INSTITUTO DE GEOCRONOLOGIA Y GEOLOGIA ISOTOPICA
Unidad Ejecutora - UE
congresos y reuniones científicas
Observed out-of-quadrant magnetotelluric phase due to large scale 3-D structure
BOOKER JOHN; AURORA BURD; RANDALL MACKIE; ALICIA FAVETTO; CRISTINA POMPOSIELLO
Workshop; 21 Electromagnetic Induction Workshop; 2012
Magnetotelluric (MT) phases substantially exceeding 90° are observed at periods greater than 1,000s along an east-west profile in Entre Rios Province of Argentina. They are not due to static distortion because phase tensor (PT) skew is large and PT phase and skew are also out-of-quadrant. Coordinate rotations inferred from PT ellipse axes, induction vectors and impedance decomposition all fail to suppress the anomalous phases. An isotropic three-dimensional (3D) inversion fits all impedance elements and the vertical to horizontal field transfer function to high precision and hence also the PT parameters. Thus anisotropy that has been suggested for similar observations in other parts of the world is not required. These anomalous phases also have nothing to do with the nearby Atlantic Ocean because they persist when the ocean is made as resistive as the continent. Although a single profile would normally not be considered sufficient to extract reliable 3D structure, the model agrees reasonably well with structure that can be inferred from regional geology. The area of anomalous phase predicted by the inversion is a narrow band that strikes 45° to the profile and is along a boundary with very large resistivity contrast. Rotating the coordinate system of the predicted responses to this strike removes the anomalous phase along most of this boundary, leaving out-of-quadrant phase only in a small region at the end of the profile where the model boundary veers north-south. This situation appears similar to out-of-quadrant phases found by Ichihara and Mogi (2009).