Monitoring geophysical surveys in engineering-geological boreholes: identification of disintegration zones, permeability, and secondary clay alteration

Abstract

This paper presents the results of time-lapse borehole geophysical surveys conducted between 2021 and 2025 in engineering-geological boreholes within the mine field of a flooded mine. The objective of this research was to develop and validate a methodology for the quantitative assessment and dynamic monitoring of zones affected by secondary geological
processes along borehole sections. A comprehensive suite of methods was employed, including wideband acoustic logging (for Vp and Vs determination), gamma-ray logging (GR), and hydrodynamic testing (HDT). An integrated interpretation of elastic and radiometric parameters enabled the development of a petrophysical model for the identification of: (1) intervals of decompacted carbonates (reduction in acoustic impedance (AI) and B elastic bulk modulus (K) by 25-40%), (2) permeable zones (porosity >10%, ∆t log >170.6 μs/m, confirmed by well testing), and (3) zones of enhanced secondary clay alteration resulting from karst-suffosion processes (VpVs >2.2, elastic bulk modulus (K) <20 GPa, 15-25% increase in GR and abnormally low shear modulus (G)). Quantitative discrimination criteria are based on threshold values of elastic parameters (Vp/Vs ratio, acoustic impedances, Young’s modulus) and clay content, as well as their temporal variations. The most intensive development of all three anomaly types was recorded in the vicinities of boreholes 1x and 9x, indicating the ongoing progression of secondary geological processes. The results confirm the high diagnostic value of time-lapse borehole geophysics for monitoring engineering-geological hazards in post-mining areas.