2D-Forward Modeling of Ground Magnetic Data of Homa-Hills Geothermal Prospect Area, Kenya
2D-Forward Modeling of Ground Magnetic Data of Homa-Hills Geothermal Prospect Area, Kenya
2D-Forward Modeling of Ground Magnetic Data of Homa-Hills Geothermal Prospect Area, Kenya
Abstract: Two dimensional (2D) Euler de-convolution techniques was applied on the selected profiles of reduced ground magnetic
data collected in Homa Hills area. Depth estimates of causative bodies were quantitatively analysed in the anomalous areas on the
residual magnetic intensity map. These depth estimates were later used as start-up parameters for 2D-forward modelling using
“mag2DC” software. Results of the analyses show that the magnetic anomalies in the region are caused by shallow-seated thermal
intrusive structures of carbonatite origin. 2D-Euler solutions revealed subsurface faulting activities up to a depth of 250m and the
presence of fluid-filled zones within the survey area which are marked by absence of magnetic sources. It is postulated from 2D-forward
modelling that the heat sources are shallow intrusive bodies such as dykes, plugs and sills taping from a deeper magmatic body and that
the thermal intrusive structures form along fracture zones.
2. Geologic setting
The Homa Mountain is a cone sheet complex comprising a
number of carbonatite cone sheets of large and small scales.
Most of carbonatite-alkaline rocks, except those composing
the carbonatite –ijolite complex in the south-eastern part of
this area, is distributed in an oval area approximately 6km
long in the NE-SW direction and 5km wide. The main
carbonatite cone sheet of Homa Mountain, the largest of all,
is located y to the southwest of the center of the oval area
and composes the major structural element of the cone sheet
complex. A series of intrusive activities of these cone sheets
have resulted in domal uplifting of the Nyanzian
Metavolcanics to an elevation 500m above the surrounding
ground. The main cone sheet of the Homa Mountain, where Figure 1: Location of Homa Hills geothermal Prospect
9962000
B'
nT
9961000
30
25
9960000 20
B D' 15
10
9959000
NORTHING (M)
5
0
9958000 -5
-10
9957000 -15
-20
C'
A' -25
9956000
-30
D -35
9955000 -40
-45
9954000
C E E'
A
9953000
664000 665000 666000 667000 668000 669000 670000 671000 672000
EASTING (M)
Figure 3: Residual magnetic map of Homa Hills
Applying the Euler’s expression to profile or line-oriented a regional value of B , and n is a measure of fall -off rate of
data (2D source), x-coordinate is a measure of the distance the magnetic field. n is directly related to the source slope
along the profile and y-coordinate is set to zero along the and is referred to as the structural index and depends on the
entire profile. Equation 3.1 is then written in form of geometry of the source (El Dawi et al., 2004). Estimating
Equation 3.2 as: depth to magnetic anomaly using Euler deconvolution
involves: i) Reduction to the pole and ii) Calculation of
horizontal and vertical gradients of magnetic field data,
( x x o ) T / x ( z z o ) T / z n ( B T ) calculated in frequency domain, iii) choosing window sizes
3.2 and iv) structural index, e.g. contact and dyke.
Volume 3 Issue 4, April 2014
Paper ID: 020131363
www.ijsr.net 95
International Journal of Science and Research (IJSR)
ISSN (Online): 2319-7064
The 2D-dimensional Euler deconvolution was generated by a faulted carbonatite dyke which is a common intrusive in
software developed by Cooper (2004) for constraining the the region. The third zone at station (667000.4-667609.4)
subsurface geometry along the profile lines. The input shows no magnetic sources. The lack of magnetic sources
parameters for the application include the geomagnetic field, coincides with faults, a possible evidence of the presence of
survey locations, inclination and declination angles. The the warm fluids.
results of the International Geomagnetic Refference Field
(IGRF) given in Table 1 were used as the inputs for the Euler solutions for profiles BB’ and CC’ are shown in
process. A window size of 11, 82.28m X-separation and Figures 5 and 6 in which high magnetic signatures, station
41.14m Y-separation were adopted. To better constrain the (66775.2-667975.2) are evident at a depth of about 350m
subsurface geology, 1.0 structural index (steep contact) below the surface for profile BB’ and this is associated with
which is an indication of faults contacts were plotted for all an intrusive body probably a dyke. In profile CC’ there is no
the five profiles; these are shown in Figures (4-8). concentration of Euler solutions at any point an indication of
little tectonic activities along the profile. Figure 7 shows two
Table 1: IGRF components of Homa Hills distinct anomalies along profile DD’. There exist no
Component Field value magnetic sources (station 9955740.4-9956240.2) and
Declination 0.9 degrees (station 9956990.4-9958115.4) an indication of fluid filled
Inclination -22.3 degrees zones. This postulates N-S trending fault in the study area.
