Probe calibration, normalisation and verification – what is the difference?
Calibrated data, this is often stipulated by a client in a contract but the calibration is commonly misunderstood.
A definition of calibration is:
A set of operations which establish under specified conditions, the relationships between values indicated by the measuring instrument and corresponding values of a quantity of a reference standard.
It is important during a calibration the measuring instrument (probe/detector) sees only the reference standard. This can be simple for some detectors such as a caliper arm where each position has been manufactured for a specific diameter. For the radioactive detectors in total gamma, density or neutron porosity probes, the volume of investigation can dictate that very large reference materials are required. For example, the Adelaide Models Uranium reference standards are 1.2 metres in diameter and the mineralised reference zone is at least 1 metre in thickness.
It is not unusual for the calibration to be undertaken at the logging equipment manufacturer’s workshop or logging contractor’s workshop or a specialised facility such as the Adelaide Models.
A definition of normalisation is:
Adjusting one set of instrument values to another set of instrument values.
In mineral geophysical logging, normalisation is achieved through logging a local reference borehole which is located on the client’s property and intersects their mineralisation.
Most of the major iron ore and coal miners have established a series of boreholes for such normalisations. The normalisation does allow some consistency of data between probes and logging contractors and repeat logging over time can be used to indicate precision of the instruments. It is not uncommon for normalisation to be required by the client every 2 weeks of continuous operation.
A definition of verification is:
An operational check of part or all of a probe’s detector functionality in well defined conditions and on an ongoing basis. In some cases, the verification could use the same apparatus as for a calibration. The caliper jig or ladder is an example. Some verifications require special apparatus in the case of a jig for a total gamma probe in a uranium program. Some verifications can employ the same boreholes used for normalisation.
At the time of a verification, if the results fall outside of an acceptable range, then normalisation or calibration is required. Verification frequency can be daily, before/after logging, fortnightly or monthly.
Each of the above processes are related to each other and allow the ongoing performance of logging equipment to be measured and form part of the data quality assurance program.