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| Titel |
Classification of Archaeological Targets by the Use of Temporary Magnetic Variations Examination |
| VerfasserIn |
Michael Finkelstein, Lev Eppelbaum |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250106827
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| Publikation (Nr.) |
 EGU/EGU2015-6504.pdf |
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| Zusammenfassung |
| Many buried magnetized archaeological and geological objects producing significant magnetic anomalies(for instance, ancient furnaces, weapon, agricultural targets and high-magnetized basalts)
may be classified without high-expensive excavations. Such a classification
may be conducted on the basis of comprehensive studying temporary magnetic
variations over these objects. It is especially significant for
archaeogeophysical investigations in the areas of world recognized religious and cultural artifacts where all excavations are forbidden (Eppelbaum, 2010).
Yanovsky's (1978) investigations laid the foundation of the magnetic
variations utilization for separation of disturbing objects with high
magnetic susceptibility (not depending on intensity of the studied magnetic anomalies). However, these procedures are inapplicable for studying low-intensive and negative magnetic anomalies, where an influence of residual magnetization may be sufficient one. At the same time the approach presented below may be used for investigation of the nature of magnetic anomalies with arbitrary intensity and origin.
In the common case (we consider for simplicity that anomalous object is a
sphere) the value of magnetic variations \textit{$\eta $ }could be estimated using the
following expression (Finkelstein and Eppelbaum, 1997):
\begin{equation}
\label{eq1}
\eta =\frac{f\left( P \right)+\delta H_a +\delta H_o }{\delta H_o },
\end{equation}
where induction parameter $P=\alpha \sqrt {\kappa & \gamma & \omega } $
(Wait, 1951), $ H_{o}$ is the initial field of magnetic variations, $H_{a}$ is
the anomalous component of magnetic variations,\textit{ $\kappa $} is the magnetic
susceptibility, \textit{$\gamma $} is the electric conductivity, \textit{$\omega $} is the frequency of
geomagnetic variations, and \textit{$\alpha $} is the radius of the sphere.
For the approximate estimation of possible values of anomalous geomagnetic
variations (AGV) over sphere within some domain $T$, we will use an expression
of the anomalous vertical magnetic component $Z$ for any point $M$ ($x$, $y$, $z)$ in the
external space (for the case of vertical magnetization) (Nepomnyaschikh, 1964):
\begin{equation}
\label{eq2}
Z_a =\frac{\left( {\kappa _1 -\kappa _2 } \right)Z_0 +\left( {1+4\pi \kappa _2 } \right)J_{RZ}^e }{1+4\pi \kappa _2 +N_{zz} \left( {\kappa _1 -\kappa _2
} \right)}.\frac{\partial ^2}{\partial z^2}\int\limits_T {\frac{d\tau }{r}}
,
\end{equation}
where \textit{$\kappa $}$_{1 }$is the$_{ }$magnetic susceptibility of the object, \textit{$\kappa $}$_{2}$ is the magnetic susceptibility of the host medium, $Z_{0 }$is the vertical
component of Earth's magnetic field, $J_{RZ}^e _{ }$is the effective
component of the vector of residual magnetization, $N_{zz}$ is the
coefficient of the demagnetization, $\frac{\partial ^2}{\partial
z^2}\int\limits_T {\frac{d\tau }{r}} $ is the second derivative of the
$z$-axis of the integral $\int\limits_T {\frac{d\tau }{r}} $, and \textit{d$\tau $} is the
volume element of the domain $T$.
Taking into account that in most cases \textit{$\kappa $}$_{2 }$is$_{ }$negligible compared
with \textit{$\kappa $}$_{1 }$of magnetic objects, as well as the fact that the residual
magnetization of $J_{RZ}^e $ when exposed to an alternating field does not
create additional fields, values \textit{$\kappa $}$_{2}$,
and $J_{RZ}^e $ in Eq. (2) can be
practically ignored in the evaluation of magnetic fields from objects. Then
for variations of the vertical component of the magnetizing field with
objects having a high content of ferromagnetic materials according to Eq. (2) we will observe abnormal values of the magnetic variations
(Finkelstein et al., 2012):
\begin{equation}
\label{eq3}
\delta Z_a =\frac{\kappa _1 \delta Z_0 }{1+N_{zz} \kappa _1 }.\frac{\partial
^2}{\partial z^2}
\int\limits_T {\frac{d\tau }{r}} ,
\end{equation}
where $\delta Z_0 $ is some increment (both positive and negative) of $Z_0 $.
