Investigations into THE Permeability and Tectonic Lineaments of Homa Hills Geothermal Prospect,using Ground Magnetic Method
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Date
2013-03-20
Authors
Otieno, Bernard Otieno
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Abstract
In this study, the tectonic lineaments and permeability around Homa Hills geothermal
prospect, Nyanzian rift were investigated in order to explore its geothermal potential
using ground magnetic method. A proton-precession magnetometer model G-856 of
accuracy of ±O.lnT was used to take total magnetic field intensity of the earth at every
station. A total of 86 magnetic stations were established over an area of about 76km2. The
magnetic data at each station was corrected for both diurnal variations and geomagnetic
corrections. The geomagnetic field was calculated using the mathematical model of
earth's magnetic field called International Geomagnetic Reference Field (LG.R.F) model
2000-2015 using PofeI1f software at each station and subsequently subtracted. This model
was calculated based on average geomagnetic field of 33420nT, inclination of -22.3° and
declination of 0.9°. Total magnetic Intensity (TMI) map and residual magnetic map of
Homa Hills were then plotted using Surfer 8.0 software. Qualitative interpretations of the
TMI map, residual magnetic map and vertical derivative map revealed fracture/fault
lineaments on the survey area trending NW-SE, NNW-SSE and N-S represented by
distinct broad negative magnetic anomalies suggesting demagnetisation due to fluid-rock
interactions, thus, showing these regions to be relatively permeable. There exist relatively
quiet magnetic anomalies on the north eastern and eastern parts of the survey area
suggesting absence of faults/fracture, thus impermeable. Modelling of selected profiles
revealed presence of intrusive bodies on the southern, central and northern regions with
the subsurface structures being as shallow as 60m and as deep as 511m. These bodies
display magnetic susceptibilities as high as 0.0298 SI units and as low as -0.113 SI units,
suggesting them to be carbonatite sills, dykes and plugs of different kinds based on
geologic units of the area. 2D Euler solutions revealed subsurface faulting activities up to
a depth of about 250m and the prese-n. c,e of fluid- filled zones within the survey area which
are marked by the absence of magnetic sources. The zones trend in the NW -SE, NNWSSE
and N-S within the northern, central and southern parts of Homa Hills. This method
confirms that faults/fractures-like structures trending NW-SE, NNW-SSE and N-S on the
northern and southern parts of the area serve as fluid conduits which support the upward
flow of the geothermal fluid and that the heat sources are shallow intrusive bodies such as
dykes, plugs and sills taping from deeper magmatic bodies and that these intrusive bodies
form along fracture zones.