Gravity Survey in Magadi Geothermal Area
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Date
1999
Authors
Waithaka, J. W.
Journal Title
Journal ISSN
Volume Title
Publisher
Kenyatta University
Abstract
Kenya has prominent geographical features such as the Rift Valley which gives
rise to geothermal reservoirs which can be exploited to produce geothermal energy for
power generation and heating. Lake Bogoria, Lake Magadi as well as other areas are
considered to have potential for geothermal energy because of the existing hot springs.
For above mentioned regions it is known that positive gravity anomalies within the Rift
Valley proper are superposed on the generally negative gravity anomaly typically for
rift structures. Research has availed that these local gravity highs can be explained as
result of high density intrusions in the upper crust. The aim of the project was to study
the gravity highs in Magadi and determine whether they could be possible sources of
geothermal energy.
The area of study comprises a part of the Rift Valley, which has geothermal
potential. The Magadi area is located in the southern part of the Kenyan Rift Valley.
The gravity prospecting method was used in the project to investigate the gravity
anomalies occurring in the area. Both qualitative and quantitative interpretations were
attempted and conclusions drawn from the findings.
A gravity survey was conducted to fill in the unsurveyed sectors. The data
collected was subjected to the necessary corrections and a Bouguer anomaly map of
the area was then drawn. The Bouguer gravity values ranges from -1800 g.u. to -1600
g.u. with the maximum peak occurring at the NNE part of the project area. The
relationship between the gravity anomalies and the topographic features of the area is
clearly noticeable.
The quantitative interpretation involved fitting theoretical two-dimensional
models to the observed data along profiles. The choice of two-dimensional models was
dictated by the shape of the anomalies to be interpreted. The anomalies were
interpreted in terms of intrusions of several bodies. The geometries of these bodies
were also determined.
Quantitative interpretation involved forward modelling along five selected
profiles named PQ, MN, UV, CD and RG. The model parameters arrived at suggests
that the causative bodies in this area are found to be dikes like structures except the
body behind the gravity profile MN which seems to be due to a shallow magma
chamber. Gravity body behind PQ is at a depth of 0.2 km to the top, 4 km to the
bottom with an average width of 1 km and dips at 110° to the horizontal. It has a
density contrast of about positive 280 to 290 kgm" and with an elongation of NNWSSE.
The depths to the top of the bodies MN, UV, CD and HG are 0.15, 0.3, 0.5
and 0.15 km respectively while the depths to the bottom are 6.1, 4, 4 and 3.6 km
respectively. The profiles mentioned above have positive density contrast ranging
between 280 kgm" and 340 kgm" , with dip angles of 90°, 90°, 82° and 90°
respectivelywhich are elongated in a nearlyN-S or NNW-SSE directions.
It was found that the densities of the rocks causing the positive anomalies in
the region is about 3 000 kgm" or slightlyless. The mode of formation of the causative
bodies and shallow depths indicate that the area is potential for the generation of
geothermal energy. Comparison of the results of this study with other studies in the
sameregion, indicate that the anomalies are caused by bodies at shallow depth.
The study of the gravity anomalies revealed that the major gravity disturbances
were caused by the rifting and the subsequent tectonic activities of the Rift Valley,
since the gravity anomalies were found to have north south trending nature which
coincidewith the north-south running features such as dykes, fractures e.t.c. that are
aligned in the direction of rifting. The agreement between the final geophysical models
Description
Department of Physics, 107p. 1999, GB 1199.8 .K4 W3