Seismic refraction is a geophysical method of obtaining an estimate of p-wave acoustical velocity of the earth’s near surface soils to depths typically less than 100 feet.  Seismic refraction surveys are commonly performed in areas where the geology is not complex or where the objective is to map the top of bedrock.

Seismic reflection is a geophysical method of obtaining structural and stratigraphic information by means of observing acoustic waves as they reflect from geologic layers.  Reflection surveys are most often performed to define complex geologic structure. Depths of penetration for reflection surveys far exceed those of refraction survey methods.

Multi-Channel Analysis Surface Waves (MASW)

MASW is a seismic method that uses the dispersive nature of surface waves to map the values of shear wave velocities (Vs) in the subsurface.  In the active method, a large sledgehammer, or other device, provides an impulse of seismic energy, with the resultant surface waves recorded by a linear array of geophones positioned along the surface.  A passive method utilizing natural and anthropogenic sources can provide additional data at greater depths than the active method.  A dispersion curve relating phase velocity and frequency is developed from the recorded data, and then modeled to a profile of Vs values.  Multiple profiles can be combined into a virtual 2-D cross-section of shear wave velocity values that can be correlated to stratigraphic and density changes.

Applications

Used to map soil and sediment thickness and elevation of subsurface interfaces, such as top of bedrock.  MASW can be used to determine shear wave velocity or density of subsurface material, and the presence and location of voids and karst features.