The low penetrating power of alpha and beta particles make them unlikely to be detected during field surveys. Instead, most radiometric surveys concentrate on detecting gamma rays. The low attenuation rate of gamma rays passing through air make both ground and airborne surveys effective.
Because radioactive decay is an ongoing process, readings must take place over long enough times to be statistically meaningful. Statistical error (σ) is proportional to the number of detected events (n): σ = 100/ √n
such that 10,000 counts produces a 1% error. This is not practical in areas with low count rates, so it is common practice to slowly traverse the area, pausing for time-consuming total-count readings when encountering radiometric anomalies.
The substantial variation in attenuation in different materials and moisture levels makes detailed field notes of surficial features where measurements are taken. The presences of bare rock, overburden, standing water (including puddles) and recent and ongoing weather conditions may all influence readings. Gamma-ray spectography is particularly effective at dry, exposed bare rock outcrops.
Some quartz gravity meters contain a small amount of radioactive material to ionize residual gas and prevent build-ups of static electricity which can produce a detectable anomaly. Thus it is unwise to simultaneously conduct a radiometric and a gravity survey with those instruments to prevent introducing noise.
Radiometric surveys are undertaken to prospect for uranium or other industrial mineral deposits, for regional geologic mapping and correlation, and to determine phosphate concentrations in-situ. Surveys can also be used to monitor buried radioactive waste for leaks, to track groundwater by adding radioactive tracers, or to establish a public health risk from radon gas exposure.
Alpha Particle Monitors
Radon produces radon gas, an odourless, colourless, upwardly buoyant gas that can easily penetrate soil and rock, and causes public health problems (including increased risk of lung cancer) when in- haled. It may be present without significant gamma radiation, and is more reliably detected through measuring alpha particles.
Alpha particles are detected by leaving a small detector on site for at least 12 hours. In field surveys, the detector is sometimes suspended within a small chamber and buried.
Nuclear logs may be taken in open or cased boreholes, above or below the water table. However, the measured radiation will change from above to below the water table even if accompanied by no change in lithology due to changes in the oxidation/reduction processes.
Natural Gamma Log
A natural gamma log is a passive measurement of naturally emitted gamma radiation using a scintillometer, usually measured in American Petroleum Institute (API) units. Lithographies with more uranium, potassium, or thorium will produce higher emitters of gamma radiation. A spectral log of the natural gamma can be made to differentiate between uranium, thorium, and potassium sources.
Natural gamma is used to define lithologies, make stratigraphic correlations between boreholes, and assess the relative sand, silt, and clay content of a unit. It is also used in well completion studies, and during uranium, coal, and shale exploration.
A gamma-gamma log is an active measurement produced by irradiating the surrounding rock with a small gamma radiation source (usually caesium, 137Cs), and recording the back-scattered gamma radiation. Because gamma rays are inversely proportional to the density of irradiated rock, a gamma-gamma log can be used to create a density log of the borehole. It is useful for defining lithology, lithological correlation between boreholes, and estimating bulk density and moisture content of the material.
A neutron-neutron log is an active measurement produced by irradiating the surrounding rock with higher-energy neutrons (from a plutonium-beryllium source), then recording the how many neutrons are recorded in a detector a short distance (30+ centimetres) away from the source. The neutrons lose energy to collisions within the rock, especially to hydrogen nucleus which have a similar mass to the neutrons, so a lower count is typically indicative of high hydrogen concentrations. Neutron-neutron logs can help define lithology, and aid in defining zones of saturated porosity where water or oil fill the pore spaces.