A method for measuring bending of extended vertically directed channels. A fibre-optic sensor having a gravity pendulum fixed at the end of a flexible hollow carrier rod is placed inside the extended vertically directed channel. A light signal is supplied via fibre-optic lines connected to the sensor to record light signals. The flexible hollow carrier rod with the fibre-optic sensor is placed along the channel and detects interference in a gas gap with a photoreceiver, said gas gap varying during the passage of the fibre-optic sensor as a result of the angular motion of the gravity pendulum away from the axis of the bowed channel. Profilograms of the variations of the gas gap for each fibre-optic line are recorded; and the magnitude and direction of bending of the channel from the vertical axis are calculated to simplify measurements of bending of a vertically directed channel while maintaining measurement accuracy.

An optical fiber includes multiple optical cores configured in the fiber including a set of primary cores and an auxiliary core. An interferometric measurement system uses measurements from the multiple primary cores to predict a response from the auxiliary core. The predicted auxiliary core response is compared with the actual auxiliary core response to determine if they differ by more than a predetermined amount, in which case the measurements from the multiple primary cores may be deemed unreliable.