Forward and inverse models of magnetically-susceptible grout in rock fracture grouting

Abstract

Rock grouting is an important technique for sealing rock fractures, but it has long been plagued by the lack of a practical approach to delineate the extent of grout propagation inside the facture flow channels. Despite previous attempts to evaluate grout performance, barely any non-intrusive way can directly and accurately reveal the grout penetration region. In this study, a new inversion method is suggested, based on a magnetic forward model, to predict the burial depth, dip angle and lateral (horizontal) projection span of a grouted area in a single rock fracture. It is assumed that the grout flow route inside a rock mass can be magnetically observable when ferromagnetic materials are added to the grout. The method commences by forward modelling the magnetic field caused by an inclined sheet of magnetic material. Based on the analytical solutions for calculating the magnetic field in the presence of the sheet, a relationship between the magnetic anomalies and the geometric parameters of the sheet is established. Following the feasibility study of the forward model, an inversion procedure is proposed to determine the geometric information of the sheet using multiple observations of magnetic data obtained at various levels above the sheet. The results show the applicability of the inverse method for estimating the burial depth at observation distances up to five times the length down dip of the sheet. Moreover, a correction nomogram is proposed to address the sources of error with known approximate parameters, and this greatly improves the model’s performance. Finally, some insights into the application of the inversion process in a three-dimensional magnetic field are presented. The current solutions for predicting the geometry of grout intrusion in a rock fracture system are shown to be efficient. Both the forward and inverse models proposed should provide valuable contributions to the problem of addressing the error involved in non-destructive, remote detection of a grout region in a fractured rock mass.