2.5D Inversion of Airborne Time Domain Electromagnetic Data through Constrained Occam and Conjugate Gradient Method to Image a Polymetallic Deposit

Articles in Press

Document Type : Research Paper

Authors

School of Mining Engineering, College of Engineering, University of Tehran, Tehran, Iran

Abstract

Porphyry deposits are major sources of copper, molybdenum, and gold. Airborne time-domain electromagnetic (TDEM) surveys play a crucial role in identifying conductive sulfide mineralization, particularly in geologically complex environments. This study employs 2.5D inversion of airborne TDEM data to image the Bell polymetallic deposit in British Columbia, Canada, offering improved accuracy over 1D methods in resolving complex geological structures, while maintaining lower computational demands than full 3D modeling. The methodology employs constrained Occam inversion with conjugate gradient optimization implemented in the ArjunGUI software. Forward modeling was conducted using finite-element methods in the wavenumber domain to simulate subsurface conductivity, and the results were subsequently transformed into time-domain using Fourier techniques.The inversion minimizes data misfit and model roughness using Lagrange multipliers and parameter bounds. Geological constraints, such as fixed background resistivities, were incorporated to ensure model accuracy. Inversion of an airborne TDEM survey line across the Bell deposit produced a resistivity model that highlights significant low-resistivity anomalies at approximately 100 m depth. These anomalies are associated with phyllic alteration zones and inferred fault structures that facilitated fluid circulation. High-resistivity areas delineate potassic alteration zones, which align with prior geophysical data and confirm the model's reliability. The 2.5D inversion provided enhanced accuracy compared to 1D methods and greater efficiency than 3D modeling, thereby facilitating targeted exploration in porphyry systems. The main limitations include high noise in late-time channels and potential mesh distortions due to topography. These limitations suggest that improvements in mesh discretization are needed for for future applications. Overall, this technique demonstrates the potential of 2.5D TDEM
inversion as a practical and efficient approach for subsurface imaging and resource evaluation in polymetallic deposits.

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Article Title [Persian]

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References

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Articles in Press, Accepted Manuscript
Available Online from 27 October 2025

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  • Receive Date: 03 August 2025
  • Revise Date: 08 October 2025
  • Accept Date: 27 October 2025

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