Well logging sonde with shielded collimated window

Claim 1

A well logging sonde adapted for generating indications of the characteristics of the formation surrounding a borehole in which a borehole mudcake may have formed, the sonde comprising: a first section including source means for irradiating the formation with gamma rays; a second section including; a shielded housing having a collimated window facing the formation, the window being substantially transparent to gamma rays including low energy gamma rays of energies below 100 keV; a near detector disposed in said housing in the path of said collimated window, the window being disposed so that the high energy gamma rays reaching the detector have been singly scattered in a first region in the formation primarily beyond the borehole mudcake; and means responsive to said near detector for generating signals representative of the density of said first region and of the photoelectric absorption of said formation surrounding said borehole; wherein said shielded housing having a collimated window comprises: an inner cylindrical shell fabricated of a gamma ray opaque material and having a conduit formed in its wall at an angle with respect to its axis; an outer substantially "U" shaped metallic outer shell partially enveloping said inner shell and having an axial length L, the open ends of said "U" shaped outer shell extending outwardly and symmetrically with respect to said inner shell conduit; an arcuate closing member, fabricated of a strong, substantially gamma ray transparent material and sealingly secured to the open ends of said "U" shaped outer shell; and an arcuate pad fabricated of a gamma ray opaque material and disposed about said closing member and about said open ends of said "U" shaped outer shell, said arcuate pad having a conduit formed at the same angle as the conduit in said inner shell, said arcuate pad conduit being aligned with said inner shell to form a collimating window interrupted by said closing member.