geophysics case study

8/3/2019 Geophysics Case Study

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Figure 1: shows Wenner’s convectional

four electrode array to measure the

subsurface resistivity.

( http://www.heritagegeophysics.com/imag

es/lokenote.pdf )

Geophysics case study

Geophysics is one of the major branches of Earth Sciences. It studies the Earth’s properties by

applying principles of Physics and using instruments for measurement. Geophysical surveys are

widely used by archaeologists to explore the areas of ground for archaeological sites. There are many

geophysical methods which are used to explore the areas. Some of the methods are shown below:

Electrical Resistivity Surveys:

They are used to determine the subsurface resistivity distribution by making measurements on

ground surface. From these measurements, we can estimate the resistivity of the subsurface and

map archaeological features. The measurements are made by

inserting four electrodes into the soil or rock. Current is

introduced in one of the pair of electrodes while voltage is

measured across the other electrodes pair. The depth of the

measurements is related to the electrode spacing.

The resistivity of soil is complicated function of permeability,

ionic content of pore fluids, clay mineralization and porosity.

The measurement of resistivity symbolizes the apparent resistivity averaged over volume of earth.

Resistivity measurements include resistivity profiling, sounding and imaging.

This method can be used to map archaeological sites and to measure bedrock and water table

depth. It can also be used to estimate metal corrosion rates and designing grounding grids.

The benefits are it has a good vertical resolution and create a detail 3D resistivity model. It has a

depth range of up to 200 feet and it has a wide variety of electrode configurations for different

applications.

Results can be affected by nearby metal fences, buried cables, pipes and soil conditions (too wet

or dry). The other problem is filtering out natural from anthropogenic anomalies.

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Figure 2: shows

cross- section of

Electrical resistivity

imaging.( http://www.technos-

inc.com/pdf/SurfaceT

echnotes.pdf )

http://www.heritagegeophysics.com/images/lokenote.pdf


http://www.technos-inc.com/pdf/SurfaceTechnotes.pdf











Electromagnetic Survey:

They are used to determine the subsurface electrical conductivity and identify subsurface metal

objects. The method measures earth’s response to electromagnetic signals transmitted by an inductioncoil. At various frequencies, the induction coil produces magnetic field which induces electric current

in the material underground; resulting in secondary magnetic field.

The intensity of these magnetic fields is measured by electromagnetic sensor. High frequency signal

respond to shallow part of the ground, while low frequency signal respond to deeper part of the earth.

Based on these responses, magnetic susceptibility and electrical conductivity are calculated for each

frequency. Electrical conductivity is determined by nature of rock, water saturation, salinity and other

parameters.

This method can also be used to map archaeological sites. Electromagnetic measurements are

applicable of mapping contaminant plumes, buried wastes, tanks, metal utilities etc.

The benefits are it provides measurements with depths ranging up to 200 feet and the measurements

are relatively easy to make. The possible limitations are the effectiveness of measurements decreases

at very low conductivities. Results can be affected by interference of nearby metal pipes, fences,

vehicles and induced noise from power lines etc.

These two methods relates to the practical which is to identify a given metal using its resistivity. The

metal which I will identify can be used as an electrode in

Resistivity Surveys or if archaeologists find ancient materials

buried underground by these surveys. They can identify different

metals e.g. metals in crowns, armours, utensils etc. by this

practical.

Sources of information:

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http://www.geovision.com/PDF/M_Resistivity.pdf


http://www.geophysical.biz/res1.htm

http://www.terraplus.com/case-histories/gpr/wynn.htm

http://physicsworld.com/cws/article/print/654

http://www.terra-au.com/Electromagnetics.asp

A J Clark 1996 Seeing Beneath the Soil: Prospecting Methods in Archaeology

(Batsford, London).

Telford, W. M., L. P. Geldart, and R. E. Sheriff, Applied Geophysics, 2nd edition ,

Cambridge University Press, 1990.

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Figure 3: shows different metal

resistivity

(http://en.wikipedia.org/wiki/Resistivity)














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