Abstract: A novel optical fiber structure is described which includes an end thereof for transmitting optical signals and substantially reducing reflections of optical signals thereat wherein the fiber end has a substantially flat end surface disposed at an angle to a plane perpendicular to the transmission axis of the fiber equal to or greater than sin.sup.-1 (NA/n) where NA is the numerical aperture and n is the index of refraction of the fiber core material, and the fiber outer surface has adjacent the end surface a flattened portion of preselected depth and length whereby substantially all paraxial and meridional optical signals propagated along the fiber axis are transmitted through the end surface and skew rays transmitted along the fiber are substantially attenuated near the fiber end.

Abstract: Magnetic material, such as discarded magnetic tape, is ground into small pieces and mixed with well drilling fluids. When the drilling fluids are lost in porous ground, cracks or fissures in the walls of the well, the magnetic tape will accumulate and serve to block the unwanted loss of the fluid. In the event that the fissures are of a size such that the tape will not block the flow, the magnetic properties of the tape may be utilized in combination with a standard well logging instrument to provide an indication of the exact location of the area of fluid loss. At that time, further remedial measures may be taken to block the undesired flow of fluid. For example, a cement slurry may be pumped into the well in the location of the crevices and thus form a permanent seal against loss of fluid.