DIY 130kHz RFID antenna design for reading tags in large area
Similar, not identical, see RFID Antenna Array Also search site for RFID. 

Also see Microchip AN710

Polling multiple readers would be easy. 

At 132 kHz, switching antenna loops would be "easy enough" using eg MOSFETS but it is unlikely to be needed. 

A long loop antenna tuned to 132 kHz should be easyish.
At high frequencies a distance of 2 metres would be large compared to the wavelength of the signal being used and building it could cause problems. However, at 132 kHz this is not the case. ie
  Wavelength in metres is given by: wavelength = (300 / Mhz) metres
 So here wavelength = 300/0.132 ~= 2300 metres = 2.3 kM
 so the wavelength is very long compared to the coil length.

so producing a resonated coil should be easy. Probably a few turns 2m long and forming a rectangular loop perhaps half way up the tunnel height = at the tunnel max width if this is a round pipe, so that the frogs travel under it at a pipe radius or less away.