Journal of Geoscience and Environment Protection, 2014, 2, 181-189
Published Online June 2014 in SciRes.
How to cite this paper: Subasinghe, N. D. et al. (2014). Analytical Signal and Reduction to Pole Interpretation of Total
Magnetic Field Data at Eppawala Phosphate Deposit. Journal of Geoscience and Environment Protection, 2, 181-189.
Analytical Signal and Reduction to Pole
Interpretation of Total Magnetic Field Data
at Eppawala Phosphate Deposit
N. D. Subasinghe1*, W. K. D. G. D. R. Charles1, S. N. De Silva2
1Institute of Fundamental Studies, Kandy, Sri Lanka
2Geological Survey and Mines Bureau, Pitakotte, Sri Lanka
Email: *
Received April 2014
A magnetic survey was carried out to find out the possibilities of demarcating a phosphate deposit
from the surrounding country rocks. It is a well-established fact that the magnetic mapping can be
utilised to investigate the subsurface objects, materials or different rock types based on their
magnetic properties. Those rocks with ferro-magnetic minerals such as magnetite generate mag-
netic anomalies which in turn help to investigate the subsurface occurrence of mineral deposits.
An economic phosphate deposit in Sri Lanka, known as Eppawala Phosphate deposit was selected
for this study. The deposit was formed as an accumulation of secondary products of an apatite-rich
carbonatite. Due to weathering of iron-rich carbonatite, magnetite and its derivatives are inti-
mately bound with the said deposit. Therefore, the magnetic signature of the phosphate body is
different to that of the surrounding country rocks. Despite some studies on different aspects of the
deposit, subsurface extents of the ore body are so far not adequately studied. Therefore, this study
was conducted to identify the boundaries of the phosphate body. The study was carried over an
area of 12 km2 5 km north from the current mining site and survey was conducted. GSM-19 Over-
houser system with integrated GPS was used to collect field data. Magnetic anomalies were plotted
using a predefined grid. The maximum positive and negative anomalies encountered in the survey
area are 690 nT and 829 nT respectively. This study showed that magnetite is not distributed
evenly in the area and the deposit extended along the north south direction. Further, processing of
analytical signal using the anomalies showed that the carbonatite occurs as a continuous body
trending in North South direction. Low magnetic latitudes magnetic data interpretation is difficult
because the vector nature of the magnetic field. Therefore,reduction to poleconcept and “ana-
lytical signal conceptwere used for the data analysis. Reduction to pole map and analytical signal
map are comparatively similar and the change of declination value has no significant effect on the
map of reduction to pole.
Magnetic Anomal y, Analytical Signal, Reduction to Pole
Corresponding author.
N. D. Subasinghe et al.
1. Introduction
Sri Lanka is situated between longitudes 79˚40'E to 81˚55'E and latitudes 5˚55'N to 9˚50'N. It has an area of ap-
proximately 65,000 km2.
Ninety percent of the country is underlain by Precambrian metamorphic rocks and rest of the country (north
and northwest) is covered by limestone of Miocene period. Precambrian crystalline rocks are divided into three
main zones (Highland complex, Wanni complex and Vijayan complex) and sub-zones (e.g. Kadugannawa com-
plex) based on rock type, metamorphic grade and isotopic characteristics, as shown in Figure 1 (Kroner et al.,
Eppawala Phosphate deposit is located in Anuradapura district of Sri Lanka as shown in Figure 2. It was first
reported by Jayawardena (1976). Among the natural resources in Sri Lanka, Eppawala phosphate deposit has a
significant place, due to its economic importance. Magmatic origin was suggested by Jayawardena (1976) for
this phosphate deposit, since the parent rock is a carbonatite intrusion. Major minerals of the deposit are calcite,
dolomite, apatite and olivine, while minor minerals include spinel, ilmonite, magnetite, amphiboles and micas
(Manthilake et al., 2008; Pitawela & Lottermoser, 2012).
