I don't believe that it is possible, that there would be no attenuation over distance.
We never said there wouldn't be attenuation. Keep in mind you're talking about something that moves at, literally, the speed of light, though. But in the one real world test that we have for evidence, in 1962, a 1.4 megaton bomb was detonated at 250 miles altitude over the Pacific Ocean. The calculated figure for the damage caused on the island of Hawaii was 5,600 V/m - volts per meter of wire - at a distance of 900 miles with the peak voltage occurring at 97 microseconds after detonation.
At that distance, approximately 300 streetlights blew their fuses, hundreds of burglar alarms went off, power lines went dead, and thousands of circuit breakers tripped, disrupting electrical power across most of the islands. In addition, about 1/3 of all low earth orbit satellites sustained electrical damage, even at a distance of 10,000 miles from the actual blast, due to the radiation traveling around the Van Allen belts and ultimately were destroyed by it.
Two reasons this wasn't more severe are simple. As you noted, the inverse cube law does apply. Second is there wasn't much chance for actual reflected energy to do damage. Most of the devices intended to measure the blast were actually destroyed because it was more powerful than anticipated.
And you can see why - you pop an airburst the same size 250 miles up over Kansas City. The effects of that single bomb would be felt all over the country. Now again in looking at that map, take your compass, put the point on Kansas City, the pencil point on Chicago, and draw a circle. If 900 miles was 6 KV/m, then 450 miles is 24 KV/m. (Again, factor of 4.) Indianapolis, Nashville, Memphis, Jackson, Little Rock, Dallas / Fort Worth, Oklahoma City, Denver, Minneapolis ... no ground damage at all. But St. Louis, Des Moines, Wichita, and many more take a 96 KV/m hit. (It would take 4 total airbursts to take out the entire continental U.S.)
For those of you down under, an airburst over the middle of NSW takes out everything on the coast from Brisbane to Adelaide. An airburst of Nagoya, Japan, takes out almost the entire island. An airburst over Liverpool takes out all of the UK and Ireland.
Oh, and that's assuming the 1.4 megaton bomb, and takes 9 of them. Our biggest bomb we currently stock is 1.2 megatons, so it may take 12 - 15 of them to cover the same territory. We have about 600 of them. Isn't that a reassuring thought?