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You are here: Home / Newsletter Archive / October 2019 Newsletter: Seismic Geophysical Surveys to Examine Bedrock Depth and Integrity

October 2019 Newsletter:
Seismic Geophysical Surveys to Examine Bedrock Depth and Integrity

The Triassic Basin soil and rock formations in the Durham, North Carolina, region provide challenges for geotechnical design and construction. These formations consist of highly heterogenous stratigraphy including sands, silts and clays overlying a weathered rock unit that can contain boulders, fractures and an inconsistent competent bedrock surface. A proposed light rail system was under design to extend through downtown Durham, and a geotechnical investigation was performed to better understand the behavior of the rock formation and its integrity.

MASW Transect Locations
MASW Survey Line Locations in Downtown Durham

Pyramid Geophysical Services was contracted to perform a Multi-Channel Analysis of Surface Waves (MASW) seismic survey across specific portions of the proposed rail alignment where geotechnical borings either could not be performed, or where significant variability in the depth of competent rock was observed. The goal of the seismic survey was to “connect the dots” between the geotechnical borings and provide a more comprehensive understanding of bedrock behavior along the alignment. Pyramid performed a total of 542 separate MASW shots resulting in 20 individual 2-Dimensional cross sections of velocity that were used to analyze the overburden, weathered rock thickness and depth to competent rock across the site.

MASW Cross Section
Example of Seismic Results Showing Interpreted Top of Competent Rock with Geotechnical Boring Overlays

The MASW results portrayed a highly variable bedrock unit in terms of both depth and competency. Isolated boulders were interpreted at various locations, and well as rock pinnacles and fractures. Pyramid analyzed the geotechnical boring data at locations where the MASW tests intersected boring locations, and overlaid these borings onto the seismic results (see above image).

The results of the seismic testing were combined with geotechnical boring data to generate plan-view contour maps of competent bedrock depths across the site (see example below). 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 is no longer moving forward into the construction phase due to other factors; however, it provides an excellent example of using both physical sampling and interpretive geophysics to accurately understand local complex geology.

Contact us today to find out more about the benefits seismic surveys can provide for geotechnical investigations!

Top of bedrock contour map
The geophysical results were used to generate contour maps of the top of competent bedrock across each project area. An example of these interpretive maps is shown above.

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Filed Under: Newsletter Archive Tagged With: Geophysics, MASW, north carolina geophysics, seismic, shear wave, top of rock

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Recent Articles

  • October 2020 Newsletter:
    Supplemental Tools for Geophysics – How We Present the Results
  • July 2020 Newsletter:
    Density Anomalies and Geophysics – Concepts and Methods
  • January 2020 Newsletter:
    More interesting information about bedrock fractures and how to analyze them
  • October 2019 Newsletter:
    Seismic Geophysical Surveys to Examine Bedrock Depth and Integrity
  • June 2019 Newsletter:
    How the Roads that lead to Rome were made

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