Cave finder !

[BENZINAS]

Hi to all , i want to tell me if can somebody built me a long range cave detector or with short range , something simple not gpr , a small device which i can work it easy , a device who can see a closed cave with rocks ( sealed ) to see the empty space behind the rocks , many caves have closed entrance and i'm looking a device to show me where those entrances are , i know that's dificult but if anyone knows to built or to show me where i can buy such a detector i would be greatfull , any help-opinion accept !