Three-dimensional inversion of gravity data using implicit neural representations

Abstract: Inversion of gravity data is an important method for investigating subsurface density variations relevant to diverse applications including mineral exploration, geothermal assessment, carbon storage, natural hydrogen, groundwater resources, and tectonic evolution. Here we present a scientific machine-learning approach for three-dimensional gravity inversion that represents subsurface density as a continuous field using an implicit neural representation (INR). The method trains a deep neural network directly through a physics-based forward-model loss, mapping spatial coordinates to a continuous density field without predefined meshes or discretisation. Positional encoding enhances the network's capacity to capture sharp contrasts and short-wavelength features that conventional coordinate-based networks tend to oversmooth due to spectral bias. We demonstrate the approach on synthetic examples including Gaussian random fields, representing realistic geological complexity, and a dipping block model to assess recovery of blocky structures. The INR framework reconstructs detailed structure and geologically plausible boundaries without explicit regularisation or depth weighting, while significantly reducing the number of inversion parameters. These results highlight the potential of implicit representations to enable scalable, flexible, and interpretable large-scale geophysical inversion. This framework could generalise to other geophysical methods and for joint/multiphysics inversion.