From one of the prior msm articles, it was shared that the initial search radius of 8 miles was recommended by experts at The National Center for Missing & Exploited Children (NCMEC) & The Federal Emergency Management Administration.
I realize that the primary initial search area works out from the abduction site in most cases, unless there is cell phone pings or other info that indicates specific areas to be searched.. Since JM is forensically linked to MH's abduction and her remains were located 10 miles from the abduction site. Depending on his window of opportunity in HG's abduction, up until her body disposal. It would be logical to conclude that HG's remains will likely be located much farther than 10 miles, and likely in an opposite direction and in a different geographical area... jmo
http://www.wset.com/story/26797046/update-search-for-hannah-graham-put-on-hold-by-weather
<Sniped from article>
Longo says once that 8 mile radius is covered, they will move the search two miles out.
He says the basis for this logic comes from professionals from the Center for Missing and Exploited Children and the Virginia Department of Emergency Management who say most missing people are discovered within a five to eight mile radius of where they were last seen, but after they've finished searching that radius thoroughly, they will follow investigative leads and tips to just search specific areas.