The Basic Principle of Seismic Survey
Seismic survey method has three principal applications which are:
- Delineation of near-surface geology for engineering studies and coal and mineral exploration within depth up to 1km. Known as engineering seismology.
- Hydrocarbon exploration and development within depth of up to 10km. Known as exploration seismology.
- Investigation of the earth’s crustal structure within depth of up to 100km. Known as earthquake seismology.
Therefore, seismic refraction survey or MASW survey can be classified as Engineering Seismology.
Basic Principle of Seismic Survey (Engineering Seismology)
The basic principle of all seismic survey methods is the controlled generation of seismic waves by a seismic source in order to obtain an image of the subsurface. Seismic wave is a transfer of energy by way of particle motion. Different types of seismic waves are characterized by their particle motion.
Seismic energy sources produce wave types known as:
a) Body waves; where the energy transport is in all directions.
b) Surface waves; where the energy travels along or near to the surface.
In shallow seismic method such as seismic refraction and/or seismic reflection survey, the prime interest are the two types of body waves:
i) P-waves (primary, longitudinal or compressional waves)
Compressional waves – particle motion is parallel to propagation direction.
ii) S-waves (secondary, shear or transverse waves)
Shear waves – particle motion is perpendicular to propagation direction.
Meanwhile, surface waves often considered to be a source of noise in seismic refraction or seismic reflection survey. However, surface waves contain valuable information about material properties of the shallow ground.
The Seismic Refraction Survey
In the seismic refraction analysis, the travel times of first arrivals on recorded seismograms are commonly used. The first arrivals have to be identified, picked and presented in travel time curves (plots of arrival times vs source-receiver distance).
Main aim of seismic refraction surveys is to determine the depths of layers, the refractor topography (dip of layers) and layer velocities.