Basic Concept: The seismic refraction method can be used to find fractures in the bedrock that might indicate voids beneath the fractures. The method involves looking at the amplitude and character of the refracted signals at each geophone. If a fracture, or more likely a fracture zone, is encountered, the amplitude of the refracted signal will be diminished. If a large open fracture is encountered, the signal may disappear completely. Figure 94 illustrates the concept, showing that traces 2 and 3 have diminished amplitudes over the fracture zone.
Figure 94. Seismic refraction across a fracture zone.
Data Acquisition: The design of the survey is the same as for any other seismic refraction survey. The length of the geophone spread should be sufficient to image the bedrock. Depending on the depth of investigation, an energy source of sufficient size is required. This may be a hammer and base plate or black powder seismic gun shots in shallow (1 or 2 feet) holes. Other methods, such as weight drop techniques, are also available. Since the fracture zone may attenuate the signals more than non-fractured bedrock, more energy may be needed than for an equivalent bedrock mapping survey.
Data Processing: Data processing is fairly simple and involves observing the seismic records for diminished amplitudes in the refraction arrival signals, as seen in Figure 94.
Data Interpretation: The locations of diminished amplitude zones can be placed on a map of the area from which the fracture zone can be outlined.
Advantages: This application of the seismic refraction method is fairly rapid to apply in the field. The seismic records are displayed on the instrument allowing potential fractures to be recognized during the field survey.
Limitations: Probably the biggest limitation is that the method is indirect in that usually fracture zones are detected and not voids, although voids may be detected if they are not too far beneath the bedrock surface. The fractures may have some other origin and not be caused by voids. However, if voids are the cause of fractures, then by having small geophone spacings, sufficient data can be obtained over the fracture zone providing good lateral resolution. Any local source of seismic noise will reduce the quality of the data, and a larger seismic energy source may be required to overcome this noise. If the ground surface is hard (concrete or asphalt), it will be difficult to plant the geophones. Surveys conducted near roads or other industrial activity will also be subject to noise from these activities. Although shear waves can be used to locate fracture zones and are less influenced by traffic noise, creating impulsive shear waves with sufficient amplitude is more difficult than for compressional waves. The method is fairly labor intensive.