Accuracy assessment of lidar-derived digital elevation models

Despite the relatively high cost of airborne lidar-derived digital elevation models (DEMs), such products are usually presented without a satisfactory associated estimate of accuracy. For the most part, DEM accuracy estimates are typically provided by comparing lidar heights against a finite sample of check point coordinates from an independent source of higher accuracy, supposing a normal distribution of the derived height differences or errors. This paper proposes a new methodology to assess the vertical accuracy of lidar DEMs using confidence intervals constructed from a finite sample of errors computed at check points. A non-parametric approach has been tested where no particular error distribution is assumed, making the proposed methodology especially applicable to non-normal error distributions of the type usually found in DEMs derived from lidar. The performance of the proposed model was experimentally validated using Monte Carlo simulation on 18 vertical error data-sets. Fifteen of these data-sets were computed from original lidar data provided by the International Society for Photogrammetry and Remote Sensing Working Group III/3, using their respective filtered reference data as ground truth. The three remaining data-sets were provided by the Natural Environment Research Council's Airborne Research and Survey Facility lidar system, together with check points acquired using high precision kinematic GPS. The results proved promising, the proposed models reproducing the statistical behaviour of vertical errors of lidar using a favourable number of check points, even in the cases of data-sets with non-normally distributed residuals. This research can therefore be considered as a potentially important step towards improving the quality control of lidar-derived DEMs.     

Implications of measurement metrics on soil freezing curves: A simulation of freeze–thaw hysteresis

Soil freeze–thaw events have important implications for water resources, flood risk, land productivity, and climate change. A property of these phenomena is the relationship between unfrozen water content and sub-freezing temperature, known as the soil freezing characteristic curve (SFC). It is documented that this relationship exhibits hysteretic behaviour when frozen soil thaws, leading to the definition of the soil thawing characteristic curve (STC). Although explanations have been given for SFC/STC hysteresis, the effect that ‘scale’ – particularly ‘measurement scale’ – may have on these curves has received little attention. The most commonly used measurement scale metric is the ‘support’, which is the spatial (or temporal) unit within which the measured variable is integrated or soil volume sampled. We show (a) measurement support can influence the range and shape of the SFC and (b) hysteresis can be attributed, in part, to the support and location of the measurements comprising the SFC/STC. We simulated lab measured temperature, volumetric water content (VWC), and permittivity from soil samples undergoing freeze–thaw transitions using Hydrus-1D and a modified Dobson permittivity model. To assess the effect of measurement support and location on SFC/STC, we masked the simulated temperature and VWC/permittivity extent to match the instrument's support and location. By creating a detailed simulation of the intra- and inter-support variability associated with the penetration of a freezing front, we demonstrate how measurement support and location can influence the temperature range over which water freezing events are captured. We show it is possible to simulate hysteresis in homogenous media with purely geometric considerations, suggesting that SFC/STC hysteresis may be more of an apparent phenomenon than mechanistically real. Lastly, we develop an understanding of how the location and support of soil temperature and VWC/permittivity measurements influence the temperature range over which water freezing events are captured.     

The volcanic and magmatic evolution of Volcán Ollagüe, a high-K, late quaternary stratovolcano in the Andean Central Volcanic Zone

