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

Abstract

A two-dimensional resistivity model was obtained for the COPROD2R data set by trialand-error forward model fitting of the apparent resistivities and phases. Two major conductivity zones were found to explain the data in the zone where the NACP anomaly is located. The first one coincides with previous interpretations of the NACP anomaly, which results in four separate conductive bodies. The second is another deeper conductor, whose top is located at 26 km depth with its base deeper than 40 km. A test removing this deeper conductor was made in order to ensure its detectability.

Electromagnetic images of the Trans-Hudson orogen: the North American Central Plains anomaly revealed

“…(1993), Oldenburg and Ellis (1993), Rasmussen (1993), Schmucker (1993), Schnegg (1993), Takasugi et al (1993), Uchida (1993), Wu et al (1993), Zhao et al (1993), Tournerie et al (1995), Ogawa and Uchida (1996), and Pous et al (1997). The models basically fall into two classes: in one class a number of anomalous conductive mid-crustal regions are electrically disconnected, and in the other class one or more conductive regions that are connected.…”

Introduction to MT-DIW2 Special Issue.

“…This is somewhat surprising, given that the stated RMS misfit is 3.8° in phase, which is larger than the feature in the data that requires the interpreter or inverse code to separate the conductors. A new result obtained by Pous et al (1997a) is the apparent existence of a conducting body in the lowermost crust and uppermost mantle beneath the centre of the profile. consider carefully the arguments put forward for the requirements for a single body, and conclude that the fine structure of the TM phase can be explained by very minor variations in the sedimentary section above the anomaly.…”

“…The first two papers in this special issue describe an interpretation (Pous et al, 1997a) and a re-evaluation of the COPROD2 dataset. This dataset comprises the off-diagonal elements from thirty-five sites crossing the North American Central Plains (NACP) conductivity anomaly (see Jones, 1993b, and references therein).…”

“…, (1993b) argued subtle features in the data required the former, citing in particular that the oneto-two degree decrease in the TM phase at sites over the anomalies could only be explained by discrete, disconnected bodies. The model presented in Pous et al (1997a), obtained using forward trial-and-error model fitting, falls into the category of the multiple-body, with four bodies, each of around 1 ftm, in the crust. This is somewhat surprising, given that the stated RMS misfit is 3.8° in phase, which is larger than the feature in the data that requires the interpreter or inverse code to separate the conductors.…”