A method is disclosed for determining the thickness of cement that has been placed in an annulus between a casing string and a wellbore penetrating a formation, wherein the method includes the steps of: logging the amount of radioactivity emitted by a radioactive material in the formation prior to emplacement of the casing and cement, including in the cement an amount of the radioactive material, placing the casing and cement in the wellbore, logging the amount of radioactivity emitted by the radioactive material and transmitted through the casing and calculating the amount of cement in the annulus by determining the amount of the radioactive material in the annulus.