URANIUM EXPLORATION - RADON SOIL SURVEY

FULL SOIL SURVEY SERVICES

Stratamodel performs full service soil surveys with experienced crews and enhanced post processing techiques delivered in GIS or client specified formats. Stratamodel uses alpha-track detection units processed by a Nationally (US) certified radon lab.

Contact Stratamodel for a radon soil survey or to purchase the most accurate radon detectors on the market.

Motorized survey with DGPS location control.

Motorized data collection

Sample site preparation.

Nigerien work crew

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RADON MEASUREMENT

Radon and helium gas are potentially the best pathfinder elements for uranium exploration. Radon, unlike helium, is relatively inexpensive to measure. All methods of radon measurement are not equal however. Two critical problems must be overcome before consistent measurements can be made.

The flow of soil gas to the atmosphere can vary by more than an order of magnitude over the course of a year and by a factor of three or more over the course of a single day. Measurements made in the morning can be quite different from those made in the afternoon at the same location. The worst possible time to measure soil radon is during daylight hours. Any system that relies on instantaneous or short term measurements (electret ion chamber, scintilation cells, and alpha probes) suffers from inconsistent measurements far more than methods that integrate radon concentration in the soil over many days or weeks.

The half life of Rn222 is 3.8 days thus methods that rely on radon capture and post deployment measurement (charcoal cannisters) only measure the average Rn222 concentration of soil for the last 36 hours they are deployed.

The best approach to these problems is a filtered, film based alpha track detector, deployed for 5-10 days. This avoids the diurnal variation in radon concentration and reduces the longer term variance due to changing weather patterns. Unlike the nitrocellulose based track-etch cups of the 70s, modern alpha track detectors use more sensitive optical grade CR39 film. The filtered detector housing excludes alpha particles that do not originate from radon gas eliminating false positive readings. Thousands of these detectors can be rapidly deployed over an exploration site then just as quickly collected for processing. Our lab techniques, detector design, and independent lab certification results in analytical errors of less than 3%. Our objective is to reduce noise due to the variance that always accompanies instantaneous or short term measurement of radon in soil.

THEORY OF RADON SOIL SURVEYS

The element radon is the radioactive daughter product of radium decay. Both radium and radon are progeny of uranium decay. Radon is a non-reactive gas that migrates away from the site of its radium parent by diffusion and advection along joints, faults and intergranular permeable pathways. If the radium source is below the watertable, offset and dilution in moving groundwater disperses the radon plume. Ultimately radon entrained in groundwater escapes to the phreatic zone where it joins the flux of soil gas to the surface.

Whether initially dispersed by groundwater transport or added directly to the soil gas above the water table, radon moves under the "pumping" influence of changes in atmospheric pressure, the Bernouli effect of wind passing over the soil surface, and increases in the water table to finally pass through the soil into the atmosphere. Daily and weekly changes in barometric pressure are an important driver of radon movement. Rainfall events that cause the water table to rise also drive soil gas toward the surface. Wind passing over the soil surface lowers the gas pressure in the upper reaches of the soil creating a gradient for gas flow toward the surface.

Several factors are working for and against detection of uranium at depth by measuring radon flux. The short half life of Rn222 means that half of the initial Rn222 concentration is lost in the first 3.8 days. The remaining amount is cut in half every 3.8 days until virtually no Rn222 is left. However, Rn222 release from its source is continuous thus long lasting plumes develop that can be detected. If the travel time of soil gas to the surface is long, this will attenuate the radon signal. Meanwhile, soil gas flux is driving atmosphere into the soil and soil gas out on a cycle similar to the tides. A series of successively 'higher tide' events where either the daily barometric pressure differential increases or a low pressure air mass is moving into the area is most favorable for radon detection.

Diurnal and weekly barometric pressure fluctuation for one month in the Sahara. The daily cycle of barometric pressure follows the changes in temperature over a 24 hour period. Soil gas containing radon flows toward the surface for several hours each day as the barometric pressure falls. The direction of flow is reversed as air is forced into the soil as the barometric pressure rises. A longer term cycle of soil gas escape is driven by changes in the weather. The reason for the inferiority of instantaeous or short term radon measurements is obvious.

Sahara barometric pressure

The last time radon detection was used in uranium exploration, most of the mechanisms responsible for radon flux were poorly understood if at all. All radon surveys suffer from the vagaries of the weather. The shorter the measurement time, the greater the risk that a representative measurement of the radon flux cannot be taken. Methods using very short time spans like direct soil gas collection (instantaneous) or Electret (several hours) are probably most vulnerable to catching the system during a 'low tide' event thus underestimating the radon flux. Even worse, measurements made at different times on different days are not always comparable.

