Establishment of Absolute Gravity Stations Referenced to International Gravity Station Network of 1971 Datum in 21 Counties in Kenya

Abstract

Gravimeters measure relative readings and to obtain absolute gravity values, they have to be referenced to a station whose gravity is accurately known. International Gravity Stations Network of 1971 (IGSN-71) consists of stations distributed all over the world whose absolute gravity has been measured accurately. In Kenya there are three IGSN-71 stations located in Nairobi, Kisumu and Mombasa. To carry out gravity survey at any location in Kenya, at least one of the three reference IGSN station has to be used as a reference station. The stations are sparsely spaced and there is need to establish more stations closer for any researcher conducting local surveys and to support; geodynamic studies, global climate change monitoring, international studies of secular variations of gravity among other applications. The IGSN-71 datum was computed based on the 1967 geodetic reference system and there was need to update new gravity values to the 1980 geodetic reference system, it also contains a Honkasalo Term that has been deemed inappropriate. A Worden Sodin gravimeter model 403 was used to determine relative gravity of the new station with reference to IGSN-71 datum. Profile method survey design was used to collect data hence ensuring efficient monitoring of gravimeters’ drift. The obtained gravity values of new stations were subject to; least squares adjustment, tidal correction, and drift correction in order to obtain a fair representation of absolute gravity at those points. The gravity values obtained are accurate to 0.15 mgal hence they are categorized as the 3rd order gravity control network. The residues of adjustment obeyed the approximately 50% positive and 50% negative rule, while normalized residues obtained were below 3 and the standard errors of unit weight of all adjusted values were below 1, showing that the weighting and adjustment process was excellent.