Hi Shokoy,
I have no experience with them, but I've seen a few "box" type detectors online that claim to go down 20 feet. One is the White's 808 Cache Relic Hunter, another is the Fisher Gemini 3. A more expensive option might be a Pulse Star II Pro which is a 1-meter or 2-meter square coil, held by one or two people while walking, but the jungle overgrowth gives reason for a pause. If you're just starting out, consider something used on eBay. Good luck on your adventure!
Shokoy, first welcome to Tnet! 30' is way in excess of what any metal detector will do, no news there. Being in a jungle setting makes things even more difficult.
Dowsing right here on our Earth might not be that crazy of a thing to try especially as a person does not even have to purchase them as you can just make a set
on your own. Dowsing Rods are simply bent metal so that might include coat hangers being cut and reshaped (you need to confirm this thought on the wire Diameter).
Just read up on it so you understand how to prepare your mind before testing out your rods. Best of Success............63bkpkr