1062 



EXPLORATION GEOPHYSICS 



It is now convenient to consider the medium around the instrument as being divided 

 into a series of thin cyHndrical shells of equal wall thickness and coaxial with the hole. 

 Each of these cylindrical shells will have a geometrical factor Gr, which, if the whole 

 medium were of uniform conductivity, would represent the relative portion of the total 

 signal contributed by that shell. Successive values of Gr may be plotted as a function 

 of the radii of the cylindrical shells. In Figure 656 such a plot is shown for the sonde 

 of Figure 654, consisting of one transmitter and one receiver coil. This curve is known 

 as the radial investigation characteristic of the sonde. 



The curve is zero at the axis of the hole, increasing rapidly to a maximum and then 

 dropping off less sharply as the radial distance is increased. If the medium were 



Plot of geometricol foctor for 

 thin horizontal beds 



+ 3L 



+ 2L 



+ L 



VERTICAL INVESTIGATION CHARACTERISTIC 

 (Two coil system spacing 'L* ) 



Fig. 657. — Curve showing vertical characteristic of two-coil 

 induction sonde. 



homogeneous, the amplitude of the curve would indicate the relative proportion of the 

 signal coming from the formation at different radial distances from the instrument. 

 The radial distance is indicated in multiples of the spacing L. 



If, as is also indicated in Figure 656 for values of the radial distance between zero 

 and ri, the medium is the mud M, and between ri and r«, the medium is an invaded zone 

 B, and beyond r2, the medium is the uninvaded formation A, the geometrical factors of 

 each of these three zones will be proportional to the corresponding areas, m, b, and a, 

 under the curve, and the relative signal response of each of these areas is the product 

 of its geometrical factor by its conductivity. 



