Mapping Geothermal Heat Source In Homa Hills Using Gravity Technique

ABSTRACT

Homa Hills geothermal prospect is located on the Nyanzian rift west of the Kenyan rift system. The manifestation of hot springs and steaming grounds in the area has revealed its geothermal potential. In order to fully asses the potential of this field, the heat source which is one of the main features of a geothermal system had to be properly located based on its perturbation on the gravity field. This enables gravity technique which is a high precision method for measuring density contrast that relates to the subsurface rocks to delineate its effect. The variation of density in a geothermal environment is as a result of magma intruding the crust in form of dykes or other structural conditions of the subsurface rocks. In Homa Hills geothermal field, reconnaissance research was done using Magnetotellurics and Transient electromagnetic by Geothermal Development Company (GDC) in an attempt to locate the heat source and establish the role of structures in the control of fluid dynamics. However, no supportive research using other geophysical methods has been done to compliment the Magnetotellurics and Transient electromagnetic methods in delineating the general extend density and depth of the heat source. In this study, Gravity survey was conducted and the data processed to remove all other effects not related to the subsurface density changes. The Bouguer gravity anomaly was interpreted qualitatively by inspecting the profiles and the grids for variations in the gravitational field and the residual anomaly was separated from the regional gravity field. The anomaly was also interpreted quantitatively by both direct and indirect methods. Direct methods involved the use of the anomalies half-width and gradient-amplitude ratios in estimating the depth of the causative body. Indirect methods involved both Euler Deconvolution and forward modeling where a two-dimensional gravity model along selected profiles was generated by computer software. This was done in order to „best fit‟ the observed gravity anomalies in an attempt to estimate depth, density and extend of the prospective heat source. Sharp dyke like structures were delineated at shallow depths of approximately 225, 10, 256, 256 and 512m with a width of 2250, 800, 750, 250 and 1250m respectively by Euler Deconvolution method. These results were found to be consistent with the results from forward modeling which shows the top of these structures at a depth of 192, 33, 206, 850 and 431m with a width of 3321, 604, 166, 394 and 821m respectively. The models obtained are in agreement with the geology of the area and the results of the Magnetotellurics and Transient electromagnetic methods conducted by the Geothermal development company. The hot springs therefore on the Southern part of the study area could be as a result of the heating effect from these shallow, high temperature dykes. As a recommendation these hot springs could be subjected to direct uses.