WELLOG Spectral Gamma Log
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WHAT IS GAMMA SPECTROSCOPY:
Gamma spectroscopy is a passive method that senses naturally occurring gamma rays having different energy levels. Gamma ray sources are comprised of many different elements , for example Uranium and Thorium, which have different energy levels. The different energy levels are identified by scintillation pulses that have different pulse heights or amplitudes. With proper system calibration, it is possible to determine the elements that are the source of gamma radiation based on pulse height.
HOW IT WORKS:
Instrumentation uses a scintillation detector, pulse amplifier, and several Single Channel Analyzers, or a Multi Channel Analyzer capable of analyzing as many as 4096 channels.
A Single Channel Analyzer (SCA) has settings for a “window” to discriminate between pulses that are above a lower pulse height and those that are below an upper pulse height. A Multi-Channel Analyzer (MCA) can be viewed as a system that separates all incoming pulses into separate energy channels and displays the total pulse count of each channel as a series of dots across the face of a CRT or in the case of a logging tool, simply counted into discrete counters. The result is a pulse energy spectrum. MCA’s may display thousands of discreet energy channels. The greater the number of channels, the better the resolution of discrete energy levels.
Gamma Spectroscopy has many interesting possibilities in Water well logging, Mining and Environmental Monitoring.
Isotopes may be added to formation fracturing fluids for definition of fracture zones to enhance secondary recovery methods.
Monitoring for Trichloroethane, Tetrachloroethane and other chlorine compounds can be accomplished using Gamma Spectral Detection methods.
Chlorine isotopes are known to emit low energy gamma rays. Using Single Channel Analyzers it is possible to discriminate low energy chlorine gamma rays from other gamma rays. The result is a log of various energy gamma rays called a Gamma Spectral Log. Gamma Spectral Logs will discriminate multiple radioactive species; Uranium, Thorium, Potassium, Chlorine and others. Logs may have one or as many as six channels. WELLOG has developed a basic six channel Spectral Gamma Logging System for logging applications. We call it a Spectral Gamma log. Available in 2011 is an improved spectral gamma logging tool that has 4096 channel resolution.
Ask about Logging While Drilling (LWD). WELLOG can Log Gamma ray data from drill cuttings while drilling. This service provides accurate information on lithology without interrupting the drilling process. LWD can improve accuracy in Horizontal Drilling operations.
Spectral Gamma logging has been used by Hydrologists for the purpose of identification of radioactive lithologies that mask potentially productive aquifers and in some cases provide an entry point for radioactive isotopes like radium and radon into water systems. Sandstones have lower gamma radiation. A potentially productive sandstone having high levels of natural radioactivity may appear as a shale or clay on a conventional gamma log. Analysis of a multi-channel Spectral Gamma Log could show a specific known radioactive isotope like U-235 or potassium that occurs in the formation.
Spectral Gamma logging can be used to isolate sources of Radium and Radon that contaminate water wells.
Spectral Gamma Logging can be used in wastewater and Land Fill perimeter monitoring.
Spectral Gamma Logging has applications in perimeter monitoring of radioactive waste or radioactive storage sites.
Spectral Gamma Logging is used in exploration for new sources of radioactive elements.
Spectral gamma Logging is used in specialized radioactive tracer surveys.
TYPICAL GAMMA SPECTRAL LOG:
The typical Gamma Spectral log measures four quantities; Total gamma counts, Uranium counts, Thorium counts, and Potassium counts. The log is presented in four tracks. Track 1 having total gamma counts in counts per minute (CPM). Each of the three other channels are presented in the space of track 2 and 3. Many other options are possible.
Spectral Gamma Logging tools can be calibrated to detect almost any discrete isotope in radioactive tracer applications.
MEAN RADIO-ELEMENT CONCENTRATIONS IN DIFFERENT CLASSES OF ROCKS: (From: Wollenberg (1978))
Rock Class: Uranium (ppm) Thorium (ppm) U/T Potassium (ppm)
Acid Extrusives 4.1 11.9 .344 31000
Acid Intrusives 4.5 25.7 .175 34000
Intermediate Extrusives 1.1 2.4 .458 11000
Intermediate Intrusives 3.2 12.2 .262 21000
Basic Extrusives .8 2.2 .364 7000
Basic Intrusives .8 2.3 .348 8000
Ultrabasic .3 1.4 .214 3000
Alkalai Feldspathoidal I E 29.7 133.9 .222 65000
Alkalai Feldspathoidal I I 55.8 132.6 .421 42000
Alkalai Feldspathoidal B E 2.4 8.2 .293 19000
Alkalai Feldspathoidal B I 2.3 8.4 .274 18000
Chemical Sedimentary Rocks 3.6 14.9 .242 6000
Carbonates 2.0 1.3 1.54 3000
Detrital Sedimentary Rocks 4.8 12.4 .387 15000
Metamorphosed Igneous Rocks 4.0 14.8 .270 25000
Metamorphosed Sedimentary 3.0 12.0 .250 21000
Using ratios of these three radioactive elements, rock classes can be determined.
Note: Radio-element ratios are determined largely from identification of daughter elements.
In the cases where U/T ratio is similar, the Potassium count will provide further discrimination.
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