The seismic methods measure the travel-time of acoustic waves propagating
through the subsurface. The shot point is the location of the source wave
and the acoustic receivers are called geophones. Source waves are generally
generated by percussion mechanisms located near the ground or water surface
(for example dropped weights or dynamite). Data are collected, stored, and
processed in a seismograph.
Seismic refraction is defined as the travel path of sound wave through an
upper medium and along an interface (at a critical angle) and then back to
the surface as shown in the figure below. The acoustic waves, like light
waves, follow Snells's Laws of Refraction.
Seismic refraction surveys are commonly used to determine the thickness of
unconsolidated materials overlying bedrock (overburden thickness) and depth
to the water table. They are used for characterizing the geological framework
of ground-water contamination studies and for assessing geologic hazards
and archaeologic studies.
An example of seismic refraction data and interpreted model.
The seismic reflection method records acoutic waves at the surface that are
reflected off of subsurface stratigrphic interfaces where changes in the
material density and conductive velocity of the acoustic waves are significant.
The reflection patterns are described by Snell's Laws of Reflection.
Seismic reflection surveys are used for determining the thickness and structure
of subsurface geology and are commonly applied in hydrocarbon and mineral
exploration, earthquake and tectonic studies, and in the marine enviromant
for resolving stratigraphic details (for example, the location and thickness
of beach-sand deposits).
Digital marine seismic profile data traversing a sand ridge approximately
6 miles off of Avalon, New Jersey. The S2 reflector marks the sequence boundary
between Holocene sediments and older Tertiary deposits. These depositional
features are primary sources for beach sand replenishment.
For more information on geophysical techniques, please refer to
Chapter 8 of the NJDEP's Field Sampling
Procedures Manual, 2005.