ABSTRACT:

We collected the TEM data using a Zonge Engineering NanoTEM system. The NanoTEM is a low-power, fast-sampling time-domain TEM system that was specifically designed to provide high resolution images of the near-surface (~50 metres depth). The NanoTEM data were collected using a 20 m x 20 m square transmitter loop with a 5 m x 5 m, single-turn receiving loop. The transmitter coil had an output current of 2 A and a turnoff of ~ 2 µs. The receiving loop sampled at 625 kHz, stacking 256 cycles at a repetition rate of 32 Hz. The stacks were averaged and then inspected to remove noisy data in the late times. The NanoTEM data were inverted using the Aarhusinv program, run using “smooth model” settings (Auken et al., 2006, 2015). The 1D inversion assumes laterally homogeneous layers. All NanoTEM soundings were inverted separately (i.e. there were no lateral constraints,) and placed next to one another and interpolated to generate pseudo 2D profiles of bulk electrical conductivity. The quality of the inversion is determined by a misfit value between the observed and modelled voltages.

ABSTRACT:

We acquired downhole NMR measurements at 0.25 m depth intervals down a 7.5 m drillhole using a Dart system (Vista Clara). The Dart quantifies water content and T2 decay times in two cylindrical shells of varying radii (12.7 and 15.2 cm) within the drillhole.

ABSTRACT:

All of the surface NMR soundings in this study used a 46 m diameter circle-eight loop, a 20 ms pulse, and two noise loops of approximately the same shape, size, and orientation. Using this configuration, we were able to achieve high-quality signal and observe exponential decay over a swath of pulse moments and we successfully removed 60 Hz harmonics caused by anthropogenic sources. Loop orientation was selected to minimize topography change over the loop. Even with noise cancellation, the signal was small, and we stacked each pulse moment between 16 and 22 times. We inverted the surface NMR data with a mono-exponential decay using MRSMatlab and a QT inversion using a magnetic inclination of 67.4°.

More information: Flinchum, B., Holbrook, W., Parsekian, A., Carr, B. (2019). Characterizing the Critical Zone Using Borehole and Surface Nuclear Magnetic Resonance Vadose Zone Journal 18(1), 1-18. https://dx.doi.org/10.2136/vzj2018.12.0209

ABSTRACT:

Fractures in Earth’s critical zone influence groundwater flow and storage and promote chemical weathering. Fractured materials are difficult to characterize on large spatial scales because they contain fractures that span a range of sizes, have complex spatial distributions, and are often inaccessible. Therefore, geophysical characterizations of the critical zone depend on the scale of measurements and on the response of the medium to impulses at that scale. Using P-wave velocities collected at two scales, we show that seismic velocities in the fractured bedrock layer of the critical zone are scale-dependent. The smaller-scale velocities, derived from sonic logs with a dominant wavelength of ~0.3 m, show substantial vertical and lateral heterogeneity in the fractured rock, with sonic velocities varying by 2,000 m/s over short lateral distances (~20 m), indicating strong spatial variations in fracture density. In contrast, the larger-scale velocities, derived from seismic refraction surveys with a dominant wavelength of ~50 m, are notably slower than the sonic velocities (a difference of ~3,000 m/s) and lack lateral heterogeneity. We show that this discrepancy is a consequence of contrasting measurement scales between the two methods; in other words, the contrast is not an artifact but rather information -- the signature of a fractured medium (weathered/fractured bedrock) when probed at vastly different scales. We explore the sample volumes of each measurement and show that surface refraction velocities provide reliable estimates of critical zone thickness but are relatively insensitive to lateral changes in fracture density at scales of a few tens of meters. At depth, converging refraction and sonic velocities likely indicate the top of unweathered bedrock, indicative of material with similar fracture density across scales.

DOI: 10.3389/frwa.2021.772185

ABSTRACT:

This is supporting data for "Low Vp/Vs Values as an Indicator for Fractures in the Critical Zone":

Poisson's ratio for earth materials is usually assumed to be positive (Vp/Vs >1.4). However, this assumption may not be valid in the critical zone because near Earth’s surface effective pressures are low (< 1 MPa), porosity has a wide range (0-60%), there are significant texture changes (e.g. unconsolidated versus fractured media), and saturation ranges from 0 to 100%. We present P-wave (Vp) and S-wave (Vs) velocities from seismic refraction profiles collected in weathered crystalline environments in South Carolina and Wyoming. Our data show that ~20% of the subsurface has negative Poisson’s ratios (Vp/Vs values < 1.4), a conclusion supported by borehole sonic logs. The low Vp/Vs values are confined to the fractured bedrock and saprolite. Our data support the hypothesis that weathering-generated microcracks can produce a negative Poisson’s ratio and that Vp/Vs values can thus provide insight into important critical zone weathering processes.

Plain Language Summary:
When a material is squeezed, the ratio between the change in height and width is described by an elastic parameter called Poisson's ratio. Most earth materials have a positive Poisson ratio, meaning the material will expand when squeezed (e.g. Playdough or wet sand). Materials with a negative Poisson’s ratio rarely occurs naturally and will shrink in all directions when squeezed. Cork is a common material with a Poisson’s ratio of approximately zero. Cork is ideal for bottling wine because its width does not change when pushing it into the bottle's narrow neck. Here we use surface-based measurements to quantify Poisson’s ratio from P-wave (Vp) and S-wave (Vs) velocities in the top 50 m of Earth’s surface. Our results show an unexpected result—material in the CZ has a negative Poisson’s ratio. We believe this unexpected behavior is caused by the combination of low effective pressures and small and irregular cracks created during rocks' transformation into soil. The cracks have a greater impact on the material’s ability to resist compression. At the same time, most of the rock is still coherent and thus only experiences a minimal loss of shear strength.

ABSTRACT:

These are the supporting data for the paper submitted to GRL Sensing a Connection: Tree Distribution is Influenced by Deep Critical Zone Structure.

ABSTRACT:

Velocity profiles from seismic refraction reveal deep critical zone (CZ) architecture under profiles hundreds of meters long. However, long transects still fail to capture CZ architecture over regional scales (1 km\textsuperscript{2} to 20 km\textsuperscript{2}). Here, we present a strategy that transforms seismic observations from individual profiles into maps of CZ architecture over tens of square kilometers. Data from 15 seismic refraction profiles (approximately line 6.6 km) collected in the weathered crystalline rocks of the South Carolina Piedmont revealed approximately 400,000 m\textsuperscript{2} of deep CZ architecture. Using casing depths from four boreholes, we show that the boundary dividing saprolite and fractured rock is 1,870 m/s. Using velocity from an outcrop in the survey area, we show that the bedrock velocity is 4,550 m/s. We used these velocities to define a three-layer CZ structure comprised of soil/saprolite, fractured bedrock, and bedrock. We developed an empirical relationship between CZ structure and minimum and maximum principal curvatures, allowing us to predict CZ architecture over approximately 17 km\textsuperscript{2}. The strong correlation between seismically inferred CZ structure and principal curvatures suggests it can predict CZ structure across larger scales.