Pseudocolour image of the ratio between thorium and potassium 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 thorium relative to potassium and warmer colours represent the opposite. Variations in Th/K ratio are caused varied mineral compositions in host rocks and soils. This statewide image was generated by merging many individual airborne radiometric surveys.