Apparatus for well logging telemetry.

Apparatus for sending pressure pulses through drilling fluid in a drill string in a well bore includes an assembly adapted to be lowered from the surface through the drill string to a position adjacent the lower end of the drill string. The external dimension of the assembly is substantially less than that of the internal diameter of the drill string, so there is always a substantial clearance between the assembly exterior and the drill string interior to permit a substantial flow of drilling fluid down the drill string, past the assembly, through a drill bit, and into an annular space between the drill string exterior and the well bore. The assembly includes an internal bore to permit drilling fluid to flow through the assembly. Means are provided for intermittently restricting flow of drilling fluid through the assembly bore to send the pressure pulses to the surface in response to the magnitude of a downhole condition to be measured. The assembly includes means to permit it to be retrieved by a wire line from the surface and without removing the drill string from the well bore.