Its fairly easy to get a good quality SLR digital camera and have the IR filter removed. Life Pixel is a company that does this at a low cost for Nikon and Canon cameras. http://www.lifepixel.com/tutorials/infrared-diy-tutorials
Depending on the camera you buy, you could get a 36 MegaPixel image in infrared, which would be extremely high resolution. Since Nikon released the D810, you can pick up a used D800 pretty cheap, and then have it converted to IR.
Thanks Patrick - that is an approach I had not considered. I was thinking more of aerial photography? But I would also need to do some site survey work; in which case this is useful advice. Many thanks for taking the time to respond.