Seismic Refraction Method for Groundwater Exploration

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Seismic Refraction Method for Groundwater
Exploration
Dr. A K Rastogi Professor, Dept. of Civil
Engineering I I T Bombay
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Ground water is not available everywhere in the
subsurface region and therefore it is very
important first step in groundwater hydrology to
identify the existence of aquifer in a particular
area.
It is only after the presence of an aquifer is
established, the scheme of groundwater
withdrawals can be planned,
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Groundwater exploration can be divided into two
major groups involving geohydrological and
geophysical methods. Geophysical Methods are
indirect methods which does not involve drilling
Seismic Refraction is a geophysical method
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The following information can be derived from the
exploration using geophysical methods 1.Depth
of occurrence and thickness of aquifers. 2.Deline
ation of aquifer boundaries of very large
groundwater basin. 3.Determination of distinctly
different water bearing layers.
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The seismic refraction method involves artificial
generation of elastic waves in the ground.
Elastic waves travel with different velocities in
different subsurface formations (2.5-6.5 km/s in
the rocks, 1-2.5 km/s in sandy aquifers, and
0.31-0.61 km/s in the non aquifer overburden.)
These waves behave similar to light waves while
passing from one medium to another. When acoustic
impedance contrast ?1V1 ? ?2V2, the elastic waves
undergo changes in their directions of onward
travel due to reflection and refraction and get
returned to the earth surface due to refraction.
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The shock waves are generally produced by
exploding dynamite (about 500 gm) in a hand
augured backfilled hole of about one meter depth.
Sometimes, the method of hammering a thin plate
placed over ground surface is also used. On the
emergence of these waves after refraction from
the subsurface boundary, these are detected by a
suitable device called as geophones. These
geophones are kept in a line at 5- 20 m interval.
On receiving the elastic wave, geophones transmit
electric signals. These signals are generally
weak, and therefore, amplified and then recorded.
Thus, the time of origin of shock waves and
their first arrival on the surface either as
direct waves or refracted waves (in distant
geophones) are noted down in the seismic
refraction.
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Components of a geophone
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Principle of Seismic Refraction Let A be a source
on the ground surface and a seismic refractor
underlies at a depth H from the source. The
seismic wave reaches the point B which falls in
other medium. If T is the total time for the
seismic wave to travel a distance APB from low
velocity medium V1 to higher velocity medium V2,

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Differentiating with respect to x and making
dT/dx 0 gives
Or
.
For the seismic refraction to take place along
the inter face boundary, r 90o, and if the
corresponding angle of incidence is ?,
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Determination of Depth to Aquifer (a) Intercept
time method
For direct waves
For refracted waves
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The slope of this segment on the travel time
curve is 1/V2 and the intercept on the time axis
is
Therefore, the depth to the formation is
given as

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(b) Cross-over distance method The cross-over
distance method is another way of finding H.
Since the field data do not exhibit an exact
straight line fit, it is advisable to cross-check
the results using other methods, such as the
cross-over distance method. As both direct
and refracted waves arrive at the geophone
together. At less than this direct wave reaches
first, at more than this refracted wave reaches
first. Hence, for X XD , TD TR
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Exploration for Aquifer Thickness
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Correction for Undulating Topography
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Determination of the Sloping Aquifer Thickness
Principle of Reverse seismic profiling is used
According to this principle time for travel along
ABCD is equal to the time for travel along the
path DCBA
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Conclusions Seismic Refraction method can be
effectively used to find the depth of occurrence
of the aquifer. It can find the thickness of the
aquifer. It can find the angle of slope of the
sloping aquifer by reverse seismic profiling. It
can take care of the undulating topography which
is normally present in many field situations. It
can not predict about the quality water quality.