Solving the expression $\frac{\partial ^2}{\partial z^2}\int\limits_T
{\frac{d\tau }{r}} $ in Eq. (3) for each particular body shape, we find that the anomalous geomagnetic variations from the body of spherical form will be
determined by the expression
\begin{equation}
\label{eq4}
\delta Z_a =\frac{4.2a^3\kappa _1 \left[ {\left( {2h^2-x^2} \right)\sin
J-3hx\cos J} \right]}{\left( {1+\kappa _1 N_{ZZ} } \right)\left( {h^2+x^2}
\right)^{5/2}}\delta Z_0 ,
\end{equation}
where $J$ is the angle between the magnetization vector and the horizon, $a$ is
the radius of the sphere, $x$ is the current coordinate, and $h$ is the depth to
its center. For a spherical body the parameter $N_{zz}$ was assumed as
$\frac{4}{3}\pi$ (Nikitsky and Glebovsky, 1990).
In accordance with Eq. (4) the relationship between abnormal to normal
variations (\textit{$\eta $}) were calculated:
\begin{equation}
\label{eq5}
\quad
\eta =\frac{\delta Z_a +\delta Z_0 }{\delta Z_0 }
\end{equation}
and plotted versus the magnetic susceptibility of a sphere with a radius $a$
(Finkelstein et al., 2012).
From Eq. (5) follows that at small values of \textit{$\delta $Z}$_{a}$ the ratio becomes close
to unity, for example granitoids - basalts, and each value of the
differential function ($\Delta ^{1-2})$ of geomagnetic variations between
the two points (1 and 2) will be close to the values of the background
level, unless there are other factors creating AGV of different origin.
The developed methodology includes: (a) estimation of influence of electric conductivity for studied objects and surrounding medium; (b) selection of the most optimal frequencies for observation of magnetic variation effect
($f(P)$ should seek to the value less than 0.6); (c) revealing relationship between observed variations (their intensity and form) and parameters of disturbing objects (their geometric and physical characteristics); (d)
calculation of magnetic susceptibility. Results obtained in the items (c)
and (d) are applied (together with other available geological,
archaeological, environmental and geophysical data) for classification of
studied ancient targets.
These procedures have been successfully tested in several ore deposits of
the Middle Asia (mainly in Kazakhstan) and Caucasus. Some preliminary
experimental observations over ancient iron-containing targets were carried
out in Israel (Eppelbaum et al., 2010).
\textbf{References}
Eppelbaum, L.V., 2010. Methodology of Detailed Geophysical
Examination of the Areas of World Recognized Religious and Cultural
Artifacts. \textit{Trans. of the 6}$^{th}$\textit{ EUG Meet.}, Geophysical Research Abstracts, Vol. \textbf{12},
EGU2010-5859, Vienna, Austria, 3 pp.
Eppelbaum, L.V., Khesin, B.E. and Itkis, S.E., 2010. Archaeological
geophysics in arid environments: Examples from Israel. \textit{Journal of Arid Environments}, \textbf{74}, No. 7,
849-860.
Finkelstein, M. and Eppelbaum, L., 1997. Classification of the disturbing
objects using interpretation of low-intensive temporary magnetic variations.
\textit{Trans. of the Conference of Geological Society of America}. Salt Lake City, \textbf{29}, No.6, p. 326.
Finkelstein, M., Price, C. and Eppelbaum, L., 2012. Is the geodynamic
process in preparation of strong earthquakes reflected in the geomagnetic
field?\textit{ Journal of Geophysics and Engineering}, \textbf{9}, 585-594.
Nepomnyaschikh, A.A., 1964. \textit{Interpretation of Geophysical Anomalies}. Nedra, Moscow (in Russian).
Nikitsky, V.E. and Glebovsky, Yu.S. (Eds.), 1990. \textit{Magnetic Prospecting HandManual}. Nedra, Moscow (in
Russian).
Yanovsky, B.M., 1978. \textit{Earth's Magnetism}. Nedra, Leningrad (in Russian).
Wait, J.R., 1951. A conducting sphere in a time varying magnetic field.
\textit{Geophysics}, \textbf{16}, 12-22. |
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