Could magnetic properties be used to image a grouted rock volume?

Lunn, R.J. and Corson, L.T. and Howell, C. and El Mountassir, G. and Reid, C. and Harley, S.L. (2018) Could magnetic properties be used to image a grouted rock volume? Journal of Applied Geophysics, 155. pp. 162-175. ISSN 0926-9851 (https://doi.org/10.1016/j.jappgeo.2018.06.015)

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Abstract

In this study, the feasibility to develop a detectable permeation grouting system is tested, based on the addition of magnetic materials to the grout, specifically, magnetite. A magnetic-based detection system is selected for development because unlike other previously trialled detection methods, magnetic fields are detectable over large distances within the subsurface, and importantly, attenuation of the magnetic field is not strongly dependent on the material properties of the surrounding rock. To test the conceptual feasibility of such a system, a finite element based numerical model is developed to simulate the magnetic field anomaly that can be achieved by the addition of magnetic materials to a cement grout. The model is verified against an analytical solution and then used to predict the magnetic field generated by a grouted cylinder of rock, assuming a fixed percentage of uniformly distributed magnetic minerals, and a central injection borehole. Two field trials are conducted to verify the detectable grouting concept, the first using a walkover survey that allowed mapping of the magnetic signal in 2D. The second is designed to mimic magnetic field measurements from a borehole monitoring array, with a single central magnetic grout block (representing the grout close to the injection point). Results of the two field trials show that the magnetic cement is detectable, even when the background magnetic noise within the surrounding soils/rocks is significant. A good agreement is obtained between the measured and the modelled magnetic anomaly. This research opens the door to the development of a ‘detectable’ magnetic grouting system, that can increase confidence in the integrity of grouted rock volumes and reduce the inefficiencies currently present in the grouting industry, enabling in-situ real-time optimisation of grouting campaigns.

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