When geophysics met geomechanics: Imaging of geomechanical properties and processes using elastic waves

Geophysics measurement techniques can be considered as non-destructive, remote methods to analyse the physical properties of geologic materials hidden at depth in the earth, within geotechnical structures or within laboratory test specimens. The objective of this paper is to highlight at least some of the potential to use geophysics methods to aid geomechanical investigations, both at the laboratory and the real-world scales, by characterising property distributions and their changes plus, eventually, the mechanisms by which they change. The focus is primarily on geophysical imaging using elastic waves, whose propagation is controlled by a material's elastic properties and density. The former can be thought of as the summation of contributions over a range of length scales: grains, discontinuities (including cemented or uncemented grain contacts), inter- or intra-granular cracks, fractures and layers, which can all be anisotropic or can produce an anisotropic aggregate material. Examples are provided, at the laboratory and hydrocarbon reservoir scales, of geophysical characterisation of these different geomechanical attributes through anisotropy analyses, time-lapse measures of deformation and "full-field" velocity imaging.