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 in- verted 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 ap- proach, it is important to collect one or more data sets for which the bulk of the subsurface is in a background or relatively unper- turbed state.Athird and commonly used approach to time-lapse inversion,invertingthedifferencebetweentwodatasets,localiz- es the regions of the model in which change has occurred; how- ever, varying noise levels between the two data sets can be prob- lematic. To further assess the various time-lapse inversion ap- proaches,weacquiredfielddatafromacatchmentwithintheDry Creek Experimental Watershed near Boise, Idaho, U.S.A. We combined the complimentary information from individual static ERTinversions,time-lapseERTimages,andavailablehydrolog- icdatainarobustinterpretationschemetoaidinquantifyingsea- sonalvariationsinsubsurfacemoisturecontent.