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=v ((K+4µ/3)/#)

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.

#= 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.

V2
 
        

V1
 
        time

                                                    distance

The arrow indicates the change in material.

The SmartSeis has the ability to calculate depth with just this information.

Related Resources

Basic statement, schematic, and advantages of the method by Technos, Inc., site characterization specialists, information on SURFACE GEOPHYSICS http://www.technos-inc.com/pdf/SurfaceTechnotes.pdf