From a technical side about all I can think you could do is to broadcast a signal back towards them at the same frequency as they are using. Right now there are only three Si frequencies in use to worry about so as long as you were broadcasting at all three frequencies at the same time you should be good. As mentioned you would have to be there or have some underwater battery powered device at each brush pile (if it were above water they could see it and want to see what it was…). I would recommend a very wide beam so that you would not have to be as exact in your aiming of the beam towards the unwanted sonar device.
Of course if you did make such a device and it worked, all they would have to do is mark a waypoint of save a trackplot and follow it back later when you weren't there to see what you were trying to hide...
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