Radiometric method - Geophysics for the Mineral Exploration Geoscientist - page 204

for a moving sensor. Topographic variations within the
circle or strip could change the detector height signifi- -
cantly, particularly if they form a large portion of the
circle/strip. For an airborne survey, terrain rising up to
the survey platform (hills) will reduce the investigation
area, and falling terrain (valleys) will increase the area.
These de nitions of the field of view are based on the
geometry of the detector and sampling system, and
assume that the radioactive source is uniformly distrib-
uted. However, the relative contributions of different
areas of the ground also depend on the distribution of
the radioactive sources. A strong source of radioactivity
located outside the nominal field of view can be detected.
The detection of smaller and weaker sources requires
closer-spaced measurements at lower survey height. It is
possible to calculate the theoretical response of zones of
increased or decreased radioactivity of a given shape, size
and position relative to the survey line, and use these for
survey design. The presence of other variable factors, such
as overburden thickness and moisture content, reduces
the usefulness of these calculations. Often, in an explor-
ation programme survey, speci cations are optimised for
the acquisition of other types of data, such as magnetics
or EM, with space and weight limitations within the
survey aircraft also a constraint on detector (crystal) size.
Consequently, the speci cations may not be optimal for
the radiometric data.

geophysical surveying typically use an integration period
of 1 s. During this period a fixed-wing survey aircraft
travels about 70 m and a helicopter around 30 m. It is
instructive to calculate the field of view for a typical semi-
regional fixed-wing airborne radiometric survey, con-
ducted 60 m above the ground with survey lines spaced
200 m apart. The length of the strip-of-investigation (see
Section 4.3.2.3 ) is 70 m and the width of the strip-of-
investigation ( Fig. 4.9c ) for measuring 50% of the radiation
from an infinite source would be approximately 75 m
(1.25 60 m), meaning that the measured radiation comes

from an area of 5250 m 2 (70 m 75 m). Furthermore, 90%

of the radiation can be measured from a strip whose width
is around 4.0 times the survey height, i.e. 240 m, or
extending 120 m each side of the survey line ( Fig. 4.9e ),
an area of 16,800 m 2

240 m). The strong bias
towards sources close to the survey line is evident, but note
how this combination of line spacing and height results in
poor sampling of the region between the survey lines.
The fact that each data point represents an

(70 m

of
the area investigated should be borne in mind when inter-
preting radiometric data, although in fact this averaging
can be useful because it reduces the in uence of insignifi- -
cant localised sources. Note that the along-line sampling
interval for radiometric measurements is much larger than
for magnetic measurements (see Section 3.5.3.1 ) where the
sampling interval of typically 0.1 s translates to a measure-
ment approximately every 7 m for a fixed-wing survey and
3 m for a helicopter survey.
Climate and weather are other important factors
affecting radiometric survey practice. The accumulation
of water on the ground surface, in the soil and in the
surface rocks, and high levels of water vapour in the air,
increase attenuation of the radiation. It is prudent not to
conduct a radiometric survey during periods of rain.
Normal survey practice is to allow the survey area to dry
out for several days following rain before conducting the
survey. Of course, in tropical and other areas subject to
large and almost continual rainfall, and where ambient
humidity levels are high, the survey must be conducted in
the prevailing conditions and the perturbing effects of
humidity, rain and surface water are accepted as unavoid-
able form of environmental noise.

' average '

4.3.3 Survey practice

The size of the detector crystal has a significant influence
on radiometric survey practice. For measurements made
on, or in, the ground, a stationary measurement can be
made over a large integration period, possibly several min-
utes, in order to minimise measurement error, albeit at the
cost of longer data acquisition time. This is particularly
advantageous when the crystal volume is small and of low
sensitivity, or when high-sensitivity measurements are
required. For reconnaissance work, it is often convenient
to select a small integration period, say 1 s, and traverse the
area in search of of elevated radioactivity worthy

of further, more detailed, investigation.
For the case of mobile platforms, the along-line sam-
pling is determined by the instrument

' hot spots '

' is integration period

4.3.3.1 Downhole measurements
Gamma-logging normally measures just the total count,
although spectral measurements can be made. The

and the platform s speed over the ground; the moving

platform travels a signi cant distance during the integra-
tion period, and this reduces measurement precision and
resolution of ground features. Instruments for airborne

'

γ

-log is
a localised measurement with the response coming from