Geology topics

Stephanie N. Phillips

Publications and source records attributed to Stephanie N. Phillips.

5 recordsLinked to original sources

USGS2025

Uncertainty quantification of geophysical and hydrologic parameters estimated from borehole nuclear magnetic resonance data

Borehole nuclear magnetic resonance (bNMR) data are typically used to infer in situ hydrologic properties. Partial water content as a function of pore size is estimated by fitting the measured NMR response to a multi-exponential T 2 distribution, and the sum of estimated T 2 amplitudes equals the total volumetric water content. From these estimated parameters, several empirical relationships are commonly used to infer hydraulic conductivity from the NMR-estimated water content and T 2 distribution. Often, parameters are estimated through deterministic inversion methods that produce a single best-fit estimate, but do not reflect uncertainties in model parameters. Here, a Bayesian Markov chain Monte Carlo (McMC) approach for analyzing bNMR data is developed that allows for comprehensive uncertainty quantification of NMR parameters and derived hydrologic properties. The underlying model that describes the T 2 distribution is defined by a set of spline interpolation points. The number of interpolation points is allowed to vary in a trans-dimensional algorithm that naturally favors simple models with fewer interpolation points, allowing the data to inform the necessary level of model complexity. Additionally, data error is estimated as an unknown parameter. Analysis of the ensemble of models output from the McMC algorithm provides useful details on the range of plausible T 2 distributions that can fit a measured bNMR decay curve, as well as uncertainty estimates of total water content. The ensemble of NMR parameters can also be propagated through commonly used relationships to produce uncertainty estimates on derived parameters such as bound/capillary/mobile water content or hydraulic conductivity.

JGR Machine Learning and Computation↗

USGS2023

Maps of elevation of top of Pierre Shale and surficial deposit thickness with hydraulic properties from borehole geophysics and aquifers tests within and near Ellsworth Air Force Base, South Dakota, 2020–21

The U.S. Geological Survey, in cooperation with the U.S. Air Force Civil Engineer Center, collected borehole geophysical data and completed simple aquifer tests to estimate the thickness and hydraulic properties of surficial deposits. The purpose of data collection was to create generalized contour maps of Pierre Shale elevation and surficial deposit thickness within and near Ellsworth Air Force Base (study area). Natural gamma and electromagnetic induction data were collected to refine or determine surficial deposit thickness at selected wells. Additionally, data from previous geophysical studies and driller logs were compiled and combined with results from natural gamma and electromagnetic induction data to provide a more spatially complete image of the subsurface. Borehole nuclear magnetic resonance (bNMR) data were collected to estimate hydraulic conductivity and water content of surficial deposits overlying Pierre Shale. Simple aquifer tests using water slugs (slug tests) were completed to estimate hydraulic conductivity of surficial deposits, and results were compared to hydraulic conductivity estimates from bNMR data. All data used to construct maps and estimate hydraulic properties are provided in an accompanying U.S. Geological Survey data release (available at https://doi.org/10.5066/P9FLR79F ). Generalized contour maps were constructed using results from 26 borehole geophysical logs, 35 geophysical transects from previous studies, and 304 wells with driller logs. Pierre Shale elevation generally followed land-surface topography, sloping from high elevations in the north to lower elevations in the south. Topographic highs of Pierre Shale, where present, could act as groundwater divides that potentially affect groundwater flow direction. Surficial deposit thickness varied spatially and ranged from 0 to 86 feet. Surficial deposits generally were thickest in higher elevation areas near ephemeral streams in the northern part of the study area. Hydraulic conductivity estimated from bNMR results using two analytical methods ranged from 0.1 to 2,314 feet per day, whereas hydraulic conductivity estimated from slug tests ranged from 0.001 to 193 feet per day. Hydraulic conductivity estimates from slug tests were plotted with surficial deposit thickness contours instead of bNMR estimates because bNMR estimates were determined to overestimate hydraulic conductivity. Hydraulic conductivity values generally were greater in the southwestern part of study area than the northeastern part.

