Three-dimensional borehole induced polarization inversion using adaptive progressive grid: Methodology and field applications

The borehole induced polarization (BIP) method, owing to its high sensitivity to water-bearing structures, demonstrates significant advantages in detecting small-scale and complex water-related hazards ahead of tunnels. However, its performance is constrained by traditional fixed inversion grid techniques, making it difficult to meet the demands of rapid and high-resolution detection. To address these limitations, this study proposes a bidirectionally guided adaptive grid optimization strategy This method combines the benefits of two popular grid optimization techniques, namely the sensitivity matrix reconstructed from highly anomalous observational data and the change in model parameters, to thoroughly identify and refine grid regions that with high sensitivity and significant parameter changes. Building upon this foundation, an adaptive progressive inversion framework for induced polarization is developed. Building on this, this paper further proposes an adaptive progressive inversion method for induced polarization. In the initial stage of inversion, a coarse grid is employed to alleviate the ill-posedness of the inverse problem. Subsequently, during the following inversion process, the grid resolution is progressively enhanced based on the bidirectionally guided grid optimization strategy, enabling high-precision imaging of resistivity and chargeability parameters. Numerical experiments demonstrate that the proposed adaptive inversion method, while reducing the number of grid cells by 20 %, generates higher-resolution inversion grids, significantly improves the accuracy of resistivity and chargeability distributions, and increases computational efficiency by 50 %. This technique has been successfully applied in a drill-and-blast tunnel project in Northwest China, where it accurately identified water-bearing structures ahead of the tunnel face, providing crucial technical support for pre-grouting operations.