but described more fully below within the instrumentation section 212 of the sonde the sensor is lowered by the wireline not shown or other means to a depth in the wellhole within the vicinity of a casing collar 214 the sensor is placed close to the casing inner wall approximately 25 mm one inch for example the sensor may be mounted on a caliper arm to maintain a fixed distance relative to the casing inner wall regardless of the casing diameter therefore sensitivity of sensor to the casing inner wall will not change once within the vicinity of the casing collar the sensor is further lowered so that it passes below the casing collar the sensor is then raised to a position above the casing collar light returning from the sensor is guided upward through the optical fiber 204 at the optical coupler or optical circulator 208 the second optical fiber 210 branches the light returning from the sensor and directs it to an optical detector 216 where it is transformed into an electrical signal 218 and subjected to analysis the invention employs a magnetooptical sensor to detect magnetic permeable anomalies caused by the presence of varying masses of ferromagnetic material present in casings tubing and pipe in the downhole environment such anomalies are sensed by a sensor comprising at least one optical collimator a faraday crystal and magnets which may be used to create a magnetic field in the vicinity of the sensor one preferred embodiment of such a sensor is shown in fig3 de polarized light 302 is guided into a sensor 304 through a first optical fiber not shown and passes through a co axially mounted magnet 306 the fiber is connected to a collimator 308 which assures that light entering a faraday crystal 310 positioned after.