NS31A-01
The detection of cavities and funnels over closed mines of Albania with the use of SP and Electrical Resistance Tomography
This study consists in the detection of cavities that could be present over closed mines of Albania with the use of SP and Electrical Resistance Tomography. In general, natural and artificial cavities represent an important investigation target in geotechnical, environmental and scientific studies. These studies could be engineering, archaeological or spelaeologal. For the detection and localization of such cavities during the latter 20 years, several geophysical studies have been used worldwide. In this field study several geophysical methods could be used such electrical, electromagnetic, gravimetric and seismic ones. Now days, the Ground Penetrating Radar Technique and Electrical Resistance Tomography (ERT) are widely used. Depending on geological formation conditions every used method is more or less effective. As a result of actual conditions of works on ore mines, the dynamic regime of the surroundings areas has undergone changes creating superficial cavities and funnels and as consequence the geological risk to the community is present. The detection of cavities is a challenge for geophysicists and in these cases the ambiguity on interpretation of geophysical data has to be solved properly. The simultaneous application of different geophysical methods and monitoring in time are necessary in order that the field data interpretation could be significant.On the frame of a collaboration with Institute of Mines of Albania, a staff from Geophysical Center of Tirana during the period 2004-2005 has undertaken one project for the detection of superficial cavities and funnels on surrounding areas over work mines which are planned to be closed. In order to make clear the presence of geological risk to community, geophysical investigation were carried out as tests over galleries of tow ore mines of Albania in Dhrovjan and Perrenjas regions. The effectiveness of the used geophysical methods was present.
NS31A-02 INVITED
UXO Detection and Characterization using new Berkeley UXO Discriminator (BUD)
An optimally designed active electromagnetic system (AEM), Berkeley UXO Discriminator, BUD, has been developed for detection and characterization of UXO in the 20 mm to 150 mm size range. The system incorporates three orthogonal transmitters, and eight pairs of differenced receivers. The transmitter-receiver assembly together with the acquisition box, as well as the battery power and GPS receiver, is mounted on a small cart to assure system mobility. BUD not only detects the object itself but also quantitatively determines its size, shape, orientation, and metal content (ferrous or non-ferrous, mixed metals). Moreover, the principal polarizabilities and size of a metallic target can be determined from a single position of the BUD platform. The search for UXO is a two-step process. The object must first be detected and its location determined then the parameters of the object must be defined. A satisfactory classification scheme is one that determines the principal dipole polarizabilities of a target. While UXO objects have a single major polarizability (principal moment) coincident with the long axis of the object and two equal transverse polarizabilities, the scrap metal has all three principal moments entirely different. This description of the inherent polarizabilities of a target is a major advance in discriminating UXO from irregular scrap metal. Our results clearly show that BUD can resolve the intrinsic polarizabilities of a target and that there are very clear distinctions between symmetric intact UXO and irregular scrap metal. Target properties are determined by an inversion algorithm, which at any given time inverts the response to yield the location (x, y, z) of the target, its attitude and its principal polarizabilities (yielding an apparent aspect ratio). Signal-to-noise estimates (or measurements) are interpreted in this inversion to yield error estimates on the location, attitude and polarizabilities. This inversion at a succession of times provides the polarizabilities as a function of time, which can in turn yield the size, true aspect ratio and estimates of the conductivity and permeability of the target. The accuracy of these property estimates depends on the time window over which the polarizability measurements, and their accuracies, are known. Initial tests at a local site over a variety of test objects and inert UXOs showed excellent detection and characterization results within the predicted size-depth range. This research was funded by the U.S. Department of Defense under ESTCP Project \# UX-0437.
NS31A-03
Microscale Controls on Ultrasonic Velocity Dispersion in Near-Surface Marine Sediments
This effort demonstrates a technique to measure poroelastic and petrophysical parameters that can be monitored over time to document diagenetic and consolidation alterations in the shallow biogeosphere. The signatures of these process effects are revealed largely through scale-dependent estimates of porosity, permeability, and the effective framework moduli that describe particle-particle mechanical interactions. Near- surface marine sediments of the Peru margin (ODP Leg 201) provide a unique dataset with which to study such near-surface processes, especially those associated with depositional, tectonic, and biogeochemical dynamics. Scanning electron microscope (SEM) image analysis and broadband (100-1000 kHz) ultrasonic compressional wave experiments are combined to interpret the microscale parameters revealed through velocity dispersion analysis. In particular, (i) back-scattered electron (BSE) images are processed to estimate the local porosity, tortuosity, and resultant permeability of the characteristic topology of each sample; and (ii) bounds for complex-valued grain and frame moduli, following an amended Biot formulation, are estimated by using the microscale imaging parameters and observed velocity dispersion. Several key results are highlighted, with regard to BSE imaging and velocity dispersion analysis, beyond the imaging and Biot parameter inversion. For example, microscale permeabilities are typically an order-of- magnitude larger than core ($\sim$2 cm) measurements. This discrepancy is critical to understanding spatial and temporal scale differences between, for example, diffusion and advection of nutrients supplying microbial communities versus tectonic dewatering and the resulting transient meter-scale pore pressure modulation. Broadband velocity dispersion analysis proves to be a powerful tool for detecting sub-wavelength sedimentological heterogeneity. Negative velocity dispersion, for example, can be used to estimate scatterer dimensions, consistent with BSE and secondary electron (SE) images, in several samples. Heterogeneities are also revealed by narrow-band spectral resonance. Such resonance phenomena likely indicate semi- continuous, cross-core laminae whose thickness can be directly constrained by the dispersion data. The concluding discussion presents sensitivity analysis that examines the potential for detecting change in baseline parameters due to possible frame and/or poroelastic variable evolution such as intergranular diagenetic precipitation or structural consolidation. {\bf Acknowledgments:} Funding for ODP postcruise research was made possible by a grant from the U.S. Science Support Program (418920-BA315). Special thanks to Nelia Dunbar (New Mexico Bureau of Geology and Mineral Resources) for her assistance with SEM imaging. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the U.S. Department of Energy under contract DE- ACOC4-94AL85000.
