Seismic Measurements for Detecting Underground High-Contrast Voids

Active seismic technology has been widely utilized for imaging and characterizing the subsurface of the earth. Its greatest success and most publicized application has been in the exploration and exploitation of oil and gas resources. The technology has also been applied in near surface situations in the field of groundwater resource management, and geoenvironmental and civil engineering. Active seismic interrogation of the first meter of the ground has also been used for unexploded ordnance (UXO) and landmine detection as well as agricultural applications. These methods are sensitive to anomalous zones in mechanical properties (bulk and shear stiffness/compliance) and densities within the subsurface because these properties control the velocity, attenuation, and impedance of the seismic waves. Although the elastic compliance and density contrasts between manmade subsurface structures and the surrounding material are high, seismic techniques have not been extensively used. Two reasons are: (1) the diameter of the structures can be small relative to the spatial sampling and the seismic wavelengths used in seismic surveying and (2) the surrounding shallow subsurface soil is highly heterogeneous with respect to its mechanical properties. In this paper, we discuss a finite-frequency seismic refraction tomography approach for detection of high-contrast voids in the shallow subsurface. Data from several field sites containing pipes at different depths will be presented. The strategies developed will soon be applied to the related problem of deeper but larger tunnels. ©2009 The Environmental and Engineering Geophysical Society