GEOWAVE
SOLUTIONS
Seismic Shearwave
Seismic shear wave analysis is the measurement of site-specific shear wave velocities to determine the site's soil classification. This is important information when building larger commercial and industrial-type structures because the site soil classification directly influences the construction engineering of the building as it conforms to the International Building Codes (IBC) and the National Earthquake Hazards Reduction Program (NEHRP).
Traditionally, shear wave velocities have been obtained by geotechnical means by using blow counts and "N values" to approximate shear wave velocity. But because of more stringent IBC regulations that have demanded more accurate measurement, a shift to geophysical measurement of shear wave velocity has occurred in the last few years.
GeoWave Solutions, Inc. offers two ways to obtain shear wave velocity on your project site: measuring surface waves for dispersion in order to derive shear wave velocity or directly measuring shear wave velocities with depth using the down hole and the cross-hole method.
Surface Wave Method
The surface wave method is probably the preferred method because of its ease, cost, and depth coverage. Other advantages include:
It only requires adequate area to lay out linear and 2D seismic arrays (preferably in direct contact with soil although not necessary)
Nearby, high ambient vibration in areas such as busy highways, industrial parks, and downtown city environments do not adversely affect the results, in fact they aid in data collection!
The results are based on the average shear wave velocity over the length of the array, not on a small, laterally constrained spot.
GeoWave Solutions, Inc. combines two types of surface wave methods together to more accurately model subsurface shear wave velocities. The first method we use is an active array using vibratory source, such as a sledgehammer, to produce surface waves. This method records predominately higher frequency data that allows us to model shallow shear wave velocities.
The second method is a passive array where we allow ambient vibration to be the surface wave source. This method records lower frequency waves that allows us to model shear wave velocities at depth. The more nearby and distant vibration that is present around the site, the better dispersion we are able to develop and the deeper we are able to model the shear wave velocity. In rural areas where ambient vibration is relatively low, we typically generate ambient vibration by using vehicles or other equipment.
By overlying the shallower actively collected data over the deeper passive data, we produce a shear wave velocity model that employs the strengths of both methods and has sufficient redundancy to confirm velocity results.
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Crosshole and Downhole Methods
The crosshole method is the most accurate way to obtain subsurface shear wave information because it is actually measuring the shear wave velocities with depth. This method, however, is also the most
expensive, time-consuming, and laterally constrained method as well.
It requires three drilled, cased, and properly grouted boreholes (per ASTM Standards) that are linearly arranged with equidistant spacing of 10 or 15 feet.
If shear wave information is required to a depth of 100 feet, an inclinometer must be used to chart borehole deviation in boreholes drilled deeper than 50 feet.
The downhole method shares many features with the crosshole method, however, it is simplified in that only one cased and grouted borehole is required. A tri-axial geophone is lowered down the borehole and measurements of surface-generated shear waves are taken every 2.5 – 3 feet, depending on the project specifications.
On larger sites, shear wave data may be required in numerous areas which increases the number of test locations that must be properly prepared. Borehole dewatering should be performed just before data collection with either method