Matt Nolan

Penetration depth as a DInSAR observable and proxy for soil moisture

We use prior theory and experimental results to construct a quantitative relationship between soil moisture and the penetration depth of synthetic aperture radar (SAR) microwaves at L-, C-, and X-bands. This relationship is nonlinear and indicates that a change of 5% volumetric water content (VWC) can cause between 1 and 50 mm of change in C-band penetration depth depending on initial VWC. Because these depths are within the range of differential interferogram SAR (DInSAR) measurement capability, penetration depth may be a viable proxy for measuring soil moisture. DInSAR is unlikely to detect a measurable change in penetration depth above 30% VWC, though certain clay rich soils may continue to cause surface deformation above that level. The possibility of using clay swelling as a proxy for soil moisture was found to be less feasible than penetration depth. Soil moisture may also be a significant, and previously unrecognized, source of noise in the measurement of subtle deformation signals or the creation of digital elevation models using repeat-pass DInSAR.

DInSAR measurement of soil moisture

Differential interferometric sythetic aperture radar (DInSAR) measurements using the European Remote Sensing 2 (ERS-2) satellite in a high-plains region of Colorado show intriguing spatial variations in millimeter-scale path-length change that may correspond to variations in soil moisture of a few percent by volume, in both farm fields and uncultivated terrain. The observed signal is hypothesized to result from both changes in penetration depth and the swelling of clay-rich soils, both due to changes in soil moisture. Comparisons with our field measurements of soil moisture cannot conclusively verify this, but strong support is found from prior and complementary research as well as the visual correlation with hydrological features such as stream channels and watershed boundaries on a 50-m scale. Detection of these subtle signals was facilitated using a digital elevation model with high vertical accuracy. If our interpretations are correct, C-band DInSAR is a promising new tool for the remote sensing of soil moisture in a variety of terrain.

Ice-thickness measurements of Taku Glacier, Alaska, U.S.A., and their relevance to its recent behavior

Using radio-echo soundings and seismic reflections, we measured cross-sections of Taku Glacier, near Juneau, Alaska, to resolve inconsistencies in previous measurements and to understand better the glaciers dynamics. The maximum thickness is about 1477 m and the minimum bed elevation is about 600 m below sea level, which establishes Taku Glacier as the thickest and deepest temperate glacier yet measured. Our data indicate that, during the 19th century, the terminus of Taku Glacier may have begun its rapid advance at a position where the ice bed was greater than 300 m below sea level and more than 25 km from the inland end of its submarine trough ; this behavior is uncharacteristic of temperate tide-water glaciers. The glacier, which no longer calves, has eroded a sediment layer 100 m thick since 1890 at an average rate of about 3 m a −1 since 1948 ; this high erosion rate retards advance by entrenching the glacier into the terminal moraine. Calculations based on ice-deformation theory indicate significant basal ice motion near the terminus and high basal shear stress (140-220 kPa) along much of its length. Estimated differences between ice flux and balance flux are consistent with observed thickening and positive net mass balance ; these data indicate that ice volume is increasing and that further advance is likely.

Seismic detection of transient changes beneath Black Rapids Glacier, Alaska, U.S.A.: I. Techniques and observations

To gain new insight into the mechanisms of basal motion, we have demonstrated the feasibility of an active seismic technique to measure temporal changes in basal conditions on sub-hourly time-scales. One region of the bed of Black Rapids Glacier, Alaska, U.S.A., was monitored for a period of 45 days using seismic reflections. The majority ofthese reflections were nearly identical. However, three significant anomalies were recorded several days apart. These corresponded with the englacial drainage of two ice-marginal lakes and one supraglacial pothole, each up-glacier of the study site, as well as dramatic increases in basal motion. Two of these seismic anomalies revealed identical changes over 1 km 2 of the bed despite the fact that their drainage events occurred at different locations. Further, these two seismic anomalies were followed by records identical to the non-anomalous state, showing that the seismic changes were reversible. In one of these events, two records taken 36 min apart revealed that the transition between the anomalous and normal states occurred completely within this short interval.

