A simple 2-D explanation for negative phases in TE magnetotelluric data

Abstract

We present magnetotelluric (MT) data collected in central Australia that display unusual negative transverse electric (TE) phases. Previous explanations for anomalous TE phases on land have relied on anisotropy, complicated 3-D geometries or coherent noise. In contrast, the central Australian data are free from coherent noise while phase tensor analysis shows that the survey region is 2-D and that the negative phases are an inductive effect. The survey was carried out in a grid that covers resistive basement rocks and conductive sedimentary cover. Stations located on the resistive basement display normal phase behaviour while stations located on the conductive cover display negative TE phases at periods of 0.01–0.1 s. Forward modelling of the region and inversion of the data shows that a shallow, laterally extensive, bounded conductor that overlies a resistor can produce the observed negative TE phases. An investigation of TE Poynting vectors for such a system shows that there is a 'collision zone' near the bounding edge of the conductor where energy that is diffusing downwards collides with energy that has been inductively coupled to the conductor and is diffusing upwards. At the base of the conductor this causes a cusp in TE apparent resistivity and phases that wrap through 360°, a phenomenon previously observed in the ocean-side of the TE coast effect. Negative phases extend to the surface of the conductor, where measurements are made in a land MT setting. The resistivity contrast between the conductive and resistive zones must be at least 1000 to produce negative phases for land MT. The period range and distance from the boundary of the conductor at which the negative phases are observed can be estimated from a combination of the thickness of the conductor and the resistivities of the two zones. The results presented here are the first example of an isotropic 2-D setting producing negative phases on land and represent an alternative explanation for observations of anomalous data.