 The Natural Gamma Ray Tool (NGT) utilizes a sodium-iodide scintillation detector to measure the natural gamma ray radiation of the formation and 5-window spectroscopy to resolve the detected spectrum into the three most common components of the naturally occurring radiation: potassium, thorium, and uranium. The high-energy part of the spectrum is divided into three energy windows, each covering a characteristic peak of the three radioactivity series. The concentration of each component is determined from the count rates in each window. Because the high-energy region contains only 10% of the total spectrum count rates, the measurements are subject to large statistical variations, even using a low logging speed. The results are considerably improved by including the contribution from the low-energy part of the spectrum. Filtering techniques are used to further reduce the statistical noise by comparing and averaging counts at a certain depth with counts sampled just before and after. The final outputs are the total gamma ray, a uranium-free gamma ray measurement, and the concentrations of potassium, thorium, and uranium.
The radius of investigation depends on several factors: hole size, mud density, formation bulk density (denser formations display a slightly lower radioactivity), and on the energy of the gamma rays; (a higher energy gamma ray can reach the detector from deeper in the formation). The vertical resolution on the log is about 1.5 ft (46 cm).
Clay typing
Potassium and thorium are the primary radioactive elements present in clays; because the result is sometimes ambiguous, it can help combining these curves or the ratios of the radioactive elements with the photoelectric effect from the lithodensity tool.
Mineralogy
Carbonates usually display a low gamma ray signature; an increase of potassium can be related to an algal origin or to the presence of glauconite, while the presence of uranium is often associated with organic matter.
Ash layer detection
Thorium is frequently found in ash layers. The ratio of Th/U can also help detect these ash layers.
Additional applications of gamma ray logs
The NGT response is affected by borehole size, mud weight, and by the presence of bentonite or KCl in the mud. In ODP boreholes KCl is sometimes added to the mud to stabilize freshwater clays which tend to swell and form bridges. This procedure takes place before logging operations start, and even though KCl is probably diluted by the time the tool reaches total depth, it can still affect the tool response. All of these effects are accounted for during the processing of the NGT data onshore.
The NGT log is routinely recorded for correlation between logging runs. To this purpose SGR (total gamma ray in API units) and CGR (computed gamma ray - SGR minus Uranium component - in API units) are usually displayed along with other curves (resistivity, sonic, density etc.). A full display of the data with SGR, CGR, and THOR (in ppm), URAN (in ppm), and POTA (in wet wt %) is usually provided separately.
Output plot of NGT data
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Temperature Rating:
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149° C / 300° F |
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Pressure Rating:
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20 kpsi (13.8 kPa) |
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Tool Diameter:
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3.625 in (9.2 cm) |
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Tool Length:
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8.58 ft (2.61 m) |
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Sampling Interval:
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6 in (15.24 cm) |
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Max. Logging Speed:
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900 ft/hr |
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Vertical Resolution:
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.75 - 1 ft (20 - 31 cm) |
| Depth of Investigation: |
1.5 ft (46 cm) |
| SGR |
Standard (total) Gamma Ray (GAPI) |
| CGR |
Corrected Gamma Ray (GAPI) |
| THOR |
Thorium (ppm) |
| URAN |
Uranium (ppm) |
| POTA |
Potassium (dec. fraction) |
| W1NG |
Window 1 (0.2 - 0.5 MEV) Counts (cps) |
| W2NG |
Window 2 (0.5 - 1.1 MEV) Counts (cps) |
| W3NG |
Window 3 (1.1 - 1.59 MEV) Counts (cps) |
| W4NG |
Window 4 (1.59 - 2.0 MEV) Counts (cps) |
| W5NG |
Window 5 (2.0 - 3.0 MEV) Counts (cps) |
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