Borehole Gravity Mining Applications Case Studies

English Abstract:

This presentation reviews the Borehole Gravity technology and shows results from exploration surveys conducted by Abitibi Geophysics. GRAVILOG is a slim-hole gravimeter developed by Scintrex (Nind et al, 2007, First Break v 25). Based on miniaturized CG-5 surface meter technology, it resolves the gravitational field to 1 microgal with a repeatable accuracy of <10 microgals. The primary uses of borehole gravity in mining are detection and estimate of tonnage of mineralization and determination of in-situ apparent density.

Now explorationists can test an electromagnetic anomaly for a coincidental excess mass prior to determining the volume of mineralization. Borehole gravity readings and density determinations are not affected by poor core recovery, washouts, or cementing because measurements have a large radius of exploration and are not affected by the casing. In-situ apparent density is useful for deposit evaluation, grade control, structure, and rock property analysis. In-situ bulk density determinations helped James Mine in Labrador to complete the orebody tonnage estimate in spite of poor core recovery due to strong alteration.

The residual borehole gravity response over a nickel deposit near Sudbury showcases a typical cross-over anomaly. From this one-hole gravity survey a tonnage estimate was made that was close to the geologically derived estimate based on many holes.

A multi-hole gravity survey on Coulon Mines Lens 44 demonstrates excellent correlation of the 3D mass model from an unconstrained Stochastic 3D Inversion (Pejman Shamsipour et al, 2010, Geophysics v 75) compared to a geological model based on drilling. The borehole gravity detects the in-hole and the off-hole excess mass and the 3D unconstrained inversion accurately positions it straddling the geological model.

Borehole gravity data collected in five holes surrounding Hudbay’s Lalor Mine show how density variations correlate with lithology and identify a potential new target.

Prudent use of borehole gravity to detect and estimate excess mass of target mineralization early in the exploration cycle can save money and time. Similarly, in-situ density measurements improve information on lithology and grade control. Multi-hole gravity data can be inverted to accurately position the excess mass model in a 3D subsurface space.