The Importance of Soil Moisture and Soil Structure for InSAR Phase and Backscatter, as Determined by FDTD Modeling

In this paper, we introduce a finite-difference time-domain simulator that accurately models the interaction of microwaves with realistic soils, specifically from spaceborne interferometric synthetic aperture radar (InSAR). The modeled soils are characterized by surface roughness, correlation length, bulk moisture content, vertical moisture gradient, and small air-filled-void content. Simulation results include both backscatter and interferometric phase, and we are particularly interested in assessing the potential of the latter as a proxy for soil moisture. We find that differences in homogeneous bulk moisture result in only small phase differences (< 5?). In contrast, combinations of vertical moisture gradients and small air-filled voids, which may typically exist in more realistic soils, can produce phase changes > 30? for HH and > 50? for VV when the soil moisture is varied from 3% to 30% in the uppermost 2 cm of the soil. Phase changes of this magnitude are easily detectable by spaceborne InSAR techniques. While a strong phase response to a change in mean bulk moisture is common to vertical moisture gradient and small air-filled-void cases, their corresponding backscatter responses are very different. A vertical moisture gradient makes the backscatter response dramatically flatter compared with the case of uniform moisture; in contrast, the introduction of air-filled voids barely alters the backscatter. Thus, it may be possible to infer near-surface soil-structure parameters such as vertical gradients or fractions of voids and inhomogeneities from combined SAR phase and backscatter data. Future SAR sensors could be optimized for this purpose. Prior theoretical work based on the assumption of vertically uniform soil-moisture distributions may need to be adjusted, and the lack of a theory that accommodates more complex soil structures may explain why backscatter inversions have yet to result in a viable operational system.

Seismic detection of transient changes beneath Black Rapids Glacier, Alaska, U.S.A. : II. Basal morphology and processes

Using changes observed in daily seismic reflections, we have investigated the basal morphology of Black Rapids Glacier, Alaska, U.S.A. The englacial drainage of ice-marginal lakes caused significant changes in the daily reflections, as well as dramatic increases in basal motion. Changes in reflection arrival times and amplitudes indicate that there is a basal till layer at least 5 m thick at some locations beneath this surge-type glacier. Rapid changes in the observed reflection coefficients during the drainage events indicate that changes in till properties must occur throughout the entire 5 m thick layer, they must last for several days following the lake drainages and they must be completely reversible over as little as 36 min. Our seismic analysis shows that changes in effective pressure of the till are unlikely to cause the required changes in the reflection coefficients, but that a decrease in till saturation is likely We therefore interpret the cause ofthe seismic anomalies as being a temporary decrease in saturation as water is input to the subglacial hydraulic system, and propose that such a change may occur quickly and reversibly by a redistribution ofoverburden pressure. Higher water pressures within the hydraulic system cause that region to support more of the glaciers weight, leaving the remaining areas to support less. Any till within these areas of decreased normal stress would experience a consequent decrease in pore-water pressure, causing gas to exolve, thus decreasing saturation. This decrease in saturation would cause a change in the strength of the basal layer and may affect basal dynamics.

Laboratory Measurement of the DInSAR Response to Spatiotemporal Variations in Soil Moisture

Differential interferometric synthetic aperture radar (DInSAR) has traditionally been used for the detection and accurate monitoring of surface movement in a scene and has found applications in fields such as mining subsidence and earthquake deformation. In these studies, the phase is understood to directly relate to the radial component of the physical deformation of the surface. In this paper, however, we use a novel combination of microwave and optical laboratory measurements to demonstrate the presence of persistent and coherent phase changes in a temporal sequence of DInSAR images, related solely to moisture change in a soil. This is confirmation of recent reports suggesting that, in some circumstances, the DInSAR signal may include a significant soil moisture signal. Laboratory measurements were used to obtain a set of high-resolution C-band DInSAR images of a sandy soil sample of an area of 2.0 m × 1.8 m and a depth of 0.2 m, with the fractional volumetric water content varying between 0.1 and 0.4. To independently monitor the soil surface for physical movement, a time-lapsed set of high-resolution digital optical images was continuously acquired. Although the soil underwent a large moisture change, the soil surface was static to within ±0.1 mm over the majority of the experiment. The DInSAR sequence displayed dynamic and complex variations of the phase, although a linear relationship with moisture change was evident when the mean phase change was considered. The work raises the possibility that DInSAR could be used for the monitoring of soil moisture change in a scene, a parameter of significant economic and environmental importance.

