Wilsonville Limestone Quarry, Northland, New Zealand
Underground Void Detection using Geophysics
Health and Safety is paramount in quarrying, where both natural and man-made hazards must be managed. Before extending quarry pits, operators must confirm whether subsurface voids, cavities, or weaknesses exist that could trigger collapse or subsidence. Our client required confidence that the quarry floor was safe for ongoing operations and future expansion.
Cook Costello was engaged to deliver a large-scale, non-intrusive geophysical survey across 15,000 m² of quarry floor. 45 earth resistivity profiles were collected, with specialised methods developed to suit the hard limestone ground. In partnership with Geocivil, over 800 small pre-drilled holes were installed to achieve reliable resistivity readings down to 20 m. The results were analysed to produce a detailed subsurface hazard map for the quarry operators.
The survey identified several large, near-surface structures filled with saline mud rather than minable limestone. These findings were confirmed through 32 targeted boreholes, which demonstrated a very high success rate in the accuracy of the geophysical data. The hazard map was then updated with both geophysical and intrusive drilling results, creating a comprehensive model of subsurface conditions.
This critical investigation directly shaped quarry planning, preventing risky and costly excavation into unsuitable ground. Key benefits included:
Worker Safety – Risks of collapse and instability were identified and avoided.
Operational Certainty – Expansion plans were refined based on reliable, site-specific data.
Proof of Accuracy – Borehole verification confirmed the reliability of non-intrusive testing in a high-risk environment.
By combining advanced geophysics with drilling verification, Cook Costello provided clear, actionable insights that safeguarded quarry workers, reduced operational risk, and supported safe, efficient development.
You can read more about this project here…
Power in Numbers
Project Gallery
