Introduction
The shallow refraction method is an important tool for the analysis of shallow subsurface structures. The method involves the analysis of seismic refraction data obtained from the subsurface in order to give an insight into the depth distribution of mechanical properties such as compressional velocity, shear velocity and rigidity. Before the advent of more sophisticated three-dimensional seismic and sonic tomography, shallow refraction provided a critical tool for viewing shallow subsurface profiles in much greater detail than was previously possible.
Description
The shallow refraction method uses surface or near surface seismic energy sources and receivers. The sources are usually vibroseis or dynamite shots and the receivers are usually geophones or seismometers. The seismic energy travels through the subsurface, where it is eventually refracted and reflected off the different boundaries present between subsurface layers of different properties, such as velocity, density and stiffness. By recording the travel time of the seismic energy through these layers and the arrivals of the refracted waves, the dips and boundaries of these layers can be determined. The method is particularly useful for mapping shallow subsurface layers which have a lower velocities than the layers below and above, such as glacial deposits, sedimentary basins or water-bearing formations. The data typically consists of one or two shooting lines, and the length of each line is normally not more than 1 km in length.
Analytical Considerations
The seismic energy sources produce seismic energy that is refracted and scattered at interfaces between layers of different density and velocity. The energy radiates outward in all directions, and the reflections and refractions can be detected in the receivers located at various positions along the shooting line. The travel time of the energy between various layers and the time of arrival of the refracted waves can be used to construct a shallow subsurface profile.
The analysis of seismic refraction data involves the solution of equations that govern the behavior of the seismic waves in an inhomogeneous medium. These equations are based on a layered structure and are characterized by an appropriate set of parameters associated with the model under consideration. The parameters of the model include the density, compressional and shear velocities, sourceto-receiver offsets and the strength of the seismic energy source. The interpretation of the refraction data is further complicated by the presence of travel time anomalies caused by multifocusing, curved raypaths and multiple reflections. These factors must also be taken into account when interpreting the seismic data.
Discussion
The shallow refraction method has been used extensively around the world to investigate shallow subsurface structures. Examples include its use in the Northwestern United States to investigate glacial deposits and its use in the United Kingdom to investigate sedimentary basins. The method has also been applied to most areas of the world where an understanding of the shallow subsurface is desired.
Conclusion
In conclusion, the shallow refraction method is an important tool for interpretation of the shallow subsurface. The method is particularly useful for mapping shallow subsurface layers with low velocities and for determining subsurface profiles. However, the method is complicated by the presence of travel time anomalies caused by multifocusing, curved raypaths, and multiple reflections. The method requires a skilled geophysicist to interpret the data and gain the full power of the method.