Bed properties and hydrological conditions underneath McCall Glacier, Alaska, USA.

Abstract During three summer field seasons (2003, 2005 and 2006) we carried out radio-echo sounding measurements with a 5MHz (central frequency) ice-penetrating radar on McCall Glacier, Arctic Alaska, USA, along the central flowline and 17 cross-profiles. Two-way travel time was, after migration, converted to ice thickness, which, in combination with a recent digital elevation model of the surface of the glaciated area, resulted in a detailed map of the bed topography. This reveals a complex basal topography in the confluence area of the different glacial cirques. The pattern of subglacial water flow following the hydraulic potential gradient was calculated for the whole glacier area and shows a confluence of subglacial water downstream from the confluence of the glacier cirques. From the ice-thickness map the total ice volume was estimated as slightly less than 0.5 km3. Bed reflection power (BRP) was determined for the glacier after correction for ice-thickness dependence. Results reveal a clear relationship between the BRP pattern and basal sliding anomalies along the central flowline.

Volume change of McCall Glacier, Arctic Alaska, USA, 1956-2003

Abstract A long history of research documents that McCall Glacier, Arctic Alaska, USA, continues to lose mass at a rate that is likely increasing with time. We present a photo comparison (1958-2003) that visually documents these volume changes, along with survey measurements that quantify these losses. Measurements of longitudinal profiles initially acquired from airborne laser altimetry, and repeated by ground-based surveys, indicate that the areally averaged rate of thinning increased between 1956-93 and 1993-2002, from 0.35 ± 0.07 m a-1 to 0.47 ± 0.03 m a-1, respectively; total volume loss was (8.3 × 107) ± (1.7 × 107) m3 and (2.7 × 107) ± (0.2 × 107) m3 (all in water equivalent) for these two time periods. These profiles also indicate that a 1 km stretch of the mid-ablation area is behaving differently from this trend, with a rate of thinning that is not changing with time. Lastly we present a comparison of several methods for calculating volume change and assess their relative errors.

Glacial-Geologic Evidence for Decreased Precipitation During The Little Ice Age in The Brooks Range, Alaska

Abstract We mapped Little Ice Age (LIA) moraines in the Brooks Range to estimate former equilibrium-line altitudes (ELAs), and combined this information with available proxy temperature estimates to infer precipitation trends because little is known about precipitation changes associated with centennial-scale climate variability in the Arctic during the late Holocene. The Brooks Range, northern Alaska (68°N), hosts hundreds of extant glaciers that exhibit geomorphic evidence for multiple fluctuations in ice extent during the past millennium. Our lichenometric age estimates for LIA moraines in the forefields of five cirque glaciers in the Sagavinerktok River valley and Oolah Valley suggest two intervals of LIA moraine formation centered around a.d. 1250 and 1650. The outermost LIA moraine was mapped on aerial photographs for 114 relatively large (1.2 ± 0.5 km) and geographically simple glaciers along a 700-km-long transect following the range crest. At their maximum extent during the LIA, these glaciers were an average of 0.2 ± 0.1 km longer than the ice margins shown on most recent U.S. Geological Survey topographic maps (1956 and 1972). The ELA was estimated using an accumulation-area-ratio method and GIS analysis. The reconstructed ELAs needed to maintain an equilibrium length for the LIA glaciers were an average of 51 ± 29 m lower than for the smaller glacier sizes of the mid 20th century. This small ELA lowering during the LIA is less than would be expected from available proxy temperature estimates from elsewhere in Alaska that indicate warm-season temperature reductions of about 1 °C. To explain this discrepancy, we suggest that precipitation decreased during the LIA. A prolonged southern displacement of the Arctic front might explain the drier conditions in the Brooks Range during the LIA.