Seismic refraction and electrical resistivity tests for fracture induced hydraulic anisotropy in a mountain watershed

Abstract:

Fractures can have a significant effect on the deep architecture of the critical zone (CZ). According to Riebe et al. (2016), fractures can be a high hydraulic conductivity pathway allowing transport of meteoric fluids into the deep unweathered zone. St. Clair et al. (2015) showed that the use of both electrical resistivity and seismic measurements can reveal highly fractured zones and, in some cases, these can be correlated with the regional stress field. Most geophysical studies in mountainous watersheds ignore fracture induced anisotropy. The Dry Creek Experimental Watershed (DCEW) is located near Boise, Idaho and has been the subject of numerous studies of the shallow hydrologic system. Despite this plethora of work, the deep subsurface hydraulic system is not yet well understood. Our study focuses on resolving the fracture induced hydraulic conductivity anisotropy in the deep CZ at the DCEW. Previous outcrop studies found the existence of a system of fractures, in alignment with the local stress field. Our objective is to test whether this alignment extends to the deep CZ and can be observed with seismic refraction and electrical resistivity (ER) measurements. For both seismic and ER measurements, we utilized a wagon wheel configuration along four different azimuths, at two adjacent ridges designated North and South ridge. Rugged topography is a complicating factor in both the data acquisition and analysis.

Preliminary results are shown in Figure 1. On the left side, the mean P-wave velocity vs azimuth is plotted; on top, North ridge, and on the bottom, South ridge. On the right side, the mean ER vs azimuth is plotted; on top, North ridge, and on the bottom, South ridge. Mean velocities and ER were calculated within 20x20 m region along each azimuth and centered at the crossing point of all profiles.

In a simple system with vertically oriented fractures, fracture orientation is aligned with the fast P-wave, and low ER direction. At the North ridge, there is significant P-wave velocity variation, however the azimuthal variation suggests two fracture sets. The ER values suggest preferential alignment of around 115°. In the South ridge, there is a clearer trend, suggesting a preferential fracture orientation of around 135°. In this case the ER and P-wave values agree.