casing by applying a varying magnetic field to the casing the eddy currents create an opposing magnetic field which interacts with the magnetic field generated by the eddy current sensor the depth of penetration of a varying magnetic field depends on the frequency of the magnetic field which is known as the skin effect the absence or presence of more or less metal in or in contact with or proximate to the well casing is detectable by monitoring the decay of the eddy currents created in the well casing this is done by monitoring the opposing magnetic field generated by the eddy currents as there is greater metal present when a control line runs along the outer wall of a well casing the eddy current readings will differ to those of the well casing where there is no control line at the outside wall the use of square wave pulses to generate eddy currents is straightforward and reliable from the operational and electronic point of view also this provides the benefit of allowing the generation and comparison of eddy current signals at and from different penetration depths simultaneously the later in time parts of the eddy currents decay curve which follow the leading or trailing edge of a square magnetic field pulse correspond to the magnetic fields of the eddy currents at a greater depth penetration than the earlier parts of the decay curve which correspond to the magnetic fields from.