(57) @ The depth at which a string of pipe is stuck in a well borehole is determined using
a tool suspended by a logging cable (22) and comprising a tool body (18) for running
in the stuck well string, the body enclosing a first electromagnetic coil (50) connected
with a suitable storage capacitor (38) and switch (36) for forming a strong magnetic
field at a specified depth on the stuck well string. A second electromagnetic coil
(48) is included in the tool and functions as a detector means. The detected signal
is output to a suitable amplifier system and a recorder (34). Upon application of
stress to the well string, the amplitude of magnetic marks above the stuck location
will either be reduced or substantially erased; the electromagnetic marks below the
stuck points will not be reduced, and the change in detected signals by the detector
means indicates the location of the well string sticking and enables location of a
free point in the string.
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