Magnetic disturbance fields were recorded during the spring and fall of 1969 with two magnetometer arrays in the Great Plains province of the United States. The purpose of these studies was to map magnetic variation anomalies arising from inhomogeneities of crustal electrical conductivity; we wished to find regions where plane layered models yield apparent resistivity curves that are adequate approximations to curves determined from magnetotelluric measurements.Variation anomalies were found to be related to lateral changes in electrical conductivity of the upper crust, changes associated with sedimentary features. In southeast Oklahoma, induced currents are concentrated in the conductivity sediments of the deep Anadarko basin. These currents give rise to attenuated vertical variation fields to the west and south of the basin and to enhanced vertical fields to the north and northeast. Reversed vertical fields are observed for stations in north central Texas, close to the Ouachita tectonic belt which separates Paleozoic sediments in West Texas from younger sediments in the Gulf Coast plains. This distribution of fields indicates concentration of induced currents in the highly conductive coastal plains sediments. A small variation anomaly is associated with the region of the midcontinent gravity high in the northern Great Plains. The anomaly is probably caused by currents in the sedimentary troughs on the steep flanks of the Precambrian basalts, which give rise to the gravity anomaly.

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