South Dakota↗

USGS2021

Nuclear magnetic resonanance logs of fractured bedrock at the Hidden Lane Landfill site, Culpeper Basin, Virginia

In May 2018, the U.S. Geological Survey (USGS) in cooperation with the U.S. Environmental Protection Agency (EPA) collected borehole nuclear magnetic resonance (bNMR) logs in three boreholes completed in sandstone and siltstone of the Balls Bluff Member of the Bull Run Formation at a Superfund Site in Culpeper Basin, Virginia. The bNMR logs were used to aid in the evaluation of the aquifer by measuring the porosity, determining the mobile and immobile fractions of water, and estimating the hydraulic conductivity, to evaluate the potential storage and transport properties at the site. The bNMR method measures the transverse (T 2 ) decay in nuclear magnetism in response to radio-frequency pulses. The relaxation decay is related to water content and the size of the pores where the water resides. In addition, the relaxation decay parameters are used to estimate hydraulic conductivity. The results were compared to other borehole logs collected at the site and to regional groundwater investigations in similar rock formations.

Virginia↗

USGS2019

Lithostratigraphic, geophysical, and hydrogeologic observations from a boring drilled to bedrock in glacial sediments near Nantucket Sound in East Falmouth, Massachusetts

In spring 2016, a 310-foot-deep boring (named MA–FSW 750) was drilled by the U.S. Geological Survey near Nantucket Sound in East Falmouth, Massachusetts, to investigate the hydrogeology of the southern coast of western Cape Cod. Few borings that are drilled to bedrock exist in the area, and the study area was selected to fill a gap between comprehensive geologic datasets inland to the north and marine geophysical data from beneath Nantucket Sound to the south. A permanent monitoring well (MA–FSW 750–0100) was installed in the boring upon the completion of the drilling and core collection. Observations from sediment cores and surface and borehole geophysical measurements were used to delineate three zones relevant to understanding groundwater flow at the study location. Shallow sands and gravels (0–107 feet [ft] below land surface [bls]) underlain by silt-rich fine and very fine sand (107–175 ft bls) form a zone of high permeability underlain by a zone of relatively lower permeability, referred to as the “shallow high-permeability” and “low-permeability” zones, respectively. A sharp lithological contact separating the shallow high-permeability and low-permeability zones may affect vertical flow of groundwater. Fine to coarse sand with intervals of clay and silt from 175 to 300 ft bls represent a deep zone of relatively high permeability, referred to as the “deep high-permeability” zone. A compacted, nonsorted unit (identified as basal till) and the bedrock surface were encountered at 300 and 305 ft bls, respectively. Hydraulic conductivity estimates from nuclear magnetic resonance logs and sediment grain-size distribution analyses indicated that the shallow high-permeability zone contributes substantially to the capacity of the aquifer to transmit groundwater at the study location. Results from geophysical surveys indicate a gradual transition from fresh to saline groundwater in the interval from 105 to 160 ft bls. Freshwater at the study site is present in the saturated unconsolidated sediments only in the 75 ft between 30 ft (the water table) and 105 ft bls in the shallow high-permeability zone. Sediments shallower than 175 ft bls closely resemble the downward fining post-Wisconsinan age deltaic and lacustrine deposits present in many parts of western Cape Cod; sediments deeper than 175 ft appear to be the product of earlier depositional processes more local to the southern coast of western Cape Cod. This study highlights how high-resolution observations of cored material coupled with a multitool geophysical approach can characterize a single boring to help better understand regional glacial history and hydrogeology.

Massachusetts↗

USGS2019

Geophysical assessment of a proposed landfill site in Fredericktown, Missouri

In June 2018, U.S. Geological Survey in cooperation with the U.S. Environmental Protection Agency (EPA) collected geophysical measurements to help evaluate the suitability of a proposed landfill site for disposing mine-waste materials in Fredericktown, MO. The geophysical investigation included electromagnetic induction, electrical resistivity tomography, horizontal-to-vertical spectral ratio passive seismic, and shear-wave refraction to characterize the unconsolidated sediment or regolith above the bedrock and the depth to crystalline bedrock. Water-based electromagnetic surveys and forward looking infrared imagery were used along the lake shoreline to identify potential groundwater-surface water interactions. Nested piezometers were installed at two locations along the shoreline to further characterize and validate the groundwater-surface water interactions identified with geophysical methods. Collectively, geophysical surveys were used to evaluate the overall suitability of the proposed site for use as a mine waste-soil and sediment repository and to evaluate this suite of geophysical methods for rapid collection and assessment of very shallow sediments.

Missouri↗