Lee D. Slater

An investigation of the ability of geophysical methods to detect and define Fulachta Fia (Burnt Mounds) on Clare Island, Co. Mayo, Ireland

Measurements of electrical resistivity and magnetic susceptibility have been made in order to determine the geophysical signature of a number of fulachta fia (burnt mounds of probable Bronze Age) on Clare Island, County Mayo, Ireland. Visible remains of fulachta fia on Clare Island frequently show a characteristic horseshoe-shaped structure at the surface, covered by a very thin layer of topsoil. Magnetic susceptibility and electrical resistivity surveys over two intact fulachta fia were carried out in order to map their lateral and vertical extent. The magnetic susceptibility surveys provided horseshoe-shaped signatures, which immediately identify the extent and orientation of both the surveyed monuments. The success of the resistivity method was found to be dependant upon the proximity of unwanted sources of resistivity variation not related to these archaeological targets, and possibly climatological and method-related influences. These results illustrate the feasibility of using geophysical methods to define a characteristic signature related to fulachta fia. The reproduction of this signature over suspected archaeological sites can provide confirmation of the existence of a monument without the need for an intrusive excavation. Resistivity surveys over two damaged fulachta fia partially exposed in the bank of a drainage channel have revealed useful information on the degree of preservation of these monuments. Resistivity anomalies were identified coincident with the location of fractured rock exposed in the channel bank, despite the terrain effect caused by the proximity to the channel edge.

Use of Induced Polarization to Characterize the Hydrogeologic Framework of the Zone of Surface‐Water/Groundwater Exchange at the Hanford 300 Area, WA

An extensive continuous waterborne electrical imaging (CWEI) survey was conducted along the Columbia River corridor adjacent to the U.S. Department of Energy (DOE) Hanford 300 Area, WA, in order to improve the conceptual model for exchange between surface water and U-contaminated groundwater. The primary objective was to determine spatial variability in the depth to the HanfordRingold (H-R) contact, an important lithologic boundary that limits vertical transport of groundwater along the river corridor. Resistivity and induced polarization (IP) measurements were performed along six survey lines parallel to the shore (each greater than 2.5 km in length), with a measurement recorded every 0.5-3.0 m depending on survey speed, resulting in approximately 65,000 measurements. The H-R contact was clearly resolved in images of the normalized chargeability along the river corridor due to the large contrast in surface area (hence polarizability) of the granular material between the two lithologic units. Cross sections of the lithologic structure along the river corridor reveal a large variation in the thickness of the overlying Hanford unit (the aquifer through which contaminated groundwater discharges to the river) and clearly identify locations along the river corridor where the underlying Ringold unit is exposed to the riverbed. Knowing the distribution of the Hanford and Ringold units along the river corridor substantially improves the conceptual model for the hydrogeologic framework regulating U exchange between groundwater and Columbia River water relative to current models based on projections of data from boreholes on land into the river.

Use of Time‐Domain Induced Polarization for Lithology Identification: A Case Study from the Hanford 300‐Area

Use of the induced polarization (IP) method, as being applied to determine the hydrostratigraphic framework at the Hanford 300-Area, Washington, offers advantages over electrical resistivity (ER) in yielding information about the physicochemical characteristics of the subsurface. The IP response in soils results from surface polarization that is largely determined by lithology. For non-metallic soils, IP typically shows a weak dependence on fluid conductivity, but strong nearlinear relationships to textural parameters (surface area, grain size, etc.). Compared to the ER method, IP has smaller signal-to-noise ratio, requiring additional care during data acquisition and processing. In particular, appropriate data weight needs to be considered for successful inversion of IP datasets.

Borehole Nuclear Magnetic Resonance (NMR): a valuable tool for environmental site management

Borehole nuclear magnetic resonance (NMR) is an emerging geophysical method being applied to hydrogeology investigations. NMR is a quantitative geophysical method that can be used to make in situ assessments of porosity, water content, mobile and immobile water fraction, and estimates of permeability. While borehole NMR is commonly used in the oil and gas industry, it is only recently that NMR tools have been designed for use in small-diameter boreholes that are typically used in groundwater studies. This video presents an overview of borehole NMR and example applications for environmental site management.

