open access

https://hdl.handle.net/2115/92980

The inclined conductive column effect: a new simple model for magnetotelluric anomalous phases

英語

Numerical modelling

2024-09

Royal Astronomical Society

Geophysical Journal International

238(3)

1825-1839

Magnetotelluric data are sometimes accompanied by ‘anomalous’ impedance phases (⁠xy and yx) in the off-diagonal components deviating from the first (0° < xy < 90°) or third (−180° < yx < −90°) quadrant, especially in long-period bands. This phenomenon is called the phases out-of-quadrant (POQ). The POQ poses a challenge in magnetotelluric modelling because simple 1-D or 2-D models cannot explain it. Previous studies have reported that strong inhomogeneity, anisotropy, or particular 3-D structures, such as the L-shaped or cross-shaped conductors, could explain the POQ. Aside from these models, we have discovered that a slanted columnar conductor also generates the POQ. Our systematic investigation through the synthetic forward modelling of an inclined conductive column with a varying geometry showed that the inclination angle and the column length may affect the POQ appearance. We investigated herein the behaviour of the electric currents around the inclined conductive column embedded in a resistive half-space. We found that the induced electric field in the region with the POQ tends to point in the opposite direction to the surrounding vectors. This result can reasonably explain the inverted phase in long-period bands. Furthermore, we confirmed that current is sucked into one end of the column, but discharged from the other end, suggesting that the column works as a current channel. The localized reverse vectors are associated with the current channelling along the inclined conductor, which generates the POQ. A volcanic conduit within a resistive host rock is one of the typical field examples of such an inclined channel. Our study suggests that the POQ is a helpful clue in imaging the geometry of a volcanic magma plumbing system through magnetotelluric surveys.

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Version of Record

This article has been accepted for publication in Geophysical Journal International ©: 2024 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.

0956-540X

1365-246X

AA10717798