Seismic Source for MASW Surveys
A seismic source generates surface waves (as well as body waves)
when it makes an impact on the ground surface. Its impact power is
directly related to the range of wavelengths for the generated surface
waves, which determines the maximum depth of investigation (Zmax)
as illustrated in a figure below. Zmax is a theoretical depth achievable
only when other conditions are met simultaneously; for example,
optimized acquisition geometry and proper use of data-analysis
algorithms. A more powerful source is needed always as a foremost
condition to increase investigation depth (Zmax).
A heavy sledge hammer (e.g., ≥ 8-lb) is the most common type for
Zmax ≤ 30 m. An accelerated weight-drop source can increase Zmax
by, for example, 30% under most favorable conditions. A projectile
source (e.g., Buffalo Gun) can increase Zmax by generating more
energy at low (long) frequencies (wavelengths) because of the source
point slightly at subsurface. Selection of a proper MASW source
should also consider other factors rather than Zmax itself; for example,
convenience in use, cost, regulation issues, etc.
A strike plate of softer material (e.g., polyethylene) can also increase
Zmax by a noticeable amount (e.g., 5-10 %), but it is vulnerable to
easy wear and tear. Nowadays, a soft plate covered with a thin metal
plate on top is often used to lengthen the life of an impact plate. Gain
in Zmax is sometimes marginal depending on the velocity and
attenuation properties of the survey site even if more powerful sources
are used. When a Zmax of 50 m or greater is needed, it is usually the
time to consider a passive survey.
The table below illustrates possible Zmax achievable for a particular
type of source commonly used for MASW surveys.