Casing Inspection - Cased-Hole Log Analysis and Reservoir Performance Monitoring - page 231

coil for eddy-current measurement, and two sensor coils wound in opposite directions
for both fl ux-leakage and eddy-current measurements. The sensor coils are wound
in such a way that for both measurements there is zero voltage so long as no anom-
aly exists, but a signal will be produced when the quality of the casing changes. The
same sensor coils can be used for both measurements, as two distinct frequencies
are involved. Typically a frequency of 2 kHz is used for measurement of the eddy
current, giving a depth of investigation of about 1 mm. The sensor pads are mounted
on springs so that they are held in contact with the casing, facilitated through cen-
tralization of the sonde. Various sizes of magnet pole pieces are available and are
selected according to the inside diameter of the casing, to optimize the signal
strength for the fl ux-leakage measurement. The fl ux-leakage data correspond to
anomalies located anywhere in the casing, while eddy-current distortion only occurs
at the inside wall of the casing.
Fig. 13.11 One embodiment
of a pipe analysis tool.
Courtesy Schlumberger