Can one use a conventional microphone as an underground acoustic sensor?
You have already asked a very similar question, and I'll give you the same answer. The common method of detecting footsteps using a detector buried in soil is a seismic sensor cable.

The frequencies of interest for footsteps are much lower than the frequencies picked up by a microphone. The energy you want to detect is not really acoustic, it is seismic. I can assure that if it was as easy as just burying a microphone we would have already done it.