An investigation of surface-to-borehole travel-time tomography for mineral exploration

Onabiyi, Oluwatosin Opeyemi (2013) An investigation of surface-to-borehole travel-time tomography for mineral exploration. Masters thesis, Memorial University of Newfoundland.

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Abstract

This thesis analyses the capabilities of the surface-to-borehole travel-time tomography method as a tool for mineral exploration and resource evaluation. Compared to electrical, electromagnetic, induced polarization and potential field methods, which are the currently preferred geophysical methods used in mineral exploration, the surface-to-borehole tomography methods offers the potential for higher resolution imaging of the subsurface. Despite the potential for improved subsurface imaging, this method has been somewhat untested for mineral exploration purposes. -- Assessment of the surface-to-borehole tomography capabilities was done using a series of increasingly realistic geological models. Acoustic finite-difference modeling was used to generate seismic travel-time data for the series of synthetic models and a minimum-structure inversion approach was used to perform the inversions on first-arrival travel-times. Evaluation of the accuracy of the inversion was done by comparing the true models to the slowness tomograms and the observed travel-times to the predicted travel times. -- Through the use of the 2-dimensional synthetic modeling experiments, this thesis successfully demonstrates the potential for deeper and higher resolution subsurface imaging than the currently used geophysical methods. Based on the acquisition parameters (vertical receiver boreholes, 50 - 60 Hz peak source frequency, and velocity contrasts expected for sulfide mineral deposits in hardrock environments), the results from the synthetic data show that resolution in the tomogram is good down to a depth of 40-50% of the borehole used. In addition, only objects on the scale of 100m - 150m or greater are resolvable with this method. Also, in comparison to other directions, it was observed that the best resolution in the tomograms is obtained in the direction perpendicular to the raypaths. Furthermore, we demonstrate that improved resolution in a tomogram is obtained by increasing the angular ray coverage of the region and by using structural and slowness constraints to perform the inversion.

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