The use of aquifers for storing gas underground has posed new problems for the natural gas industry. One does not have the advantages of converting an existing gas field into a storage project. In such cases, the original gas accumulation is sufficient proof that a reservoir exists in which gas can be stored. In the case of aquifers, however, the operator must first find an adequate geologic structure in which a suitable reservoir rock is located. This usually is solved by the application of existing exploration techniques, although the search for an aquifer storage site is normally carried out in much more detail than would ever be considered necessary in exploring for oil or gas. The most critical problem that the operator must face in aquifer storage is that of determining the tightness of the caprock layers overlying the intended storage zone. Having found a suitable site, the operator must attempt to answer the question: "Will gas leak through the caprock?". A number of aquifer storage projects have been hampered by migration of gas out of the intended storage zone.