Refraction microtremor testing offers a variety of applications, especially in site characterization, according to James Niehoff, PE, in his Heiland lecture “The Use of Refraction Microtremor Testing in Site Characterization. Niehoff briefly explained the testing technique before providing a plethora of miniature case studies.
Niehoff began by discussing traditional methods of geotechnical studies. One mentioned was soil test boring, which is “where we drill holes in the ground and then take samples of the material below grade.” Niehoff noted the importance of soil test boring, stating, “The geophysical testing we’re going to be talking about does not replace soil test borings, but is an adjunct to this testing technique.” The other technique mentioned was backhoe test pits, which Niehoff described as “primitive, but certainly useful.”
According to Niehoff, traditional exploration can have serious limitations, mostly pertaining to data collection. “We typically drill borings throughout the building area… on one hundred foot centers.” He approximated that for a typical building, these borings represent only 0.014% of the total building area, “so obviously geotechnical engineers have to extrapolate that data and make a lot of estimates and guesses about what happens in that boring area.” Also, these techniques do not provide enough reliable data for evaluation of seismic characteristics. Aside from data issues, traditional field study techniques can experience access difficulties.
After establishing flaws in the traditional methods, Niehoff described the refraction microtremor procedure. “It’s very similar to typical refraction testing. However, in this case, we’re not using a specific energy source,” said Niehoff. The procedure involves putting several geophones into the ground ten to twenty-five feet apart and using already existent background noise. “Oftentimes, background vibration from traffic is more than sufficient to generate enough noise,” Niehoff explained. Niehoff and his team “typically take fifteen to twenty samples for each location and combine those to reinforce the signal.” The data collected from the geophones is fed into a computer, where it is converted into a frequency slowness plot. Points from the frequency slowness plot are then selected. “Once those dispersion picks are selected, we put that into a dispersion curve and from that dispersion curve… we can develop a shear wave velocity profile down to and up to about 300 feet,” Niehoff said.
Niehoff also reported on several cases where refraction microtremor techniques have made planning more efficient. In planning the fifty story Devon Energy facility in Oklahoma City, Oklahoma, the techniques enabled accuracy in a tight spot. The site for the building was covered by a two-story parking garage, and the technique allowed geotechnical surveys to be conducted before demolishing the parking garage.
In another case, a proposed shopping center in Erie, Colorado, was known to be underlain by coal mines, but the extent of the mines was not known. Carefully patterned testing revealed potential problem areas. This in turn allowed Niehoff and his team to establish the extent of the mines and make recommendations for mitigating risks.
The process can also be of assistance in sensitive situations, either with equipment or geology. When applied to the 33-story J.W. Marriott Hotel in Indianapolis, Indiana, increased accuracy allowed a different seismic classification of the building. This in turn reduced the amount of steel needed for the foundation and the cost.
Niehoff concluded, “In summary, refraction microtremor testing and other geophysical testing techniques allow for a more complete understanding of sub-surface profiles.”