False radon anomalies flagged by low radon/thoron ratios.
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Radon-222, from the uranium-238 decay series, has a half-life of 3.8 days. Thoron (radon-220),
from the thorium decay series, has a half-live of 55 seconds.
It is easy to resolve these two radon isotopes in the field, just by looking at the drop
in count rate over the first few minutes.
The ratio of radon to thoron can tell us a lot about soil permeability and weed out false
anomalies.
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In 1972 I ran an east-west radon-in-soil-gas line across and perpendicular to
the boulder train just south of Midwest Lake in the Athabasca of Saskatchewan.
As expected, I noted high radon readings at the boulder train.
The ground here is low, flat and damp. The boulders give a strong gamma-ray signal as well.
At the west end of this soil line is a north-south sand ridge, about two meters high and
parallel to the nearby esker. This sand ridge is removed from any uranium mineralization.
On the top this sand ridge I was surprised to find high radon readings.
However, the count rate here dropped off rapidly in the first few minutes, indicating thoron.
The explanation of the thoron anomaly is that the dry, permeable, sandy soil gives up its gases
readily, all its gases, methane, carbon dioxide, nitrogen, oxygen etc., as well and radon and
thoron. So, it's not really a radon or thoron anomaly, but a permeability anomaly.
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I learned of a recent survey using a different system (not ours) which can
measure radon only, not thoron. This survey was in a glaciated terrain similar to Midwest
Lake. They found a strong radon anomaly on a small sand hill, which
turned out to be a false anomaly, not associated with uranium mineralization. Most likely this
anomaly is due to the ease of gas migration out of this dry, sandy, permeable soil. These
mobile gases would include thoron, as well as radon, and we hope that our
instrument would have identified high thoron levels (low radon/thoron ratios), and flagged
it immediately as a false amomaly.
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These "high" thoron levels in soil gas are only trace amounts and cannot be detected by
a gamma spectrometer. The half-life of thoron is only 55 seconds, and the amount that
can get into the gas phase in this short time is too low relative to the gamma background for
it to be detectable. With our Lucas cell, however, we only need a few dozen thoron atoms, and
with the short half-life we can count most of them in the first three minutes.
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High readings on the thorium channel of the gamma spectrometer indicate thorium minerals in
the rock or soil. Our Lucas cell measures alpha radioactivity (radon and thoron) in the gas
phase.
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In order to weed out thoron anomalies we have developed a formula to resolve radon from thoron
giving on-the-spot results with our field instruments.
We have been making this discrimination routinely in our field surveys ever since.
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In 1984 we ran an orientation survey in the Karoo of Madagascar
link.
By calculating radon/thoron ratios and radon, we were able to recognize anomalies due to
mineralization buried as deep as 18 meters.
Simply stripping thoron out and plotting pure radon is not enough:
you have to use the radon/thoron ratios as well.
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In 2008, one of our clients reported very good results in glacial soils.
They made an important drill intersection at least three meters under anomalous surface radon
and high radon/thoron ratios.
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To our knowledge, radon/thoron ratios have not been applied to oil and gas exploration;
they could prove very powerful.
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INSTRUMENTS
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We measure: radon - radium - thoron - radon daughters - alpha radiation.
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The
Lucas cell
is recognized as the most sensitive and reliable method for these elements.
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Our instruments are used around the world in exploration for uranium, oil & gas, groundwater and hydrothermal,
and in environmental protection, health physics, earthquake prediction, and evaluation of hydrocarbon and NAPL contamination.
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In the radon business since 1968, our latest major instrument update was 2015.
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Modern, low-power, field-rugged electronics. Some earlier versions still working after 35 years.
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Continuous real-time monitoring and data recording.
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Winter and summer, from the Sahara Desert to the Canadian Shield, our instruments have faced up to severe field conditions.
- Intrinsically safe functions.
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Sensitive to geochemical trace levels necessary for radon in lake water and for radon-thoron isotope ratios.
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Can work in a tent without electricity or be carried from point to point in the field.
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50 readings per day. Results available immediately.
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Rechargeable battery pack good for a long day in the field and recharges in a few hours.
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Can be operated by junior personnel if carefully supervised.
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Same instruments used for radon and radium in soil, sediment,
plant parts, rocks, water, soil gas, air, and snow, and for radon daughters in air.
- EPA compliant.
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Click here for more details of instruments.
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Click here for or other instruments, components and
accessories we provide
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Technical specification sheets and pictures of our instruments provided on request.
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Multilingual consulting and training (if required).
For instruments contact
R.H. Morse & Associates Ltd.
1-416-269-9979
morse@finderschoice.com
skype: robert.morse.toronto |
Robert H. Morse, Ph.D., P.Eng.
November 28, 2008
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