Monitoring in Situ Bioremediation at the Rifle, Colorado IFRC Site with Nuclear Magnetic Resonance and Magnetic Susceptibility Measurements

Nuclear magnetic resonance (NMR) and magnetic susceptibility (MS) borehole logging measurements were collected at the Rifle Integrated Field Research Challenge (IFRC) site. The Rifle IFRC site is located at a former uranium oreprocessing facility in Rifle, Colorado. Although removed from the site by 1996, leachate from spent mill tailings has resulted in residual uranium contamination of both groundwater and sediments within the local aquifer. Since 2002, research at the site has primarily focused on quantifying uranium mobility associated with stimulated biogeochemical processes.

A Geophysical Characterization & Monitoring Strategy for Determining Hydrologic Processes in the Hyporheic Corridor at the Hanford 300-Area

The primary objective of this research was to advance the prediction of solute transport between the Uranium contaminated Hanford aquifer and the Columbia River at the Hanford 300 Area by improving understanding of how fluctuations in river stage, combined with subsurface heterogeneity, impart spatiotemporal complexity to solute exchange along the Columbia River corridor. Our work explored the use of continuous waterborne electrical imaging (CWEI), in conjunction with fiber-optic distributed temperature sensor (FO-DTS) and time-lapse resistivity monitoring, to improve the conceptual model for how groundwater/surface water exchange regulates uranium transport. We also investigated how resistivity and induced polarization can be used to generate spatially rich estimates of the variation in depth to the Hanford-Ringold (H-R) contact between the river and the 300 Area Integrated Field Research Challenge (IFRC) site. Inversion of the CWEI datasets (a data rich survey containing {approx}60,000 measurements) provided predictions of the distributions of electrical resistivity and polarizability, from which the spatial complexity of the primary hydrogeologic units along the river corridor was reconstructed. Variation in the depth to the interface between the overlying coarse-grained, high permeability Hanford Formation and the underlying finer-grained, less permeable Ringold Formation, an important contact that limits vertical migration of contaminants, has been resolved along {approx}3 km of the river corridor centered on the IFRC site in the Hanford 300 Area. Spatial variability in the thickness of the Hanford Formation captured in the CWEI datasets indicates that previous studies based on borehole projections and drive-point and multi-level sampling likely overestimate the contributing area for uranium exchange within the Columbia River at the Hanford 300 Area. Resistivity and induced polarization imaging between the river and the 300 Area IFRC further imaged spatial variability in the depth to the Hanford-Ringold inland over a critical region where borehole information is absent, identifying evidence for a continuous depression in the H-R contact between the IFRC and the river corridor. Strong natural contrasts in temperature and specific conductance of river water compared to groundwater at this site, along with periodic river stage fluctuations driven by dam operations, were exploited to yield new insights into the dynamics of groundwater-surface water interaction. Whereas FO-DTS datasets have provided meter-scale measurements of focused groundwater discharge at the riverbed along the corridor, continuous resistivity monitoring has non-invasively imaged spatiotemporal variation in the resistivity inland driven by river stage fluctuations. Time series and time-frequency analysis of FO-DTS and 3D resistivity datasets has provided insights into the role of forcing variables, primarily daily dam operations, in regulating the occurrence of focused exchange at the riverbed and its extension inland. High amplitudes in the DTS and 3D resistivity signals for long periods that dominate the stage time series identify regions along the corridor where stage-driven exchange is preferentially focused. Our work has demonstrated how time-series analysis of both time-lapse resistivity and DTS datasets, in conjunction with resistivity/IP imaging of lithology, can improve understanding of groundwater-surface water exchange along river corridors, offering unique opportunities to connect stage-driven groundwater discharge observed with DTS on the riverbed to stage-driven groundwater and solute fluctuations captured with resistivity inland.

