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The rapid and accurate quantification of radioactivity in the ground is important in many instances to enable appropriate decisions to be made about limiting its use. In-situ gamma spectrometry has been shown to be a potentially powerful technique for providing this information but its accuracy is dependent on knowledge of the activity variation with depth which is generally unavailable. Methods that allow this distribution to be rapidly determined have previously been published but all rely on a fixed depth-distribution model which is unrealistic. A novel technique using submerged collimated detector measurements termed the "submerged detector method" (SDM) has been proposed, and theoretically assessed in earlier work and has given promising results. In this work, the SDM is experimentally assessed at 19 sites in Wales, U.K., of variable /sup 137/Cs activity levels and profile shapes. Practicality and accuracy both in the derived activity per unit area and the profile shapes were assessed against the best of the other methods. The results confirm that the SDM gives more accurate results with the derived activity per unit area on average within /spl sim/40% of the true value and the depth distributions being more accurately determined.