Seismic Constraints on Critical Zone Architecture, Boulder Creek Watershed, Front Range, Colorado

Abstract

We use a minimally invasive, shallow geophysical technique to image the structure of the criti cal zone from surface to bedrock (0-20 m) in two small drainages within the Boulder Creek Criti cal Zone Observatory (BcCZO). Shallow seismic refracti on (SSR) surveys provide a three-dimensional network of two-dimensional cross-secti ons (termed quasi-3D) of criti cal zone compressional wave velocity (V p ) structure within each catchment, yielding a spati al descripti on of the current criti cal zone structure. The two catchments, Betasso and Gordon Gulch, represent contrasti ng geomorphic histories within the Front Range: Betasso shows hillslope response to a late Cenozoic increase in fl uvial incision of Boulder Creek, while Gordon Gulch represents more steady erosion. The mean depth to fresh bedrock in both catchments is roughly 15 m. Unique subsurface features in each catchment refl ect acti ve geomorphic processes not suggested by similariti es in mean interface depths. Betasso contains thick disaggregated materials high in the drainage that are nearly absent near the outlet. This presumably refl ects the impact of base-level lowering, which we suggest has progressed roughly 500 to 1000 m up into the catchment. Aspect-driven diff erences in the subsurface within each catchment add complexity and overprint the broader geomorphic signals. Shallow seismic refracti on subsurface structure models will guide future investi gati ons of criti cal zone processes from landscape to hydrologic modeling and are invaluable as connecti ons between ti me-consuming point measurements of physical, chemical, and biological processes.Abbreviati ons: BcCZO, Boulder Creek Criti cal Zone Observatory; ERT, electrical resisti vity tomography; GPR, ground-penetrati ng radar; Ma, megaannum; quasi-3D, a three-dimensional grouping of two-dimensional surveys; SSR, shallow seismic refracti on.Shallow seismic refracti on allows minimally invasive, broad spatial investigations of the subsurface (Leopold et al., 2008b). We use this geophysical technique to pair surficial evidence of geomorphic processes with physical and chemical weathering signatures interpreted from SSR surveys.Th e interactions between weathering and transport processes sculpt terrestrial landscapes. Hillslope processes that include downslope movement of material by rain splash, frost creep, biological activity, and other gravity-driven mechanisms serve to redistribute material that is freed from the underlying bedrock by weathering processes. Th e hillslopes are in turn coupled to adjacent streams, which serve as their boundary conditions. Th is highly coupled geomorphic system therefore encompasses the majority of hydrological, geochemical, and biological activities that can all change in effi ciency and in dominance through time.Recently, the term critical zone (CZ) has been applied to the shallow terrestrial environment spanning from the lowest extent of groundwater to the top of the vegetation canopy (Brantley et al., 2007;Anderson et al., 2007) (Fig. 1). Th e defi nition...

Designing a network of critical zone observatories to explore the living skin of the terrestrial Earth

“…One example of this is the initiative in which every CZO in the United States pursued geophysical measurements. Many papers have been published exemplifying this approach to map out the subsurface architecture (Befus et al, 2011;Holbrook et al, 2014;Orlando et al, 2015;Olyphant et al, 2016). Now, geophysicists travel among CZOs to image the subsurface with a battery of instruments to image the belowground landscape (St. Clair et al, 2015).…”

“…Under hills, imaging has revealed locally consistent patterns of subsurface CZ structure that can relate depth, fracture density, porosity, and weathering (e.g., Befus et al, 2011;Brantley et al, 2013;Orlando et al, 2015;St. Clair et al, 2015).…”

Seismic Refraction and Electrical Resistivity Tests for Fracture Induced Anisotropy in a Mountain Watershed

“…The Critical zone (CZ) can be defined as the portion of earth that extends from the top of trees to a depth that is no longer affected by meteoric fluids (Befus et al, 2011;Riebe et al, 2016). Delineating the upper part of the CZ is easy, but characterizing the lower part is still a challenge.…”

“…Geophysical methods help us better understand fracture induced azimuthal anisotropy, specifically, 2D seismic refraction, and ER methods (Yeboah-Forson and Whitman, 2014;Busby, 2000;Zhu et al, 2012;Holbrook et al, 2014;Befus et al, 2011;Babcock et al, 2015;Greenhalgh et al, 2009;Matias, 2002;Li and Uren, 1997). Fracture presence introduces seismic azimuthal anisotropy (Tsvankin and Grechka, 2011;Lynn and Michelena, 2011;Burns et al, 2007;Crampin, 1985), and electrical azimuthal anisotropy (Berryman and Hoversten, 2013;Greenhalgh et al, 2010;Yeboah-Forson and Whitman, 2014;Taylor and Fleming, 1989).…”

“…Most CZ-hydrogeophysics surveys are collected using only a single azimuth (Olona et al, 2010;Befus et al, 2011;Holbrook et al, 2014;Yamakawa et al, 2012;Yamakawa et al, 2010). The results might vary dramatically depending on the direction of the survey with respect to the fracture orientation.…”