Investigating the stratigraphy of an alluvial aquifer using crosswell seismic traveltime tomography

Abstract

In this study, we investigate the use of crosswell P-wave seismic tomography to obtain spatially extensive information about subsurface sedimentary architecture and heterogeneity in alluvial aquifers. Our field site was a research wellfield in an unconfined aquifer near Boise, Idaho. The aquifer consists of a ∼ 20-m-thick sequence of alluvial cobble-and-sand deposits, which have been subdivided into five stratigraphic units based on neutron porosity logs, grain-size analysis, and radar reflection data. We collected crosswell and borehole-to-surface seismic data in wells [Formula: see text] apart. We carefully considered the impact of well deviation, data quality control, and the choice of inversion parameters. Our linearized inverse routine had a curved-ray forward model and used different grids for forward modeling and inversion. An analysis of the model covariance and resolution matrices showed that the velocity models had an uncertainty of [Formula: see text], a vertical resolution of [Formula: see text], and a horizontal resolution of [Formula: see text]. The velocity in the saturated zone varied between [Formula: see text] and [Formula: see text]. Inclusion of the borehole-to-surface data eliminated the X- shaped pattern that is a common artifact in crosswell tomography, and the increased angular coverage also improved the accuracy of the model near the top of the tomogram. The final velocity model is consistent with previous stratigraphic analyses of the site, although the locations of some of the unit boundaries differ by as much as [Formula: see text] in places. The results of this study demonstrate that seismic tomography can be used to image the sedimentary architecture of unconsolidated alluvial aquifers, even when the lithologic contrasts between units are subtle.