Water detection in Underground Utility line

We need to listen sounds from underground utility water line 8ft underground UPVC 4 inch dia. Connected with the 1/2 inch UPVC pipe which is also underground but its one end is out of ground.The total length of the 1/2 inch Pipe is 20 meters horizontal along the ground level. We need to listen water sound/vibration in the utility line from the 1/2 inch pipe which is connected in order to start our suction pump when there is water in the utility line.

Your responses will be highly appreciated.