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Appendix D. Seismic Methods
There are a variety of seismic methods used. The objective of the survey controls which
specific method to use. This section will give you an overview of some commonly used
methods. Please refer to the bibliography at the end of the manual. If a more detailed
and thorough description of seismic methods is needed then Butler (2005) can be
recommended as it is relatively new and has an extensive and updated listing of
references.
In the refraction and reflection methods there is usually a division between shallow and
deep surveys.
Refraction
The objective is to find out the arrival times of the head waves to map the depth to the
refractors in which the waves travel. The refraction method is based on the assumption
that the earth is made of layers of materials that increase in seismic velocity with each
successively deeper layer. The key element is that an incident ray is critically refracted
along the boundaries between layers, before returning to the surface. From the first
arrival times it is possible to calculate the seismic velocity for each layer and the depth
to the boundaries. The seismic velocity gives information about material properties, and
what kind of material comprises each layer. Additionally, frequency analysis of the
recorded signals can give more information about the material properties.
The principles for seismic refraction techniques can be found in most geophysical
textbooks. For a more detailed description of both theory and practice, see Sjögren
(1984).
Investigations performed with the refraction method can yield a variety of reliable data
such as depth of various overburden layers, depth to bedrock, rock quality, soil
compositions and solidity, rip ability, excavatability, water tables and rock structure.
The refraction seismic method can be used for a wide range of applications, for example:
Underground Tunnels and their entrances, machinery halls, gas and oil
storage facilities, air raid shelters
Foundations Heavy industrial buildings, bridges, harbor quays and
breakwaters, dams, piling, airfields
Excavations Harbor basins and entrances, pipelines, canals, roads, railways
Resource searches Gravel, sand and quarry sites
Water prospecting Groundwater table in the overburden, water bearing sections of
rock
Ore prospecting Mineralized weathered zones, buried channels with high
mineral content
Reflection
In this method, the arrival time events are attributed to seismic waves that have been
reflected from interfaces where changes in acoustic impedance occur, and of wave
shape changes.