The inversion of data from complex 3-D resistivity and I.P. surveys

The search and recovery for base and precious metals in recent years has led to surveys in more challenging areas over complex deposits and in extreme terrains. Such deposits frequently have accessory minerals that can be detected by induced polarization (I.P.) surveys. Due to their complex shapes and host terrains, 3-D surveys and inversion models are necessary to accurately resolve them. However, in some cases, the survey lines are not arranged rectilinearly. To accommodate an arbitrary arrangement of the electrodes, a model discretisation that is independent of the electrode positions is used. The rugged terrain can be accurately modelled by the use of the finite-element method where the surface of the mesh matches the topography. Innovative arrays such as the offset pole-dipole array have been used to rapidly survey large areas at a lower cost compared to traditional dipole-dipole arrays. Such arrays frequently have large geometric factors that make it difficult to accurately calculate the I.P. anomalies with the conventional linear perturbation approach that uses the difference of two resistivity calculations. The complex resistivity method, where the I.P. component becomes the imaginary component of the resistivity model, avoids this problem as it effectively decouples the resistivity and I.P. calculations. Furthermore, time-lapse 3-D surveys using surface and borehole electrodes have been conducted to monitor the flow of sodium cyanide solution directly injected in steep-sided ore rock piles for secondary recovery of gold. A 4-D resistivity inversion method is used to map the flow of the solution during the injection process.