entrance alert

[digsys]

As can be seen the number of incident variations posted, and enviromental considerations (snow etc), this is unlikely to ever be 100% solved / accurate.
It's looking like my suggestion of - grandpa with a shotgun - is probably the most relaible.

[Brumby]

This is not only a sensor, but also an actuator .

LOL

A little less discriminating than grandpa with a shotgun - but lower maintenance cost.

[NottheDan]

My employer uses a software-based tracking system that seems to work well for them, something like this. (not affiliated, just the first example I found)

[Jeroen3]

This is not only a sensor, but also an actuator .

Best solution. Even works when the power goes down. Although you might want another brand.

[darinsquared]

Two star pickets, some decent fishing line and a cow bell, that's all I have to offer up today.   

I was contemplating something like this but a little more technical.  Using a fishing line connected to a switch that would sound an alarm if the line is "disconnected".  If would need to be physically reset everyday.

For one entrance I am now considering a gate.  When the gate is closed it will line up with a proximity switch shutting off an alarm.  I need to figure out a way to open the gate remotely for guests and open it from a vehicle with RFID (or similar method).  Both will have to disable alarm.  All other entrances are too wide for this method. 

I prefer an invisisble barrier.  I am playing with using a straight sound sensor on both perimeters.  If my intruders drive electric cars I might be hooped. 

[C]

According to Vishay you can do 30m for a beam break IR sensor.

No idea how wide your other entrance(s) are, but sliding gates do close very large widths. Gate slides behind current fence as it opens.
Some the gate does not touch the ground. Some have support wheels on gate. Some hang from overhead beam.  Can use cable to pull open or closed.