Very nice, an experimenter's wet dream. 
As a side note for possible improvement, I recall playing with an exterminators Ultrasonic kit that Polaroid sold in the early 80s. In their receiving circuit they added a AGC (automatic gain control) function to the op-amp stages. Seems the return echo amplitude decreases with distance, so their amp stages would start up with a lower gain and increase gain over time to better detect the received signal and have better signal to noise ratio overall. I'm sure this would increase distance capability but at some added circuitry complexity.
Lefty