NS31A-04
GPR Investigation of an Earlier Romanesque Foundation Beneath the Gothic Abbey Church of Valmagne, Villeveyrac, South of France
Ground-penetrating radar (GPR) is widely used in archeology as it is a very high resolution and nondestructive geophysical method. Information obtained from GPR provides detailed images of near-surface buried objects. GPR or other nondestructive imaging techniques are essential to permit minimal excavation of a historically sensitive building for which archeological investigation is planned. A 3-D GPR survey with 500- and 800-MHz antennas was performed on the fine gravel floor of the Gothic Abbey Church of Valmagne, near Montpellier, south of France. The whole floor area of the abbey including aisle, choir, transept, and nave was surveyed. Processed 2-D GPR profiles and time slice images of both 500- and 800-MHz data on a known buried limestone pier of earlier Romanesque construction within the survey area show distinct anomalies corresponding to the location of the object. The Romanesque pier abuts a present-day Gothic pillar inside the transept of the abbey. Several other GPR anomalies were also found. For example, n intriguing row of periodically-spaced buried objects can be seen down the center of nave which may be related to an earlier Romanesque wall. Imaging the foundations is very challenging due to low dielectric constant between foundations and host soil. Therefore the next stage is to attempt to apply 3-D seismic coherency techniques to the 3-D GPR data to better image the near subsurface structures inside the abbey. Our results shall be used to guide archeological ground-truthing and provide constraints on architectural historical models of Cistercian abbey construction.
NS31A-05
GPR Waves at the Interface: An Investigation of Electromagnetic Wave Polarization Attributes for Fracture Characterization
Remote detection and characterization of vertical fractures is of significant importance to the near-surface environment. Previous research showed that high-angle of incidence electromagnetic (EM) wave propagation through fractures introduces detectable changes to the phase and amplitude of linearly polarized EM waves. These changes were exploited using ground-penetrating radar (GPR) signals to detect the location of vertical, non-reflecting fractures in geologic formations. The work presented here employs analytical and numerical modeling of a discrete vertical fracture in homogeneous matrix to quantitatively relate EM wave polarization properties to fracture properties. We investigate the amplitude and phase response of variable polarization EM waves propagating through vertical fractures of variable aperture and fluid content. Air-filled fractures introduce increasing phase-lead difference between perpendicularly and parallel polarized signals as the frequency of the signal increases. For a specific frequency, the perpendicular polarization phase-lead difference increases as the fracture aperture increases, for apertures less than one wavelength. Perpendicularly polarized signals are consistently lower in amplitude compared to parallel polarized signals and both polarization signal amplitudes decrease as fracture aperture increases. However, propagation through water-filled fractures results in the amplitude of perpendicularly polarized signals to be consistently greater than parallel polarized signals and, similarly to air-filled fractures, both polarization signal amplitudes decrease as fracture aperture increases. Phase relationships between variable polarization EM waves through water-filled fractures display similar responses as air-filled fractures for fracture apertures significantly smaller than signal wavelength. However, as fracture aperture increases, the phase-lead relationship reverses resulting in parallel polarized signals leading in phase. GPR observations of vertical fractures in concrete blocks are in support of the findings of this study. The characteristic EM signal responses to fracture properties predicted by analytical solutions and modeled using numerical simulations indicate that fracture properties can be characterized quantitatively using variable polarization GPR observations.