Volcán Ollagüe is a high-K, calc-alkaline composite volcano constructed upon extremely thick crust in the Andean Central Volcanic Zone. Volcanic activity commenced with the construction of an andesitic to dacitic composite cone composed of numerous lava flows and pyroclastic deposits of the Vinta Loma series and an overlying coalescing dome and coulée sequence of the Chasca Orkho series. Following cone construction, the upper western flank of Ollagüe collapsed toward the west leaving a collapse-amphitheater about 3.5 km in diameter and a debris avalanche deposit on the lower western flank of the volcano. The deposit is similar to the debris avalanche deposit produced during the May 18, 1980 eruption of Mount St. Helens, U.S.A., and was probably formed in a similar manner. It presently covers an area of 100 km² and extends 16 km from the summit. Subsequent to the collapse event, the upper western flank was reformed via eruption of several small andesitic lava flows from vents located near the western summit and growth of an andesitic dome within the collapse-amphitheater. Additional post-collapse activity included construction of a dacitic dome and coulée of the La Celosa series on the northwest flank. Field relations indicate that vents for the Vinta Loma and post-collapse series were located at or near the summit of the cone. The Vinta Loma series is characterized by an anhydrous, two-pyroxene assemblage. Vents for the La Celosa and Chasca Orkho series are located on the flanks and strike N55 W, radial to the volcano. The pattern of flank eruptions coincides with the distribution in the abundance of amphibole and biotite as the main mafic phenocryst phases in the rocks. A possible explanation for this coincidence is that an unexposed fracture or fault beneath the volcano served as a conduit for both magma ascent and groundwater circulation. In addition to the lava flows at Ollagüe, magmas are also present as blobs of vesiculated basaltic andesite and mafic andesite that occur as inclusions in nearly all of the lavas. All eruptive activity at Ollagüe predates the last glacial episode ( 11.000 a B.P.), because post-collapse lava flows are overlain by moraine and are incised by glacial valleys. Present activity is restricted to emission of a persistent, 100-m-high fumarolic steam plume from a vent located within the summit andesite dome.Sr and Nd isotope ratios for the basaltic andesite and mafic andesite inclusions and lavas suggest that they have assimilated large amounts of crust during crystal fractionation. In contrast, narrow ranges in ¹⁴³Nd/¹⁴⁴Nd and ⁸⁷Sr/⁸⁶Sr in the andesitic and dacitic lavas are enigmatic with respect to crustal contamination.     

Using LM‐OSL of quartz to distinguish sediments derived from surface‐soil and channel erosion

This study describes the use of linearly modulated optically stimulated luminescence (LM‐OSL) to distinguish surface‐soil derived sediments from those derived from channel bank erosion. LM‐OSL signals from quartz extracted from 15 surface‐soil and five channel bank samples were analysed and compared to signals from samples collected from two downstream river sites. Discriminant analysis showed that the detrapping probabilities of fast, first slow and second slow components of the LM‐OSL signal can be used to differentiate between the samples collected from the channel bank and surface‐soil sources. We show that for each of these source end members these components are all normally distributed. These distributions are then used to estimate the relative contribution of surface‐soil derived and channel bank derived sediment to the river bed sediments. The results indicate that channel bank derived sediments dominate the sediment sources at both sites, with 90.1 ± 3% and 91.9 ± 1.9% contributions. These results are in agreement with a previous study which used measurements of ¹³⁷Cs and ²¹⁰Pb_ex fallout radionuclides to estimate the relative contribution from these two sources. This result shows that LM‐OSL may be a useful method, at least in the studied catchment, to estimate the relative contribution of surface soil and channel erosion to river sediments. However, further research in different settings is required to test the difference of OSL signals in distinguishing these sediment sources. And if generally acceptable, this technique may provide an alternative to the use of fallout radionuclides for source tracing. Copyright © 2015 John Wiley & Sons, Ltd.     

Strong slope-aspect control of regolith thickness by bedrock foliation

The porous near-surface layer of the Earth's crust – the critical zone – constitutes a vital reservoir of water for ecosystems, provides baseflow to streams, guides recharge to deep aquifers, filters contaminants from groundwater, and regulates the long-term evolution of landscapes. Recent work suggests that the controls on regolith thickness include climate, tectonics, lithology, and vegetation. However, the relative paucity of observations of regolith structure and properties at landscape scales means that theoretical models of critical zone structure are incompletely tested. Here we present seismic refraction and electrical resistivity surveys that thoroughly characterize subsurface structure in a small catchment in the Santa Catalina Mountains, Arizona, USA, where slope-aspect effects on regolith structure are expected based on differences in vegetation. Our results show a stark contrast in physical properties and inferred regolith thickness on opposing slopes, but in the opposite sense of that expected from environmental models and observed vegetation patterns. Although vegetation (as expressed by normalized difference vegetation index [NDVI]) is denser on the north-facing slope, regolith on the south-facing slope is four times thicker (as indicated by lower seismic velocities and resistivities). This contrast cannot be explained by variations in topographic stress or conventional hillslope morphology models. Instead, regolith thickness appears to be controlled by metamorphic foliation: regolith is thicker where foliation dips into the topography, and thinner where foliation is nearly parallel to the surface. We hypothesize that, in this catchment, hydraulic conductivity and infiltration capacity control weathering: infiltration is hindered and regolith is thin where foliation is parallel to the surface topography, whereas water infiltrates deeper and regolith is thicker where foliation intersects topography at a substantial angle. These results suggest that bedrock foliation, and perhaps by extension sedimentary layering, can control regolith thickness and must be accounted for in models of critical zone development. © 2020 John Wiley & Sons, Ltd.     

