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A Magnetotelluric Investigation of Hawaiian Groundwater Systems in Kona, Hawai‘i Island, Hawai‘i, including Uncertainty Quantification

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Title:A Magnetotelluric Investigation of Hawaiian Groundwater Systems in Kona, Hawai‘i Island, Hawai‘i, including Uncertainty Quantification
Authors:Viti, Taylor Mata
Contributors:Grobbe, Niels (advisor)
Geology and Geophysics (department)
Archie's law
uncertainty quantification
Date Issued:2019
Publisher:University of Hawaiʻi at Mānoa
Abstract:We investigate the spatial distribution of groundwater at high elevations in
Hawaiʻi Island's Keauhou aquifer system, where a sparse distribution of
water wells indicates anomalously high head levels that are likely caused by
a geological structure of unknown nature. To study this system, we have
conducted a MT survey on a transect climbing the southwestern flank of the
Hualālai shield volcano. We carry-out a 2D deterministic inversion using
MARE2DEM (a freely available MT modeling program), producing an image of the
bulk electrical resistivity distribution in the subsurface. We explore the
use of 1D, station-by-station, Bayesian inversions (using our own software),
in an attempt to supplement the limited amount of water wells at higher
elevations, by creating `virtual water wells' from MT stations that can be
used to infer e.g.~head-level information. Even though we can infer
information about the subsurface groundwater distribution with reasonable
accuracy, we infer that the vertical resolution of the MT data is most
likely insufficient to be used as actual virtual water well head-level
data. However, the MT results are anticipated to be useful for roughly
validating and constraining hydrological models in terms of e.g.~different
hydrogeological regimes.
Comparing the 1D inversion results with the 2D results, we find that the 1D
inversion results match the 2D results reasonably well for most stations,
especially until the depth at which seawater associated resistivities are
encountered. Using the uncertainty quantification of the Bayesian
inversions, we determine that depths of up to approximately 1 km beneath
mean sea level are well constrained by the data. To first order, the 2D
image exhibits a layered resistivity structure. Based on literature values
for the resistivities of Hawaiian basalts under various saturation
conditions, these layers are consistent with (in order of increasing depth),
unsaturated Hawaiian basalt, freshwater-saturated Hawaiian basalt, and
seawater-saturated Hawaiian basalt. A large head gradient delineating inland
high-level heads from coastal low-level heads is also evident in the image
as an apparent discontinuity in the freshwater valued resistivities.
Resistivities associated with seawater-saturated basalts occur in the
high-level aquifer starting at a depth of approximately 1 km beneath mean
sea level, suggesting that the high-level aquifer is a vertically extensive
body of fresh water that is hydraulically continuous with seawater, most
likely via a brackish water mixing zone.
In light of the virtual water-well approach, the resistivities from the 2D
image are inverted for porosity and saturant salinity, using an Archie's
law-based Bayesian inversion routine that we have developed. The resulting
salinity profile reflects an increase in salinity with depth, but with very
few samples meeting or exceeding the salinity of seawater. Due to the
non-identifiability of the Archie's law parameters, incorporating
sufficient, high-quality prior information is probably key when trying to
obtain useful outcomes, for example by constraining parameter ranges in
terms of depth range, using prior porosity information from core samples, or
using prior salinity information from water wells in the region.
Pages/Duration:48 pages
Rights:All UHM dissertations and theses are protected by copyright. They may be viewed from this source for any purpose, but reproduction or distribution in any format is prohibited without written permission from the copyright owner.
Appears in Collections: M.S. - Geology and Geophysics

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