Topographic correlations with soil and regolith thickness from shallow-seismic refraction constraints across upland hillslopes in the Valles Caldera, New Mexico
Authors
How rock is weathered physically and chemically into transportable material is a fundamental question in critical-zone science. In addition, the distribution of weathered material (soil and intact regolith) across upland landscapes exerts a first-order control on the hydrology of watersheds. In this paper we present the results of six shallow seismic-refraction surveys in the Redondo Mountain region of the Valles Caldera, New Mexico. The P-wave velocities corresponding to soil (≤ 0.6 km s−1) were inferred from a seventh seismic survey where soil-thickness data were determined by pit excavation. Using multivariable regression, we quantified the relationships among slope gradient, aspect, and topographic wetness index (TWI) on soil and regolith (soil plus intact regolith) thicknesses. Our results show that both soil and regolith thicknesses vary inversely with TWI in all six survey areas while varying directly with slope aspect (i.e. thicker beneath north-facing slopes) and inversely with slope gradient (i.e. thinner beneath steep slopes) in the majority of the survey areas. An empirical model based on power-law relationships between regolith thickness and its correlative variables can fit our inferred thicknesses with R2-values up to 0.880 for soil and 0.831 for regolith in areas with significant topographic variations. These results further demonstrate the efficacy of shallow seismic refraction for mapping and determining how soil and regolith variations correlate with topography across upland landscapes. Copyright © 2016 John Wiley & Sons, Ltd.
Figure 9: (A) Velocity-depth result of the inversion (top) for Line E (upper Jaramillo) and its standard deviation (bottom). The solid black contour beneath the topography shows the soil thickness with white error bars showing the uncertainty at intervals of every fourth station. The solid black contour at depth depicts the mean regolith thickness as inferred by the 2.95 km s−1 velocity contour. The dashed lines above and below are the minimum (2.5 km s−1) and maximum (3.4 km s−1) contours, respectively, from which the white error bars were determined. The inverted blue triangles show a subset of the receiver stations. Note the sharp horizontal change in velocities at x = ~110 m. This an imaging artifact from having a sharp kink in the line at the base of the valley. Vertical exaggeration is 2:1. (B) An identical display, however for the Line F (lower Jaramillo) results with error bars plotted every fourth station. Vertical exaggeration is 2:1.
