Subsurface architecture of the Boulder Creek Critical Zone Observatory from electrical resistivity tomography

Abstract

The architecture of the critical zone includes the distribution, thickness, and contacts of various types of slope deposits and weathering products such as saprolite and weathered bedrock resting on solid bedrock. A quantitative analysis of architecture is necessary for many model-driven approaches used by pedologic, geomorphic, hydrologic or biologic studies. We have used electrical resistivity tomography, a well-established geophysical technique causing minimum surficial disturbance, to portray the subsurface electrical resistivity differences at three study sites (Green Lakes Valley; Gordon Gulch; Betasso) at the Boulder Creek Critical Zone Observatory (BcCZO). Possible limitations of the technique are discussed. Interpretation of the specific resistivity values using natural outcrops, pits, roadcuts and drilling data as ground truth information allows us to image the critical zone architecture of each site. Green Lakes Valley (3700 MASL), a glacially eroded alpine basin, shows a rather simple, split configuration with coarse blockfields and sediments, partly containing permafrost above bedrock. The critical zone in Gordon Gulch (2650 MASL), a montane basin with rolling hills, and Betasso (1925 MASL), a lower montane basin with v-shaped valleys, is more variable due to a complex Quaternary geomorphic history. Boundaries between overlying stratified slope deposits and saprolite were identified at mean depths of 3.0 AE 2.2 m and 4.1 AE 3.6 m in the respective sites. The boundary between saprolite and weathered bedrock is deeper in Betasso at 5.8 AE 3.7 m, compared with 4.3 AE 3.0 m in Gordon Gulch. In general, the data are consistent with results from seismic studies, but electrical resistivity tomography documents a 0.5-1.5 m shallower critical zone above the weathered bedrock on average. Additionally, we document high lateral variability, which results from the weathering and sedimentation history and seems to be a consistent aspect of critical zone architecture within the BcCZO.

Influence of glacier advance on the development of the multipart Riffeltal rock glacier, Central Austrian Alps

“…Measurements were taken at a frequency of 4·16 Hz and current of 0·1 to 5·0 mA which results in potentials up to 750 mV. In general we followed the protocol used and described in detail in Leopold et al (2013). If the repetition was outside the limit of 3%, the point was measured six times with a subsequent averaging.…”

Critical Zone Structure Under a Granite Ridge Inferred From Drilling and Three‐Dimensional Seismic Refraction Data

“…In contrast, geophysical measurements can be gathered in focused surveys designed to systematically and noninvasively explore the deep CZ over broad spatial scales (e.g., Parsekian et al, 2015;Robinson et al, 2008). CZ researchers have increasingly used them to document lateral and vertical variability in regolith properties (Befus et al, 2011;Holbrook et al, 2014;Leopold et al, 2008Leopold et al, , 2013Olona et al, 2010;Orlando et al, 2016), to estimate bedrock fracture density (Clarke & Burbank, 2011;Heincke et al, 2006), to determine shallow aquifer structure (Garambois et al, 2002;Gburek et al, 1999;Gburek & Urban, 1990;McClymont et al, 2011;Pasquet et al, 2015), to constrain depth to bedrock (Mills, 1990;Nyquist et al, 1996;Pavich, 1986), and to explore the relationship between topographic stress and fracture density . One drawback is that indirect measurements from geophysical tools can be ambiguous about variations in subsurface properties unless boreholes are drilled along the survey lines for calibration through direct observations.…”

From on high: Geochemistry of alpine springs, Niwot Ridge, Colorado Front Range, USA

“…Periglacial processes episodically sorted regolith, creating lenses and local channels composed of pebbles to cobble gravel separated by more poorly sorted layers of fines, rubble derived from fractured bedrock, and aeolian material (Leopold et al, ). The shallow subsurface thus is vertically and laterally variable and includes buried zones of higher permeability (paleochannels) in the regolith that are the primary conduits for subsurface water in the alpine zone (Leopold, Huber, Dethier, & Weihenstephan, ; Leopold, Lewis, Dethier, Caine, & Williams, ; Williams et al, ) and in at least some adjacent areas (Figure ).…”