THE COPROD2 DATASET - TECTONIC SETTING, RECORDED MT DATA, AND COMPARISON OF MODELS

To facilitate a comparative study of approaches and algorithms for two -dimensional interpretation of magnetotelluric (MT) data, a dataset of thirty-five sites, called COPROD2, was distributed to the electromagn etic induction community. The data are from stations along a 400 km ea st-west profile in southern Saskatchewan and Manitoba, Canada, crossin g the thick Paleozoic sediments of the Williston basin. Within the bas ement beneath the sediments lies one of the world's longest and most e nigmatic crustal conductivity features-the North American Central Plai ns (NACP) conductivity anomaly. Also, at the eastern extremity of the profile is a second basement anomaly (TOBE) interpreted to be associat ed with the Thompson Nickel Belt at the Superior-Churchill boundary. T he MT data were corrected for static shifts, and only the off-diagonal impedance data together with the transfer function data, and their er rors, were made available. These MT data are of wide bandwidth (384 Hz -1820 s) and high quality (impedance errors typically <2%), and requir e sophisticated modelling and inversion in order to extract as much me aningful information as possible from them. A challenge for those inte rpreting these data is that there is a very small, but critical, respo nse in the B-polarization data to the presence of the basement anomali es. In this introductory paper, the previous electromagnetic, and othe r geophysical, studies of the NACP anomaly, and its tectonic setting w ithin the Trans-Hudson orogen, are described. Representative data from the COPROD2 data are illustrated, and models derived by various group s are shown. Comparisons of these models reveals that a single measure of misfit is an inadequate description of how well a model fits a dat aset; one must compare the model fit to the data at virtually each dat um to avoid trends in the misfit residuals. Additionally, reliable and consistent error and static shift estimates are essential in order to obtain high resolution images of the Earth's conductivity structure. Finally, the COPROD2 data high-light the sensitivity in B-polarization mode data to breaks in the resistive layer, whereas the E-polarizatio n data sense regions of enhanced conductivity.