Page 189 - Well Logging and Formation Evaluation

Homing-in Techniques 179

0.00025

0.0002

0.00015

Field gradient (T/m) 0.00005 DFax
0.0001

0
950 1000 1050 1100 1150

–0.00005

–0.0001
Depth

Figure 12.1.3 Field Due to a Monopole: DF ax

x = D F xy *.652 ). (12.1.21)
(
0
Similarly, the separation of the two stationary points (zero gradient) on
the F ax curve is related to x by:

x =D F ax 2 . (12.1.22)

This may also be measured by finding the separation of the two zero
crossing points of the DF ax curve. If the separation of the two points on
the AX dir curve on either side of the point representing AX dir = 90 degrees
at which AX dir = 135 degrees and 45 degrees is measured, x may be found
from:

x =DAX dir 2. (12.1.23)