Monitoring Of Vertical Deformations At Olkaria Geothermal Power Station

Abstract

There is concrete scientific evidence that shows the presence of vertical deformations within and in the environs of geothermal fields around the world. In this study, an investigation was done on the Olkaria Geothermal Power Station. An overview of various techniques of acquiring data for monitoring earth deformations is presented. Of these methods, precise levelling was selected as a suitable technique to this study. Since the level surfaces are not parallel, the measured height differences depend on the path taken. A definitive determination of height is thus possible only by considering geopotential height. This necessitated measurements of gravity at monitoring points. The basic principle in the deformation monitoring is that one observes the temporal changes of geodetic points with respect to others that are assumed stable. In this study, three epochs of observations were done but all the points were assumed to be unstable. The Gauss-Markov model under the least squares principle was adopted as the basis for the estimation of the required parameters. Since all the monitoring points were assumed unstable, single epoch adjustment was performed using free network adjustment model. The problem of deformation monitoring is delicate in that the deformations to be detected are almost of the same order of magnitude as the accuracy of the observations from which they are derived. Moreover, the existence of outliers can easily be mistaken for deformations. Thus several statistical tests were performed to reduce chances of making wrong conclusions. According to the data observed and the method used, Olkaria Geothermal Power Station was found to be experiencing a maximum ground subsidence rate of about 100 millimetres per year. The areas next to the wells that produce steam were found to be fairly stable and the most likely reason may be the re-injection of water into these wells. The areas that have not been utilized and where steam and other hot gases are being emitted were found to be experiencing uplift. The areas that were slightly far from the Geothermal Station and which were suspected to be stable were, however, found to be unstable