Mapping the mechanical properties of rocks using automated microindentation tests

A microindentation scanner is constructed that measures the spatial fluctuation in the elastic properties of natural rocks. This novel instrument performs automated indentation tests on the surface of a rock slab and outputs 2‐D maps of the indentation modulus at submillimeter resolution. Maps obtained for clean, well‐consolidated, sandstone are presented and demonstrate the capabilities of the instrument. We observe that the elastic structure of sandstones correlates well with their visual appearance. Further, we show that the probability distribution of the indentation modulus fluctuations across the slab surfaces can be modeled using a lognormal probability density function. To illustrate possible use of the data obtained with the microindentation scanner, we use roughly 10 cm × 10 cm scans with millimeter resolution over four sandstone planar slabs to numerically compute the overall drained elastic moduli for each sandstone sample. We show that such numerically computed moduli are well modeled using the multicomponent form of the Hashin‐Shtrikman lower bound that employs the observed lognormal probability distribution for the mesoscopic‐scale moduli (the geometric mean works almost the same). We also compute the seismic attenuation versus frequency associated with wave‐induced fluid flow between the heterogeneities in the scanned sandstones and observe relatively small values for the inverse quality factor ( Q −1 <10 −2 ) in the seismic frequency band 10 2  Hz < f < 10 4  Hz. The numerically computed frequency dependence in the attenuation varies from one type of sandstone to another, and we observe significant anisotropy in the attenuation associated with waves propagating in different directions.