Existing ccable you simply use a 4-5W Rf transmitter, probably around 13MHz, which is in the ISM band, and a loop antenna for the receiver. Loop antenna is small at 13MHz, so will detect well. The air hose to blow through the pipe is better, you have to block off the house end, with the pipe in there (rags soaked with water stuffed into the hole, and rammed tight work well) and blow the air through, which at least gets you to which pipe in the pit is yours. then you use a heavy fishing line, with a long flag of rag attached to the end, and blow it through, using a small diameter pipe to pass through the rag fill. Thin pipe just a bit bigger in diameter than the line, so the air loss is not too bad, and you will get the flag at the pit after a few minutes. When pulling the fibre optic cable back attach a second fishing line to it as well, so as to leave a pull for any further cable upgrades.
If the compressor you use is not enough air flow rent one for a day, it is a lot cheaper than trying to dig up 100m of yard and lay a new conduit.