Spectral Induced Polarization Monitoring during Microbial Enhanced Oil Recovery

Microbial Enhanced Oil Recovery (MEOR) has been established as a cost effective method for enhancing tertiary oil recovery. Although not commonly used for shallow heavy oils, it could be a viable alternative since it can offer sustainable economic recovery and minimal environmental impact. Successful MEOR treatments require accurate, real time monitoring of the biodegradation processes resulting from the Injection of microbial communities into the formation. Results of recent biogeophysical research suggest that minimally-invasive geophysical methods could significantly contribute to such monitoring efforts. Here we present results of laboratory experiments to assess the sensitivity of the spectral Induced polarization method (SIP) to MEOR treatments. We used heavy oil from a shallow oilfield in SW Missouri to saturate three sand columns. We then followed common industry procedures and used a commercially available microbial consortia (Para-Bac/STM) to treat the oil columns. the active MEOR experiment was performed in duplicate while a control column maintained similar conditions, without promoting microbial activity. We monitored the SIP signatures, between 0.001 Hz and 1000 Hz, for a period of six months. to support the geophysical measurements we also monitored geochemical parameters, including pH, Eh and fluid conductivity, and collected weekly fluid samples from the outflow and inflow which were analyzed to confirm that microbes actively degraded the heavy oils in the column. Destructive analysis of the solid materials was performed upon completion of the experiment, Preliminary analysis of the results suggests that SIP is sensitive to MEOR processes. in both inoculated columns, we recorded an increase in the low frequency polarization with time, where there were measurable changes up to 3.5 mrads in the phase shift recorded for both active columns, while no change was observed in the control column for the duration of the experiment. these results may indicate that remote geophysical methods could successfully complement current MEOR monitoring schemes for promoting sustainable oil recovery.

Magnetic Susceptibility Measurements as a Proxy for Bioremediation at Hydrocarbon Contaminated Sites

Magnetic susceptibility (MS) of sediments affected by hydrocarbon contaminated groundwater was studied at two sites. the sites used in the study are Bermidji MN, and Carson City, MI. Two cores were retrieved from Bemidji site; one from the contaminated area and the other from the background area. Three cores were collected from the Carson site; two within the contaminated area and one from the background area. All the cores that we collected extended from the unsaturated zone into the saturated zone. for the Bemidji site, there is a 0.9m hydrocarbon smear zone due to groundwater level fluctuations. for the Carson site, hydrocarbon smear zone is approximately 1-2 m. MS and Grain size data were collected from the cores from both sites. Our results show that MS increased towards the top of the GWT within the contaminated area of both sites. in contrast, the MS does not show any changes around the GWT within the clean cores of both sites. We postulate that this increase in MS is due to iron reducing microbes creating magnetite as a byproduct of hydrocarbon breakdown. Based on these results we conclude that the MS measurements can be used as a tool to investigate microbial activity within hydrocarbon contaminated zones.

Electrical Imaging of Permeable Reactive Barrier (PRB) Integrity

The permeable reactive barrier (PRB) is a promising in-situ technology for treatment of hydrocarbon contaminated groundwater. A PRB is typically composed of granular iron, which degrades chlorinated organics into potentially nontoxic dehalogenated organic compounds and inorganic chloride. Geophysical methods may assist assessment of in-situ barrier integrity and evaluate long term barrier performance. The highly conductive granular iron makes the PRB an excellent target for electrical imaging methods. Surface and cross-borehole electrical imaging was conducted at the PRB installed at the US Department of Energy Kansas City plant. The poor signal strength and insensitivity at depth, which results from current channeling in the highly conductive iron, limited surface imaging. Crossborehole electrical measurements were highly effective at defining an accurate cross-sectional image of the barrier in-situ. Cross-borehole images obtained for seven panels along the barrier indicate significant variability in barrier integrity along the installation. In addition, the images suggest variability in the integrity of the contact between PRB and bedrock. This non-invasive, in-situ evaluation of barrier geometry has broad implications for the evaluation of PRB performance as a passive method for hydrocarbon treatment.