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Spectral Gamma Logging

What is the spectral gamma method?

Spectral gamma borehole geophysical methods measure natural-gamma energy spectra, which are caused by the decay of uranium, thorium, potassium-40, and anthropogenic radioactive isotopes. Each of these isotopes has a spectral signature that enables its presence to be identified. Regular natural gamma tools provide a total count of natural gamma emissions from these isotopes. The spectral gamma tool measures the energy of the gamma emissions and counts the number of gamma emissions associated with each energy level.

Why collect spectral gamma data?

Spectral gamma data can be used to identify and quantify the amount of uranium, thorium, and potassium-40 isotopes detected in boreholes. These data can be used to

• differentiate between shales and potassium salts
• recognize and estimate percentages of mineral types in bedrock surrounding a borehole
• compute shale percentages
• detect arkosic sand units
• determine clay mineral types
• identify clay depositional environments
• detrital: high thorium/low uranium ratios
• marine: low thorium/high uranium ratios
• correlate structural and stratigraphic features
• study water-rock interactions
• map clay content

How does the spectral gamma tool work?

Spectral gamma logging builds on a fundamental understanding of nuclear geophysics. In the spectral gamma tool, gamma radiation is measured by converting gamma rays (or "gamma photons") to electronic pulses that are then measured and counted. The gamma radiation is detected when it directly or indirectly causes ionization in a medium through which it is passed. In a spectral gamma tool, this medium is usually a sodium iodide or cesium iodide scintillation detector (or "crystal"), which emits a pulse of light when struck by a gamma ray. The crystal is optically coupled to a photomultiplier tube that amplifies the pulse of light and outputs a current pulse. The energy of the pulse is proportional to the energy of the gamma radiation that caused the pulse. The spectral gamma tool records both the number of pulses and the energy level of each pulse.

To record the full spectrum of naturally occurring gamma emitters, spectral gamma probes record the spectrum from 0 to 3 megaelectronvolts (MeV) and count the emissions in 256, 512, or 1024 channels across the spectrum.

With proper probe calibration, estimations of the concentration of potassium, uranium, and thorium can be made. For the best data counts, it is necessary to keep the tool at a fixed location in the borehole for a period of several minutes to collect a statistically representative sample of the surrounding formation. The lower the total gamma radiation at a selected depth, the longer the recommended period of measurement.

Data are typically presented in a total gamma ray log and (or) as a weight fraction of potassium, uranium, and thorium (KUT log).

The gamma spectral method has a high vertical resolution and can be used in boreholes that are:

• air-filled,
• water-filled,
• mud-filled,
• open,
• plastic-cased, or
• steel-cased.

Sample Spectral Gamma Data

Data are collected in a fixed location in the borehole or by logging at very slow speeds. The rate of logging speed is dependent on the rate of gamma emissions: the lower the gamma emissions, the lower the logging speed or the longer the time needed at a fixed measurement location in the borehole. Data collected at a single depth are typically presented in a plot that shows the counts of emissions for each channel in the spectrum. Data collected while moving the tool in the borehole are generally shown as the total count of emissions falling within the energy windows of potassium, uranium, and thorium.

In the Figure 1, the spectrum of gamma energy is plotted along the horizontal axis and the counts per second are plotted along the vertical axis. The energy ranges from 0.1 MeV to 3.0 MeV and is divided into 1024 channels.

Figure 1. Plot of sample gamma energy spectrum.

Gamma emissions plotted in figure 1 at different energy levels are interpreted to correspond to the presence of different elements in the formation surrounding the borehole: (1) the peak between the cesium (Cs) and potassium (K) windows is interpreted to correspond with bismuth, which is a decay product of uranium and thorium (such as in the peak between the Cs and K bars); (2) the peak overlapping the potassium (K) window is interpreted to correspond to potassium-40; and (3) the peak within the uranium (U) window is interpreted to correspond to disintegrations from uranium. Post-processing software can be used along with the data to determine the concentration of these elements in the formation.

Interpretation of spectral gamma emissions data can be done with basis spectrum models or spectral stripping to determine elemental concentration. A qualitative interpretation of potassium, uranium, and thorium can be made by comparing the observed gamma emissions to spectral gamma logs collected in a calibrated site. Careful calibration of the spectral gamma probe in a test pit of known isotope concentrations permits the process of spectral stripping to be done. (Spectral stripping identifies the individual elements contributing to the total gamma count through the comparison of the total spectrum against known standards for individual elements.)

For More Information:

USGS offices and cooperators can contact Carole Johnson (OGW BG) at cjohnson or (860) 487-7402 x17 to learn more about using the OGW BG spectral gamma tool or to discuss related training needs.

For more in-depth information on the spectral gamma logging method, see:

Hearst, J.R., Nelson, P.H., and Paillet, F.L., 2000, Well logging for physical properties, 2nd edition: Wiley and Sons, Inc., New York, 492 p.
Keys, W. Scott, 1990, Borehole geophysics applied to ground-water investigations: Technical Water-Resources Investigations of the United States Geological Survey, Book 2, Chapter E2, 150 p.
Serra, O., 1984. Fundamentals of well-log interpretation (vol. 1) -- the acquisition of logging data: Elsevier, Amsterdam, Dev. Pet. Sci, 15A.
USGS WRD National Research Program Borehole Geophysics Group's online bibliography of gamma spectral logging references.

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