Underground storage caverns are widely used for the bulk storage of petroleum products, in particular, crude oil. These caverns can store millions of barrels of hydrocarbon liquid safely for long periods of time at low cost. The caverns are accessed through a casing in a borehole down to the cavern. The lower end of the casing opens into an upper region of the cavern termed the chimney. The chimney provides a transition from the casing into the cavern body. The lower end of the casing has a structure termed a casing shoe which seals the earth formation around the casing from the interior of the chimney. The chimney region needs to be periodically inspected to determine if it is stable. The salt structure of the cavern is subject to erosion and collapse of sections of the wall. Deterioration of the physical structure of the chimney can also lead to the opening of cracks which may allow the stored liquid to leak into the formations around the casing and the chimney. The invention presents a process of injecting a gas into the well while measuring the gas pressure and optionally measuring the volume of injected gas. The gas drives down an interface between the gas and hydrocarbon liquid. By monitoring the rate of change of the gas pressure, and detecting a sudden decrease in the rate of change, it can be determined when the interface has been driven down to the region immediately below the bottom of the casing at the upper end of the chimney. This determines a critical location for the interface. Further steps of injecting gas and measuring gas pressure are used to determine the profile of the chimney.