Sound travels in air with a known speed, a little over 1000 feet a second.  In a thunderstorm, every five seconds you count between seeing the lightning flash and hearing the thunder means about a mile between you and the lightning bolt.

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If you generate a sound at the top of a well, it will travel down the well casing to the water level, reflect off, and return an echo back to the top.  This is what a sonic well level instrument does. The length of time between the original sound and the return echo is used to calculate the water surface depth.  

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The advantage of a sonic instrument over a conductivity probe, which takes a single level reading, is that it can make repeated readings, once a second.  This allows the water level drop during pumping to be tracked, in most cases.  The data logging feature of the instrument means it can be installed and left to record the water level for a day to observe in more detail how the pattern of water use, and the condition of the well, is affecting the well level. 

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If the minimum level gets close to the pump, that's one indication of the possibility of running out of water.  When the pump stops, the sonic instrument measures how fast the well refills.  If the refill is slow, that could indicate a low yield well.  A low yield well could run out of water with too much demand, even if there is plenty of groundwater.

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For all the wells ECMC has tested to date, the sonic instrument has worked well.  Its limitations are:

1. The top of the well must have a hole, called a vent opening, for the sonic instrument to be installed.  Most wells have one, but a few older ones do not.

2. If there are solid obstructions in the well between the sonic instrument and the water, they will return false echoes.  If their depth is known, the instrument can be instructed to ignore these false echoes.  However, if the obstructions block too much of the echo from the water surface, the instrument will not be able to read the water level. 

3. If the well casing is rough or porous, it may dampen the sound conduction and weaken the water surface echo below the point of detection.

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