Radial and vertical responses of the differential probe

As is well known, coil induction probes used in induction logging have various arrangements of coils. It is appropriate to distinguish in every differential probe the basic (main) two-coil probe having a maximal product of transmitter (T) and receiver (R) coil moments. Other coils are considered to be focusing coils and, they form several additional coil probes which provide focusing features of the induction tool. 

From the point of view of the location of the focusing coils with respect to the center of the basic probe, the multi-coil probes can be divided in symmetrical and nonsymmetrical ones. In symmetrical probes focusing coils are located in such a way that for every pair transmitter-receiver coils, displayed with respect to the center, there is another pair with 

It is also obvious that symmetrical probes have symmetrical profiling curves with respect to the formation center provided that the resistivity of the medium from both sides of the formation is the same. From the point of view of the position of focusing coils with respect to basic ones multi-coil probes are classified as probes with internal, external and mixed focusing. 

For internal focusing additional coils are placed between basic ones for external focusing they are located outside the main probe, and finally for mixed focusing additional coils are placed inside as well as outside the basic probe. The configuration of symmetric differential probes is shown in Fig. 7.4. 

In addition it is appropriate to notice that in symmetrical multi-coil probes moments of transmitter and receiver coils of corresponding pairs for example, basic probe, are the same, i.e. Mj- = Mr, Mtp = Mr. . 

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