|Year : 2018 | Volume
| Issue : 1 | Page : 42-46
Spectral gamma ray logging: A cost-effective method for uranium exploration
G Jegannathan1, V Veluswamy1, B Ram Mohan Reddy1, Pravin Kumar Sharma2
1 Department of Atomic Energy, Atomic Minerals Directorate for Exploration and Research, Bengaluru, Karnataka, India
2 Department of Atomic Energy, Atomic Minerals Directorate for Exploration and Research, Hyderabad, Telangana, India
|Date of Submission||31-Jan-2018|
|Date of Decision||21-Feb-2018|
|Date of Acceptance||13-Mar-2018|
|Date of Web Publication||31-May-2018|
Department of Atomic Energy, Atomic Minerals Directorate for Exploration and Research, Bengaluru, Karnataka
Source of Support: None, Conflict of Interest: None
The most useful technique in uranium exploration program is undoubtedly radiometric surveys. This is due to the fact that uranium emits gamma rays ranging from as low as 47kev to 2.2Mev, which can be detected and quantified using suitable radiation detector. Combination of aerial radiometric surveys, ground examination of the detected anomalies, followed by drilling and gamma ray logging of drilled boreholes has resulted in the identification of large uranium resources. Borehole logging provides the most important subsurface information required for the uranium exploration program. An area known to contain only uranium, computed gamma ray logging with a Geiger Muller (GM) Detector rapidly gives the required subsurface radioactivity information whereas, in a heterogeneously mineralized area of uranium with thorium, logging data using GM detector may mislead to wrong interpretation. Under such condition, using the principle of gamma ray spectrometry, scintillation detector-based spectral gamma ray logging is carried out. Identifying uranium in the presence of thorium is a complex process and this paper deals with a case study on the spectral gamma ray logging carried out to locate the subsurface uraniferous zone in Pakkanadu area, Salem district of Tamil nadu, where the surface anomaly indicated the presence of high thorium content. The various limitations such as small detector size, large sample volume, high-correction factor required for quantifying the individual elements, and the study carried out for optimizing the time required for data acquisition are discussed.
Keywords: Correction factor, data acquisition time, spectral logging, uranium exploration
|How to cite this article:|
Jegannathan G, Veluswamy V, Reddy B R, Sharma PK. Spectral gamma ray logging: A cost-effective method for uranium exploration. Radiat Prot Environ 2018;41:42-6
|How to cite this URL:|
Jegannathan G, Veluswamy V, Reddy B R, Sharma PK. Spectral gamma ray logging: A cost-effective method for uranium exploration. Radiat Prot Environ [serial online] 2018 [cited 2020 Jan 18];41:42-6. Available from: http://www.rpe.org.in/text.asp?2018/41/1/42/233648
| Introduction|| |
Wide-ranging application of Gamma ray spectrometry includes geological mapping of bedrock and surface geology, environment concerns (mine tailing, radioactive waste), detection of lost sources, radiological emergency response, and medical uses. The uranium-thorium ratios of rocks, which are important in strata recognition and correlation studies, can be determined directly by gamma ray-spectrometry without the necessity of making individual uranium or thorium assays. Gamma ray spectrometry is also used for finding rare metals. Geological applications rely on the natural radioactivity of rocks due to the presence of natural isotopes, potassium–40 K (0.012% of natural potassium), uranium–238 U (99.27% of natural uranium), and thorium–232 Th (99.98% of natural thorium) and their decay products. Potassium gives off monoenergetic gamma rays whereas U and Th decay to other elements called daughters, which are radioactive and emits gamma rays raging from 47keV to 2.62MeV shown in [Figure 1].
Direct measurements of potassium, uranium, and thorium concentrations provide valuable geochemical, mineralogical, and ore deposit data information. Spectral gamma ray logging makes use of unique energies 1.46MeV for K, 2.62MeV for Th, and 1.76MeV for U (Ra.) in their determination.
The instrumentation used for logging is shown in [Figure 2]. The logging system uses a 1″ diameter (thickness in case of borehole logging as most of the gamma ray to the detector comes from the walls of the borehole [Figure 3] and 4″height detector, 256 channel MCA, personal computer, winch, and pulley. The logging cable is single-core double armoured steel cable which transmits the digital data from the logging probe to the winch on the surface. The winch is interfaced through a serial interface RS-232 to P. C [Figure 2] which displays the digital data. The whole system is powered by a pencil type nickel-metal hydride 9.6V (1.2VX8) rechargeable batteries integrated with the logging probe assembly which slides along the borehole column. Gamma ray interaction with the detector [Figure 3] results in an electrical pulse of amplitude proportional to the energy of the captured gamma ray. These pulses according to their amplitude (energy) are stored in various memory locations called channel numbers, and a spectrum equivalent to the energy of the incoming gamma ray is produced as shown in [Figure 4].
Calibration is an important aspect of spectral logging, as the response of the scintillation detector varies with temperature.(Light output of the scintillation detector decreases with the increase in temperature and may cause a drift in the spectrum.) Initial calibration will be carried out in three independent boreholes containing known concentration of uranium, thorium, and potassium. This will determine the sensitivity of logging system for individual channels (Sk, Su, and Sth) along with total channel sensitivity (Stot). The stripping or correction factors α, β, γ, a (the contribution of high energy Th in U and K Channels, the contribution of U in K and Th channels) will also be determined and is shown in [Table 1]. These constants will be stored in the memory.
Just before logging, peak centroids of uranium, thorium, and potassium are verified with respective standards.
