So did you have the county come out and give you an idea of underwater reservoirs? That can give you a place to start. Then get a water loving tree...willow and witch hazel are the best...and get a good Y shaped branch. You want to hold it loosely. I prefer fist up, thumbs toward me...and start walking... when the Y falls, water. Mark...do again and again until you pinpoint the strongest res. Old timers can do it once or twice and know. But this is a tried and true method