Separated spherical harmonic analysis coefficients of the external and internal parts of the observed quiet-day geomagnetic field variations (Sq) for the North Central Nigeria were used to determine the conductivity profile to depths of about 873 km by Schmucker equivalent substitute conductor method. Within the crust, the conductivity increased from 0.027 S/m at a depth of 7.4 km to 0.074 S/m at 15.5 km and 0.098 S/m at 24 km depth. It suddenly rose to 0.181 S/m at 26.5 km and then decreased to 0.131 S/m at 37.1 km depth. The conductivity within the upper mantle rose gradually from 0.043 S/m at 60.4 km to 0.045 S/m at 100.7 km and reached 0.071 S/m at 220.6 km. It fluctuated from 0.092 S/m at 273.6 km to 0.105 S/m at 457.5 km and got to 0.118 S/m at 523.1 km. Finally, it reached 0.163 S/m at 601.9 km and 0.271 S/m at 727.3 km depth. There seemed to be some evidence of discontinuities near 71-165 km, 165-221 km, 221-405 km and 405-666 km. The region showed a roughly exponential increase of conductivity with depth. The profile gave evidence of a less steep increase in conductivity with depth to about 405 km and very steep increase in conductivity thereafter.
Key words: North Central Nigeria, crust, upper mantle, spherical harmonic analysis coefficient (SHA), geomagnetic field variation, quiet day, electrical conductivity-depth structure.
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