Measuring ground-borne vibration with an inaccurate seismograph
Recently, staff at an international laboratory specialising in the calibration of geophones, were shocked by the highly inaccurate seismograph that had been sent to them for calibration. The instrument, built in the UK, had been previously calibrated by the manufacturer and passed off as being accurate. When inspected by a senior electronics engineer, the seismograph’s design meant that it was incapable of being accurate. Instead, it read up to 8mm/s lower than it should in all three axes and could never accurately measure in compliance with any British or International Standards.
Manufacturers that self-certificate their own equipment and issue calibration certificates, without their kit ever having been independently tested and calibrated, cannot necessarily be trusted to be supplying instrumentation that is accurate.
As an example, if the above seismograph was used to measure ground-borne vibration from a hammer driven piling rig, where the maximum permissible level was 15mm/s, readings measured at 8Hz would have been 3mm/s below what they really were. And at 4Hz the levels would have been 5.73 mm/s lower. In other words when the seismograph was reading 15mm/s at 4Hz, in reality the level would be 20.73mm/s. At 4Hz, the seismograph would have indicated a level 5.48mm/s lower than it actually was.
Unlike for example sound level meters, there are no ISO build standards for seismographs, so manufacturers of seismographs try to ensure that their instruments measure in compliance with methodology standards such as BS 5228-2:2009+A1:2014.
Clearly there are manufacturers who cannot design and build their seismographs with the required accuracy, yet they claim to be able to meet the requirements of relevant standards and even issue certificates of calibration that are completely false.
The moral of the story is that buyers of seismographs should ensure that the make and model that they are proposing to purchase should at some stage have been calibrated by an independent laboratory and checked for accuracy at multiple points between 4Hz and 100Hz.
By: Andy Maslin, Accudata Limited. 19th September 2019.
