I'm planning to make a device that's going to consist of two things:
transmitter
receiver
The transmitter will be underground and I need the receiver to be able to localize it and measure the distance between them.
I'm not sure how to start and what to use to be able to do this. I would be grateful if anyone could suggest something.
Electromagnetic waves do not penetrate the ground very well and the propagation is very dependent on the ground composition and amount of moisture, so a simple signal strength is not going to cut it.
Underground communication for things like cave rescue use magnetic inductive loops and these penetrate the ground a lot better than EM waves, but again you have the same problem of propagation. As you would with any seismic methods.
This sounds interesting but as people have said you need to give more information, is this a caving application or are you talking about a buried object? Is this some sort of "pig" in a pipe that you are trying to track? What sort of distances and depths are you talking about?
Metal detectors can obviously localize metallic objects that are not too deeply buried. If the object was of a known mass and material then perhaps the technology could also be modified to give an indication of depth. Perhaps the object could also actively stimulate the detector.