Seismic Refraction
Basic Concepts
Seismic Waves
Using a shot source, sound waves are sent
out in a wave front much like the ripples from a rock being dropped in a
pond. These waves propagate out in
all directions, but many travel through the ground and the time it takes this
wave to travel the distance from point source to the geophone, and back to the
recording source. Seismic waves
are P-waves initially, but as it travels it creates a refracted P and S wave.
Refraction concepts and critical
angle
When critical refraction of a wave occurs,
the wave energy will travel parallel to the two differing mediums, however, as
this wave propagates, smaller waves called the Head wave will move towards the
surface again at the critical angle, which it first entered the ground at. These head waves are picked up by the
geophones and sent to the seismograph.
The critical angle is simply an angle at which the angle of incidence= V1/V2>1. This allows the wave to propagate
parallel to the two different surfaces.
However, if the angles are increase past the special angle allowing this
to happen, complete reflection of the wave occurs.
Velocity equations and properties
of materials
Vp=√ ((K+4μ/3)/ρ)
K= the incompressibility of a material.
Incompressibility refers to a materials
change of volume due to change in pressure. The relation between pressure
change versus volume change is how K is measured. A good example of this idea is the properties of a
sponge. With increase pressure,
the size of the sponge decreases, or compresses.
μ = Shear Modulus. Relates to
the rigidity of a material.
This variable refers to the
amount of deformation experienced by a material, and how much it resists the
shear stress.
Sheared stress sheared solid
Material and the deformation due to stress:
This also deals with uniaxial compression, which refers to how much a material
can elongate in the x and y directions.
ρ=
the density of the material
The greater the density of a material, the
slower the velocity. Each type
of material can have a different density, for example saturated soil in
comparison to unsaturated soil, or gneiss in comparison to a sedimentary rock
unit.
Interpreting refraction data
When the seismograph gives a print out
or reading it may seem like a bunch of squiggly lines, but each line has a relation
between time and velocity, which will ultimately give you depth of the unconformity,
or where velocity changes. Note that
there is a slope-like appearance between each of the signals given by
consecutive geophones. If you draw
a line between each of the beginning of these points or bumps you will have the
slope of the velocity. If there
are two of these, then it is a clear indication of a velocity change, and where
this slope break occurs, there is an unconformity, or change in material, i.e.
if you changed from saturated soil to bedrock.
time
distance
The
arrow indicates the change in material.
The SmartSeis has
the ability to calculate depth with just this information.
The Colorado School of Mines has an
excellent outline of the basic concepts associated with the seismic refraction
method http://talus.Mines.EDU/fs_home/tboyd/GP311/
Basic statement, schematic, and advantages
of the method by Technos, Inc. - Site Characterization specialists -
information on SURFACE GEOPHYSICS http://www.technos-inc.com/Surface.html#I19
