Transient induction logging

As is well known, an increase of the depth of investigation in induction logging is usually realized with the help of multi-coil differential probes that in many cases permit us to eliminate the influence of currents in the borehole and in the invasion zone. 

However, the theory and numerous experiences show that for a given length of the basic probe, L, the depth of investigation in the radial direction does not exceed (0.5 0.6)L, and with an increase of the conductivity of the borehole and the invasion zone it becomes less. 

All these factors, which restrict to a certain degree the application of induction logging with differential probes, impel us to investigate the possibilities of the transient method, which is successfully applied in other areas of exploration geophysics. 

As is well known, if the moment of the transmitter coil changes as a step function with time, induced currents appear in the surrounding medium. At the beginning they are concentrated in the borehole, but with an increase of time their intensity increases at larger distances from the coil, and always there is such moment when they are practically located in the formation and their magnitude is defined by the conductivity of the external area, i.e. the formation. For this reason the magnetic field measured on the borehole axis, starting from this moment, does not practically differ from that in a uniform medium with the formation conductivity. 

A similar situation occurs in the presence of the invasion zone even when its conductivity is greater than that of the formation. Inasmuch as at the late stage of the transient response, induced currents are practically absent in the borehole and in the invasion zone the magnetic field, measured by a receiver, is not dependent of the position of the probe with respect to the borehole axis and caverns, as well as of the distance between the coils of the probe. At relatively large times induced currents are located far away from the 

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We will now investigate the relationship between frequency domain and time domain responses. In most cases considered in this section a transient electromagnetic field is excited by a step function current in the source. Moreover the theory of the transient induction logging described in this monograph will be developed for this type of excitation. For this reason the relationship between frequency response and transient response corresponding to this single type of excitation will be our principal concern. The information we need is obtained through use of the Fourier transform which takes the well... 

T = (2-Kpt X 10 ) scaled variable used in transient induction logging... 

P. TYGELAND P. HUBRAL —TRANSIENT WAVES IN LAYERED MEDIA A.A. KAUFMAN AND GV. KELLER — INDUCTION LOGGING... 

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 this chapter we will consider the theory of the transient method of induction logging in a uniform conducting medium, as well as in media with cylindrical and horizontal interfaces. Also one section will be devoted to transient responses of the electric dipole. 

Kaufman, A. A., Sokolov, V. P. Theory of induction logging based on the use of a transient field. Monograph, Nauka, 1972. 

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