Regions of anomalously large telluric interference in IP: The Cobar syndrome

Abstract

Regional differences are observed in the severity of telluric noise interference in induced polarization surveys. In some areas, of which Cobar (New South Wales) is an extreme example, surveys must often be postponed or confined to particular times of day. Using simple resistivity and magnetotelluric models, and typical magnetic micropulsation spectra, we show how this arises. The situation is worst when a very resistive basement is overlain by a thick conductive cover. In that case the IP signal is small on account of the conductive cover, while telluric voltages are large because of the resistive basement. For typical IP frequencies, and for the values of overburden resistivity found at Cobar, noise to signal ratio is shown to be greatest for overburden thicknesses of 200?300 m. This compares with the 80?150 m thicknesses actually observed. Noise to signal ratio is relatively insensitive to dipole length for a given separation of dipole centres.