Robert W. Jacob

Properties of precipitation-induced multilayer surface waveguides derived from inversion of dispersive TE and TM GPR data

Precipitation events result in a high water content of the shallow subsurface. When the water remains in the shallow subsurface, a shallow low-velocity layer can be generated which acts as a waveguide when the contrasts in permittivity are large enough. The electromagnetic waves emitted by ground-penetrating radar (GPR) systems are trapped within these thin surface layers and show pronounced dispersion, which depends on the velocities and thicknesses of the surface waveguide layers and the velocity of the material below it. Conventional traveltime techniques cannot be reliably applied because the different phases cannot be clearly identified. Recently developed techniques for inverting dispersed waveforms in transverse electric (TE) and transversemagnetic (TM) GPR data are used to provide information on the thickness and permittivity of a single-layer low-velocity waveguide induced by precipitation events. Repeated measurements at one location after two precipitation events show that, for increasing soil ...

Identifying dispersive GPR signals and inverting for surface wave-guide properties

The vadose zone is a dynamic environment in which water is retained or transferred into the saturated zone or atmosphere. Knowledge of water content in the near-surface soil layers is important for improving our understanding of groundwater recharge, evaporation, and uptake by crops or natural vegetation (Vereecken et al., 2008). Ground-penetrating radar (GPR) measurements are capable of estimating the subsurface radar velocity, which can be converted to water content using Topps equation. The direct ground wave is often used to provide radar velocities for the very shallow part of the vadose zone (Huisman 2003). However, this method can only be applied when the subsurface can be approximated by a homogeneous half-space (e.g., a subsurface with a uniform water content profile). Due to the large permittivity of water (∊r = 80), the permittivity of soils can change dramatically when precipitation or irrigation is occurring at a site.

GPR reflection and dispersion analysis methods for water content: Multi-year study of GPR estimates and soil core measurements of water content

A multi-year series of ground penetrating radar (GPR) measurements, complemented by contemporaneous soil cores, were collected at a single location in south-eastern New England, USA. The shallow subsurface is characterized by a 0.9 m thick sandy soil layer which overlies a gravelly-sand layer. Over a number of months, and different soil moisture conditions, 30 common midpoint (CMP) soundings were collected, and on each of the days, co-located soil cores were taken for analysis and comparison with the soil water content from the GPR velocities. GPR velocities were estimated using two independent methods: standard normal moveout (NMO) analysis of reflected traveltimes, and the analysis of frequency-dependent velocity dispersion of shallow GPR waveguide modes. A comparison of GPR estimated water content in the field versus gravimetrically measured water content in the lab provides a site-specific Topp-like empirical basis for predicting soil water conditions from GPR data. Although such site-specific relations may be useful, we find that for our site comparing the results of this study with the conventional Topp relation reconfirms the continuing utility of the latter.

Geomorphology of icy debris fans: Delivery of ice and sediment to valley glaciers decoupled from icecaps

The pace and volume of mass flow processes contributing ice and sediment to icy debris fans (IDFs) were documented at sites in Alaska and New Zealand by integrating field observations, drone and time-lapse imagery, ground penetrating radar, and terrestrial laser scanning. Largely unstudied, IDFs are supraglacial landforms at the mouths of bedrock catchments between valley glaciers and icecaps. Time-lapse imagery recorded 300–2300 events reaching 15 fans during intervals from nine months to two years. Field observations noted hundreds of deposits trapped within catchments weekly that were later remobilized onto fans. Deposits were mapped on images taken three to four times per day. Most events were ice avalanches (58%–100%). Slush avalanches and/or flows were common in spring and fall (0%–65%). Icy debris flows were <5% of the events, observed only at sites with geomorphically complex catchments. Rockfalls were common within catchments; few directly reached a fan. Site selection provided a spectrum of catchment relationships between icecaps and fans. The largest most active fans occur below hanging glaciers or short chutes between the icecap and glacier and were dominated by ice avalanches, slush avalanches, and slush flows. Larger, complex catchments allowed temporary storage of ice and sediment that were later remobilized into ice and slush avalanches and debris flows. Unlike alluvial settings where larger fans are associated with larger catchments, there are variable relationships between IDF area and catchment area. Exceptionally active and dynamic compared to alluvial fans, the studied IDFs exhibited annual resurfacing rates of 300%–>4000%. Annual contributions by mass flows ranged from 133,200 to 5,200,000 m3, representing 3%–56% of fan volume. Although ablation occurred, mainly during summers, significant ice transfer occurred through fan subsurface areas to adjacent valley glaciers. Icy debris fans annually contributed <1%–~24% of the mass of adja cent valley glaciers. Small glaciers (e.g., McCarthy Glacier, Alaska) showed minor thinning (<1 m/yr) compared to larger glaciers (e.g., La Perouse, Douglas, and Mueller Glaciers, New Zealand) that lost >5–10 m/yr over the hundreds of meters of valley glacier adjacent to the IDFs studied. Some IDFs lengthened in response to thinning of valley glaciers. Icy debris fans supplied significant ice and sediment to valley glaciers, slowing the rate of deglaciation. Results of this study have implications toward managing hazards and predicting glacial mass balance in alpine regions. For example, having quantitative information about the role of ice contribution from IDFs to valley glaciers may result in forecasting a lower rate of deglaciation than traditionally recognized for some glaciers decoupled from icecaps.