Refraction

Seismic refraction profiles the subsurface by determining the path and velocity of compressional or shear waves. The waves are created by shot, hammer, weight drop, or some comparable method to put energy into the ground.

Detectors are laid out at regular intervals in a line to measure the first arrival energy and the time of arrival.

The data are plotted in time distance graphs, from which velocity of and depth to layers can be calculated. This is possible because the compression or shear waves are refracted across layer boundaries where there is a difference in elastic and density properties.

MASW

Multi-channel analysis of surface wave (MASW) seismic methods are useful where the survey goal or target of interest might not be detected using standard refraction seismic methods, or where site conditions preclude the use of seismic refraction, or where a combined MASW and refraction microtremor (ReMi) approach is needed to develop a detailed site specific seismic Site Class for purposes of engineering design calculations.

In situations where expected caliche or cemented soil layer(s) overlie less dense soils, or where bedrock including varying layers of soft and hard rocks (example: claystone interbedded with sandstone units), or if surface or buried pavements or other infrastructure like existing slab on grade foundation or buried concrete box culverts are present within or under the planned survey location, standard refraction seismic methods cannot be performed because such existing materials likely have a higher P-wave velocity than the underlying soil materials of interest. These situations create a “velocity inversion” situation that precludes the seismic refraction method from working properly. Such velocity inversion conditions are usually either recognizable in the field during data collection, or from a desktop study of the geologic background for a site before field work is initiated, or from a combination of both.

When such conditions are recognized we often propose that (MASW) roll-along seismic methods be used. The data is collected using a Geometrics StrataView or Geode 24 channel digital seismograph unit and a 24-channel landstreamer unit with 4.5 hertz (Hz) center frequency geophones. The MASW technique uses recorded surface waves (specifically Rayleigh waves) to develop an approximate shear wave velocity profile of the site. Unlike the conventional refraction method, the MASW method does not require an increase of material velocity with depth. Therefore, low velocity zones (velocity inversions) are often detectable using MASW, and existing layer velocity inversions do not preclude collection of useful data. Recorded MASW data is processed and analyzed using either Geometrics SeisImager/SW software package, and/or SurfSeis 6 (Kansas Geological Survey) software for comparison of seismic model results. The resulting two-dimensional (2-D) MASW model is presented as a model of s-wave velocity versus depth, which can be correlated to approximate P-wave velocities, and associated estimated material rippability, depth to caliche, depth to cemented soils, and/or bedrock.

In some situations using MASW methods it is also possible to image and map positions of buried channels, earth fissure zones, voids, waste trenches, and landfills, especially when MASW is combined with results from other geophysical methods conducted over the same study area.

1-D Refraction Microtremor (ReMi)

Our geophysical services include the performance of one-dimensional (1-D) refraction microtremor (ReMi) seismic evaluation profiles at project sites. The goal for proposed 1-D ReMi evaluations is to obtain IBC and ASCE Vs100 or Vs30 seismic Site Class, and to provide site specific seismic parameters useful for calculating seismic site response and risk analyses conducted by other consultants or project engineers.

The 1-D ReMi seismic technique uses recorded surface waves (specifically Rayleigh waves) that are contained in passive “background noise” to develop an S-wave 1-D velocity profile of the study area down to a depth, planned in this case to be up to approximately 100 feet bgs. The ReMi data analysis also provides an estimate of seismic Site Class as per IBC Vs30, ASCE, and NEHRP definitions. We often also supplement 1-D ReMi studies with use of 1-D MASW methods which provide an active seismic source, this helps to avoid misinterpreting shear wave data that is biased due to an indeterminant phase angle. 1-D ReMi and 1-D MASW data are inverted together in a robust modeling algorithm to increase the confidence level in obtained results.

2-D Refraction Microtremor (ReMi)

Where man-made pavements or other relatively high velocity layers are present at ground surface or in the near subsurface, such as caliche cemented soils or packed cobbles and boulders, standard seismic refraction evaluations may produce erroneous results. Therefore, an optional scope of services might include the performance of a surface wave study using a roll-along two-dimensional (2-D) refraction microtremor (ReMi) seismic evaluation profiles at specified project locations. The 2-D ReMi method is insensitive to pavements and other near surface “fast layers” and can resolve information where seismic refraction methods might not work.

Often, the purpose of 2-D ReMi evaluations is detection of approximate depth to bedrock, which typically has significantly higher surface wave velocities than the overlying alluvium, the seismic velocities of the subsurface for use in estimating rippability, IBC Vs100 (feet) or NEHRP Vs30 (meters) and seismic Site Class, detecting the presence of soil layers that might be at risk for liquefaction during maximum credible seismic events (MCE), and for providing site specific parameters useful for calculating seismic site response conducted by other consultants or project engineers.