The BC87 Dataset: Application of Hypothetical Event Analysis on Distorted GDS Response Functions and Some Thin Sheet Modelling Studies of the Deep Crustal Conductor.

Abstract

Galvanic distortion of the regional MT responses was inferred from the analyses of the BC87 dataset presented at the MT-DIW1 in 1992. Since perturbation currents also produce an anomalous magnetic field, the GDS response functions were suspected to be distorted in the long period range. In order to reveal the correct regional strike direction, hypothetical event analysis was applied to the data from all sites. With this technique, common regional information, such as strike and impedance phase, can be recovered from distorted response functions. Although the BC87 GDS data are noisy, and the distortion is more of an inductive than of a purely galvanic nature, the resulting regional strike direction of 60° agrees well with the azimuth suggested from previous MT decomposition methods. Thin sheet modelling studies of the geometry of the deep crustal conductor (L-shaped or oblique) at 1000 s imply that along the BC87 profile induction arrows are generally produced by current concentrations around a resistive region associated with the Nelson Batholith. Comparison of the model induction arrows with those determined from the BC87 data indicates that the model featuring the oblique, NE striking conductor (SABC) may be favoured.