A Gravity Survey Of The Jombo Hill Area, South Coast Kenya

Abstract

The aeromagnetic survey of the Jombo Hill Area conducted in l977 revealed an anomaly of 2000 gammas in amplitude in the area of the intrusive rocks. This value gtands,out as one of the highest values ever recorded in Kenya. In order to investigate the anomaly further, a gravity survey was conducted in the area within 40 km radius_of Jombo hill. After the necessary corrections to the observed data they were compiled into a Bouguer gravity map of the area. The Bouguer gravity anomalies range from - 40 to over 50 mgals. The peak of the anomaly occurs immediately to the south of the hill and most of the contours close on this peak. It is noticeable that the magnetic and gravity anomalies are roughly coincident. The relative ease and convenience of gravity 3D modelling relative to magnetic 3D modelling, in the context of this study, dictated the placing of a greater emphasis on the former in the quantitative interpretation. The gravity anomaly is interpreted in terms of an inverted funnel like body of density 3.1 gcm-3, with its top surface at about l km depth. Estimates from the magnetic map yield a similar value for the depth to the top of the body. Most of the magnetization in these rocks is the induced type and is in the direction of the ambient field. The large grain size has an appreciable effect on the magnetization in these rocks. It appears from the study, that both the magnetic and gravity anomalies are caused by a single body. The gravity anomaly depicts the effect of the entire body while the magnetic anomaly that due to a relatively small central and shallow portion of it. The exact geometry of this portion was not determined. However a sphere fitted to the anomaly gave satisfactory results. By comparison with the geology of the area and with the characteristics of other known alkaline complexes in Eastern Africa, the gravity model is considered geologically reasonable.