I built up an inventory of over 20 level 1 xmp bursters, and thought that would be enough to take over a low level portal - not even close. However, I did finally notice that I did different amounts of damage to the resonators depending on where I stood. I started off standing near the portal, but only did about 1% damage to each of the surrounding resonators. But if I moved closer to the resonators, I did up to 8% damage to that one resonator, and a little bit to the others resonators in range.
Is it faster to detonate xmp bursters on the resonators or on the portals for maximum damage?