Abstract: A system for determining a characteristic of a stream of gas flowing through a branch wellbore of a multilateral wellbore system including a main wellbore extending from surface into the earth formation and a plurality of branch wellbores, each branch wellbore being provided with a conduit for passage of a stream of hydrocarbon gas from the earth formation to the main wellbore, the system comprising a plurality of sound generating devices, each sound generating device being arranged in a corresponding one of said conduits and being operable to produce a sound wave of selected frequency upon flow of the stream of gas along the sound generating device, the frequencies of the sound waves produced by the different sound generating devices being mutually different, the system further comprising a sound receiver capable of receiving each sound wave of selected frequency.