|SEISMIC METHODS |
Seismic methods depend on the elastic properties and density of rocks and the ability of geologic materials to transmit energy waves. The velocity of transmission of impressed energy pulses is dependent on the elastic properties and density of the rock.
Seismic methods are applied in two different modes: reflection and refraction. Both modes make use of the longitudinal velocity of the energy wave. The longitudinal velocity, VL, is related to the elastic properties and density of the rock medium by:
E = Young's modulus of elasticity, dynes/cm2
s = Poisson's ratio
r = rock density, gm/cm3
For ground-water exploration seismic survey methods will be most useful in distinguishing boundaries between valley fill and underlying older and perhaps denser bedrock.
On a larger scale, seismic methods may be used to trace regional aquifers in the subsurface.
The major disadvantage of seismic methods is they are very slow and very costly.
SEISMIC REFLECTION METHODS
Seismic reflection methods make use of the times it takes for a seismic energy pulse generated at or near the land surface by hammer, shotgun or explosion to return to the surface after reflection from subsurface rock. Reflections take place where their is a contrast in density between subsurface rock.
The reflected energy pulses are recorded at the surface by geophones that are sensitive to ground motion. Geophones are laid along lines and the variation in time for the return of the energy pulses is interpreted in terms of subsurface geological structure. The depth to the reflecting surfaces is determined by the elapsed time from the energy pulse to its return provided the velocity of the seismic pulse for various rock types is known or can be modeled.
The seismic reflection technique provides better information on subsurface geological structure than any other geophysical method. Where density contrast between subsurface geological materials is poor, seismic reflection methods may not be justified.
SEISMIC REFRACTION METHODS
Seismic refraction methods require the geophone arrays to be located at farther distances from the point of the energy input pulse. The energy pulse travels downward and horizontally through the earth before returning to the geophones. The method is useful for determining the travel times for the seismic shock wave to move known distances through earthen materials. Variations in the return time as shot points and geophone detectors are moved can be interpreted in terms of subsurface structure.
The seismic refraction method does not provide the spatial accuracy provided by seismic reflection, it does provide information on seismic wave velocity within different geologic materials that can assist the geophysicist in interpretation of seismic reflection data.
Seismic reflection and refraction data are analyzed based on assumptions of subsurface structure and physical properties. Recent developments in computer processing have made it possible to construct models and to test field data against model calculations. The interpretation, however, requires geologic information and good judgment by the geophysicist making the interpretation.
For ground-water exploration, seismic methods can only indicate overall geologic structure and the form of lithologic boundaries. Seismic methods cannot locate zones of rapid ground-water movement and optimum wells sites.
This website contains case studies of the application of seismic methods to ground water exploration. As new case studies are developed they will be added.