I am currently working with airborne gamma-ray surveys of Brazilian Amazon region.
With the same array of surveying, the same kind of detector, aicraft, etc... the data from Potassium channel of the Amazon region has much more noise than the others surveys from central Brazil, or southern Brazil.
What comes to my mind is something related to the weathering process. But, even in fresh rock areas from the Amazonia, the signal-to-noise rate is very poor, and I don't really think that's the cause.
Anyone has a guess?
I'm sending two maps for comparison purposes. The first is from Rondonia state, northern Brazil, and the second is from Minas Gerais, eastern Brazil. Both maps are from some previous works of mine.
The calibration is up to date. The values of a terrestrial survey are compared to those acquired in airborne survey. But this is made in another setting in terms of moisture and temperature that those found in Amazonia.
May this be the cause?
Dear Guilherme Ferreira, did you realize this in what projects?
In fact, the Amazon region is a problem for airborne surveys, where there is much influence of radon and emission of radionuclides referring to vegetation. In some cases you can also observe this in regions of fresh rock because there is a previous contamination in the sensor. But I have worked with several projects in the Amazon Craton and this contamination does not present a considerable portion of the data.
Best wishes!
Iago Sousa Lima Costa Geophysicist, Laboratory of Seismology Geological Survey of Brazil - CPRM +55 61 2108-8413
Guilherme,
I tend to agree with Iago. The variety and radionulcide transport in vegetation can give wide response to radiation monitoring equipment. Our very sensitive body counting instrumentation can detect if someone has recently eaten a banana for instance, based on the accumulation of K-40 in their body. So you might be seeing a real effect of differing concentration of potassium.
You might also be seeing random error and interpreting it as a real effect. It would be interesting to know if it is true noise (at very low "nanocurie" levels). You could check that by directing the instrument over know areas, like a body of water for instance.
Cheers, Rick
It is almost impossible to give an answer to the question, having no knowledge about the local conditions and about Amazonia in general, but also no information about the measurement device and the treatment of the data. Only some hints can be suggested.
Usually, airborne surveys use a large scintillator , and the K channel is corrected for the U and Th contributions.
Here the K concentration seems quite low, which means that the relative importance of the U/Th corrections could be high, and this might be a source of noise in the K result, because of the statistical noise in the U and Th channels.
Another important aspect is the role of the forest cover. In principle the forest can act as a source in the K channel, and this is important where the soil is poor in K. Otherwise the forest mostly acts as a shield against radiation emitted by the soil. Irregularities in the forest cover appear as noise in the map.
The two aspects can be combined, as the forest will create "noise" by shielding the U and Th channels, which will be reflected in the corrections to the K channel.
Biogenic Potassium Salt Particles as Seeds for Secondary Organic, DOI: 10.1126/science.1223264
Science 337, 1075 (2012);
Christopher Pöhlker et al.
You should complement your KUT measurements (Potassium40, U and Th) by these measurements. Also, as someone said above, usually it's easy to distinguish K40 and the other peaks. I can send you this paper if you give me your email address. It's too hard to attach it on RG. [email protected]
Normally, potassium channel data become noisy when either thorium or uranium or both concentrations in ground are high. This high concentrations make enough contribution to K channel through Compton scattering. Now uranium measurement is done through Bi-214 emissions which is large when radon in the area is high. So you need to look into radon concentration also.
The answer of why potassium channel data become noisy , i think it is belong to either Thorium or Uranium concentrations are high
The gist of the Pohlker et al. article is that leaves exude (excrete) potassium. They couldn't make a detailed analysis (like if it's changing with time - very likely it does), however, it clearly is an unknown that could also cause the variation you observe. What the other colleagues above suggested is certainly true, it could affect your K40 variability (including Rn, which is very variable, of course)... It's an interesting problem, quite a difficult one. Have you published these results?
Dear Guilherme,
You asked the question a little while ago, and maybe you resolved this by now, but I'll add my answer in any case.
The signal-to-noise ratio of the top map is low because the concentration of K is very low. You probably only have a very few counts per second (cps) in the potassium window, and the intrinsic statistical uncertainty is very high. In a perfect environment it should approach sqrt(N) where N is number of counts; That's a law of the physics of radioactive decays.
So your question is more about why the concentration of Potassium is so low in this area.
The first thing you should clear out is that there hasn't been an issue with data acquisition and processing. There may have been a spectral drift during data collection causing the potassium K-40 peak to sit not where it is expected. e.g. it may be in lower channels in the 1024-channels spectra. This would obviously compromise the data processing. Although geophysical gamma-ray system are very robust nowadays, this should be checked anyway. If the natural radioactivity levels are so low, the system could indeed have issue stabilizing.
Next, that could really be the expression of a natural phenomena. What do you know about the local lithology and soil genesis ? You may have a rock type that is very low in potassium, likely a sedimentary rock, can this be the case ? and how did it contributed to the formation of the local soil ? is this area wet, either high soil moisture, or even a wetland area with some standing water ? this will absorb a lot of the gamma-rays coming from the ground.
hope this helps,
Richard Fortin
Geological Survey of Canada
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