Seismic refraction maps contrasts in seismic velocity – the speed at which seismic energy travels through soil and rock. This parameter typically correlates well with rock hardness and density, which in turn tend to correlate with changes in lithology, degree of fracturing, water content, and weathering.
There are two basic approaches to seismic refraction data analysis: layer-cake and tomographic inversion. The former is the more traditional approach, although tomography has become more popular as faster computers have made it much more feasible than in the past.
Especially in the near-surface, it is not always the case that seismic velocities are divided into high-contrast, discrete layers. Nor is it the case that velocities are constant horizontally. Conventional layer-cake inversion techniques, such as the delay-time method, assume both, and require the geophysicist to provide layer assignments before the data inversion can be completed.
Tomography is less constrained in this sense; it does not “think” in terms of layers, and it better accommodates horizontal velocity variations. If discrete layering is not apparent in the raw data, the tomographic approach is generally more appropriate. As such, Geometrics’ SeisImager Refraction Analysis software offers both options.
The depth of penetration in a seismic refraction survey is approximately 1/5th of the length of the geophone spread, including offset shots. So if you need to see 10m deep, you will need room to lay out a (minimum) 50m seismic spread, as measured from offset shot to offset shot.
For most engineering refraction work, the best possible source is a 14 or 16lb sledgehammer. A downhole seisgun is not a good refraction source in general, except in cases where the surface is too soft to use a hammer effectively. An accelerated weight drop can be a good source, but is not portable and requires vehicle access to the shot points. Small explosives, such as Kinepak, are ideal when portability is required and the depth of interest is greater than what can be reached with a hammer.
Seismic refraction requires that velocities increase with depth. A lower velocity layer beneath a higher velocity layer will not be detected by seismic refraction, and will lead to errors in depth calculations. Fortunately, this is a fairly uncommon occurrence in the shallow subsurface.
The seismic source employed must match the desired depth of penetration. For hammer and plate work, the maximum depth you can expect to explore to is about 15-20m; however, this can vary significantly depending on geology, surface conditions, cultural noise, and the person swinging the hammer. Refraction is a relatively broad-brush technique – it looks at gross velocity differences, and you should not expect to be able to map more than 3-4 individual velocity layers.
Cultural noise can be a problem – it is more difficult to conduct a seismic survey in an urban environment than in a rural one. Surveying along busy roadways should be avoided when possible. Shooting at night is sometimes necessary in order to achieve acceptable signal-to-noise ratio in busy areas.
The final product of a refraction survey is a velocity model, such as the layer-cake inversion shown on the left, or a tomographic inversion as shown below