Apollo Geophysics ~ Applied Geological and Geophysical Exploration, Non-Destructive Testing (NDT), Environmental Services, and Equipment Rental Services ~ Bellingham, WA


					Apollo Geophysics 314 East Holly Street, Suite 207 Bellingham, WA 98225 Ph: 360.647.8303, Fax: 425.671.0865 Email:

Thanks again for your help on this site - you guys keyed us in to two good (positive UST) locations.

-Yen-Vy Van, P.G.
Senior Geologist

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Seismic Refraction

Identifying soil and rock types and depth to the local bedrock is an important step to determine foundation design prior to beginning any project to assure the most successful outcome.
Seismic Refraction is the most commonly used geophysical method. It is based on the measurement of seismic energy travel time through the subsurface and the development of an interpretive model of velocity and thickness for various layers of earth materials through which the seismic wave has passed in its travel from source to detector.
Interpreted results from a Seismic Refraction survey yield Seismic Velocity Profiles that show the distribution, shape, and thickness of subsurface layers having different seismic velocities. There is usually a closed relationship between seismic velocity and soil or rock types. With adequate correlation from other, more direct methods of exploration, it is usually possible to relate the seismic results to geologic conditions.

The Seismic Refraction Method is most frequently employed to determine depth to rock for classification of excavation and for foundation design studies. There are other uses of this method that are less frequently considered, such as using Seismic Velocity to identify rock formations; to determine the relative quality of rock; and to locate areas or zones of poor rock.
Seismic Refraction has been used in Geotechnical Investigations for several decades to delineate the top of rock and provide information with regard to the Rock Quality Designation (RQD) for rock. The principal behind Seismic Refraction assume that the velocity increases with depth and that only the refracting surface of each seismic layer is defined. Thus, Seismic Refraction seemingly works well to define lateral variations in the rock quality such as shear zones.

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