Following the challenges highlighted in my earlier post, I mobilized to my project site to do geophysical surveys the second week of June. This is the first time in many years that our geotechnical team has done our own geophysical survey work. I think we were successful. Unfortunately, I won’t know one way or the other until we’ve processed and interpreted the data.
For the purposes of our survey, we employed the Multichannel Analysis of Surface Waves (MASW) method. For us, MASW has two primary advantages:
- It works well on noisy sites
- We already own the data processing and interpretation software, and have some training in how to use it
Geophysical theory is not something I’m an expert in. My rudimentary understanding of MASW is that we can look at the travel times of various frequencies of seismic waves to generate a velocity profile of the subsurface. The frequency of the surface wave determines how deep the wave penetrates. So, if we look at the difference in travel time for waves of various frequencies, we can understand what the wave velocities are for various depths of the subsurface. The velocities are then correlated to other material properties (primarily density and strength), and we can make some interpretations about the subsurface.
In this case, we have some existing rock exposures near our MASW survey, so that gives us a baseline to begin building a model. Our goals for the survey were to map the top of bedrock surface and evaluate rippability of the rock. Provided that we have good data, both of these goals should be attainable.
I stated that we have some training in using our processing and interpretation software, but this is still the first time that our team has processed geophysical data in several years. We’re facing some technical challenges, but I’m hopeful that we’ll be able to push through with some technical assistance; and that I will have some fun pictures to show. For now, here are a few pictures of us working to collect the data.
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