An investigation to determine the relationship between the depth of contamination within the concrete lining of a nuclear pond and the ratio of x-ray to gamma-ray photopeak intensity obtained from sodium iodide detector is described. The environment was simulated using established radiation transport codes. Caesium-137 is assumed to be the sole source of contamination. A detector was simulated across the energy spectrum of a source, and rastered across its location from ±10cm at intervals of 0.5cm. A combination of point and distributed sources were simulated within a concrete structure, as well as in free air to create a comprehensive set of data. The results were then validated against experimental data and demonstrate a high level of agreement. This investigation indicates that the x-ray / gamma-ray ratio has the potential to localise contamination on the surface of the concrete lining; further
investigation is required to locate the contamination in three dimensions.