Nuclear Borehole Geophysics
Nuclear borehole geophysics investigate borehole-intersected formations by employing principles of nuclear physics, which studies the constituents and interactions within atomic nuclei. Thus, the methods discussed consider and exploit phenomena that occur within nuclei of subsurface atoms either as a result of natural mechanisms or external forces. These subatomic interactions are directly or indirectly measured, and, because measurement characteristics depend upon material properties, nuclear borehole methods are used to quantitatively estimate such properties.
Nuclear borehole geophysics is generally used to detect physical and chemical material properties and variations of materials adjacent to the borehole. Certain methods (e.g., gamma-gamma, NMR, and neutron) can estimate the density or porosity of borehole-intersected formations. Other tools (e.g., natural gamma and spectral gamma) consider specific chemical element concentrations, which are relevant to lithologic distinctions. When absolute values cannot be determined due to tool limitations, calibration issues, or borehole construction, qualitative measurements may still be used to differentiate the properties with depth.
Nuclear borehole methods involve different types of radiation, which is, in general, the emission or transmission of energized particles or waves through a medium. Radiation is categorized as either ionizing or non-ionizing based on its energy level. Ionizing radiation, which can break chemical bonds and ionize atoms or molecules, exceeds 10 electronvolts (eV) and is relevant to most nuclear methods. Non-ionizing radiation, however, is pertinent primarily to the nuclear magnetic resonance (NMR) method.
The classic nuclear methods (i.e., gamma and neutron) involve radioactive decay, whereby an excited, unstable nucleus emits radiation that allows for atomic deexcitation. The two radiation types applicable to these “nuclear radioactivity” methods are gamma photon radiation, which is a type of electromagnetic radiation, and neutron particle radiation. These radiation emissions can occur naturally due to the presence of certain radioisotopes or be produced by the interactions between nuclear radiation and subsurface earth materials.
Thus, the “nuclear radioactivity” borehole methods can be further classified as either passive or active. Passive methods (e.g., natural total gamma and spectral gamma) detect gamma radiation from naturally occurring radioactive sources, which are primarily radioisotopes of potassium, thorium, uranium, and their decay products. Active methods (e.g. gamma-gamma and neutron density) employ a source of gamma or neutron radiation. These methods measure radiation that is either emitted or scattered as a result of interactions between source radiation and earth materials.
The NMR method, which is similar to magnetic resonance imaging (MRI), was developed using discoveries in nuclear physics. Unlike the preceding methods, NMR is concerned with proton behavior exhibited within hydrogen nuclei and does not involve ionizing radiation. The NMR method can estimate the volumetric percentage of water and pore-size distribution of the formation water. No active sources are required for the borehole NMR method. Instead, NMR exploits strong magnetic fields and radio waves (i.e., non-ionizing, electromagnetic radiation).
Neutron and gamma radiations are highly penetrative, so nuclear borehole methods (minus NMR) are unique and can be used in any type of borehole casing. However, because of this attribute, active-source nuclear logging requires special handling and a permit or be prohibited by some sites altogether. Thus, the NMR method, which excludes nuclear radiation and involves more in-depth hydrological analysis, has become an up-and-coming substitute for the active-source logs. Further method discussions are linked in the following:
- Natural Gamma and Spectral Gamma Logging
- Gamma-Gamma Density Logging
- Neutron Borehole Logging
- Borehole Nuclear Magnetic Resonance (bNMR)
References
International Atomic Energy Agency, 1999, Nuclear geophysics and its applications: IAEA Publications Technical Reports Series no. 393, 200 p.
Wightman, W.E., Jalinoos, F., Sirles, P., and Hanna, K., 2003, Application of Geophysical Methods to Highway Related Problems: Lakewood, CO, Federal Highway Administration, Office of Bridge Technology, 742 p.