A caliper-behind casing apparatus and method determines a location and size of a washout (i.e., a void) behind casing in a borehole for a salt cavern used for storing hydrocarbons. The cavern and tubing in the casing are filled with brine. Gaseous nitrogen is used to fill an annulus between the tubing and the casing above and below a casing shoe to obtain image responses from operating a pulsed neutron logging tool in the tubing along the borehole. Analysis of burst ratios of near and far detectors on the pulsed neutron logging tool from these passes is used to detect and estimate a void or washout in the formation behind the casing.