Occam ’ s Inversion and the North American Central Plains Electrical Anomaly

The COPROD2 magnetotelluric (MT) data set obtained over the North American central plains electromagnetic anomaly in the Canadian shield features a predominantly onedimensional response at periods shorter than 14 s and a predominantly two-dimensional response at longer periods (to 910 s). A subset of the COPROD2 data consisting of both strikeparallel and strike-perpendicular components of the MT apparent resistivity and phase data at periods between 14–910 s was subjected to the Occam’s inversion process, which attempts to find maximally smooth models which fit a data set to a specified misfit. After 9 iterations a model was found fitting these data to 10% in resistivity and 2.9◦ in phase, and after a few more iterations excess structure was removed to reveal three discrete conductive zones of resistivity less than 1 Ωm at depths of 8–22 km in a relatively resistive background layer of 100–1000 Ωm. The inversion algorithm was modified to take advantage of the 1D structure of the shorter periods, which were inverted to obtain a model representative of surface sediments which extend to a depth of about 2 km. This surface structure was included in the long-period inversion by imposing a second penalty term in the regularized inversion, and the resulting model featured a broader, more complex conductive anomaly and a pronounced, westward-dipping fabric in the midto lower-crustal rocks. Graphitic rocks can account for the highly conductive parts of the models.