THE NIGERIAN STANDARD GRAVIMETER CALIBRATION LINE

Abstract

The Nigerian Standard Gravimeter Calibration Line (NSGCL) is derived from a series of precise gravity measurements made with three LaCoste and Romberg Model G gravity meters in order to establish a primary gravity network for Nigeria. The NSGCL extends from Jos in central Nigeria to Illela in Northwestern Nigeria, thus covering a total distance of about 1483 km. Its extreme absolute gravity values are 977844.608 mGal at Jos and 978221.324 mGal at Illela which yield a maximum interval gravity value of 376.716 mGal. The ten base stations along the line are fairly uniformly spaced both in terms of their spatial separations and the interval gravity values between consecutive base stations. The absolute gravity values along the calibration line are referred to the IGSN 71 datum. The average accuracy of the interval gravity values within the calibration line is ± 15 μGal with worst and best values of ± l8 μGal and ± 9 μGal respectively.Due to the large length of the NSGCL, a supplementary calibration line which runs from Jos to Katsina was appended to it. Besides its relatively short length of 579 km, the appended calibration line has a comparative range of 304.811 mGal.An evaluation of the existing calibration line shows that it can no longer meet the requirements for the field calibration of gravimeters due to its internal inconsistencies. It is therefore concluded that the NSGCL should effectively replace the existing calibration line.