An ultrasonic scanner tool employs a transducer that excites a target plate at a known distance from the transducer. One side of the target plate forms a chamber that in operation is filled with wellbore fluid. On the other side of the target plate, a chamber is formed containing a vacuum or a fluid with a significantly lower acoustic impedance than the wellbore fluid and the target plate, allowing an improved measurement of the acoustic impedance of the wellbore fluid.