Leila Abbasian, PhD

Accurate time-depth conversion for electromagnetic (E-M) geophysical methods is crucial for effective subsurface exploration and geotechnical assessments. The accuracy of depth measurements is affected by variations in E-M wave velocity across various media. Conventionally, in common offset ground penetration radar (GPR) surveys, this conversion utilizes underground point reflectors to determine E-M wave velocity, translating two-way travel time (TWTT) into depth measurements. However, the absence of such reflectors in many field conditions limits reliable depth estimation. This study presents the development of a methodology for the direct, in-field measurement of E-M wave velocity in drilled rock core samples that are representative of the media where the E-M survey is conducted, thereby eliminating the need for underground point reflectors and reducing reliance on time-consuming laboratory analyses.

The developed methodology was evaluated by measuring the E-M wave velocity for specimens of uniform lithology with varying lengths. Advanced signal processing techniques were employed, including dynamic corrections, DC-shift, Dewow and background removal filtering to minimize low-frequency noise, median filtering to remove spikes, frequency-wavenumber (FK) migration to enhance signal resolution, and spectral whitening to improve primary reflections visibility. The FDTD simulations complemented the experimental work, aiding in radargram interpretation and optimizing the experimental design. The measured wave velocities were compared with predictive wave velocities from mineralogy analysis of the specimens with good results.  Experimental and simulation results indicate that this method offers a repeatable, precise way to measure E-M wave velocity directly in the field, enhancing the speed and reliability of time-depth conversions in E-M geophysical methods. 

The developed methodology was applied for high resolution borehole imaging using GPR for delineation of a narrow vein gold deposit. Subsequent mining operations showed good agreement between predicted and measured deposit subsurface geology.