Geology Reference
In-Depth Information
An observer is an important possible source of spurious radiation, es-
pecially if the sensor is carried in a backpack. In these circumstances the
absorption of radiation by the observer's body must also be taken into ac-
count, usually by direct experiment. Watches with radioactive luminous
dials are now rare, but compasses need to be carefully checked. Obviously,
a calibration source should not be carried.
A small amount of radioactive material is included in the vacuum cham-
bers of some gravity meters to prevent build-ups of static electricity on the
springs. Radiometric and gravity surveys are occasionally done together,
and absurd conclusions have been reached.
4.3.4 Recording radiometric data
Because gamma radiation is strongly absorbed by both rock and soil, com-
prehensive notes should be taken during radiometric surveys. Radiation
comes from a very thin surface layer, and a source with a lateral extent that
is small compared with the distance to the detector will produce only a small
anomaly. If, on the other hand, the source is extensive and at the surface, the
height of the detector should not greatly affect the count rate. Generally, this
condition (2
π
geometry) is achieved if the lateral extent of the source is ten
or more times its distance below the detector. Some other possible source
geometries and factors for correction to standard
2
π
values are shown in
Figure 4.2.
Source
geometry
is always important in radiometric surveys and espe-
cially so in assay work. Any departures from 2
π
geometry must be noted,
together with details of soil cover. If bare rock cannot be seen, some attempt
should be made to decide whether the overburden developed
in situ
(and is
therefore likely to be radiometrically similar to the bedrock) or was trans-
ported into place, and to estimate its thickness. Weather conditions can also
be important. In particular, since absorption is much greater in wet than in
Figure 4.2
Geometries and correction factors for radiometric surveys over
ground with uniform concentrations of radioactive elements.
