Deep learning electromagnetic inversion with convolutional neural networks

Geophysical inversion attempts to estimate the distribution of physical properties in the Earth's interior from observations collected at or above the surface. Inverse problems are commonly posed as least-squares optimization problems in high-dimensional parameter spaces. Existing approaches are largely based on deterministic gradient-based methods, which are limited by non-linearity and non-uniqueness of the inverse problem. Probabilistic inversion methods, despite their great potential in uncertainty quantification, still remain a formidable computational task. In this paper, I explore the potential of deep learning (DL) methods for electromagnetic (EM) inversion. This approach does not require calculation of the gradient and, once the network is trained, provides results instantaneously. Deep neural networks based on fully convolutional architecture are trained on large synthetic data sets obtained by full 3-D simulations. The performance of the method is demonstrated on models of strong practical relevance representing an onshore controlled source electromagnetic CO2 monitoring scenario. The pre-trained networks can reliably estimate the position and lateral dimensions of the anomalies, as well as their resistivity properties. Several fully convolutional network architectures are compared in terms of their accuracy, generalization and cost of training. Examples with different survey geometry and noise levels confirm the feasibility of the DL inversion, opening the possibility to estimate the subsurface resistivity distribution in real time.