Pseudocolour image of the ratio between uranium and thorium within the upper 20 centimetres of the ground. This image was generated using normalised input grids to avoid ‘divide by zero’ errors. Cooler colours indicate lower abundances of uranium relative to thorium and warmer colours represent the opposite. Variations in U/Th ratio are caused varied mineral compositions in host rocks and soils. This statewide image was generated by merging many individual airborne radiometric surveys.