3D electrical conductivity tomography of volcanoes

Abstract

Electrical conductivity tomography is a well-established galvanometric method for imaging the subsurface electrical conductivity distribution. We characterize the conductivity distribution of a set of volcanic structures that are different in terms of activity and morphology. For that purpose, we developed a large-scale inversion code named ECT-3D aimed at handling complex topographical effects like those encountered in volcanic areas. In addition, ECT-3D offers the possibility of using as input data the two components of the electrical field recorded at independent stations. Without prior information, a Gauss-Newton method with roughness constraints is used to solve the inverse problem. The roughening operator used to impose constraints is computed on unstructured tetrahedral elements to map complex geometries. We first benchmark ECT-3D on two synthetic tests. A first test using the topography of Mt. St Helens volcano (Washington, USA) demonstrates that we can successfully reconstruct the electrical conductivity field of an edifice marked by a strong topography and strong variations in the resistivity distribution. A second case study is used to demonstrate the versatility of the code in using the two components of the electrical field recorded on independent stations along the ground surface. Then, we apply our code to real data sets recorded at (i) a thermally active area of Yellowstone caldera (Wyoming, USA), (ii) a monogenetic dome on Furnas volcano (the Azores, Portugal), and (iii) the upper portion of the caldera of Kīlauea (Hawai'i, USA). The tomographies reveal some of the major structures of these volcanoes as well as identifying alteration associated with high surface conductivities. We also review the petrophysics underlying the interpretation of the electrical conductivity of fresh and altered volcanic rocks and molten rocks to show that electrical conductivity tomography cannot be used as a stand-alone technique due to the non-uniqueness in interpreting electrical conductivity tomograms. That said, new experimental data provide evidence regarding the strong role of alteration in the vicinity of preferential fluid flow paths including magmatic conduits and hydrothermal vents.

Hydrothermal alteration of andesitic lava domes can lead to explosive volcanic behaviour

“…We further note that hydrothermal alteration typically weakens volcanic rock 19,56–58,64 and that such weakening could reduce the stability of the dome and further increase the likelihood of unexpected dome explosions and associated hazardous pyroclastic density currents 4,19,65–67 . On the basis of our findings, mapping the extent and evolution of hydrothermal alteration at active lava domes using geophysical methods such as electrical 20,21,68,69 and muon tomography 70–72 , spectroscopic methods such as visible and infrared spectroscopy 36,73 , and gas monitoring 26 emerge as an important tools to help anticipate dome explosions at otherwise unpredictable dome-forming volcanoes.…”

Three‐Dimensional Electrical Conductivity and Induced Polarization Tomography of a Rock Glacier

“…Journal of Geophysical Research: Solid Earth method on synthetic and true field data has already been performed in Soueid Ahmed, Revil, Jardani, et al (2018) and Soueid Ahmed, Revil, Byrdina, et al (2018) for the resistivity and chargeability problems and by Qi et al (2018).…”

Imagery of internal structure and destabilization features of active volcano by 3D high resolution airborne electromagnetism

“…Among others, ground resistivity imagery techniques, such as 2D or 3D electrical resistivity tomography (ERT) 10,14,15 , magnetotellurics (MT) and transient electromagnetic soundings (TDEM) 16,17 , are renowned for being well-suited and reliable in such settings. While ERT oftenly provides high resolution imagery of the first hundred meters 18,19 , MT and TDEM provide a smoother image of the bulk resistivity up to few kilometres depth 20 . All these methods have been improved during the last decades to image resistivity variations in 3D 19,20 .…”

“…While ERT oftenly provides high resolution imagery of the first hundred meters 18,19 , MT and TDEM provide a smoother image of the bulk resistivity up to few kilometres depth 20 . All these methods have been improved during the last decades to image resistivity variations in 3D 19,20 . However, ground acquisitions in steep and harsh environments, as it is the case on the Piton de la Fournaise volcano (Fig.…”