Finding the Resonance Frequency of a Piezo
Drive the piezo with a 50 Ohm sweep generator but only sense current such as a 1 Ohm resistor , this will give you I =V/R and large  peak at low series resistance at resonance.

If you know how to measure a resistor by voltage ratios or impedance matching or a current ratio ratio, then it is simple to get min or max resistance.

Just using a 10M scope probe is simply the wrong reference tool for a series piezo application.  A log impedance analyzer is the best way.  

my early experience

When lacking test equipment, in my early days as an Eng, I would  use the scope x sweep to control an FM gen. then either measure current or voltage with a reference R near what I expected. This could easily display series (or parallel) resonant impedance.

Today there are more clever ways.  Here you want high power low Z , so use a low Z current shunt.




  Above  shows a piezo with a log scale of 10 Ohms at top and 20k at bottom using a log impedance analyzer.