Imagerie des infrastructures souterraines et des cibles d'exploration minérale par les méthodes géoradar, électriques et électromagnétiques

This thesis focuses on the application of geophysical imaging techniques, specifically with electrical and electromagnetic methods, for the identification of underground infrastructure, and the exploration of deep mineral resources. The advantage of most geophysical methods is the ability to penetrate the ground non-destructively, although each method has its own detection depth limit. Since the present study targeted both near-surface structures (underground infrastructure) and deeply buried structures (mineral exploration), a strategy combining multiple geophysical methods and numerical modeling is adopted for the realization. First, Ground Penetrating Radar (GPR) and Electrical Resistivity Tomography (ERT) methods were used together to determine the depth of bedrock to assist in installing a sewer pipe network. GPR measurements with different frequencies and ERT measures with different configurations were performed. Specific site information, such as geological descriptions of boreholes and the exact location of some civil infrastructures (pipes) are also integrated in our study in order to reduce ambiguity in the interpretation of geophysical observations. The two geophysical techniques proved to be complementary. The high resolution provided by GPR allowed the identification of underground pipes, which were used to calibrate the velocity of the radar wave in the subsurface. In addition, the ERT technique compensated for the shallow depth of investigation of the GPR and allowed the delineation of the overburden-bedrock interface by the variation of resistivity. If in the first study the targeted depth of investigation was less than 10 m, the mining infrastructures can be located hundreds of meters underground, so the challenge is even more consequent. In the second stage, the three techniques (TDEM, ERT, and GPR) were combined together, starting with numerical modeling to establish the detectability of these old galleries. Considering the conditions in which these infrastructures can be found: empty, filled with fresh water, contaminated by acid mine drainage, or backfilled. An approach of using simulation methods is used to design the optimal parameters for field measurements first, and then a field validation procedure was developed. Based on the simulation results, the potential contribution of the TDEM method to detect deep galleries will be possible at hundreds of meters, if they are filled with conductive water. ERT would be more suitable for empty (resistive) or conductive water-filled galleries but would require the transmission of a high current intensity. Finally, with GPR measures in boreholes, galleries could be detected if they are located at a maximum of 10 meters around the borehole. Due to the lack of measuring instruments, only the ERT method approach was validated by carrying out 2D and 3D measurements on the site of the former Quemont and Horne mines in Rouyn-Noranda (Quebec, Canada). The 3D surveys were carried out with a measuring system that allows the transmission of a high current intensity, ranging from 2.5 to 7.6 A, and allowed the detection of galleries at a depth of 100 m. In the third part of this thesis, the problem of deep mineral resources exploration has been addressed. TDEM measurement protocol was developed through the design of a small transmitter loop for use in underground galleries, by performing numerical modeling of the primary and secondary magnetic fields respectively transmitted by the loop and induced by the conductors representing the mineralized bodies. Finally, this loop was tested with TDEM measurements in a gallery of the LaRonde mine (Agnico Eagle Mines Ltd) located 3 km underground. These measurements were made using a down-hole receiver in two available boreholes near the gallery. They allowed the validation of the protocol by detecting two massive sulphide lenses located in the zone.