The COPROD2 Dataset: Tectonic Setting, Recorded MT Data, and Comparison of Models.

Abstract

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 electromagnetic induction community. The data are from stations along a 400 km east-west profile in southern Saskatchewan and Manitoba, Canada, crossing the thick Paleozoic sediments of the Williston basin. Within the basement beneath the sediments lies one of the world's longest and most enigmatic crustal conductivity features-the North American Central Plains (NACP) conductivity anomaly. Also, at the eastern extremity of the profile is a second basement anomaly (TOBE) interpreted to be associated with the Thompson Nickel Belt at the Superior-Churchill boundary. The MT data were corrected for static shifts, and only the ofd diagonal impedance data together with the transfer function data, and their errors, were made available. These MT data are of wide bandwidth (384 Hz-1820 s) and high quality (impedance errors typically <2%), and require sophisticated modelling and inversion in order to extract as much meaningful information as possible from them. A challenge for those interpreting these data is that there is a very small, but critical, response in the B-polarization data to the presence of the basement anomalies. In this introductory paper, the previous electromagnetic, and other geophysical, studies of the NACP anomaly, and its tectonic setting within the Trans-Hudson orogen, are described. Representative data from the COPROD2 data are illustrated, and models derived by various groups are shown. Comparisons of these models reveals that a single measure of misfit is an inadequate description of how well a model fits a dataset; one must compare the model fit to the data at virtually each datum 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 highlight the sensitivity in B-polarization mode data to breaks in the resistive layer, whereas the E-polarization data sense regions of enhanced conductivity.

COPROD2 Revisited: Can the B-Polarization Phase Distinguish between Single- and Multi-Body Anomalies?

“…If the second value is adjusted to, say, 3.7 Q m, the B-polarization phase responses at 59.6 s and 85.3 s become those shown in Fig. 1 where they are compared with the corresponding responses for the original agarwal model, which have also been reproduced with error bars by Jones (1993) in his Fig. 9.…”

“…In his discussion of the COPROD2 dataset, Jones (1993) has stressed the importance of a "very small but critical" minimum in the B-polarization phase responses for periods 56.9 s and 85.3 s in the region stretching from coordinate -50 km to the origin. He claims this is the key to deciding whether the NACP anomaly is a horizontally continuous structure or whether it comprises a number of electrically isolated conductive bodies.…”

Two-Dimensional Forward Modeling of the NACP Anomaly (COPROD2R Data).

“…These constitute a structure which is similar to the "multi body" (JONES 2) model of the NACP anomaly (Jones, 1993). Although these conductors explain the divergence between both polarizations around 20 s, they are not the only two-dimensional structures, since the divergence remains at longer periods.…”

“…The main difference with respect to previous models of the COPROD2 data set (see Jones, 1993) is the presence of the deepest conductor. To ensure that it is detected by the MT data, a test of sensitivity to its removal from the model was performed.…”

“…Apparent resistivities corrected for static shift, phases, and geomagnetic transfer functions were provided in a period range from 0.0026 s to 1820 s. The electrical structure was predominantly two-dimensional (Jones, 1993), the measurement axes being NS and EW, so that the E-polarization direction is the northward-directed electric field. A major conductive anomaly, the North American Central Plains (NACP) conductivity anomaly (Jones and Savage, 1986), was detected within the basement, and another one on the eastern part of the profile called the TOBE conductive anomaly.…”