A blind geophysical target refers to a mineral deposit that is hidden under barren country rocks and has no surface features that can be detected only by anomalous geophysical signatures. The most effective testing method entails the use of reverse circulation (RC) and diamond drilling (DD). Reverse circulation employs double-wall drill pipes and uses air to bring rock chips to the surface at high speed (Cao et al., 2019). On the other hand, diamond drilling creates a cylindrical core of the rock formation.
The process of rigorous definition of the target is one of the initial steps in the whole programme. Exploration geologists use 3D inversions created from magnetometric, gravimetric, or electromagnetic surveys to determine the shape and depth of the target. Information about its position allows identifying collar points and azimuths needed to test the blind target. The use of comprehensive geophysics is essential to minimize financial risks related to deep mineral prospecting (Okada, 2022).
An economic exploration program is usually performed using the “RC pre-collar with a DD tail” method. It is too costly and slow to drill through the depths completely using DD. In such a situation, RC drilling, which ensures extremely high rates of penetration along with excellent cuttings transport efficiency (Zhang et al., 2022), is used first to punch through the barren overburden. When the drilling approaches the horizon of modeled interest, a switch from RC to DD is made.
The transition from RC to DD drilling requires precise control over drilling operations. With the aid of MWD instruments and downhole survey equipment, it can be ensured that drilling occurs according to the planned profile and there are no deviations (Hopper et al., 2019). Once the drilling reaches the transition depth, the hole is cased so that it will not cave in. Further, the process of DD drilling using mud fluids begins.
It is crucial to maintain meticulous sampling throughout both stages for successful geological confirmation. The RC chips will be geochemically tested, usually through fast analysis equipment like the laser-induced breakdown spectroscopy (Harmon et al., 2019). In the next stage involving the DD holes, the core will be oriented, logged, and taken for laboratory analysis. This process ensures that there was either mineralization or no mineralization associated with the anomaly found.
Finally, the post-drilling step involves integrating the hard drilling information into the preliminary geophysical model. The physical parameters like specific gravity and magnetic susceptibility of the DD core obtained from the drill hole will then be measured. This will involve iterating on the 3D inverse modeling to ensure the exact characterization of the blind target.
References
Cao, P., Zhao, Q., Chen, Z., Cao, H., & Chen, B. (2019). Orthogonal experimental research on the structural parameters of a novel drill bit used for ice core drilling with air reverse circulation. Journal of Glaciology, 65(254), 1011–1022. https://doi.org/10.1017/jog.2019.76
Harmon, R. S., Lawley, C. J., Watts, J., Harraden, C. L., Somers, A. M., & Hark, R. R. (2019). Laser-induced breakdown spectroscopy—An emerging analytical tool for mineral exploration. Minerals, 9(12), 718. https://doi.org/10.3390/min9120718
Hopper, T., Schubert, M., Joldes, G., Moran, B., Kalisch, P., Crumby, M., Buters, N., Ott, K., Rogozinski, K., Morgan, Q., Neville, T., Shanmugam, H., Zaeper, R., Lowe, J., & Prince, A. (2019). Downhole geophysical data acquisition during reverse circulation drilling for mining. ASEG Extended Abstracts, 2019(1), 1–2. https://doi.org/10.1080/22020586.2019.12072916
Okada, K. (2022). Breakthrough technologies for mineral exploration. Mineral Economics, 35(2), 429–454. https://doi.org/10.1007/s13563-022-00317-3
Zhang, X., Wu, Z., Zhao, Z., Sun, P., Tang, L., & Shabir, U. (2022). Insight into dust control performance of a reverse circulation drill bit using multiphase flow simulation. Engineering Applications of Computational Fluid Mechanics, 16(1), 841–857. https://doi.org/10.1080/19942060.2022.2047110
