Induction logging characteristics

In many cases the parameters of geoelectric section defining the field behavior are very small. For example, if the length of the induction probe is 1 m, the frequency of the field is 20 x 10 Hz and the resistivity of the medium is 5 ohm-m, we have p = L//i 0.12. In a more resistive medium, parameter p is even smaller for this frequency. Correspondingly, for a relatively resistive medium this theory of induction logging provides sufficiently accurate information about radial characteristics of this method. 

After these comments we will consider radial characteristics of induction logging proceeding from the geometric factor Gi. 

A choice of frequency or frequencies for induction logging cannot be done by using the results of calculation of the field in a medium with only cylindrical interfaces. However, these data allow us to investigate radial characteristics of two-coil induction probes, as well as other types of probes, consisting of several coils. In particular, comparison of calculations basted on exact and approximate methods permits us to establish a range of frequencies and resistivities of a medium where it is reasonable to apply so-called focusing multi-coil probes. 

Arbitrarily, a multi-coil induction probe can be presented as a sum of two-coil induction probes. Currently, there are known symmetrical and non-symmetrical multi-coil induction probes, and their characteristics will be considered in detail in the next sections. As a rule in induction logging using one frequency the electromotive force, caused by the current in the transmitter coil or coils, is significantly greater than that generated by induced currents in the medium. For this reason an additional compensating coil to increase the accuracy of measurement is installed. Due to this the electromotive force caused by the primary field is practically equal to zero. It is also appropriate to notice that some differential probes do not require a compensation coil. 

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