Patent ID: 8902704

Claim:
A method for logging the location of a fluid depth boundary layer in a wellbore during deep drilling comprising the steps of: emitting, by means of a signal generator, an input acoustic signal at the ground surface of the wellbore, said acoustic signal generating pressure waves that travel downward in the wellbore; said acoustic signal emitted into a gaseous medium within the wellbore that is disposed above a wellbore fluid that is non-gaseous; receiving a return acoustic signal in which the pressure waves traveling into the wellbore are reflected at least at the fluid depth; said step of emitting including generating a signal pattern having a predetermined, time-variable frequency spectrum; identifying features in the return acoustic signal as reflections from respective pipe joints that are disposed along the wellbore by sensing the return signal patterns; identifying a feature in the return acoustic signal as a reflection at the boundary layer within the wellbore; analyzing the reflected pressure waves by calculating, from the input acoustic signal and the return signal patterns by correlation in the time-variable frequency spectrum, the number of pipe joints above the boundary layer, and from it, in turn, the distance between the signal generator and the boundary layer.