Deep Conductivity Structure Research Articles

Magnetotelluric and Magnetovariational Studies in Atlantic Canada

Summary The geographic pattern of the Atlantic Canada coastal geomagnetic induction anomaly is delineated using both the vertical transfer function and the attenuation of the ‘normal’ vertical component. Numerical transfer function modelling in the spatial-time domain over the frequency range 5- to 120-min period indicates anomalous induction due to the transition from highly conductive deep crust and mantle beneath the Scotian Shelf and Grand Banks to more resistive structures inland. The enhanced deep crustal conductivities may arise from hydration processes associated with long term low grade tectonic subsidence. Additional observations in eastern North America suggest the Atlantic Canada anomaly is related to a large region of high conductivity underlying the seaward side of the Appalachian system. High frequency (<10-min period) local magnetic induction anomalies adjacent to shallow salt water bodies are quantitatively approximated by Cagniard theory current flow within the water masses. Anomaly intensification by electrical current funnelling is apparent at one station. Telluric fields on land are strongly perturbed by superficial conductivity contrasts making them comparatively less useful than geomagnetic soundings for the study of deep conductivity structure.

Anomalies of Geomagnetic Variations in the Southwestern United States

A survey with temporary geomagnetic field stations (Askania Variographs) has been conducted in the southwestern United States in order to study local differences of geomagnetic variations. Anomalous large Z-variations, accompanied with a slight reduction of the variations in D and H, have been recorded along the California coastline for long-period (Sq) and short-period (bays, ssc's) variations. This Californian coastal anomaly has been interpreted as edge-effect of the Pacific Ocean and its effect upon the induction within the highly conductive substratum in the upper mantle, thereby allowing conclusions about the deep conductivity structure Less pronounced anomalous variations further inland in central California (Sierra anomaly) seem to be caused by conductivity differences in the continental surface layers. A third anomaly between Tuscon, Arizona, and Sweetwater, Texas (Texas anomaly), has been interpreted as a change in the deep conductivity structure which occurs along the eastern margin of the Rocky Mountains.