We learned that the concentration of natural occurring radionuclides correlates strongly to the concentration of fine material (clay) in sediment. These radionuclides (40K, 232Th and 238U) are present in rocks and sediments since the origin of the earth. Most sediments are composed of sand, clay and organic matter. Due to the differences in chemical composition of these minerals, the concentration of radionuclides varies for each fraction. This relation between the concentration of radionuclides and sediments is established by calibration in the laboratory.

Our wiki page:  Texture of soil and sediment describes the application.