Nuclear techniques for borehole logging in mineral exploration

A new approach provides opportunities for spectral gamma analysis in boreholes for mineral exploration

“…The probability of photoelectric absorption is roughly proportional to the fifth power of the atomic number, Z, of the element with which the gamma-ray interacts, and inversely proportional to the gamma-ray energy, E. The probability of Compton scattering is roughly proportional to Z/E (Killeen, 1997).…”

“…The low energy window is affected by both density and Z while the high energy window is mostly affected by density , thus the backscattered spectrum based on the ratio of the count rates in two energy windows, one at high energy and one at low energy can be applied to obtain information about the elemental composition of the rock (Figure 1) (Killeen, 1997). Since the (PE/Compton) ratio increases in zones containing high Z materials, this concept can be used to indicate a heavy element to produce assays such as the lead assays (Killeen, 1988, Killeen, 1987 and (Killeen, 1989).…”

“…and Nelson P.H., 1979), (Czubek J.A., 1979), (Glenn W.E. and Hohmann G. W., 1981) and (Killeen, 1997). Real commercial LWD tools introduced in the 1970's and in the 1980's have produced a greater variety of measurements, such as Gamma-ray, resistivity, NMR and neutron logs became available for larger drilling systems used in oil and gas exploration (Crain, 2014).…”

“…The most common technique of measuring density in-situ within the borehole is the gamma-gamma log which is a natural gamma-ray logging tool with a weak radioactive source on the nose of probe (Killeen, 1997). Using a radioactive source makes this method less favourable in mining industry due the possibility of losing the source.…”

“…Most of the recent advances related to gamma-ray spectrometry indicate that it is possible to determine the concentrations of Th, U and K with proper calibration (Killeen, 1997) and high quality of collected data. Generally spectral gamma logging performs poorly in mineral wireline logging due to the small hole diameters that limits the size of scintillation crystals making it simply too small to efficiently stop the gamma rays to produce a true representation of the gamma spectrum, plus logging speeds are too fast to acquire sufficient gamma rays to form a representative spectrum (Killeen, 1997). However, shuttle approach will provide sufficient data quality as the sensor progresses at drilling speed that is 100 times slower than wireline pull rates.…”