Fault detection
In the article “Gamma-Ray Spectrometry in Radioactive Prospecting: Application Tool as Detecting Fault Trace”, Imaizumi Masayuki writes about the potential to map active and inactive faults by radioactivity prospecting. The text below gives a brief overview of these results.
Behavior of K, U, and Th around Faults
Structure of Fault Fracture Zones
A geological fault is a planar or gently curved fracture in the Earth's crust where the rocks on one side have moved relative to the rocks on the other side. It is a break or discontinuity in the Earth's lithosphere (crust and uppermost mantle) along which displacement or movement has occurred. Faults can manifest in complex structures. During large-scale shear fractures, a mixture of breccia and clay forms as the rock mass breaks over a broad area, creating what is known as a fault fracture zone. Beyond a certain thickness of these materials, a highly fractured rock mass grades into less fractured, normal rock with increasing distance from the fracture zone's core. Intrafault clay minerals generally develop through chemical weathering and alteration, driven by interaction with groundwater post-fracture (Tanaka, 1990).
As a fresult, outcrops of fault fracture zones typically reveal a banded structure, featuring zones of fault gouge and fault breccia running parallel to the fault. The clay zones, rich in uranium due to the affinity of clays to enrich in U, are positioned on both sides of the fracture zone. This can form anomalies in the concentration of 238U.
Geochemistry of Fault Fracture Zones
The clay formation within fault fracture zones is significantly influenced by the host rock's composition, as well as the prevailing temperature and pressure conditions (Yoshida et al., 1992).
Non-equilibrium uranium-series data suggest that uranium leaches very slowly from primary minerals (Smellie et al., 1995). Additionally, fault cracks are often filled with secondary minerals like hematite and goethite, which are frequently associated with concentrated uranium. Studies reveal that total Fe, Fe3+, and U levels typically increase towards fault cracks, whereas Th concentrations remain relatively stable (Smellie et al., 1995). These enriched concentrations of uranium lead to increased levels of radon (Rn) that eminate from the fault (Bena et. al., 2022).
Case studies in Japan
The article “Gamma-Ray Spectrometry in Radioactive Prospecting: Application Tool as Detecting Fault Trace” validates the potential for fault detection using Gamma-Ray Spectrometry (GRS). A study incorporating radioactivity prospecting—including soil radon gas surveys—was conducted across four known fault areas. These included active faults such as the Adera fault, Abashiri Lake east coast fault group, and Hongū fault, as well as the non-active Tanagura east marginal fault. The results indicate that fault traces can be reliably detected using the following three indices:
Faults inferred from the continuity of anomaly points exhibiting an increased Bi/Tl ratio (effectively the U/Th ratio) above a predefined threshold for each study area.
Faults identified from elevated concentratios in radon.
Faults can be inferred from the continuity of points where the average value of radioactive elements changes in a stepped fluctuation pattern, indicating different rock compositions in contact due to faulting.
Each of these indices shows a respective error margin of 0–30 m, 0–180 m, and 0–100 m compared to the published fault positions, which themselves may be inaccurate by up to 100 m or more. Given this, the estimated fault positions derived from the three indices lie within an acceptable error range. Hence, it is concluded that radioactivity exploration for fault detection should utilise these three indices for robustness.
Advancement in the analysis technology for radioactivity prospecting not only requires precise GRS and radon gas concentration measurements but also the integration of Artificial Intelligence (AI) technologies. This underscores the necessity for developing sophisticated analytical tools to enhance the accuracy and reliability of fault detection methods.
References
Tanaka, T. (1990). Geological Structures and Fault Mechanics.
Yoshida, H. et al. (1992). Clay Mineral Composition in Fault Zones.
Smellie, J.A.T., Laaksoharju, M., and Tullborg, E.-L. (1995). Uranium and Iron Distribution in Fault Zones.
Kingston, D.R. (1989). Techniques in Spectral γ-ray Logging in Carbonate Formations.