your project
I put a driveway sensor down in the ground eons ago and my first choice was a piezo disc. My first problem was protecting the disc from the elements, while allowing the vibrations to get through. The piezo disc has a great high frequency response and lousy low freq response but a lot of voltage output. So to make it bullet proof you have to protect it with something and that invariably lowers the resonant frequency and at the same time cuts the output.
If you go that route - "Plasti-Dip" tool coating works best - minimum extra mass added to the transducer, minimum fuss. Mineral oil is great too, but a pain to contain - but keeps the high frequency response there. Paraffin wax is pretty good but cuts the highs - but is quick and easy and reversible. Epoxy is bullet proof, but kills the output (but I cast mine into a block and a coating might be better).
Something like latex mold material might be the best of all choices, but I didn't have any back then.
The "target" foot falls or vehicles invariably produces a lot of low frequency vibration in the ground and not a lot of high frequency - it is the nature of the medium, it just transmits low frequency much better than high. So the piezo isn't the ideal sensor - easier and cheaper to match the signal you're trying to capture with a single sensor. than resort to lots of sensors with very limited range.
One thing I did find with the piezo devices was "critters": hibernating snakes, "ground bees" in summer, and moles or voles foraging through the leaves would trigger false alarms.
If you are doing sensor research I'd like to hear your experiences.
I think a central geophone and three or four outlying sensors could "watch" more than an acre of land with no problem - but what kind of range were you looking for? Pretty sure a rabbit couldn't get through.
I built my own geophone(s) with a bobbin from a cup core transformer, and a small cylindrical magnet. Two springs I wound from some SS fishing leader suspended the magnet, and I put it in a small jar and sealed the leads with some hot melt glue. It worked and never had a problem.
Before you jump onto the piezo solution . . . look at others. It only takes a few minutes to spin wire on a bobbin and minimal electronics to get a good (low frequency) signal output. If one sensor can replace many, and do a better job . . . I was getting 500 mv of signal with someone walking over the transducer, and 30 mv peaks at 100 feet away. Deeper sensor and better range - but at lower output.
That was a quick Google check. Any cheap sources?