The second one is charging hourly. We tend to prefer hourly fees on projects that are particularly open ended or very small scope. Our hourly rates are currently between $60 and $120 per hour depending on who is doing the work.
The word sonder exists, though. It means to probe, and in a more specific use, to test the depth of something. The word sound (as in the water feature) is a cognate, both related to the Old English sund meaning to swim, or the sea. I can't really think of an English equivalent that's specific beyond mere awareness.
Sonography is a non-invasive medical procedure that uses the echoes of high-frequency sound waves (ultrasound) to construct an image (sonogram) of internal organs or body structures. In sonography, a transmitting device (the transducer) sends out high-frequency ultrasound waves. Harmless sound waves, which contain no radiation, bounce off the surfaces of the object they hit. The reflected sound forms an echo which is visualized on the screen.