Induction logging radial

At the end of the 1960 s serious attention was paid to other modifications to induction logging. One of them is based on the use of transient fields, when measurements are performed in the absence of the primary magnetic field (Kaufman and Sokolov, 1972). The study of the secondary fields, caused by induced currents in a medium with either cylindrical or horizontal interfaces allowed one to describe the radial and vertical responses of the two-coil probe and find the most optimal range of time for measurements. 

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. 

It is appropriate to notice here that in logging methods based on application of direct current the direction of current lines depends on the relation between resistivities of borehole, invasion zone, and bed. At the same time in induction logging, when the source of the field is a vertical magnetic dipole located on the borehole axis, current lines, regardless of the distribution of resistivity in a radial direction, present themselves as circles located in horizontal planes with their centers on the borehole axis, and they do not intersect the cylindrical surface between media with different conductivity. 

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. 

This method of superposition of radial responses of two-coil induction probes does not practically allow us to find parameters of probes which are sensitive only to certain parts of medium, similar to special probes in electrical logging as micrologs. 

In such cases when the depth of investigation of the multi-coil probe is not sufficient in the radial direction, and correspondingly the apparent conductivity differs from the formation conductivity, in order to perform interpretation it is necessary to have additional information derived from either other induction probes or applying different logging methods. It is obvious that similar types of problems arise when the influence of the surrounding medium becomes essential. 

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