sensors will be manufactured or adapted, characterised and finally calibrated. For low frequencies, the methods to determine whether a measurement device complies with legal requirements, have not yet been developed and Task 2.2 will focus on development of the early realisation of such methods for application in legal metrology. This can also be understood as a particular application of the newly developed primary calibration methods from Task 1.1 to address a specific metrological requirement, namely the testing of sound measuring devices for the regulated European market. Testing, however, includes a variety of methods and parameters and acoustic measurements only are one element of many. 

Traceability for global seismic and acoustic environmental sensor networks by novel on-site calibration and improved knowledge of operational sensor behaviour

The aim of this work package is to maintain traceability for permanently deployed sensors within regional and global networks by novel on-site calibration and improved knowledge about operational sensor behaviour. These networks specifically include the seismic and acoustic sensor networks of the IMS deployed to monitor compliance with the CTBT and the GSN which serves as a multi-use scientific facility and societal resource for monitoring, research, and education.  Task 3.1 will identify feasible excitation sources for performing on-site calibrations, develop strategies and analysis procedures for that purpose and, ultimately and where possible in Task 3.2 perform an exemplary calibration for testing and demonstration purposes.  Once some calibration methods are in place, they will provide the base for further evaluation of properties such as stability, drift or self-noise of the monitoring station’s sensors in Task 3.3, which are mission critical for the work. Those properties will be characterised in relation to the variability of on-site environmental conditions such as temperature, ambient pressure or humidity.

Improvements in current environmental measurement station deployment strategies gained by traceable calibration, known measurement uncertainties and improved knowledge of operational sensor behaviour 

Improving the level of confidence and quality of measurements produced by current and future regional and international environmental monitoring networks, as well as improving analytical and predictive models that use these data, is a major challenge in better understanding climate change and its effects and other sources of non-natural critical events. The aim of WP4 is to contribute to this improvement by utilising the results obtained in WP1, WP2 and WP3, namely the establishment of measurement traceability chains perfectly suited to low frequencies in the fields of airborne acoustics, seismic and underwater acoustics, linking the primary standards present in NMIs to the measurement devices at the heart of operational monitoring networks, and a better knowledge of the performance of long-term measurements consistent with the time scale of events to be monitored and by developing robust uncertainty estimation and propagation models for the calibration chain from primary standard to on-site calibration. The measurement uncertainties associated with all calibration methods developed in WP1, WP2, and WP3 will be jointly evaluated in Task 4.1. A type of case study related to a legal metrology application will be undertaken in Task 4.2 where a comparison between the use of calibrated versus uncalibrated equipment will be performed. In Task 4.3 the consequences of a known measurement uncertainty for users of the IMS measurement data will be showcased by the investigation of uncertainty propagation in modelling. Ultimately in Task 4.4 the knowledge and experience gained from the project, together with conclusions for the station operators regarding improved deployment, will be collected and published in a good practice guide.