A case study on Pakkanadu
Tube-well logging carried out in Pakkanadu area of Tamil nadu using Geiger Muller (GM) Detector indicated a high subsurface radioactivity. As the surface anomaly indicated presence of thorium, spectral gamma ray logging was carried out to ascertain the nature of subsurface radioactivity in the boreholes drilled in that area. Because of the small detector (1” × 4” NaI (Tl)), the logging system had a lower sensitivity. Small detector size, combined with large sample volume will result in a high correction for uranium and potassium channels [Table 1].
The presence of higher thorium will also affect U peaks. [Figure 5]a and [Figure 5]b shows the effect on uranium peak due to thorium concentration. The black and red lines in the display indicate the centroid for uranium and thorium peaks whereas the data in the right-hand corner show the channel number and the channel count for the uranium peak.
|Figure 5: (a) U peak in the absence of Th. (b) U-peak getting smeared by the presence of Th|
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Thus, the error in uranium values will increase with the increasing thorium content. Similarly, K estimation is very difficult in the presence of high uranium and thorium. Considering the current sourcing capacity of the battery and also the variation of the detector response with temperature, it is difficult to carry out logging for a longer duration (>4 h.).(Inside the borehole temperature varies as 1°C/100 mts. and in Pakkanadu area of Tamil Nadu the surface temperature varies from 28°C to 38°C from morning to noon.)
From the data acquired by GM logging in PKD-02, three different mineralized zones, namely, 60.00–70.00 m, 110.00–115.00 m, and 200–210.00 m., were selected for carrying out a study on time required for data collection at each depth by spectral logging. Three different time constants, namely, 10s, 50s, and100s, were used with a logging interval of 20 cm. The data acquired from 110.80 m–114.20 m for the above-mentioned time constants is shown in [Figure 6].
From this study, an acquisition time of 10 s and a logging interval of 20 cm was selected which enabled us to carryout out logging of the entire borehole of nearly 200.00 mts deep in 2 days. To have minimum contribution on lower energy channels from higher energies, four channel integration was carried out for elemental computation.
[Figure 7]a shows the concentration of radium and thorium in bore hole PKD-02 as a function of depth. But for the anomalous zone from depth 110.00 to 123.00 mts, the mineralization is completely thoriferous. The anomalous zone (uranium, mixed and thorium favouring zones) from drilled depth 110.00 mts to 123.00 mts of the borehole is shown separately in [Figure 7]b.
|Figure 7: (a) Radium, thorium profile in Borehole PKD-02. (b) Logging profile of PKD-02 from 110.00 to 123.00 m|
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The core samples from drilled depth 110.50–123.0 m of the bore hole were analyzed in the laboratory for comparing spectral logging data. Analysis of the sample results showed profile similar to that of logging [Figure 7]b. and is shown in [Figure 8]a and [Figure 8]b.
|Figure 8: (a) Sample assay data 110.50 m–117.30 m showing uranium favoring and mixed zone. (b) Sample sssay showing thorium zone from 120.80 m to 123.00 m|
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The radium and thorium distribution in bore hole No. PKD-01 is shown in [Figure 9].
The mixed (U + Th) anomalous zone indicated from the drilled depth 153.80–154.60 m of the borehole was reported in the core samples PKD-1/CA/24 as 0.062%of Ra (eq), 0.094% of ThO2 and in PKD-1/CA-25 as 0.25% of Ra (eq) and 0.14%of Th02.
The potassium response of the logging system was studied in borehole PKD-04, in which logging reported an average of 9% K from drilled depth 150.00–250.00 m in the absence of U and Th. Samples analyzed from same region of the borehole reported 9.7%K.
Bore hole No. JLK/1of Jalakandapuram an adjacent area to Pakkanadu indicated thorium-free uranium mineralization of 0.018% Ra (eq) from drilled depth 250.40–254.40 m and the core samples analyzed from the same depth reported 0.013%Ra (eq) with negligible thorium.
| Conclusions|| |
The spectral logging data and sample analysis data from drilled depth 110–123.00 m of PKD-02 show a good agreement in thorium values, whereas uranium values slightly vary in mixed zones. This may be attributed to the fact that thorium being at the higher end of the spectrum, their window counts gets less affected whereas in a mixed zone, uranium window counts requires correction due to small detector size and presence of high thorium. However, spectral gamma ray borehole logging in Pakkanadu area has clearly demarcated uranium, mixed, and thorium-favoring zones in the boreholes.
In the absence of uranium and thorium, potassium zone was also identified. This has helped to carry out selective sampling for further studies. Only 300.00 m of core samples were analyzed from nearly 2500 m of drilled core. This has avoided nearly 10–12 months of workforce and unnecessary radiation exposure for those involved in sampling at different stages. (Health physicist recommends exposure rate as minimum as possible).
Eight bore holes drilled in pakkanadu area indicated subsurface thorium favouring mineralisation, may be a reason for surface thorium exposure in this area. But spectral logging in bore hole no. JLK/1 of Jalagandapuram, an adjacent area of Pakkanadu has indicated low order uranium activity free of thorium. This gives rise to the possibility of uranium occurrences in near by areas although Pakkanadu area is thoriferous, providing a valuable qualitative idea for future exploration in that area.
The authors are very grateful to Sri L.K. Nanda Director, Atomic Minerals Directorate for Exploration and Research for giving permission to publish this paper. Sri. M. B. Verma Additional Director (OP-I) for his consistent support to carry out this work. Sri A.K. Bhatt, Regional Director and Sri B. Saravanan, Deputy Regional Director, Southern Region, Bengaluru for their encouragement. Sincere thanks to all my colleagues for their valuable suggestions.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9]