1:45 PM - 3:15 PM
[SEM14-P11] A 3-D model explaining magnetotelluric anomalous phases: an oblique low conductive column
Keywords:Magnetotelluric method, 3-D resistivity modeling, Anomalous phases
Magnetotelluric (MT) data is sometimes accompanied by “anomalous” impedance phases (φxy and φyx) in the off diagonal components that deviate from the first (0 < φxy < 90) or third (-180 < φyx < -90) quadrants, especially in long period bands. This phenomenon is called the phases out-of-quadrant (POQ). The POQ has been one of the challenges in MT modeling, as simple 1-D or 2-D models cannot explain it. Previous studies reported that strong inhomogeneity, anisotropy, or particular 3-D structures, such as an L-shaped conductor, could explain the POQ. Besides such models, we discovered that a slanted columnar conductor also generates the POQ. This study shows our systematic investigation of the conditions accommodating the POQ.
For the synthetic tests, we introduced an oblique conductive column (1 Ωm) in the uniform medium of 1000 Ωm. The model space comprised cubic cells with a dimension of 1×1×1 km each. We used the ModEM code (Egbert and Kelbert, 2012; Kelbert et al., 2014) to perform forward calculations and obtain the MT impedance on the ground surface or at any depth by frequency. Then we tried different combinations of vertical length and inclination of the oblique column.
One of our findings is that the inclination angle and length of the column affect the appearance of the POQ. Principally, it only appears on the sloping side. However, the component (xy or yx) accompanied by the POQ depends on the slanting direction of the column. For example, the POQ is seen only in the φyx when the conductor dips to the west. Next, we calculated the induced electric field from the impedance tensor to investigate the behavior of the electric currents around the conductive column. We found that the induced electric field in the region with the POQ tended to point in opposite directions to the surrounding vectors. It is why we have an inverted phase in the long period bands. Furthermore, we observed that current was sucked into the top, while it was discharged from the bottom of the column. These characteristics were common to our models accompanied by the POQ.
A volcanic conduit is one of the typical field examples of such an oblique columnar conductor. Therefore, our study implies that the POQ can be a helpful clue in imaging the geometry of a volcanic magma plumbing system by MT method.
For the synthetic tests, we introduced an oblique conductive column (1 Ωm) in the uniform medium of 1000 Ωm. The model space comprised cubic cells with a dimension of 1×1×1 km each. We used the ModEM code (Egbert and Kelbert, 2012; Kelbert et al., 2014) to perform forward calculations and obtain the MT impedance on the ground surface or at any depth by frequency. Then we tried different combinations of vertical length and inclination of the oblique column.
One of our findings is that the inclination angle and length of the column affect the appearance of the POQ. Principally, it only appears on the sloping side. However, the component (xy or yx) accompanied by the POQ depends on the slanting direction of the column. For example, the POQ is seen only in the φyx when the conductor dips to the west. Next, we calculated the induced electric field from the impedance tensor to investigate the behavior of the electric currents around the conductive column. We found that the induced electric field in the region with the POQ tended to point in opposite directions to the surrounding vectors. It is why we have an inverted phase in the long period bands. Furthermore, we observed that current was sucked into the top, while it was discharged from the bottom of the column. These characteristics were common to our models accompanied by the POQ.
A volcanic conduit is one of the typical field examples of such an oblique columnar conductor. Therefore, our study implies that the POQ can be a helpful clue in imaging the geometry of a volcanic magma plumbing system by MT method.