From AMS Glossary
A technique to determine the water level in a well, lake, or any water body with a free surface, using the principle of measuring back-pressure in a pressurized air line that extends downward below the water surface to a known elevation.
When air under pressure is introduced to the air line, it forces the water in the line out until the air bubbles from the bottom of the tube. At this time, noting the air pressure in the line, the pressure is converted mathematically into a height-of-water volume. Air-line sounding is a very effective means of measuring water levels in very deep wells.