Aikons, "15 meters" is about 49 ft. deep! Why/what the heck do believe that some treasure is .... of neccessity, buried that deep? You'd need heavy equipment to dig a hole that deep! And if this is someone's stashed/hidden cache of olden-times, then why oh why oh why would you think it needs to be that deep? I mean, whether it was put at 3 ft. (1 meter) or 30 or 40 ft, all that would matter is that the surface is indiscernable.
And what size object do you think this is? Individual coins? A jar of coins? A chest of coins? A mack-truck sized object full of coins? No, there's no detector that detects "49 ft. deep", unless you're talking a volkswagon sized object.
Let me venture a guess here: You're not from the USA right? Such is the lore and superstition of certain cultures. That they believe that goodies are of necessity always buried at insane depths. And humorously, when you show them that there is no signal forth-coming from a 2-box unit in the floor of the cave they brought you to (or wherever), it's NEVER because there's not a treasure there. It's always because "it must be deeper". So if you dig 10 ft. and nothings there, well then by golly, it must be 20 ft. deep. And if you dig 20 ft. deep and nothing's there, then by golly it must be 30 ft. And so on, and so forth, till you're miles deep. But at no time is there *not* a treasure there mind you.