NS31A-06
Geophysical Identification of Filled Sinkholes and Buried Fractures in Glaciated Karst
Karst aquifers are highly susceptible to contamination, with numerous points of entry for contaminants through recharge features such as sinkholes, swallow holes and soil pipes. These recharge features may be filled or obscured at the surface, requiring the use of geophysical techniques or remote sensing for their identification. This problem is particularly acute in recently glaciated terrain, where surface topography is often unrelated to underlying active karst features, as illustrated by three Illinois examples in this study. Approximately 10 km south of Rockford, IL, a network-type cave of interconnected solutionally-enlarged fractures extends over an area of approximately 1 ha, beneath a low Ordovician dolomite ridge covered with approximately 5 m of glacial till and outwash. Ground-penetrating radar (GPR) profiles along the ridge top, utilizing 50 and 100 MHz antennas, showed strong diffractions over unsaturated caves and karst conduits. A trough-like feature approximately 20 m wide and 3 m deep was also identified in GPR profiles approximately 20 m west of the mapped cave passages. It probably represents an elongate filled sinkhole, similar to those observed along the wall of a quarry less than 1 km west of the cave. GPR profiles and two-dimensional electrical resistivity imaging identified clay-filled sinkholes 2-3 m wide and about 3 m deep in Bourbonnais, IL, 100 km south of Chicago, where 1 to 4 meters of loess and till overlie karstic Silurian dolomite. Penetrometer surveys revealed deeply weathered bedrock zones containing vertical clay-filled caves beneath the filled sinkholes. Surface water infiltration estimates are being made for these features, utilizing the geophysically-derived geometries, along with hydraulic conductivity and gradient data. EM conductivity profiles were combined with resistivity soundings at landfill in southern Illinois near St. Louis, where approximately 5 m of till overlies karstic limestone. Linear high conductivity anomalies appear to coincide with major bedrock fractures - the intersection of which were selected for monitoring wells. Well logs confirm that bedrock is considerably deeper along these anomalous zones, which appear to be percolation conduits.
NS31A-07
Capacitively Coupled Resistivity Survey of the Sea Ice Near Barrow, Alaska
Capacitively coupled resistivity methods have the ability to image areas of high resistivity such as the arctic sea ice. In addition, due to their mobility, capacitive arrays typically take data faster than other resistivity methods and can thus cover a wider survey area in a given amount of time. Surveys using capacitive systems have been carried out across permafrost terrain (see, e.g., Timofeev, et al, 1994 and Calvert, 1992) using dipole-dipole spacings on the order of tens of meters, giving depths of penetration of the same magnitude and showing features on the order of meters in extent. Results of a new capacitively coupled resistivity survey carried out near Barrow, Alaska will be presented. This survey uses much smaller dipole-dipole spacings in order to image the one- to two-meter thick sea ice. The data density will be such that sub-meter-sized features within the ice may be discerned. Data will be taken along multiple closely-spaced lines so that two- and three- dimensional inversion may be done yielding a more accurate representation of the sea ice.
NS31A-08
Relationships Between Electrical Conductivity - Water Content, Water Potential and Unsaturated Hydraulic Conductivity for Three Soils
In soil physics, water retention and hydraulic conductivity are key parameters for predicting water fluxes in soils. Determination of these hydrodynamic characteristics in the lab, particularly unsaturated hydraulic conductivity, is most often complicated, time consuming and error-prone. These difficulties often prohibit the examination of numerous soil samples for determining these parameters as would be necessary to get a good estimation of the field variability. In this case, an indirect and easy to measure variable, closely linked to water retention or hydraulic conductivity, would be helpful in the assessment of these parameters. Electrical conductivity (EC) is a good candidate for such a variable because, in a porous medium, its magnitude is largely determined by the number of water filled pores and their connectivity. Relationships between water content (or saturation) and EC have been established both from empirical or theoretical point of view for some time. However, relationships between EC and unsaturated hydraulic conductivity are much more scarce, as are experimental data. We present relationships between EC and water content or water potential for three soil types: a clay loam, a sandy loam and a sand. We also present experimental relationships between EC and unsaturated hydraulic conductivity. The soil were cored undisturbed in the field and water retention was measured together with E. Hydraulic conductivity was calculated from the Wind evaporation method and from steady state measurements for low suctions. Mercury porosimetry measurements were also performed after the experiments. Water saturation reasonably follows a power-law relationship with relative EC (EC/ECsat). Exponents of the power law being around 3 - 3.5 for the clay loam, 1.1 - 1.7 for the sandy loam and 8 - 12 for the sand (for matric potentials between 0 and -80 cm in the latter case). Variation of the relative EC with the log of water potential shows a S-shape, with an almost linear part in a range potential depending on the soil type. The hydraulic conductivity is a decreasing function of the relative EC, the saturated hydraulic conductivity being in agreement with the model of Katz and Thomson.
NS31A-09
Effects of Local Meteorological Variability on Surface and Subsurface Seismic-Acoustic Signals
Seismometer, microphone and other geophysical instrument performance is sensitive to the material in which the instrument is installed. Since monitoring operations often depend on geophysical instruments to identify near-surface features of interest, it is important to quantify how local meteorological variability in precipitation, wind, or temperature in particular geo-environments can either improve or degrade instrument capability. Data from recent tunneling activity monitoring show a significant increase in the recorded signal amplitude before and after 36 hrs of steady precipitation. This improvement applied to both the signals of interest and the continuous noise field. This post-precipitation increase in amplitude would make these types of signals of interest identifiable at greater distances from the source, effectively improving the range of the instrument. The enhanced capability would diminish as the near-surface environment dried out. Near-surface variability would potentially create cyclic variations in capability/discrimination that could introduce false positives in the data or be interpreted as instrument failure.