Foreword

A consortium of National Measurement Institutes, and other national authorities carrying out research in geoscience, have been collaborating to establish measurement traceability for low frequency sound and vibration measurements. The purpose of this document is to describe how developments in metrology delivered by the project can add value to geophysical applications exploiting Infrasound and seismic measurements both on land and in the ocean.

Calibration has long been recognised as an essential part of establishing a measurement system. The purpose of calibration is simply to transform the output of the measurement system into meaningful information about the quantity of interest, such as infrasound pressure or ground velocity in the first instance. Therefore, the calibration must be linked to an absolute realisation of that quantity, known as a primary standard. Primary standards are typically realised in the laboratory and transferred to sensor systems used in the field via secondary calibration methods and sensors capable of transferring that calibration on to other sensors, known as transfer standards. Geophysical measurement applications are somewhat unusual in that the sensor systems to be calibrated often cannot be removed from the field, and therefore require and additional field-calibration step (or steps).

This document focussed on sensor systems deployed in the International Monitoring System (IMS) supporting the provisional Comprehensive Nuclear Test-Ban Treaty, but is also relevant in other geophysical monitoring applications. It provides technical and practical details of each of the calibration steps, including those established in the IMS operational procedures, to link field measurement data all the way back to the fundamental physical realisation of the particular quantity. The document is organised by technology, with stand-alone sections for Seismic, Infrasound and Hydroacoustic sensor systems. Consequently there is some commonality between the sections in both structure and content. Some information is, with good intention, repeated across the sections. This provides the reader interested in one of the featured technologies with all of the relevant information presented as one coherent narrative.