Physical principles of multi-coil differential probes

Physical Principles of Multi-coil Differential Probes 

Until now it was assumed that interaction between currents is absent, i.e. all currents induced in a conducting nonuniform medium, regardless of the distance from the source, are shifted in phase by 90°. For this reason electromotive forces induced in measuring coils of a probe are in phase with each other, and they are added and subtracted in the same way as scalars. If only one component of the electromotive force, for instance the quadrature component, is measnred it is subjected to the same operations as scalars, regardless of whether currents are shifted in phase by 90°, or the internal skin effect manifests itself and due to it at every point of a medium there are both quadrature and inphase components of the induced current. 

For this reason we can say that a necessary condition for the application of differential probes is the absence of interaction between currents in those parts of a medium the influence of which should be significantly reduced. In other words, those parts of the medium have to correspond to Doll s area where the current density is defined by the primary magnetic flux and the conductivity at a given point. It is natural that in choosing parameters of probes in order to increase the depth of investigation in the radial direction, it is important to eliminate the influence of parts of the medium located relatively close to the source. As calculations show, this condition usually takes place even for sufficiently high frequencies. 

However, absence of the skin effect in the area, the influence of which it is necessary to reduce, is not sufficient for the efficiency of multi-coil induction probes. Let us present a signal measured by the probe as a sum of two signals, namely, one which is caused by currents in the area where there is not practically interaction between currents and a 

As was shown in previous chapters with an increase of the distance from the source the skin effect is more strongly manifested, and in a general case the distribution of currents in the external area (the formation) depends on the magnitude of currents in the first area, which is closer to the probe. 

---