A Comparison of Downhole Seismics and Multi Channel Analysis of Surface Waves MASW

The graphic below shows the basic components of the two techniques.  Downhole seimics involves measuring the time a compressional and a shear wave take to travel from a source on the surface to a receiver in a borehole.   It is straight forward and requires no assumptions but requires a borehole or well.  MASW involves measuring the velocity and dispersion of a surface wave as it travels past a string of geophones.  The shear wave velocity with depth is calculated from the measurements.  The technique requires an assumption of Poisson's ratio but does not require a borehole or well.



The graphic below is a comparison of the results of the two techniques.  The data were collected near Blythe California in the summer of 2009.  The dowhole data were collected in the upper 75 feet of a 500-foot deep six-inch diameter test well.  The MASW data were collected along a 300-foot long string of geophones located near the well.  The data were collected in a clay soil above the water table and were used to help determine potential liquefaction for a proposed power plant.


For practical purposes the results from the two techniques were the same.  Both techniques indicated typical shear wave velocities for unconsolidated clays lying above the water table, between 750 and 1600 feet per second.  Both techniques also showed the clays at a depth between 30 and 50 feet had a slightly lower shear wave velocity than the clays above and below.  

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