Abstract: Method and an apparatus for determining a vertical distance between a first marker and a second marker embedded in a formation traversed by a borehole so as to quantify the occurrence of earth layer compaction or subsidence. The markers are implanted within a formation and their relative position is monitored over time to detect the presence of formation subsidence and compaction. A tool having three or more detectors adapted to sense signals emitted from the markers is positioned proximate the markers, where the detectors are separated from each other by a known vertical spacing. The tool is positioned at least at three elevations such that a reference elevation of a reference portion of the tool is determined when (a) the first detector detects a signal emitted from the first marker, (b) the second detector senses a signal emitted from the second marker, and (c) the third detector detects a signal emitted from one of the markers.