METHODS AND SYSTEMS FOR USE WITH A POSITIVE DISPLACEMENT MOTOR

A method for determining a rotational frequency of a positive displacement motor positioned in a downhole portion of a drill string, the method including: providing at least one measurement device for measuring a property of a part of the drill string; providing a processor for processing a signal output by the at least one measurement device; operating the positive displacement motor whilst the part of the drill string is in a non-driven state; receiving the signal at the processor; determining, using the processor, a frequency spectrum of the signal; and determining the rotational frequency of the positive displacement motor by: identifying a peak in the frequency spectrum in a pre-determined frequency range associated with an expected rotational frequency of the positive displacement motor; and/or identifying a peak in the frequency spectrum having a pre-determined peak width associated with an expected rotational frequency of the positive displacement motor.