Surface Wave Methods to Determine Vs down to 100 Feet Depth
The new Refraction Microtremor (ReMi) method has basically replaced the Spectral Analysis of Surface Waves (SASW) method here in Las Vegas, NV. I would estimate as many as 1,000 tests (surveys) have been run, by geotechnical companies, in the last 2 years to determine the IBC class site, average Vs (shear wave velocity) down to 100’.
The big advantage of ReMi over SASW is it uses 12 or more sensors (geophones) while SASW usually uses 2 sensors which keep expanding out. Also ReMi uses background noise which generates the Rayleigh waves which are the ReMi signal but noise to the SASW. Another method called Multiple Analysis of Surface Waves, (MASW), uses multiple sensors like ReMi but has its own source like SASW. SASW/MASW must use a hammer, 2 ton weight drop, vibrator like Vibroseis, etc, to generate the Rayleigh waves for a source that must overcome the background noise. In other words- SASW/MASW requires an active source while ReMi normally uses a passive source- just background noise. An active source such as a vibrator can be used with ReMi to determine the shallow geology Vs, 0 to ~30’ more accurately, than using just the passive natural background noise.
Incidentally, all three methods measure the vertical Rayleigh wave component to get the Rayleigh Dispersion curve to obtain the Vs model. The Vs model is usually a 1-D model showing Vs changing with depth, but new ReMi programs can show a 2-D model showing the horizontal lateral changes.
I suggest you try and attend the Association of Engineering Geologists (AEG) meeting, which will be held in Las Vegas during September 2005, as I and others will present the results of many ReMi tests run in the Las Vegas area.
