Abstract: The present invention relates to a method and apparatus for measuring subsidence by evaluating the distance between radioactive markers which have previously been implanted in the formation traversed by a borehole. The apparatus comprises an elongate body containing at least two nuclear detection assemblies which are spaced apart along the longitudinal axis of the body. Each detection assembly inludes at least one position sensitive detector for detecting position along the longitudinal axis of the body. Measurements are performed while the apparatus is held stationary opposite the markers.

Abstract: The present invention relates to a device for placing a radioactive source into a formation through which a borehole passes. The device includes a gun barrel, a bullet containing a radioactive source and engaged in the barrel, and an explosive device for propelling the bullet towards the formation. The device further includes two items which reduce the speed at which the bullet, when fired, penetrates into the formation. These items are a spacer, placed in the barrel between the bullet and the explosive device, and a braking shield placed at the port end of the barrel. The spacer reduces the effect of the gases produced by the explosive device upon detonation. The braking shield is entrained with the fired bullet, the front end of the shield producing drag which also acts to retard the forward propulsion of the bullet. The shield is designed to break away from the bullet upon formation impact, further reducing the kinetic energy of the bullet. The invention is applicable to soft formations.