Rock mass characterization using ground penetrating radar (GPR), rotary-percussion drilling performance, and indentation test

Li, Zijian (2023) Rock mass characterization using ground penetrating radar (GPR), rotary-percussion drilling performance, and indentation test. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

This thesis aims to develop a comprehensive rock characterization method for the rock's mechanical and Electromagnetic (EM) properties. In the mining industry's exploration and excavation stage, the rock properties' characterization plays a vital role. The precise mapping of the subsurface geometry and the characterization of the spatial lithology distribution is crucial for establishing the mining development plan. However, several problems regarding rock characterization in both EM and mechanical scope remain to be investigated. For subsurface geology mapping, shielded mono-hole Ground Penetrating Radar (GPR) has been a helpful tool. The limitation of the borehole GPR was identified in our previous study in that the groundwater significantly impacts its signal quality. The precise GPR data interpretation depends on the detailed EM properties (especially the dielectric permittivity) characterization of the target rock formation. A rapid and economical method to obtain the dielectric permittivity of the rock is required to be developed to provide solid proof for the subsurface geology mapping through GPR data interpretation. The accuracy was verified to be satisfactory for the field trial application. The invention of a new fluid system was proposed to eliminate the groundwater problem in the borehole GPR survey, which is required to possess appropriate EM properties and the ability to replace the groundwater in the borehole during the GPR survey. For the rock mechanical properties characterization, the in-situ unconfined compressive strength (UCS) plays an important role, and its measurement has always been problematic in field operation. For the highly fractured formation, the highly developed fractures and joints system in the rock mass limit the selection of the applicable rock samples for the UCS test because there is a strict requirement on the sample size. An alternative method that lowers the sample size requirement and presents reliable UCS result needs to be investigated. Regarding the rock EM properties characterization, A quick and economic measurement method of the dielectric permittivity was proposed, which presented acceptable accuracy, straightforward operation procedures, and low requirement of the test rock sample. A series of field tests proved the ability of the mono-hole shielded antenna to map the subsurface geometry with the borehole filled with air. The impact of the wellbore groundwater on the borehole GPR imaging quality was illustrated in detail in both theory and experiment. Then a practical solution was proposed by substituting the groundwater in the borehole with a newly developed fluid. The effectiveness of that fluid was proved through numerical simulation, lab tests, and field trials. These three innovations helped identify and remove the obstacles that hinder the application of the shielded mono-hole GPR for subsurface imaging. A Data Acquisition System (DAQ) was developed for the rock mechanical property characterization to collect the real-time Logging While Drilling (LWD) data of the Rotary-Percussion Drilling in the mining field. An analytical model was developed to predict the in-situ UCS and gold grade of the ore formation based on the real-time LWD data, which directly reveals the spatial distribution of the lithology and provides valuable information for the gold mine excavation and development. A series of indentation laboratory tests were conducted on the NQ size rock core as an alternative lab test to the traditional UCS test. Two analytical models were developed to predict the UCS value from the indentation test result. In the indentation test, the required sample size is much smaller than the traditional method, which indicates that it is more suitable for highly fractured formation. Its sample size requirement and the experiment's Standard of Procedures (SOP) were presented, proving its feasibility in determining the rock formation's intact UCS. Then, a Discrete Element Method (DEM) simulation was carried out to elaborate the sample preparation layout and optimize the sample preparation procedure. After calibration against the lab test result, through the DEM simulation, the indentation test was optimized where the minimum required size of the rock sample was obtained. It is also proposed that the particle size of the DEM rock model is a fundamental influence parameter on the rock failure model and the stress-strain response during its compressional failure process. Overall, this study provides practical solutions to rock characterization in EM and mechanical scopes and facilitates subsurface imaging and stability and safety evaluation.

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