Active gamma techniques are generally transmission based using an active source (e.g. Cs-137) and detected using a gamma-ray spectrometer. As the gamma radiation passes through the soil, photons are transmitted following Beer-Lambert’s law with signal attenuation determined by the transmission length, mass density and transmission coefficient of the substance.
The transmission coefficient is material dependant (through absorption, scattering etc) and with mass density effects the technique can be used for measuring soil water content [1] and soil bulk density.
Requires an active gamma source.
Research.
1. Chanasyk, D. S. & Naeth, M. A. (1996) Field measurement of soil moisture using neutron probes. Canadian Journal of Soil Science, 76, 317-323.