Much initial oil well prospecting is accomplished by analyzing the shock waves produced by scientifically placed explosive charges.  Various sensors are strategically located and connected to instruments to record the seismic waves.

This is an expensive procedure.  Therefore, before the explosives are detonated the data logging system is checked for function and accuracy.

Accurate, repeatable dc millivolts are fed into the analog-to-digital converters.  These voltages are simulating the inputs from the various transducers and represents a significant part of the pre-test. Thus, the data logging system has been verified, and the test can proceed.

(Note the similarity to the jet engine test.)