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.


  • \$\begingroup\$ There was a question about water level detection on here recently - may be worth you searching for it.... \$\endgroup\$
    – Solar Mike
    May 31, 2018 at 5:25
  • \$\begingroup\$ I did searched about them and found a couple of posts however there were some suggestions of a contact type or placing a sensor at the bottom, however my problem is that the borewell can only be accessed from the top and i need to measure the level from there, i have attached an image in the link above if you want to take a look. so i can't attach any sensor to the walls or at the bottom, this makes this project difficult and rather interesting. \$\endgroup\$ May 31, 2018 at 5:33
  • \$\begingroup\$ another approach would be to find the resonant frequency of the bore. \$\endgroup\$ May 31, 2018 at 8:56

3 Answers 3


The easiest way to get started is to cobble a proof of concept (PoC) system out of consumer audio speakers, amplifiers, microphones.

To rephrase what you want to do: send a sound pulse from the top of long and narrow well, the pulse would travel down the well, reflect from the water level, calculate the distance to water from the round trip time of flight.

It's challenging - if at all possible - to make this work in a thin and narrow wells. The challenge is that the sought reflection from the surface of the water will be neither first nor the last reflection you receive. The sound will not travel in a narrow beam like a laser. It will propagate as a cone. At some point, this cone will become wider than the diameter of the well. Some sound is going to reflect back from the walls, and that's going to be the first reflection you receive. You will continue to receive reflections. At some point, you will receive the reflection from the water at the bottom of the well. The reflections will not stop there. You will continue to receive multipath reflections.

Record echoes from well with known diameter and water level. Maybe there will be a consistent feature in the echos which you can clearly identify as the water surface. Maybe you'll see more if get several echoes at several frequencies from the same well.

p.s. If the wells are always straight, a laser rangefinder might work.

  • 1
    \$\begingroup\$ Having played with ultrasonic rangefinders, I bet that you get excellent results in a narrow well. At least if you choose the right wavelength. A pipe acts as a waveguide, causing the sound to experience total internal reflection. You may want to use a narrowband pulse, as different frequencies may have different propagation modes (and so different velocities). \$\endgroup\$
    – jms
    May 31, 2018 at 6:45
  • \$\begingroup\$ The idea of a PoC si what i am trying to obtain, starting with a piezo buzzer and trying to operate over several frequency, however i thought of the idea of using a laser rangefinder however what i found was they rather penetrate the water surface and follow a refraction rather than a reflection and the ratio of reflected wave is comparatively less to be detected. \$\endgroup\$ May 31, 2018 at 7:20
  • \$\begingroup\$ @jms Can you suggest any frequency range that i can start from for getting a better spreading angle i..e. a lesser angle. I also found that while increasing frequency we can achieve a lesser angle but the range gets sacrificed. \$\endgroup\$ May 31, 2018 at 7:23
  • \$\begingroup\$ @shwetankvishnu I'm really not qualified to be any more specific (read: further advice might lead to more harm than good) \$\endgroup\$
    – jms
    May 31, 2018 at 7:49
  • \$\begingroup\$ for 4" wavelength the frequency is 3300hz, ultrasonic would be too high for the bore to be a good waveguide, and remeber the bore is not empty, there's a power cable and a water hose/pipe in there too. \$\endgroup\$ May 31, 2018 at 10:04

I see a lot of problems with this concept:

  • first how smooth are walls? If they aren't you can get many unwanted echos
  • attenuation will be main problem, especially with reflection as main signal
  • speed of sound will change significantly with temperature and humidity
  • it will be loud

Now to solutions/your questions:

  • I would use loudspeaker (easier to get it working)
  • for signal use low frequency sine sweep, for example 1-5 kHz (and use cross correlation for detection)
  • I would use pc with sound card and amplifier
  • if you use piezo you can also use him as microphone, otherwise condenser mic is good
  • for loudspeaker, microphone and amplifier I would look in music shop or similar

P.S. I don't know your exact situation, but I would try to solve this problem by dropping pressure sensor to bottom of bore and read absolute pressure there.

  • \$\begingroup\$ Thank you for the answer, I was a bit confused between the piezo speaker and the magnetic coil based but i think i would go with the latter, and use a condenser mic along. I thought of using a microcontroller to generate the wave however i think i can start with a smartphone with amplified output through the 3.5mm to my speaker unit. The pressure sensor is an option but it is again a contact type measurement. Thank you anyway for all the suggestions, appreciate it..... \$\endgroup\$ May 31, 2018 at 10:32

A few months late, but if you are using sound waves in a narrow well you could treat it either as a waveguide and use either time-domain reflectometry or as a resonant pipe and use the resonant frequencies to estimate the distance.

Given your pipe diameter sounds of ~1.5kHz and higher will propagate down the bore, much higher than this and you will start dealing with multiple modes of propagation, much lower than this and you have a resonant cavity instead of a waveguide.

I would put a water-resistant speaker sealed against the pipe at the top and a water-resistant microphone further down the bore at a known distance, so that you can better estimate and correct for sound propagation speed. Given changing temperatures and humidity levels these will not be constants. Remember that a speaker emits sound from both sides, so it either has to be in an enclosed rigid box or its top surface has to be in open air while the driving surface seals the well.

  • time-domain reflectometry.

Emit a single pulse (e.g., one or a few cycles of a 2kHz sine wave) and measure the received signal. This would consist of several attenuated repetitions of the transmitted signal. An auto-correlation would give you the time delay between the multiple echoes, the delay between the transmitted signal and the first signal will provide you with the needed information to calculate sound speed.

  • resonant frequencies

Emit a continuous sinusoidal frequency that is slowly swept through a few octaves (e.g, 100Hz-1.6kHz). Note that in this case the intent is to produce a standing pressure wave; so, just like in an organ, the wavelength is given by the length of the pipe.

The pipe will act as a resonator that will produce a comb of response peaks at frequencies directly proportional to the wavelength of the signal supported by the pipe. Although some resonant frequencies will be the same, this comb of frequencies will be uniquely determined for any length of pipe, and its maximum common denominator will give the actual pipe length.

As before, the delay between the excitation and the signal with respect to the distance between the microphone and speaker would allow you to calibrate the measurement.


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