Hi Charles,
I think that what you are seeing is a patch of less positive conductivity, rather than negative. Wet beaches do give a signal due to the salt water content and this signal is normally fairly constant over quite large areas. The threshhold can be adjusted, either manually or automatically, depending on the detector, to cancel out this background signal. If the beach conductivity should lessen for any reason then the threshold will go negative. Things that can cause a localised area of less conductivity are:- a buried rock. Fresh water from a spring, stream or river, percolating through the sand. Water from an outfall pipe. A non porous rock is virtually non conducting and will interrupt the currents set up in the beach. Fresh water is also a poor conductor and will dilute the conductivity of the sea water.
The XL500 has a manual threshold control so one would expect that this would need adjustment from time to time to compensate for beach conductivity changes. Detectors like the Surfmaster PI, Barracuda or Deepstar have self adjusting threshold so that the electronics compensates automatically for normal changes. The other point about the XL500 is that the search coil is unshielded so the capacitance between the coil and the beach will cause a negative signal. This is especially so if the beach is touched with the coil or you brush against wet seaweed. Once the coil is immersed in the water you are OK because the capacitance then stays constant.
Eric.
