Underground storage caverns are used for the bulk storage of hydrocarbon liquids, such as crude oil and gases. The caverns are typically formed in salt formations by dissolving the salt and removing it with a flow of water. The cavern is accessed through a bore hole which has casing and internal tubing with an annulus between the casing and tubing. Some cavern bore holes may have casing, but no tubing. The cavern typically has hydrocarbon liquid above brine with an inert gas above the hydrocarbon liquid. In order to use the cavern, and periodically check it for physical integrity, it is necessary to test the cavern to determine if there is leakage from the cavern or the bore hole. The interface of the hydrocarbon liquid and overriding gas is moved downward by injecting gas into the annulus. Acoustic pulses are sent down the annulus through the gas to determine when the interface is located just below the end of the casing in the top of the cavern chimney (a reference level) by examining the return reflection pulse from the interface for a polarity inversion. When this is detected, a measured volume of gas is injected into the annulus. After a waiting period, the gas is released from the annulus and measured until the interface is detected by acoustic pulses to again be at the reference level. The volumes of injected gas and released gas are compared to determine if there has been leakage from the cavern. Alternative, the interface can be driven by gas pressure from the surface down to the casing bottom and back to the surface with gas volumes detecting leakage.