The dictionary definition of calibration is; to determine by measurement or comparison with a standard, the correct value for each scale reading on a device. As a consultant or blast contractor, you assess risk every day. But, have you considered the importance of, and the risks associated with the calibration of your seismograph and the accuracy that is provided? Can your calibration provider back you up in times of trouble and provide documented evidence that the equipment was functioning within specification? Can their calibration procedures withstand independent scrutiny and be shown to be traceable to a National Standards for accuracy of measurement? These are important documents that in court provide tremendous support that blasting and monitoring activities are conducted professionally and competently.
The recommended interval for calibration of most measuring equipment including seismographs is one year. Across most of North America and many parts of Europe regulatory authorities actually require seismographs to be calibrated on an annual basis. This requirement has been or is being adopted by several other countries around the world and some specific projects even require that seismographs be calibrated every three or six months. Calibration is important to ensure the instrument is performing as it was designed to, and measures accurately the true ground vibration and air-blast. Although seismographs are designed to be used in a rugged environment, they are still a sophisticated electronic monitoring device. Therefore, preventative maintenance becomes an important part of the annual calibration process. Many manufacturers and their authorized calibration facilities will perform upgrades and preventative maintenance on your instruments, often free of charge, providing the units are regularly serviced. The upgrades often include product enhancements of both hardware and software and result in expanding the functionality of the equipment.
During the calibration process the geophones are mounted on a shake table with a reference sensor and excited at a specific frequency and amplitude. At this point the sensors should be checked for the “as found” conditions of each channel. This must be performed before any adjustments have been made. These results will allow the user to assess all of the vibration records that have been recorded since the last time the seismograph was calibrated and determine the impact, if any, on these records. The seismograph is then adjusted to match the level being recorded by the reference sensor. The geophones are then checked to ensure they are within specification across the required frequency range of the equipment. Microphones are calibrated in a similar manner. They are exposed to a sound source at a specific frequency and sound pressure level and the “as found” conditions are recorded. The microphone is then adjusted to match a reference microphone. Then, like the geophone, they are checked to ensure they meet the specifications for the required frequency range. To ensure compliance, it is very important that all sensors are tested at several frequencies within their required range. For example, if a calibration service provider uses a device called a piston phone to calibrate microphones, the microphone is only checked at one frequency and one pressure level. This frequency is typically about 250 Hz and is at the high end of most seismographs frequency response. This is not a valid method of calibration, because the singlepoint calibration does not provide any indication of the response at other frequencies. The seismograph may be reading high, low or nothing at all at other frequencies within its specified frequency range. Some models of seismographs are meant to have the geophone, microphone and data acquisition unit calibrated as a system. Generally this allows the entire system to be calibrated more accurately. However, a limitation of instruments that are calibrated using this method is that the geophone and the microphone are matched to the acquisition unit and the sensors may not be interchangeable with other instruments, even if they are the same model from the same manufacturer, without a decrease in overall accuracy. Other models of seismographs may have the geophone, microphone and the data acquisition unit calibrated as independent assemblies. This type of seismograph should maintain its accuracy when sensors from compatible models are interchanged. To help maintain the integrity of the recorded data, most seismographs have a sensor checking function. Some users may confuse the purpose of this function with that of the yearly calibration. This sensor check can provide valuable information about the sensors and their set up. If a sensor has not been installed or connected properly, the sensor check function would provide some indication of a failed sensor. In general, the sensor check will induce an electrical pulse into the sensor that will cause the mechanical components in the sensor to move. The seismograph in turn measures this movement, just as if it was a true vibration, and the response is recorded. This response is then analyzed to make sure the sensors are operating within an acceptable range. This is a very good indicator that the sensors and unit are working properly. However, the sensor check is NOT a calibration check and cannot replace the annual calibration process. The sensor check does not compare the measured result against an external traceable reference sensor, nor does it test the entire electronic circuits that are integral to the geophone response.
When your seismograph is calibrated by an authorized calibration facility, the facility will issue a Calibration Certificate. The calibration certificate provides the list of reference equipment used in the calibration process. In order for seismograph. Using unauthorized facilities may cause problems in court if the reference equipment is not traceable, or if the certification documents are incomplete. In some instances, unauthorized facilities have even been found to calibrate equipment without ever updating the calibration date within the instrument. If an improper calibration date is printed on vibration records, it may call into question the validity of the report itself. Also, if a seismograph does require repair, the manufacturer has the best experience and knowledge to find and fix the problem. They will also ensure that any replacement parts that are used meet the specifications for the equipment. Unauthorized facilities do not have access to test specifications, procedures or parts lists and some of these facilities may use substitute parts that do not meet the functional requirements. This can ultimately lead to inaccurate vibration levels being reported.