In this dissertation I have studied the effect of partially conducting overburden
on the VLF anomalies of an underlying inclined conducting half-plane. In tropical
climate, overburden is normally well developed due to climatic conditions. This study is
realized through numerical modeling using finite elements and algorithms computing the
results employing the eGs developed by Luiz Rijo of the Department of Geophysics of
the Federal University of Pará, Brazil. In these numerical experiments two geoelectrical
situations between the overburden and the underlying half-plane are considered: one
when they are in galvanic contact called as “galvanic overburden” and the other when
they are not in contact called as “inductive overburden”. The depth, dip and conductance
of the half-plane is varied alongwith the conductance of the overburden in this study. The results are presented in the form of Tilt-Angle and Ellipticity profiles. The Ellipticity profiles are highly affected by the presence of both types of the
conducting overburden. The minor positive and negative peaks in the central part of the
profile disappear as the overburden conductance increases. Initially there is an
enhancement in the ellipticity anomaly at the lower conductance of the overburden but
then it decreases as the overburden conductance increases. However, the tilt-angle
anomalies decrease monotonously. Also, in both the ellipticity and the tilt-angle profiles,
the distance between the peaks become shorter due the overburden. Consequently, the
target depth is interpreted as shallower than the real. The index of asymmetry, used to
determine the dip of the half-plane, is also affected by the overburden. Therefore, the
dip determination is hampered in the presence of the overburden. An anticlock wise
rotation in Argand diagram due to increase in the overburden conductance, result in
interpretation a target of less conductance than the real value. Therefore, the presence
of the conducting overburden should be taken into the consideration during
interpretation of the VLF anomalies.
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