The present disclosure relates to apparatus and methods for evaluating an earth formation though a drillstring during tripping of the drillstring, particularly locating an area of circulation loss in a wellbore. The apparatus may include a neutron source and at least one radiation detector, which may be disposed on a logging instrument, configured to be dropped or pumped into a drillstring at the end of drilling. The logging instrument may be configured to make measurements through a homogenous portion of the drillstring while the drillstring is being tripped. The apparatus may include a memory and processor for logging data for later retrieval. The method may include adding a radiation absorbing substance to drilling fluid in the wellbore; exposing the earth formation to neutron radiation; and making a plurality of measurement indicative of radiation measurements of sigma or distinct energy peaks along the wellbore.