Electrical conductivity of mantle in the North Central region of Nigeria

The mantle electrical conductivity profile of the North Central region of Nigeria was determined using the quiet day ionospheric current variations (Sq). The employed magnetic averaged hourly data were obtained from Magnetic Data Acquisition System (MAGDAS) ground based observatories at two Nigerian stations located at Ilorin (8°30'N, 4°33'E) and Abuja (8°59'N, 7°23'E) for the year 2009 and 2010. The magnetometer data from Pankshin (9°20'N, 9°27'E) and Katsina-Ala (7°10'N, 9°17'E) for the same years were equally employed. The separation of both the internal and external field contributions to the Sq variations was successfully carried out employing spherical harmonic analysis (SHA). Transfer function was performed in computing the conductivity–depth profile for North Central region of Nigeria from the paired external and internal coefficients of the SHA. The conductivity value of approximately 0.039S/m was estimated at a depth of 100km which rose gradually to 0.087S/m at 207km depth and 0.142S/m at 367km (close to the base of upper mantle). Subsequently, the conductivity profile continued rising to a value of 0.144S/m at 442km, 0.164S/m at 653km and 0.174S/m at 710km. Finally, value of approximately 0.195S/m at a depth of 881km and 0.240S/m at 1100km depth were recorded at the lower mantle with no indication of leveling off. Some evidence of discontinuities near 100–214km, 214–420km, 420–640km, 640–900km and 900–1100km were clearly obvious. The sharp increase in conductivity from about 100km depth to 230km was interpreted to correspond to the global seismic low velocity zone – the asthenosphere.