Shallow seismic investigations of fracture densities and distributions within the deep critical zone
Abstract
The strength and coherence of bedrock plays a key role in shaping landscapes, resisting erosion, and modulating the efficiency of critical zone processes. Whereas most intact, unfractured bedrock is relatively strong, fractures and discontinuities within the rock mass greatly reduce this mechanical strength and increase hydrologic permeability, which in turn increase susceptibility to erosion, physical and chemical weathering, biologic activity, and gravitational collapse. Therefore, the key factor is not solely the strength of intact rock, but rather the effective strength and competency of the entire rock-mass within the near-surface where it interacts with climatic, topographic, and biotic variables. Quantifying subsurface characteristics over geomorphically relevant scales (10s to 100s of meters), however, has proven exceedingly difficult. Because variations in near-surface velocity reflect changes in the mechanical properties of the subsurface, shallow seismic surveys provide a promising method for quantitatively assessing rock-mass strength and subsurface fracture distributions. Here we present a suit of studies that reveal strong correlations between field-based measurements of seismic velocity and geomorphic-scale measures of rock-mass properties. We show a direct relationship between seismic velocities and fracture distributions observed in outcrops and drill-cores. Furthermore, we show that the rate of bedrock extraction from industrial mining sites (an analog for erodibility) correlates with rock-mass velocities, whereas depth-dependent velocity variations strongly modulate landslide processes and hillslope stability. We demonstrate that field-based measurements of p-wave velocities provide an accurate first-order assessment of rock-mass strength, which is largely invariant with rock type. This relationship suggests that a strong, but highly fractured rock can have an equivalent seismic velocity and rock-mass strength to a weak rock that has been less densely fractured. In general there is a nonlinear relationship between seismic velocity and rock-mass strength, however, a distinct geomorphic threshold exists at ~2.5-3.5 km/s that delineates between unstable, more readily eroded rock masses (low velocities) versus stable, highly resistant rock masses (fast velocities). In contrast, reductions in seismic velocity and elastic moduli (calculated from p- and s-wave velocities), relative to unfractured rock, provide insight into the density and distribution of subsurface fractures, which may be more relevant for assessing the impact of fractures on hydrologic permeability, "deep" bedrock weathering, and rates of regolith production. We show that greater reductions in both seismic velocity and elastic moduli correlate nonlinearly with greater porosity (more fracture space) and increased density of fracture intersections (lower elastic properties).
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFMEP41I..07C
- Keywords:
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- 0935 EXPLORATION GEOPHYSICS / Seismic methods;
- 1815 HYDROLOGY / Erosion;
- 1859 HYDROLOGY / Rocks: physical properties