CAN MAGNETOTELLURIC DATA RESOLVE LOWER-CRUSTAL CONDUCTORS

It is commonly accepted that the lower crust is electrically conductive. The most commonly used measurement technique is magnetotellurics (MT) using natural plane waves, but also a few controlled-source experiments have been carried out to study this problem. Previous work using MT and controlled-source data has shown that in areas devoid of a sedimentary layer a hypothetical homogeneous lower crust can be resolved accurately. Adding a conductive surface layer degrades the resolving power of MT and controlled-source data. We have studied this problem quantitatively by varying the conductances (product of the conductivity and thickness) of the sedimentary and lower-crustal layers. Both MT data and the data calculated with a controlled-source (grounded dipole) technique are used for investigation. We show that for sedimentary conductances of 200-2000 S, MT data combined with controlled source data may identify a conducting lower crust provided its resistivity is less than a few hundred OMEGA m, but its thickness and resistivity cannot be resolved.