Seismic Refraction Survey

Some people love it, some hate it, most barely understand it. Seismic refraction survey is a method that is often used to determine excavation characteristics. You'll notice that the term "rock" was omitted in the previous sentence. That's because seismic refraction really doesn't know the difference between materials like soil, water, rock, plastic, or concrete. What seismic refraction measures is the velocity that a compression wave travels through a material. The velocity of the compression wave in typical subsurface materials in the Piedmont region varies from low values of about 1,000 feet per second (fps) to high values over 5,000 to 10,000 fps.

The compression wave velocity is measured by placing sensitive motion detectors (geophones) on the ground surface along a line at roughly 10-to 20-foot spacings, covering a total of 100 to 200 feet. The ground is impacted with a sledgehammer, which triggers an oscilloscope, which then measures the travel time of the compression wave to each geophone. Harder materials located below softer ones will cause the compression wave to refract and affect the travel time of the wave to a surface geophone. Mathematical analysis of the travel times will produce a profile of changes in compression wave velocity. Through past study and experience the compression wave velocity can be correlated with excavation characteristics and material type. Materials with compression wave velocities less than 3,000 fps are interpreted as soil and can usually be excavated with small backhoes, scrapers, and front-end loaders. From about 3,000 to 5,000 fps the material is interpreted as weathered rock, and ripping is usually required to pre-loosen the materials to facilitate excavation. Above 5,000 fps it becomes more likely that blasting will be required, and the material is interpreted as rock.

Remember that seismic refraction survey alone really knows nothing about the physical makeup of the subsurface material, and a geotechnical engineer's judgment is needed to make the interpretation. To compound the problem, realize that soil is really just very soft rock, and rock can be thought of as very hard soil. In the Piedmont region changes in material hardness are often gradual, and the spectrum between soil and rock is continuous. Groundwater can also play a key role in interpretation. The seismic compression wave velocity in water is 5000 fps, and even experienced geotechnical engineers have mistakenly interpreted groundwater as rock. It is often very valuable to perform a few soil test borings to correlate actual subsurface conditions with seismic refraction survey.

Where is seismic refraction the most useful? If proposed excavation is greater than 20 feet, accessibility to the site is limited, and you have flexibility to adjust proposed grades, seismic refraction survey can represent your best tool to evaluate excavation characteristics.