7MHz Transducer Measuring Doppler Shift
Signal level directly from the transducer is probably tens of mV at most, and may be getting lost in the scope noise. Try to reflect or transmit from only a few mm and (if not already doing so) use your scope probe in X1 mode, which reduces the input-referred noise floor by an order of magnitude.  

For more serious work you will need an appropriate preamplifier.