TM 5-6635-386-12&P
Figure 1-4. BS and AC Positions
NOTE
The tester’s raised (collimated) BS position blocks shallow gamma rays to provide deeper reading for soils. Lower AC
position permits utilization of shallow rays to decrease depth of effective reading for shallower materials. (Surface
roughness effect increases with AC position, however, and site prep requires care).
If the radioactive source is NOT collimated, the source
being positioned at the bottom of the tester permits
gamma rays to stream at an angle backwards towards
the detector, the tester will pick up 90% of its returned
radiation from approximately 1.5 - 1.75 inches deep.
Both positions will read the same overall depth for 99%
of its returned radiation.
Thus, if the intention of the reading is to read a soil
embankment of more than 2-inches thickness, then
either BS or transmission should be used to insure the
deepest possible reading.
If the density of a thin lift overlay is desired or any lift of
less than 2-inches thickness, then it would be desirable
to use the AC position and to place the additional surface
seating care required in these shallow positions.
Figure 1-5. Effective Depth-Inches
1-17. TRANSMISSION ROUGHNESS ERROR
Transmission
is
inherently
an
accurate
mode
of
measurement in all respects. It is a linear measurement
with depth and is not fooled by varying densities near the
soil surface and is comparatively immune to surface
roughness problems.
1-14
