One-Dimensional Model Study of the PNG Dataset Using Site-Independent Groom-Bailey Decomposition

Conventional magnetotelluric (MT) analyses of the Papua New Guinea (PNG) dataset reveal that it is a very good two-dimensional (2D) dataset with relatively high S/N ratios up to 10 2 s. However, traditional Swift's angles are unstable at some sites and differ slightly site to site. This implies that weak but significant distortions are present at all sites. Hence, the regional 2D strike angle was sought by minimizing the global X 2 misfits derived from the site- and frequency-independent Groom-Bailey (GB) decomposition at seven selected sites. The other three sites were eliminated by X 2 -test. The strike angle was determined to be 60° west of geographic north. One-dimensional (1D) Monte-Carlo inversion was performed using the Berdichevsky averages of the decomposed 2D principal tensor elements. The 2D model simply based on the 1D inversions was able to reproduce the observed impedances, except for the systematically high values of TM phase in the period range of 10 -1 to 10 s. This indicates that a 1D approximation of the electrical conductivity structure beneath this region is reasonably valid. The 1D models indicated the existence of a thin (up to 2.5 km) resistive layer just below the surface, suggesting the 'single' Darai limestone.