Geophysical investigations such as gravity and magnetic
methods can provide both qualitative and quantitative subsurface information
for site. These two methods are passive as they do not depend on controlled
sources yet discovering the natural occurrence of variations in the earth’s
gravity and magnetic fields.
Magnetic survey is the oldest and one of the most widely
used geophysical investigation in delineating the geology such as lithology and
subsurface structure of basement. It measures the magnitude and the orientation
of Earth’s magnetic field. When a ferrous material is located within the
Earth’s magnetic field, it will produces an induced magnetic field where the
induced field is superimposed on the Earth’s field at that area, creating a
Generally, the magnetic content of rocks varies much
depending on the rock types and the environment it is in. Most of the rocks are
not magnetic, yet few types of rocks contain enough minerals to originate the
magnetic anomalies. The common causes of magnetic anomalies are mafic dykes,
fault and lava flows. Sedimentary formations are usually nonmagnetic and have
smaller variation while mafic and ultramafic igneous rocks exhibit greater
effect and are useful in exploring the bedrock geology concealed below cover
formations (Mekonnen, 2004). Hence, the interpreted information that reflects
differences in local abundance of magnetization is useful in mapping the
geologic structures of the basement rocks.
The aims of this research are to investigate the subsurface
geology and the possibility of granitic body in Gunung Semanggol based on the
anomalies of Earth’s magnetic field which results from the magnetic
susceptibility of the underlying rocks.
The study area, Gunung Semanggol is located at the Kerian
distinct, north of Perak Darul Ridzuan (Figure 1). It situated along the side
of North-South Expressway. It consists of northern and southern part of hill
which the highest peak is about 390m above sea level at the southern range. The
total study area covered the whole Gunung Semanggol which was around 40km2.
Figure 1 Study Area – Gunung Semanggol,
Bukit Merah (From Google Map)
Gunung Semanggol area in Bukit Merah is part of Semanggol
Formation which located at the Western Belt in Peninsular Malaysia. The
formation is named after Gunung Semanggol (Alexandar, 1959). Burton (1973a)
divided the formation in Gunung Semanggol into two informal members, namely
Rhythmite Member and Conglomerate Member. However, Foo (1990)
classified into rudaceous-arenaceous facies of intraformational conglomerate
and sandstone, and argillo-arenacous facies of rhythmically bedded sandstone
and shale. Yet, in this present day, it is surrounded by recent alluvium and
Semanggol Formation has been studied widely and published by
many, yet, just covering areas of the northern and southeast Kedah. Less data
have been studied and published on Perak area especially Gunung Semanggol.
There is no previous study mentioning about the igneous and volcanic evidence
around the Gunung Semanggol. But the new evidence that was found by UTP around
the Gunung Semanggol complex, relatively gave a possibility existence of
igneous and volcanic (Mansor & Sokiman, NGC 2015).
MATERIALS AND METHODS
In this research, proton free-precession magnetometer
(Scintex ENVI) was used to detect the total field intensity that are within
vicinity by means of discrete measurement at sampling intervals of few seconds.
When the magnetic field of a rock is measured, the result is actually a measure
of the field as it is being effected by the earth’s magnetic field, as well as
any other large bodies of magnetic rock which are nearby. GPS and compass tool
were used to obtain the bearing and coordinates at each station as well.
Total intensity magnetic field measurement was conducted by
proton free-precession magnetometer which measures the horizontal and vertical
components of the magnetic field or the total field (Fig. 2).
Figure 2 Magnetometer Equipment
The magnetic survey was acquired by loop technique along
Gunung Semanggol. A main base station was set up before and after data acquisition
which consist few base stations. Along the transverse, a total of 167 magnetic
stations were established with approximately 0.5 to 1 km spacing. The collected
readings included coordinates of station, altitude, time taken and magnetic
variations in nano-Tesla (nT). Repetition measurement of a base station at
frequent intervals was carried out in correcting the diurnal drift.
With the acquired data, the magnetic observations were
corrected for diurnal variation and referenced to the base station. Earth’s
magnetic varies in day and night times, due to the interactions of solar wind
with Earth’s ionosphere. This variation is called as diurnal variation. Hence,
repetition measurement of a base station at frequent intervals was conducted
for diurnal effect correction. After subtracting the diurnal effect from the
raw magnetic data obtained, the regional field was calculated by applying the
standard mathematical model of Earth’s main magnetic field, known as
International Geomagnetic Reference Field (IGRF). It is calculated based on the
dates, elevation, latitudes and longitudes of the obtained magnetic data,
generating the result of geomagnetic field 41,638nT, inclination -6.155? and
declination -0.21?. In order to determine the residual magnetic field, the IGRF
value was subtracted from the obtained magnetic data for each station.