Cleanup at the Savannah River Site brings with it the need to clean out and close down the radioactive waste tanks constructed in support of the fuel rod dissolution process. An innovative technique for assaying waste tanks has been developed at the Savannah River Site. The technique uses a gamma detector in the annular space between the inner and outer walls of double walled tanks. Unique shielding, counting electronics, and deployment techniques were developed to facilitate mapping interstitial liquid levels, sludge layers and other structures in the waste tank located near the tank walls. The techniques used, results, and lessons learned will be discussed.