SUMMARY Thanks to the emergence of new technologies developed with the goal of performing large-scale galvanometric induced polarization surveys and thanks a better understanding of the underlying physics of induced polarization, this geophysical method can now be applied in the field of volcanology and geothermal resources assessment. A new approach is developed here for directly inverting the primary and secondary electric fields recorded at a set of independent stations when injecting a primary current. The use of independent stations to measure the primary and secondary electrical fields improves the quality of the data by reducing the capacitive coupling effects inherent to systems based on long cables. It avoids issues associated with using the same electrodes for both current injection and voltage measurements and negative apparent resistivity and chargeability values. With such acquisitions, we can perform true 3-D surveys in areas characterized by complex topography such as volcanoes. The numerical scheme we developed returns as output the electrical conductivity and chargeability fields. The implemented methodology presents several advantages. The first is the use of data types at the stations, for example the electric field intensity, that are independent from the local geometrical station parameters such as electrode spacing and dipole orientation. The second advantage lies in the suitability of the proposed approach to perform large-scale applications since we use a matrix-free approach that does not require the assembly of the Jacobian matrices. The third concerns the possibility of performing the inversion on complex geometries through a consistent use of the finite element method on unstructured meshes in combination with algebraic multigrid preconditioning for the regularization and the solution of the forward and adjoint problems. The computation of 3-D sensitivity maps can also be a real asset in survey design. After validating our approach with a benchmark synthetic case study, we test it on a large-scale induced polarization survey that mimic true field conditions on a volcanic environment with rough topography. Our tests demonstrate the high potential of this electric field approach in volcanology especially for deep (3 km) imagining of the internal structure of volcanoes, which in turn could improve our understanding of hydrothermal systems and allow the monitoring of active volcanoes and the potential risk of collapse.

中文翻译：

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火山岩的诱导极化。4. 大规模诱导极化成像

总结 由于以进行大规模电流计诱发极化调查为目标而开发的新技术的出现，以及对诱发极化的基本物理特性的更好理解，这种地球物理方法现在可以应用于火山学和地热资源评估领域. 这里开发了一种新方法，用于在注入初级电流时直接反转在一组独立站记录的初级和次级电场。使用独立站来测量初级和次级电场通过减少基于长电缆的系统固有的电容耦合效应来提高数据质量。它避免了与使用相同电极进行电流注入和电压测量以及负表观电阻率和可充电性值相关的问题。通过这样的采集，我们可以在火山等复杂地形的区域进行真正的 3-D 勘测。我们开发的数值方案返回电导率和可充电性场作为输出。实施的方法具有几个优点。首先是在站点使用数据类型，例如电场强度，这些数据类型独立于局部几何站点参数，例如电极间距和偶极子方向。第二个优点在于所提出的方法适用于执行大规模应用，因为我们使用了不需要组合雅可比矩阵的无矩阵方法。第三个问题是通过在非结构化网格上持续使用有限元方法结合代数多重网格预处理来对复杂几何进行反演的可能性，用于正则化和解决前向和伴随问题。3-D 敏感度图的计算也可以成为调查设计中的一项真正资产。在通过基准综合案例研究验证了我们的方法后，我们在大规模诱导极化调查中对其进行了测试，该调查模拟了具有粗糙地形的火山环境中的真实现场条件。