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DOWNHOLE SEISMIC MEASUREMENT
The transmission of seismic survey is common method which is
used to obtain sub-
surface geotechnical data as complement data to site
investigation techniques,
especially in obtaining mechanical rock or soil quality, such as
physical and mechanical
properties. Those values later will be valuable for rock mass
quality designation.
Seismic velocity has more attention than the geological
interface in this survey.
Compressional (p! and shear (s! wave velocities are used for
defining mechanical
properties or engineering parameters of the material.
The seismic theory is based on the physical principle that an
elastic shock wave
generated in homogeneous material will travel as a function of
the density and elastic
modulus.
p "
E( !
( !( !
#
# $ #
+
s "E#
$ # . ( !+
where % & " Youngs modulus of deformation
" densitygr'cm
" Poissons ratio
p " Compressional wave velocity (m/sec)
s " Shear wave velocity (m/sec)
Theoretically, every rock has its own specific compressional and
shear velocities for
seismic waves to travel.
The type of seismic waves of particular interest are )rimary ()!
and Shear (S! waves.
The ) and S waves are called *body+ waves because they travel
through inside material.
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The ove and ayleigh waves appear only at the surface. The ) wave
is a pressure
wave (dilation!. ts front wave consists of compression and
dilation particle motion in
the particle. The shear wave is a shearing motion (transversal!.
The particle motion isper pendicular to the direction of wave+s
path.
The velocity of S wave is slower than ) one, but the ove and
ayleigh are even
slower than ).
Theoretically, by determining the velocities of the body waves
and the density of the
material, )oisson+s ratio, shear modulus, /oung modulus, and
0ulk modulus can be
evaluated.
1or calculating, it uses the standard formula from 2ST3 4 $567,
and the applicable
equations are%
)oisson ratio
"
#
# $
$
$
( ' !
( ' !
Vp Vs
Vp Vs
Shear modulus
8 " s$
" 6
$ $Vp Vs
/oung modulus
& " s$ 6
#
$
$
( ' !
( ' !
Vp Vs
Vp Vs
0ulk modulus
9 "#
# $
E
( !
where %
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p " compressional velocity (m/sec)
s " shear velocity (m/sec)
" density (gr'cm
!
These figures are linear and dynamic characteristics and need
not always be identical to
static modulus performance of foundation material. :evertheless,
some of them can be
proved by empirical data. )articularly, some low strength
material (such as water! has
a high confined compressive strength and corresponds with high )
wave velocity.
Therefore, shear wave determination is also often done.
