During the calibration process the geophone's 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 geophone's 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 single-point 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.