Total Intensity 33420nT Figure 8 shows a high magnetic signature at shallower depth
of about 205m (station 667694.8) an indication of magmatic
3.2.1 Interpretation of Euler solutions intrusive body, and a very low magnetic signature (station
Figure 4 shows magnetic anomaly along profile AA’. Three 668194.8-668494.8) could be an indication of a fluid filled
distinct trends are evident which coincide with the location zone. These undulating signatures and the Euler
of dykes and faults within the study area. The profile begins deconvolution solutions clearly show shallower subsurface
with a relatively low magnetic anomaly points at station intrusions and faulting/contacts pattern within the geological
(666000-666509) which could be possibly a sedimentary units.
layer. This signature is followed to the south by high
signatures at station (666600-666700) and is postulated to be
Fluids
Fault Fault
Figure 4: Processed ground magnetic data with 2D Euler solutions obtained along profile AA’. Plus (+) signs are Euler
solutions for 1.0 structural index.
Figure 5: Processed ground magnetic data with 2D Euler solutions obtained along profile BB’. Plus (+) signs are Euler
solutions for 1.0 structural index.
Figure 6: Processed ground magnetic data with 2D Euler solutions obtained along profile CC’. Plus (+) signs are Euler
solutions for 1.0 structural index.
Fluids
Fault
Fault
Figure 7: Processed ground magnetic data with 2D Euler solutions obtained along profile DD’. Plus (+) signs are Euler
solutions for 1.0 structural index.
Volume 3 Issue 4, April 2014
Paper ID: 020131363
www.ijsr.net 97
International Journal of Science and Research (IJSR)
ISSN (Online): 2319-7064
Fault
Fault
Figure 8: Processed ground magnetic data with 2D Euler solutions obtained along profile EE’. Plus (+) signs are Euler
solutions for 1.0 structural index.
Forward modelling was done using “mag2DC” computer program. “mag2DC” calculates the anomalous field caused by an
assemblage of 2 – dimensional magnetic bodies defined by a polygonal outline. The description of the method of the program
“mag2DC” can be found in the work of Talwani and Heirtzler (1964). The use of this program involves a trial and error
procedure to obtain a good fit to the observed anomalies. Depth estimates of the possible causative bodies determined from
euler deconvolution were used as start-up parameters in the “mag2DC” software. Figures 9 to 13 show the modeled bodies of
the subsurface structures causing anomalies on the selected profiles.
that Homa Hills prospect is generally characterised by a [9] M. Talwani, J.R. Heirtzler, “Computation of magnetic
broad and low magnetic signatures at the southern and anomalies caused by two dimensional structures of
northern parts surrounded by high magnetic belt from the arbitrary shape, in Computers in the mineral industries,”
NE and SE. Besides, it includes surficial or local anomalies part 1: Stanford University publications, Geol. Sciences,
of shallow seated origins, with orientations in the direction 9: 464-480, 1964.
N-S, NW-SE and NNW-SSE. The average modeled depth [10] S.E. Williams, et al. “Comparison of grid Euler
for the near surface magnetic anomaly sources (postulated to deconvolution with and without 2D constraints using a
be a carbonatite dyke) of the area is 205m, while that of the realistic 3D magnetic basement model.” Geophysics,
deep-seated anomaly sources is 511m. The results further 70, 13-21, 2005.
support the delineation of faults/fractures trending N-W, [11] Zhang et al. “Euler deconvolution of gravity tensor
NW-SE, NNW-SSE and NE-SW, and heat sources gradient data.” Geophysics, 65, 512-520, 2000.
associated with shallow intrusive along structures.
Author Profile
The ground magnetic study of this area has helped in a
number of ways to delineate lineaments and target zones Adero Bernard received his B. Ed. (Science) degree
with intrusives. Firstly, the major subsurface structures from Moi University and MSc. (Physics) degree from
delineated (faults/fractures, sills and dykes) will aid the Kenyatta University in the year 2012. He is currently a
geothermal exploration work in the area. Secondly, the Petroleum Geophysicist with National Oil Corporation
of Kenya.
linear nature of the anomalies in this part suggests that the
rocks may be bounded and offset by fault. The results
Ambusso Willis received his B.Sc. (Physics) from the
further support the delineation of faults/fractures trending N- University of Nairobi, M.Sc. (Pet. Eng.) from Stanford
W, NW-SE, NNW-SSE and NE-SW, and heat sources and Ph.D. (Physics) from Kenyatta University. He is
associated with shallow intrusive along structures. Since now a senior lecturer at the department of Physics,
geothermal exploration requires multi-disciplinary approach, Kenyatta University.
other exploration methods such as detailed gravity survey
done in the prospect area during the same period need to be Abuga Vincent received his B.Ed. (Sc.) degree from
analysed together with this piece of work in order to discern University of Nairobi 2006; M Sc. in Physics from
deeper tectonic lineaments in this prospect area. Kenyatta University in 2013.