The magnitude of a radon anomaly associated with a parent concentration of uranium will be only partly due to the size and grade of the parent body. Dispersion and dilution along the pathways to the surface increase the size of the radon footprint but also reduce its magnitude. The location of the anomaly relative to the parent body will be strongly influenced by the orientation of the pathways to the surface and the velocity of the groundwater if the parent body is below the water table.

Radon emits alpha particles as it undergoes radioactive decay. There are several common methods of measuring the emission of alpha particles but the best method for use in soil surveys is filtered alpha track detection. Alpha particles create streaks or tracks on sensitive film. Track density on the film is directly proportional to the radon concentration within the design parameters of the detector that is deployed in the field. When the film is "developed", the tracks are counted with an optical reader linked to a computer. The track density is then calculated and related to the radon activity which is reported as tracks per unit area per day. Background levels of radon activity are used as a basis to plot anomalous values that may then be used to refine the search for drill targets.

Filtered alpha track detectors are the best means of economically measuring radon in soil. This view of the interior and exterior of a detector shows the major design elements. The filtered aperture admits radon gas but prevents measurement of alpha radiation from sources other than radon. The optical grade CR-39 detetor foil is more sensitive than older nitro-cellulose based detection media. The symetrical chamber design has supior counting statistics to the sloped alpha track cups used in the 70s.

Alpha detector

PRACTICE OF RADON SOIL SURVEYS

Theory is only a guide to what can and cannot be expected from a real soil survey. Survey success depends on the reduction of three sources of errors and interpretation in the context of the underlying pedology, geology, and hydrogeology. Analytical error is generated during sample processing. Sampling error arises while the detectors are deployed in the field. The object of the soil survey is to narrow the search for a uranium target. The interpretation of the results are the most crucial step and also the most likely to be in error.

Well designed surveys attempt to quantify errors, eliminate their sources when possible and compensate for those that cannot be avoided. When the sampling and analysis are complete, geologic, pedologic, hydrogeologic, and geochemical reasoning must be applied to interpret the results. Drilling on the bright colors of a shaded contour map may or may not be the best use of a completed survey.

Optimal depth deployment of a radon detector. Radon concentration is rapidly attenuated as the soil surface is approached. Rigorous deployment to constant depth is critical to reducing sample error and measuring the true radon concentration in soil.

Alpha detector in soil

Analytical error, arises in the lab that processes the film at the end of a survey. This analytical error can be very low if the film has not been over exposed. Tracks are counted by an optical reader and tabulated by computer. Repeated reading of the same sample film can have very low variance (high precision). If the film is left in proximity to radon for extended periods, the track density becomes high enough to make measurement when the film is developed difficult. Track densities in excess of 150 tracks per square millimeter can no longer be read by optical scanning. An experienced lab technician with reference films for higher track densities can estimate within an order of magnitude higher densities. Never the less, 'dark' films should not constitue any more than 5-10% of a survey. A larger proportion degrades survey accuracy by obscuring the background values.

Photomicrograph of a radon detector film showing alpha particle damage. Molecular scale defects created by alpha particle collisions with the detector foil are enhanced by etching in a caustic solution then optically scanned. As track density increases, individual tracks coalesce and multiple tracks, are counted as a single track by the scanner. Estimates made by an experienced lab technician with a series of reference images is possible but the level of precision is at the order of magnitude level.

Alpha track density

Labs do not attempt to link track density to soil radon concentration because there is no standard to measure against and the sampling environment is so varied. This means that the accuracy of this method can only be inferred. Labs in the United States that measure radon for homes and businesses are certified every two years. Even though labs will not quantify the accuracy of measurements of detectors used in soil surveys, a certified lab is preferable to an uncertified lab. This absence of quantifiable accuracy can be compensated in part by a deployment method that approaches the design parameters of the detectors used. In addition, duplicates are deployed for quality control, one component of sampling error, and exposure control. Replicates are run on random samples to quantify precision. Blanks are run to determine detector background.

Sampling error has many sources, some of which can be avoided by careful survey design and implementation. Soil depth, soil moisture, gas volume around the detector, seal of the detector from the atmosphere and time of year all affect results. Over exposure of the film can damage or destroy all or part of a survey. Careless record keeping, locational errors and other human error are other major sources of sampling error.

Interpretation of the results of a radon soil survey in the context of the soil conditions at each sample point, the local groundwater regime, and the underlying geology have the best chance of turning alpha-track measurement into a useful tool for uranium exploration. Underlying geology must not only be favorable as a uranium host but active pathways to the surface must exist over mineralization. Translation of a radon anomaly by advection in moving groundwater must be considered therefore a conceptual understanding of the local hydrogeology is critical. Soil is the medium tested in a radon soil survey and pedological conditions have a significant impact on radon retention and flux. Soil conditions must be taken into account when deploying radon detectors and interpreting the results. A radon soil survey used as a uranium exploration tool is only as good as our understanding of what it tells us about the presence or absence of uranium ore in the subsurface.

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