Despite having a number of studies on geological, geochemical and petrological aspects of the deposit, litera-
ture on geophysical aspects are rare. Apart from some bore hole studies conducted in 1970’s no systematic sur-
vey has been conducted to estimate the extent and the boundaries of the Eppawala phosphate deposit. The de-
marcation of the apatite deposit and country rock is economically important for mining and planning activities.
Gravity method and resistivity method are the widely used geophysical techniques for resource exploration in
general. Magnetic method is widely used for exploring ore bodies containing magnetic minerals such as iron,
nickel and cobalt bearing minerals. In this project, we used magnetic method as the geophysical technique, be-
cause Eppawala phosphate deposit contains magnetite (Dahanayake & S uba singh e, 1988; Subasinghe, 1998),
which is a common accessory mineral found in many other apatite deposits around the world (Rajesh et al.,
2006). This ferromagnetic mineral produces an induced magnetic field influenced by earth’s magnetic field,
which is useful in demarcating the magnetite-bearing rocks from the others. Since Sri Lanka is located near the
magnetic equator, “reduce to pole” concept is employed for data analysis (MacLeod et al., 1998).
2. Theory
Nabighian (1972 &1974) developed the concept of 2-D analytical signal, or energy envelope, of magnetic
anomalies. The amplitude of the 3-D analytic magnetic field signal at location (x,y) can be expressed as:
|( ,)|dT dT dT
Axydx dydz
 
= ++
 
 
(, )Axy
is the amplitude of the analytical signal at (x,y); T is the observed magnetic field at (x,y).
The purpose of the reduction to the pole is to take an observed total magnetic field map and produce a
magnetic map that would result, had an area been surveyed at the magnetic pole. Assumpsion is, all the observed
magnetic fields of the survey area are due to induced magnetic effects.
[ ]
() sin( )cos( )cos()
Θ= + −Θ
is the wavenumber direction;
I is the magnetic inclination;
D is the magnetic declination.
3. Methodology
GSM-19 portable magnetometer with integrated GPS (Global positioning System) system was used for the
magnetic survey, with a base-station for diurnal correction.
Based on the literature and preliminary studies, the survey was started at a point 5 km away from the current
northern quarry of the deposit (Figure 1). The survey lines were extended up to 3.0 km in the east-west direction
N. D. Subasinghe et al.
Figure 1. Main precambrian lithotectonic units.
Figure 2. Apatite deposit and area of study.
with a 500 m distance between them to make up the grid. The base station was located either on the survey line
or on the adjacent one.
4. Area of Study
Eppawala apatite deposit was located about 200 km away from capital city of Colombo, in Anuradapura district
(Figure 2 inset).
The carbonatite complex forms six hillocks and seconday apatite deposit which is leached from carbonatite
N. D. Subasinghe et al.
complex (Jayawardena, 1976). Our study area was located about 5 km north of the current mining site (Figure
2). Present mining process is based on the map made in 1976.
5. Results and Discussion
Magnetic anomalies were calculated using walking magnetometer data and base-station data. Distribution of the
magnetic anomaly is shown in the Figure 3. Location 01 and 02 showed maximum positive anomaly where lo-
cation 03 showed maximum negative anomaly. Earth induced magnetic field of the magnetite in the carbonatite
causes the anomaly. According to magnetic anomalies, locations 01 and 02 may have the maximum concentra-
tion of magnetite. Magnetic anomalies show that magnetite is neither distributed evenly in the area nor continu-
An object with two magnetic poles can produce positive and negative anomalies. In Figure 3, pink areas
show the positive anomaly, whereas blue areas show the negative anomaly. If a grain of magnetite is assumed to
be a bar-magnet, each side of the grain act as either North or South Pole.
