The Complicated Link Between Material Properties and Microfracture Density for an Underground Explosion in Granite

Geophysical techniques are often implemented as quick and inexpensive ways to locate and characterize fractures in the subsurface, which is important for a number of geoscience fields. Seismic velocities are the most widely used proxies for identification of fractures, but the correlation is not always well defined. Here we present material property data: unconfined compressive strength, bulk density (ρ), Young's modulus (E), Poisson's ratio (ν), P wave velocity (Vp), and S wave velocity (Vs), in conjunction with microfracture densities measured on samples of granite collected before and after underground chemical explosions. Results indicate the relationship between fractures and material properties is complex, even in this single‐lithology environment. We interpret that this complexity arises from varying fracture mechanisms (e.g., dilation‐inducing fractures vs. compression‐inducing fractures) in different parts of the core, due to differences in stress conditions. Additional complexity may result from chemical interactions between the fresh fractures and the fluids in the area. Water content appears to have a significant, if not dominant, role in the unconfined compressive strength of the samples. We suggest caution when using elastic property measurements as a proxy for fracturing in areas of explosion‐induced damage, or in other areas where a variety of mechanisms induce fracturing. Key Points The effect of explosion‐induced microfractures on material properties is complex and may differ between dilation and compression regimes We show an example of damage (increased microfractures) where wave velocities are not decreased and may be increased Minor differences in water content appear to have a strong influence on unconfined compressive strength of granite