82
of radiation observed in the test sample to the radiation intensity
measured in air. This relationship can be expressed as the count ratio
varying as a decreasing exponential with respect to the density of the
material. The count ratio is defined by the manufacturer as the ratio
of counts per minute observed for the test material to the counts per
minute observed in the magnesium standard (Troxler,1972). The observed
density of the soil obtained from the gamma measurement represents the
wet bulk density (Equation 3-1).
Pwet = A B ln(CR) (3-1)
where:
Pwet : wet bulk density of the soil [g-cm-3
A, B : constants of regression
CR : count ratio dimensionlesss]
count per min in test material
count per min in standard
If the dry bulk density is known, then the volumetric water content
can be calculated by
0 ( Pwet" dry) 3-2
Pw
where:
8 : volumetric water content [cm3cm-3]
-3
Pwet : wet bulk density of soil [gcm-3]
Pdry : dry bulk density of soil [gcm-3]