RIASSUNTO
The idea is to send a wave down the hole from surface – either hydraulically through the annulus mud column, or through the string mechanically. This wave would travel downhole with known speed, will be reflected at a reflector, and travel back. The travel time is measured and with travel speed the reflection depth calculated. For a hydraulic pressure pulse down the annulus, this reflector could be the top of a gas bubble, of a cement column, or of a fish. (For a torsional or axial pulse along the string, the reflector could be a stuck point or the broken-off end of a twist-off.)