TITLE:
Analytical Signal and Reduction to Pole Interpretation of Total Magnetic Field Data at Eppawala Phosphate Deposit
AUTHORS:
N. D. Subasinghe, W. K. D. G. D. R. Charles, S. N. De Silva
KEYWORDS:
Magnetic Anomaly, Analytical Signal, Reduction to Pole
JOURNAL NAME:
Journal of Geoscience and Environment Protection,
Vol.2 No.3,
June
13,
2014
ABSTRACT: 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 magnetic 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 intimately 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 Overhouser
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 pole” concept and “analytical signal concept”
were 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.