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Advantages and Disadvantages of the Refraction and Reflection Methods

On the previous page, we attempted to describe some of the advantages and disadvantages of the seismic methods when compared to other geophysical methods. Like the electrical methods, the seismic method encompasses a broad range of activities, and generalizations such as those made on the previous page are dangerous. A better feel for the inherent strengths and weaknesses of the seismic approach can be obtained by comparing and contrasting the two predominant seismic methods, refraction and reflection, with each other.


Refraction Methods Reflection Methods
Advantage Disadvantage Advantage Disadvantage
Refraction observations generally employ fewer source and receiver locations and are thus relatively cheap to acquire. Because many source and receiver locations must be used to produce meaningful images of the Earth's subsurface, reflection seismic observations can be expensive to acquire.
Little processing is done on refraction observations with the exception of trace scaling or filtering to help in the process of picking the arrival times of the initial ground motion. Reflection seismic processing can be very computer intensive, requiring sophisticated computer hardware and a relatively high-level of expertise. Thus, the processing of reflection seismic observations is relatively expensive.
Because such a small portion of the recorded ground motion is used, developing models and interpretations is no more difficult than our previous efforts with other geophysical surveys. Because of the overwhelming amount of data collected, the possible complications imposed by the propagation of ground motion through a complex earth, and the complications imposed by some of the necessary simplifications required by the data processing schemes, interpretations of the reflection seismic observations require more sophistication and knowledge of the process.
Refraction seismic observations require relatively large source-receiver offsets (distances between the source and where the ground motion is recorded, the receiver). Reflection seismic observations are collected at small source-receiver offsets.
Refraction seismic only works if the speed at which motions propagate through the Earth increases with depth. Reflection seismic methods can work no matter how the speed at which motions propagate through the Earth varies with depth.
Refraction seismic observations are generally interpreted in terms of layers. These layers can have dip and topography. Reflection seismic observations can be more readily interpreted in terms of complex geology.
Refraction seismic observations only use the arrival time of the initial ground motion at different distances from the source (i.e., offsets). Reflection seismic observations use the entire reflected wavefield (i.e., the time-history of ground motion at different distances between the source and the receiver).
A model for the subsurface is constructed by attempting to reproduce the observed arrival times. The subsurface is directly imaged from the acquired observations.

As you can see from the above list, the reflection technique has the potential for being more powerful in terms of its ability to generate interpretable observations over complex geologic structures. As stated before, however, this comes at a cost. This cost is primarily economic. Reflection surveys are more expensive to conduct than refraction surveys. As a consequence, environmental and engineering concerns generally opt for performing refraction surveys when possible. On the other hand, the petroleum industry uses reflection seismic techniques almost to the exclusion of other geophysical methods.

In this set of notes, we will only consider seismic refraction methods.


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Seismology