circuit design - Water level detection using non contact sonic sensors

I have been working on a project, where I need to find the water level of underground borewells with a diameter of around 6-10 inches and a depth of up to 100 meters. I have been researching a lot about the methods that I can use for this and after ruling out the contact method of a using a probe I settled for a non contact sonic method where a sound wave would be sent and the time of flight of echo would be used to calculate the distance. Now I was thinking about making a circuit out for it and had a couple of questions in mind:-

1. Which sonic transmitter can I use i.e. a piezo or a simple loudspeaker version?



2. What circuitry would be exactly required to work this out? As far as I know I would produce a PWM of varying frequency until I get an optimum value, fed from a controller and amplify it to feed it to the transmitter.


3. For the receiving sensor I think the condenser mic would serve the purpose, correct me if I am wrong and any suggestions would be appreciated.


4. Where can I find these sensors in order to use them for my project? My local vendors seem to be unaware about any of these.

Here is the link to the discussion a started for knowing about the frequency and spreding angle concepts related to sonic beams.

https://physics.stackexchange.com/questions/408894/sound-ultrasonic-waves-distance-travelled-and-spreading-angle