Locating copper-nickel ore bodies using multi-parameter inversion of transient electromagnetic data

As a vital tool for locating metal deposits, the transient electromagnetic method (TEM) is extensively employed in geophysical surveys, but traditional TEM relying solely on resistivity parameters struggles to distinguish ore bodies from non-ore anomalies. TEM detection in metal mining areas is particularly susceptible to interference from the induced polarization (IP) effect of highly polarizable minerals, leading to distorted interpretations. A case study from a typical mining area in Xinjiang demonstrates that the proposed multi-parameter TEM inversion technique, which jointly utilizes resistivity and polarizability, can achieve precise delineation of copper-nickel ore bodies. Using the Cole-Cole model to characterize the IP effect, transient responses incorporating IP effects are solved through frequency-domain forward modeling combined with discrete cosine and sine transforms. The inversion employs a hybrid approach, integrating Particle Swarm Optimization (PSO) with a Genetic Algorithm (GA). The objective function incorporates minimum length constraints for polarizability and first-order smoothing regularization for resistivity, frequency dependence, and time constant, along with lateral continuity control to enhance profile consistency. A 600 m×600 m transmitter loop was deployed at the research area, completing measurements along 7 survey lines. Multi-parameter inversion demonstrated excellent data fitting. Resistivity sections revealed a deep low-resistivity layer exhibiting a structure dipping northward on the west side and relatively flat on the east side, buried at 150∼250 m. High polarizability anomalies were concentrated at depths of 50∼200 m, spatially correlated with low-resistivity zones. Resistivity and polarizability thresholds for ore body delineation were established based on an analysis of the local geological context. The delineated ore bodies exhibit discontinuous distribution, primarily located between 50∼200 m depth. The mineral body extends only to a limited depth, but the non-extinction at the periphery indicates the potential for mineral exploration in the periphery. The localization results are consistent with drill hole verification and significantly narrowed the anomaly range compared to single low-resistivity anomalies. The study demonstrates that the TEM multi-parameter inversion method can effectively process TEM data from metal mining areas, simultaneously obtaining subsurface resistivity and polarizability parameters. The combination of these parameters enables high-precision 3D localization of copper-nickel ore bodies. This research provides a methodology for direct ore body localization using TEM and holds reference value for the exploration and localization of concealed deposits using TEM.