Case Study – Copper Mine in North America

HGI has worked on a number of copper mines in Arizona, New Mexico, Sonora, Chile, and elsewhere to characterize the movement of pregnant leach solution (PLS) through copper heaps. Copper heaps are relatively easy to characterize with electrical resistivity geophysics, because the copper sulfate solution is extremely electrically conductive compared to other leach solutions (e.g. cyanide solution used in gold mines). The electrical conductivity of the PLS is high because of the high solution grade and ionic strength, and can be an advantage for copper heaps because the large electrical target contrast compared to dry ore. Common problems encountered in copper heaps include pooling, compaction, side slope seeps, slow percolation, and other adverse hydraulic processes, are easily solved with the electrical resistivity method.

Solution pooling and slow percolation is a common problem in copper heap leach pads. HGI has the experience to diagnose and identify ways to help mitigate against adverse hydraulic processes.

An example resistivity section from a geophysical case study is shown below. The survey included both the top and the side slopes – HGI has a dedicated rope crew that can work within your safety constraints to ensure a full coverage of the heap. The example shows a relatively shallow copper heap leach pad and our electrical resistivity line included a continuous collection of over 2000ft across the surface of the pad. Due to the shallow nature, accurate representation of the liner depth is very important to ensure that features near the liner are properly characterized.

The copper heap characterization figure above is annotated to help highlight important features within the resistivity line. For example, near the liner, there are both high and low resistivity zones. These are attributed to compaction and adequate drainage, respectively. Near the surface, there appears to be a thick layer of solution pooling, likely because the drainage rate is less than the surface application rate. The pooling is causing a perched water table to form, with a hydraulic gradient that is towards the side slope. Along this particular section, a seep was forming along the side slope and it was hypothesized that when the topography intersected the perched zone, a seep will form. There is a lot of information to be gleaned from a resistivity section, like the one above. You can be assured that our interpretation will conform to standard hydraulic processes.

The distinct advantage of having HGI collect and analyze the data for you is the investment we place in equipment, logistics, and having a unique expertise in hydrogeology.

Another valuable way of visualizing a resistivity section from a heap is from a bird’s eye view. Parallel 2D survey lines can be combined and modeled together in 3D space, allowing you to see a more complete picture and identify hydraulic processes across a broader portion of the heap. An example from another geophysical case study is shown below, highlighting horizontal resistivity sections from a survey encompassing one complete heap cell and partial coverage over an adjacent cell. From this vantage one can see clear differences in geophysical properties both laterally and vertically within this region of the heap, highlighting areas of potential solution pooling and compacted zones that may negatively impact the heap’s performance.