All magnetite grains in the area act as a single magnetite body and produce anomalies in the area. Figure 4
shows the magnetic anomaly map after assuming all magnetite grains act as one magnetite body. Location 01 in
Figure 4 shows the body of the magnetite. According to the literature, country rock in the area is mainly granitic
gneiss, while magnetite is mostly found in the carbonatite rock, which is the parent rock of the phosphate de-
posit. Therefore, the phosphate deposit extends along north south direction. The carbonatite host rock extends
further to the north from our surveyed area. Though there are no outcrops in the area, the data suggests a sub-
surface deposit. Central part of the deposit is close to the surface and depth of the deposit is increasing along the
East-West direction.
Magnetic data were further analysed using reduce to pole (RTP) method and the resulted maps are shown in
Figure 5. The inclination was kept in constant and declination was change from 30˚ to 90˚. Maps of RTP were
compared with map of the analytical signal. Comparison of maps of RTP and map of analytical, no significance
difference is shown. Area of “A” in the map of analytical signal shows gradual decrease of the anomaly from
middle to left side of the map. However, in maps of RTP, drastic change of the anomaly from higher to lower
values is evident. Declination of the induced field was changed from 30˚ to 90˚, but there is no significant dif-
ference in the middle area of the maps. This suggests that the declination has no significant effect on this in-
duced field.
The natural inclination and declination in Sri Lanka are 1.2 ˚ and 2.4˚ respectively. Figure 6 shows a map
derived by substituting those inclination and declination values in Sri Lanka.
In Figure 7, inclination of 30˚ is kept constant and declination was changed from 30˚ to 90˚ by 30 degree in-
tervals. Relatively these three maps are similar. Even the maps with inclination 0˚ (in Figure 5) show similari-
ties with the maps with inclination 30˚. According to the literature, most suitable approach is the analytical sig-
nal concept because very large amplitude correction is required for north-south features at low magnetic lati-
Contour map of analytical signal is shown in Figure 8.
Contour values were extracted where the horizontal lines cross the contours and plot the contour value as a
function of distance (Figure 9).
A cross section of the deposit can be constructed using the above graphs. It is a well established fact that the
parent rock of the Eppawala phosphate deposit is carbonatite, which is an igneous rock. This igneous body has
been intruded as an intrusive body into the surrounding meta-sedimentary country rocks. Therefore, most prob-
able the shape of the cross sectionalong the line 4 is as shown in Figure 10.
6. Conclusion
Map of RTP and map of analytical signal show comparatively similar map and declination has no significant ef-
fect on this map of RTP. The magnetic anomalies are not evenly distributed in the area and so is the magnetite.
Highest positive magnetic anomaly value is 690.29 nT and the negative value is 825.66 nT. The phosphate de-
posit extends along the north south direction in the subsurface confirming the supposition made by the previous
workers. However, the extent of the phosphate deposit extends further to the north than the earlier workers sug-
N. D. Subasinghe et al.
Figure 3. Magnetic anomaly map.
Figure 4. Map of analytical signal.
(a) (b)
N. D. Subasinghe et al.
Figure 5. Map of reduce to pole (a) inclination 0˚ and declination 30˚ (b) inclination 0˚ and declination 60˚ (c) inclination 0˚
and declination 90˚.
Figure 6. Map of reduction to pole with inclination 1.2˚ and
declination 2.4˚.
(a) (b)
N. D. Subasinghe et al.
Figure 7. Map of reduce to pole (a) inclination 30˚ and declination 30˚ (b) inclination 30˚ and declination 60˚ (c) inclination
30˚ and declination 90˚.
Figure 8. Contour map of analytical signal.
(a) (b)
(c) (d) (e)
Figure 9. Variation of contour values as a function of distance (a) line one, (b) line two, (c) line three, (d) line four, (e) line
N. D. Subasinghe et al.
Figure 10. Cross section along line four.
Authors wish to thank the Director of Institute of Fundamental Studies (Sri Lanka), Geological survey and
mines bureau (GSMB Sri Lanka), Dr. A. Pitawela Ms. A. Samaranayake and Mr. A. Tennakoon. Grant RG/
2012/NRB/03 from National Science Foundation, Sri Lanka is acknowledged for the financial support.
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