The North American Central Plains (NACP) conductivity anomaly, originally located by widely-spaced geomagnetic array studies, was more precisely located 75 km east of the previous position by the dense magnetotelluric survey undertaken by contract for PanCanadian Petroleum Limited (Jones and Savage 1986). However, Maidens and Paulson (1988) interpret from their data that an anomaly in electrical conductivity does indeed exist at 105 degrees W, which is believed to be the western boundary of the proposed NACP zone of Alabi et al. (1975). Maidens and Paulson state ``Although there is no question about the existence of the major anomaly reported by Jones and Savage, there is a question about the name assigned to it''.
The purpose of this comment is fourfold:-
1: To discuss the resolution of the GDS observations by Alabi et
al. in southern Saskatchewan, and show that
their data are very ambiguous in the vicinity of the MT
profiles, and accordingly little confidence should be given
to the precise location of the NACP from these GDS observations.
2: To demonstrate that on the PanCanadian
data no anomaly exists at 105 degrees W longitude after the MT responses
have been corrected for static shift,
3: To discuss the resolution on Maidens and Paulson's data
of the parameters of their 1D models presented, and show that their
interpretation of an anomaly near 105 degrees W longitude appears to rely
solely on their long period phase estimates at one of their sites.
Such ``single station'' anomalies should always be treated with utmost
caution.
4: To illustrate that E-polarization 1D modelling of 2D data, as
undertaken by Maidens and Paulson, can result in ``false conducting
layers''.
It will be concluded that there is only one anomaly in this region and that it is centred on 103 degrees W longitude. It is suggested that the supposed anomaly in Maidens and Paulson's data will disappear after considered interpretation of the data.
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