Abstract: A method for determining the density of substrata by means of a radiation source K comprising a collimator (1) and of a detector comprising a collimator (3), by which method the direction of radiation may optionally be changed in relation to the direction of detection, and the change of the detected signal may be measured. According to the invention the direction of radiation and the direction of detection are situated in substantially the same plane. By deducting the signal of the desirable depth of measurement from a somewhat greater depth of measurement, substantially only the signal originating from singly scattered radiation is obtained, and as a result it will be possible to measure the density at greater depth than previously. On the basis of a spectrum analysis of the spectrum originating from the measurement, the variation of the density with the depth could be obtained.