Borehole fluid flow monitoring apparatus

A well logging sonde carries first and second electro-acoustic transducers (23,33) and control means (52,54) operable to control the transducers so that during a first measurement step the first transducer (23) transmits a pulse and the second transducer (33) receives a pulse that has passed through the fluid and so that during a second measurement step the second transducer (33) transmits a pulse and the first transducer (23) receives a pulse that has passed through the fluid. Signal means (60,64,69) provides travel time signals representative of the travel times of a pulse between the transducers in the two measurement steps, and deriving means (80) at the surface is connected to the signal means through a cable (56), and derives a signal representative of the velocity of the fluids in accordance with the travel time signals. The sonde may also carry caliper means (43) to measure the borehole diameter, and the deriving means can then also derive the volume flow rate of the fluid.