it will be l z u00b7 2 u2062 u2062 u03c0 u00b7 r p 2 1 where l is the length of the deployed fiber z the depth or axial position r is the radius of the helix and p is a constant giving the vertical separation of the helix 39 s loops in the same way a relative position can be defined the key advantage of this technique is the possibility to obtain the actual position or depth of the plug another aspect of the apparatus is that it can allow determining a displacement of the plug effectively as said above the winding of the fiber ensures that the deployed fiber part 10 a has a known trajectory or curve and the fbl measures the length of the deployed fiber up to the plug therefore a speed or an acceleration of the plug can be determined function of time or function of depth the skilled in the art who performs cement job will also appreciate the use of this method even when precision of the position is not exact effectively for cement job exact localization of the plug is not required a precision of 1 meter is exaggerated of 10 meters is excellent and of 100 meters is enough it is understand that even if the trajectory of the deployed fiber is slightly changed or modified within the wellbore it will have a low impact on the cement job because only importance will be to know.