Troy R. Brosten

Application of time-lapse ERT imaging to watershed characterization

Time-lapse electrical resistivity tomography ERT has many practical applications to the study of subsurface properties and processes. When inverting time-lapse ERT data, it is useful to proceed beyond straightforward inversion of data differences andtakeadvantageofthetime-lapsenatureofthedata.Weassess various approaches for inverting and interpreting time-lapse ERTdataanddeterminethattwoapproachesworkwell.Thefirst approachismodelsubtractionafterseparateinversionofthedata from two time periods, and the second approach is to use the inverted model from a base data set as the reference model or prior information for subsequent time periods. We prefer this second approach. Data inversion methodology should be considered when designing data acquisition; i.e., to utilize the second approach, it is important to collect one or more data sets for which the bulk of the subsurface is in a background or relatively unperturbed state.Athird and commonly used approach to time-lapse inversion,invertingthedifferencebetweentwodatasets,localizes the regions of the model in which change has occurred; however, varying noise levels between the two data sets can be problematic. To further assess the various time-lapse inversion approaches,weacquiredfielddatafromacatchmentwithintheDry Creek Experimental Watershed near Boise, Idaho, U.S.A. We combined the complimentary information from individual static ERTinversions,time-lapseERTimages,andavailablehydrologicdatainarobustinterpretationschemetoaidinquantifyingseasonalvariationsinsubsurfacemoisturecontent.

Hyporheic exchange and water chemistry of two arctic tundra streams of contrasting geomorphology

Received 11 July 2007; revised 29 December 2007; accepted 7 March 2008; published 11 June 2008. [1] The North Slope of Alaska’s Brooks Range is underlain by continuous permafrost, but an active layer of thawed sediments develops at the tundra surface and beneath streambeds during the summer, facilitating hyporheic exchange. Our goal was to understand how active layer extent and stream geomorphology influence hyporheic exchange and nutrient chemistry. We studied two arctic tundra streams of contrasting geomorphology: a highgradient, alluvial stream with riffle-pool sequences and a low-gradient, peat-bottomed stream with large deep pools connected by deep runs. Hyporheic exchange occurred to � 50 cm beneath the alluvial streambed and to only � 15 cm beneath the peat streambed. The thaw bulb was deeper than the hyporheic exchange zone in both stream types. The hyporheic zone was a net source of ammonium and soluble reactive phosphorus in both stream types. The hyporheic zone was a net source of nitrate in the alluvial stream, but a net nitrate sink in the peat stream. The mass flux of nutrients regenerated from the hyporheic zones in these two streams was a small portion of the surface water mass flux. Although small, hyporheic sources of regenerated nutrients help maintain the in-stream nutrient balance. If future warming in the arctic increases the depth of the thaw bulb, it may not increase the vertical extent of hyporheic exchange. The greater impacts on annual contributions of hyporheic regeneration are likely to be due to longer thawed seasons, increased sediment temperatures or changes in geomorphology.

Multi-offset GPR methods for hyporheic zone investigations

Porosity of stream sediments has a direct effect on hyporheic exchange patterns and rates. Improved estimates of porosity heterogeneity will yield enhanced simulation of hyporheic exchange processes. Ground-penetrating radar (GPR) velocity measurements are strongly controlled by water content thus accurate measures of GPR velocity in saturated sediments provides estimates of porosity beneath stream channels using petrophysical relationships. Imaging the substream system using surface based reflection measurements is particularly challenging due to large velocity gradients that occur at the transition from open water to saturated sediments. The continuous multi-offset method improves the quality of subsurface images through stacking and provides measurements of vertical and lateral velocity distributions. We applied the continuous multi-offset method to stream sites on the North Slope, Alaska and the Sawtooth Mountains near Boise, Idaho, USA. From the continuous multi-offset data, we measure velocity using reflection tomography then estimate water content and porosity using the Topp equation. These values provide detailed measurements for improved stream channel hydraulic and thermal modelling.

Watershed characterization using seasonal time‐lapse DC resistivity data

Changes in water saturation can cause significant changes in the electrical conductivity of near surface materials. Thus mapping changes in the conductivity distribution during wet and dry periods provides an indirect way to quantify changes in the saturation of the near-surface medium. This, in turn, leads to better understanding of the water mass balance in a watershed characterization problem. The conductivity imaging can also provide other information such as fracture orientations within the nearsurface medium. At Dry Creek watershed; situated at 1830m elevation near Boise, Idaho; we study seasonal changes in saturation using the time-lapse DC resistivity method. Four DC resistivity data sets were acquired between October 2005 and July 2006 to monitor the changes in conductivity. The results indicate that DC resistivity is a cost-effective tool to characterize a watershed and aids in the interpretation of other data collected in the watershed.

Site Characterization For Groundwater Using Controlled-Source Electromagnetics

Controlled source electromagnetic data were acquired at two sites in Southern California in order to characterize the subsurface and identify structures that would influence water flow and distribution. The electromagnetic data were inverted to ascertain the subsurface electrical conductivity structure of the sites. Due to higher data quality and validity of layered Earth assumption, 1D inversion was sufficient for characterizing the Anza, California field site. The Joshua Tree, California site presented a greater challenge due to more subsurface heterogeneities and variable electrode coupling conditions. 1D inversion was attempted for these data, but fitting the data with the 1D inversion proved to be nearly impossible. These data were inverted in 3D at lower frequencies in order to characterize the deep structure of the site. The 3D inversion results are relatively consistent with what is known about the subsurface geology for the site.