A generalized transformation approach for simulating steady-state variably-saturated subsurface flow

A numerical method is proposed to accurately and efficiently compute a direct steady-state solution of the nonlinear Richards equation. In the proposed method, the Kirchhoff integral transformation and a complementary transformation are applied to the governing equation in order to separate the nonlinear hyperbolic characteristic from the linear parabolic part. The separation allows the transformed governing equation to be applied to partially- to fully-saturated systems with arbitrary constitutive relations between primary (pressure head) and secondary variables (relative permeability). The transformed governing equation is then discretized with control volume finite difference/finite element approximations, followed by inverse transformation. The approach is compared to analytical and other numerical approaches for variably-saturated flow in 1-D and 3-D domains. The results clearly demonstrate that the approach is not only more computationally efficient but also more accurate than traditional numerical solutions. The approach is also applied to an example flow problem involving a regional-scale variably-saturated heterogeneous system, where the vadose zone is up to 1 km thick. The performance, stability, and effectiveness of the transform approach is exemplified for this complex heterogeneous example, which is typical of many problems encountered in the field. It is shown that computational performance can be enhanced by several orders of magnitude with the described integral transformation approach.     

Structure proof and significance of stereoisomeric 28,30-bisnorhopanes in petroleum and petroleum source rocks

Two C₂₈H₄₈-pentacyclic triterpanes were isolated from Monterey shale. X-ray crystallography of a crystal containing both compounds proved their structures as 17β,18α,21α(H)-28,30-bisnorhopane and 17β,18α,21β(H)-28,30-bisnorhopane. Several differences are found between 28,30-bisnorhopanes and the regular hopanes. Unlike the regular hopane epimers, for practical purposes the three epimeric 28,30-bisnorhopanes [17α,21β(H)-, 17β,21α(H)-, and 17β,21β(H)-]cannot be distinguished by their mass spectra. Special conditions are needed to separate them by gas chromatography. The diagenetically first-formed epimer is thought to be 17α,21β(H)- because it predominates in immature shales. The order of thermodynamic stability is 17β,2lα(H) < > 17α,21β(H) > 17β,21β(H), and all three epimers are present in petroleum. 25,28,30-Trisnorhopanes can be analyzed in similar fashion and are found to have similar thermodynamic characteristics. The percent of the ring D/E cis epimer of 28,30-bisnorhopane and/or 25,28,30-trisnorhopane is a useful maturation parameter similar to the 20S/20R sterane ratio. Evidence indicates 25-demethylation of 28,30-bisnorhopane to 25,28,30-trisnorhopane during advanced stages of biodegradation. Hence, percent ring $\text{D}\text{E}$cis 25,28,30-trisnorhopane has an application to maturation assessment in heavily biodegraded oils.     

Isotopic and trace-element profiles across the New Britain island arc,Papua New Guinea

New Pb-, Sr-, and Nd-isotopic data have been obtained for the rocks of volcanoes overlying a wide range of depths (100–580 km) to the Wadati-Benioff Zone (WBZ) in the New Britain island arc, Papua New Guinea. Well-defined trends consistent with two-component mixing are observed in combined Pb-isotope/trace-element plots. One of the components is believed to represent a slab contribution whose isotopic signature, unlike those noted for several other arcs, appears to be dominated by subducted, altered, oceanic crust rather than by sediment. This conclusion is consistent with the results of a recent Be–B study of New Britain rocks. The influence of the slab component is considered to decrease as depth to the WBZ increases. Higher abundances of high-field-strength elements correlate with increasing depths to the WBZ, and may be indicative of smaller degrees of partial melting of the mantle wedge as WBZ depths increase. Abundances of other incompatible elements appear to reflect a complex interplay between the slab-derived flux and melting process.