The seismic survey is one form of geophysical survey that aims at measuring the earth’s (geo-) properties by means of physical (-physics) principles such as magnetic,
electric, gravitational, thermal, and elastic theories.  It is based on the theory of elasticity and therefore tries to deduce elastic properties of materials by measuring their
response to elastic disturbances called seismic (or elastic) waves.    

What Are Seismic Waves?
A seismic source-such as sledgehammer-is used to generate seismic waves, sensed
by receivers deployed along a preset geometry (called receiver array), and then recorded
by a digital device called seismograph (
Fig. 1).  Based on a typical propagation
mechanism used in a seismic survey, seismic waves are grouped primarily into direct,
reflected, refracted, and surface waves (
Fig. 2).      

There are three major types of seismic surveys: refraction, reflection, and surface-wave,
depending on the specific type of waves being utilized.  Each type of seismic survey
utilizes a specific type of wave (for example, reflected waves for reflection survey) and its
specific arrival pattern on a multichannel record (
Fig. 3).  Seismic waves for the survey
can be generated in two ways: actively or passively.  They can be generated actively by
using an impact source like a sledgehammer or passively by natural (for example, tidal
motion and thunder) and cultural (for example, traffic) activities.  Most of the seismic
surveys historically implemented have been the active type.  Seismic waves propagating
within the vertical plane holding both source and receivers are also called inline waves,
whereas those coming off the plane are called offline waves (
Fig. 4).    

Refraction Survey     Reflection Survey     Surface-Wave Survey
Fig. 1.  Schematic of overall field setup for a seismic survey.
Fig. 2.  Major types of seismic waves based on propagation characteristics.
Fig. 3.  (Right) A field record and interpretation of different seismic events based on
the arrival pattern.  Fig. 4. (Below) Illustration of active versus passive waves and
inline versus offline waves.
What is seismic survey?