Radiometrics

The purpose of gamma-ray spectrometry is to provide information about the distribution of the three radioactive elements, uranium, thorium, and potassium. The distribution of these elements with regard to hydrocarbon exploration is significant. As with many other elements, uranium, thorium, and potassium, and their isotopes, are affected by the alteration effects of hydrocarbon microseepage. Radiometrics is a term applied to the measurement of the gamma ray spectrum at three specific windows where emissions for uranium, thorium, and potassium are located. Bismuth 214  represents the Uranium window at 1760 Kev (thousand electron volts), Thallium 208 represents the Thorium window  at 2620 Kev, and Potassium which has a single emission energy at 1460 Kev. A standard interpretation for hydrocarbon exploration consists of looking for decreases in gamma emissions from all of these windows or a decrease in the total count. seepage anomalies from those related to lithology. Changes in radiometric response can also be attributed to road surface changes, outcrops, drainages, road cuts, and road fill.  Keen observation is the key to culling the false anomalies from the seepage anomalies.

Radiometric surveys can be run as a continuous profile or as discrete points. Traverses run as continuous data can yield a considerable amount of detail with regard to signal character by showing the location of faults and fractures, radiation halos, and traditional seepage anomalies. Though the character of the signature is missing the discrete points yield a more visual interpretation of the spatial extent of the radiometric highs and lows. As with any survey, the greater the sample density the better the interpretation.

Radiometrics is a first wave culling tool for reconnaissance geochemical surveys. It should be used to delineate anomalies that can be tested for the presence of hydrocarbons using other surface geochemical techniques.

A gamma ray spectrometer is a device that separates gamma radiation into two or more energy components. Spectrometers require a detector and a device to analyze the signal. The detector, normally a sodium iodide crystal, absorbs the gamma radiation and converts it to a light flash or scintillation. The light is received by a photomultiplier tube which converts the light flash to a voltage proportional to the intensity of the light flash. The counting device then separates the voltage into a number of magnitude dependent classes which represents the energy spectrum of the incident gamma rays.

Most of the useful gamma emissions, for petroleum exploration, are located in the low energy range of the spectrum below Potassium 40.

Acquistion of radiometric data does require a knowledge of the variables that can affect the gamma ray signal. In addition to the normal variables of geometry and physical property contrasts, it is necessary to consider the size and the efficiency of the detector, the speed at which the detector moves, the effects of meteorlogical variables, topography, and cultural influences.

Large crystal volume, 112 in3 or more, is the most important aspect of radiometric surveying. The larger the volume the higher the number of gamma counts that can be collected. This equates to the greater sensitivity that is required to detect the secondary alteration of near surface microseepage anomalies.

The count observed during any specified period of time in any particular radiation environment is directly proportional to the volume of the crystal detector and the minimum speed of the vehicle.

Moisture content in the soil or air may cause variations in the gamma ray readings. Standing water or snow will yield strong lows due to absorption. Small crystal volumes require a recovery time of about 3 hours after a rain storm, though such affects are negligable when using large detector volumes.

Depending on the source of the radiation, barometric pressure might be a variable. Radon can influence radiometric readings such that high pressure might suppress gamma counts or low pressure might enhance the number of counts.

Ideally radiometric surveys should be run over flat featureless terrain or areas with few outcrops or lithologic changes. Difficulty of interpretation is proportional to the ruggedness of the topography making it harder to discern seepage anomalies from those related to lithology. Changes in radiometric response can also be attributed to road surface changes, outcrops, drainages, road cuts, and road fill.  Keen observation is the key to culling the false anomalies from the seepage anomalies.

Radiometric surveys can be run as a continuous profile or as discrete points. Traverses run as continuous data can yield a considerable amount of detail with regard to signal character by showing the location of faults and fractures, radiation halos, and traditional seepage anomalies. Though the character of the signature is missing, the discrete points yield a more visual interpretation of the spatial extent of the radiometric highs and lows. As with any survey, the greater the sample density the better the interpretation.

Radiometrics is a first wave culling tool for reconnaissance geochemical surveys. It should be used to delineate anomalies that can be tested for the presence of  hydrocarbons using other surface geochemical techniques.