Pyramid staff have taken advantage of this relatively new technique in the recent past for the purposes of geologic mapping, identifying depth to rock, analyzing subsurface velocity and density profiles, and geotechnical investigations. We utilize the Geometrics Geode seismograph with a 24 channel geophone system to collect subsurface velocity (shear wave) data at our project sites. The MASW method is useful in areas where shallow buried utilities and other metal or electrical objects would cause interference when attempting to use other geophysical methods (such as MER or EM).
MASW is an effective tool in delineating geologic features in the subsurface, including major stratigraphic changes (i.e. sand to silt to clay) as well as the top of rock and the integrity of the upper rock formation. The velocity data obtained from an MASW survey can also be used by geotechnical engineers in foundation design and structural remediation plans.
Pyramid also incorporates traditional seismic refraction techniques for projects requiring information such as depth to bedrock, depth to groundwater, and lithologic characteristics. Refraction provides excellent subsurface information regarding significant bedding changes and transitions into saturated conditions. Pyramid has utilized the seismic refraction method for the purposes of assisting in construction design as well as mapping basement profiles for hazardous waste sites.
Downhole Seismic Testing
Pyramid provides downhole seismic testing to our clients for specific geotechnical needs. Downhole seismic testing uses a single downhole geophone and a prepared borehole to investigate seismic behavior changes at high resolution in the subsurface at a single point location. Both P-waves and S-Waves are collected during a downhole survey. P-Waves are collected by using a sound source such as a sledgehammer to vertically strike a metal plate at the ground surface. S-Waves are collected by striking a shear beam at the ground surface on its left and right sides. P-wave and S-Wave measurements are taken at a specified depth interval in the borehole, such as every three feet. The results provide geotechnical engineers with velocity information that can be used for design purposes.