One of the most common applications of near-surface seismic testing is to identify the depth of the top of a bedrock unit in the subsurface. The two most effective seismic methods associated with this application are: 1) Seismic refraction, and 2) Multi-Channel Analysis of Surface Waves (MASW). Most people who are somewhat familiar with geophysics immediately think of refraction as the best tool to identify the top of rock. However, this method has its limitations. Specifically, refraction is based on the fundamental principal that the geologic layers (i.e. clay, sand, rock) increase in density/seismic velocity with increasing depth, and that there is a distinct boundary between each layer. So seismic refraction will not be able to depict a low density layer below a higher density unit (such as a zone of in-filled soils within a rock fracture, or a void). This type of situation is called a velocity inversion. Additionally, seismic refraction is generally not effective in a situation where concrete or asphalt is present at the ground surface.
MASW provides an alternative method to map density changes in the subsurface that is not limited by the principles described above. We won’t bore you with the details, but in brief, MASW measures the velocity of seismic surface waves and uses an inversion process to calculate the shear wave velocity of subsurface geologic units. In contrast to refraction, MASW can depict velocity inversions, and therefore can be used to make interpretations regarding voids, fractures, weathered rock, and can be performed on hard ground surfaces such as concrete and asphalt.
Pyramid Geophysical Services performed an MASW survey in downtown Durham, North Carolina to identify the top of a relatively weathered rock unit. The survey was associated with a geotechnical investigation for the design of a tunnel associated with a proposed light rail. MASW was chosen because the rock unit at the site was known to be weathered, and the geotechnical borings provided evidence of significant variability in the depth to competent/refusal bedrock.
Contour maps of the depth of competent rock were generated for the various portions of the MASW survey area.
Correlations between the geophysical interpretations and boring data were exceptionally high, and the resultant data set provided engineers with a clear understanding of geologic conditions with which to finalize design plans. This particular project provides an excellent example of using both physical sampling and interpretive geophysics to accurately understand local complex geology.
