Detecting device

A detecting apparatus has a body incorporating three spaced infra red light detectors around its periphery. Three light emitting diodes are associated with respective detectors. The body is lowered by a wire cable and when the body reaches a section, where underground blasting has already occurred, the intensity of radiation picked up by the detectors significantly changes. In this way the depth of a bore hole can be measured by the length of cable then extending from an upper roadway to the body. Thus, the depth is measured without endangering personnel who would otherwise have to enter a lower roadway below an unsupported section of roof