Optimal Spread Design for Passive MASW

The passive surface wave method has the potential to reach investigation depths significantly greater than active sources and can be effectively used in very noisy settings. In this study the optimal spread layout and orientation is investigated and determined that would allow surface wave penetration deep enough into the bedrock to permit studies of stress and strain conditions. An array of 336 receivers was deployed in a 2-D spread. The 4.5 Hz vertical geophones were orientated at parallel and orthogonal alignments to nearby railroad tracks. The dominant energy source were trains passing both along the north and west sides of the site. By selecting the appropriate recording times it was possible to uniquely capture trains in a wide range of azimuthal orientations. The relatively large overall spread made it possible to compare various sub-arrays by processing only selected receivers. Arrays orientated parallel to the direction of incident low frequency waves, resolve frequencies as low as 4 Hz even using small number of receivers. These lower frequencies allow a marked increase in the maximum depth of investigation, thereby, helping to solve engineering problems at greater depths than possible with active source investigations.