Well Log Stratigraphic Interpretations in the Columbia Plateau, Idaho, Oregon, and Washington (sir2010-5246_strat)
Description
As part of a U.S. Geological Survey Groundwater Resources Program study, a three-dimensional
geologic model was constructed for approximately 53,000 square miles of the Columbia Plateau
in Idaho, Oregon, and Washington. This model was constructed to define the general aquifer
system geometry for use in a regional numerical groundwater flow model. Simplifications and
assumptions consistent with this ultimate goal and with the uncertainty in the available data
were made. The model units consist of Miocene-age Columbia River Basalt Group strata and
younger sedimentary overburden covering approximately 44,000 square miles. Data were
compiled from numerous databases and detailed studies that were completed during the past
three decades. These data include stratigraphic interpretations of more than 13,000 wells and
a contiguous compilation of surficial geology and structural features in the study area. These
data were simplified and used to construct piecewise-smooth trend surfaces that represent
upper and lower subsurface model unit boundaries in this complex folded and faulted terrain.
The smoothness of the surfaces implicitly represents uncertainty in prediction of each surface
resulting from data gaps, errors in borehole interpretations, errors in mapped contact locations,
and uncertainty in the shape of the paleosurface upon which flood basalts were emplaced.
Surfaces were recombined using a rule-based algorithm to construct a fully three-dimensional
model with a 500-foot grid resolution that is consistent with the data and for which error estimates
may be made. The modeling process yielded improved estimates of unit volumes, refinement of
location of large structural features, and identification of features that may be important for ongoing
groundwater studies.