North Korea’s Punggye-ri Nuclear Test Site: Significant Acceleration in Excavation Activity; No Test Indicators but significant increase in soil excavated by comparing size of "Dec 2013 Spoil Addition" (frame on left) with "Jan 2014 Spoil Addition" (frame on right).
It can be concluded that North Korea will conduct its fourth nuclear test in 2014 and probably from late April 2014 - based on data provided by Johns Hopkins University's US-Korea Institute. The US has a wide range of intelligence platforms to predict the (virtually exact) timing of the fourth test - including satellites (of course) and RQ-170 Sentinel stealth drones deployed in the region.
Agency France Press (AFP) via Australia's 7 News, reported, February 14, 2014 http://au.news.yahoo.com/world/a/21464288/excavation-work-at-n-korea-nuclear-test-site-us-think-tank-claims/ :
"Excavation work at N.Korea nuclear test site, US think-tank claims
Seoul (AFP) - Recent satellite imagery suggests North Korea has stepped up excavation work at its main nuclear test site, but there are no signs of an imminent test, a US think-tank said on Friday.
The images indicate a "significant acceleration in excavation activity" at the remote Punggye-ri test site in the northeast, said the closely-followed 38 North website of the Johns Hopkins University's US-Korea Institute. [see original 38 North article at http://38north.org/2014/02/punggye021314/figure1-punggye-021314/ ]
The debris excavated from a new tunnel at the site appears to have doubled in little over a month since last viewed in December [2013], the institute said.
The purpose of the excavation was unclear, it said, adding that Pyongyang was unlikely to use this tunnel for its next test since two other tunnels at the site already appear complete.
Visible indications of an imminent test -- camouflage netting, special vehicles -- can usually be detected four to six weeks in advance.
Based on the most recent imagery, "there are no signs that a test is in preparation," the analysis concluded.
North Korea staged its third -- and most powerful -- nuclear test [on February 12, 2013] last year after two previous tests in [October 9, 2006] and [May 25, 2009].
Two months later, it boasted that it would restart its five megawatt reactor at its Yongbyon nuclear compound -- the source of weapons-grade plutonium for its previous tests.
Pyongyang's current stockpile of fissile material is variously estimated as being enough for six to 10 bombs.
38 North said in December that satellite images suggested North Korea was following through on "wide-ranging, extensive" efforts to fully reactivate its Yongbyon facilities."
Connect with my article on North Korea's third nuclear test of February 2013 http://gentleseas.blogspot.com.au/2013/02/north-koreas-3rd-nuclear-test-yield.html
Background
For further background see http://en.wikipedia.org/wiki/List_of_North_Korean_nuclear_tests