NS33A-01
4-D imaging of ground condition change using resistivity data
Geophysical methods are commonly used to obtain ground condition changes in various application fields of geophysical methods. These monitoring of ground condition needs accurate data measurements as well as proper data evaluation method to obtain or image the change of the physical property of the subsurface with time. In this study, we extended the three-dimensional (3-D) resistivity imaging algorithm to four-dimensional (4-D) imaging algorithm, where measured resistivity data and geoelectric structure of subsurface is assumed to be 4-D (i.e., 3-D in space and varies with time). Compared to the conventional time lapse imaging based on the separate inversion of data set acquired at different time of monitoring survey, 4-D inversion algorithm provided much clear image of the change of the ground with much less inversion artifacts. The algorithm was applied to the field data set and we could show that our 4-D imaging algorithm provides more quantitative means to evaluate the change of the ground conditions and the cause of these changes.
NS33A-02
Electrical Imaging of Infiltration in Agricultural Soils on Long Island, New York
High resolution electrical resistivity imaging of vadose zone infiltration experiments was conducted on agricultural soils by the City College and Graduate Center of CUNY, in cooperation with Cornell University's Agricultural Stewardship Program and Long Island Horticultural Research and Extension Center (LIHREC) in Riverhead, New York. Measurements were made in active vineyards with a commercial resistivity imaging system, using a half- meter electrode spacing. Soils considered were Riverhead sandy loam (RdA), Haven loam (HaA), and Bridgehampton silty loam (BgA). The Riverhead and Haven soils are the most common types found on eastern Long Island. The Bridgehampton is considered the most fertile. Soil samples and measurements of soil compaction were collected at the same time as the geophysical measurements. In addition, remote sensing data were obtained for the three sites and processed to produce normalized difference vegetation index (NDVI) data to evaluate potential correlations between vegetation vigor, soil texture and water migration patterns. Applications of this study include continuous water content monitoring in high value cash crops (precision agriculture). Changes in electrical resistivity during infiltration are clearly visible at all three locations. Preliminary analysis of the results shows correlations of baseline resistivity with particle size distributions and correlations between changes in resistivity during infiltration and soil compaction data. Time-lapse electrical images of the three sites will also be compared with published properties for these soils, including particle size distribution, saturated hydraulic conductivity, available water capacity, and surface texture.
NS33A-03
Time-Lapse Electrical Resistivity Monitoring of the Remediation of a Saline Groundwater Plume
Time-lapse electrical resistivity (ER) surveys are used to monitor the movement and remediation of a saline contaminant plume over the span of three years. Temperature is responsible for a large proportion of the observed ER changes. A temperature resistivity relationship is developed through laboratory testing on core from the site and analysis of petrophysical models. We present a protocol for removing the effects of temperature from ER difference images. Saturation changes also contribute to changes in the ER images. However, at this site, saturation is too spatially heterogeneous to apply a simple correction as was done with temperature. We show that the magnitude of the saturation changes are low and present an analysis of the uncertainty this introduces into the image interpretation. After accounting for temperature variations, we observe changes in the subsurface ER distribution that are consistent with depression focused recharge, solute flushing, and spatially variable effectiveness of the remediation program.
NS33A-04
Estimation of Site-Scale Temporal Variations in Seismic Response as a Function of Geophone Placement
The use of high-resolution geophysical investigations in conjunction with environmental remediation (e.g., separating climate-driven variations from subsurface variations due to anthropogenic manipulations; characterization of infiltration; characterization of subsurface flow and contaminant transport) typically requires temporal resolution not commonly associated with near-surface seismic techniques. Additionally, the estimation of variations in physical and hydraulic properties (i.e., water content; porosity; permeability) using, for example, seismic refraction p-wave velocity tomograms, is controlled by the spatial resolution and signal-to-noise (S/N) of the measured waveform. We therefore investigate the frequency, amplitude, and natural variability of geophone response over time using multiple strategies in geophone placement (e.g., re-coupled on multiple time-scales; buried at varying depths in cylindrical boreholes) in unsaturated unconsolidated fluvial sediments to better define and constrain standard operating procedures for future time-lapse 2D and time-lapse 3D (i.e., 4D) near-surface seismic surveys. In order to delineate natural source variation from subsurface variation, a separate control transect (i.e., geophones were removed and replaced each acquisition period) offset from the static experimental line by 1 meter was acquired simultaneously. Under the assumption the unconsolidated fluvial medium is isotropic at the meter scale, source-receiver positioning is considered identical by locating shots along the median plane of the experimental and control lines. The parallel transects were re-acquired on day, week, and month time-scales coupled with daily climate measurements (i.e., maximum and minimum temperature, precipitation). Of particular interest are periods immediately preceding and following both major and minor climate events that indicate both the rate and effect of climate-driven subsurface recharge.
NS33A-05
Effects of initial velocity model and spread system geometry on tomographic images
The success of tomography is well known in medical science and the most important reason is the ray coverage directly connected to the data acquisition geometry. On the other hand, due to the limited ray coverage of the conventional data acquisition systems, the construction of the proper initial models should be taken into consideration to reach the optimum image of the subsurface. At the first stage, to understand the achievement of medical tomography, a data acquisition geometry which is designed similar to the medical one is tested and compared with the conventional data acquisition geometries. Then initial model importance on the final image reconstruction results are investigated on both model studies and the field data. Additionally, it is focused on how to construct the proper initial velocity models in seismic tomography studies in the areas of environmental, geotechnical and engineering geophysics. In this study, two important subjects can be concluded: 1) The data acquisition system should be fit to the geological structure and 2) The proper initial velocity model, which is taken as close to the reality gives the optimum reconstructed image in the seismic tomography studies.
NS33A-06
High Resolution Geophysical Characterization of Fractures within a Granitic Pluton
The FEBEX underground gallery was excavated in the Aar Granite (Switzerland), a heterogeneous granite containing from very leucocratic facies to granodiorites. The geology of the gallery shows the existence of various sets of fractures with different attributes: geometry, kinematics, fracture infilling, etc. The study of the structural data, new observations on the FEBEX gallery itself and borehole televiewer data acquired in the newly drilled boreholes, have allowed to identify four sets of fractures. The first group of fractures has a typical distribution and characteristics of en echelon tension fractures and were formed in late magmatic stages, according with the paragenesis of the minerals that filled the craks. The main strike is around 300 (280-300). These fractures are deformed and displaced by the other group of faults. The second group corresponds to the lamprophyre dikes, of mantelic origin, with an orientation oblique to the tunnel, and slightly oblique to the first group of fractures (strike, 310-330). They were formed during an extension event well evidenced by their irregular margins and flame structures into the granite. The margins of these dikes show several reactivations as strike slip faults. Geophysical data has been acquired to characterized the fracture network of the surrounding volume within the FEBEX gallery. The geophysic data include new borehole logging such as Natural Gamma and Borehole Ground Penetrating radar. The processing and integration of these different data sets indicates that the GPR record can provide images of a third set of fractures, which are probably fluid filled. This set of fractures a subparallel to the tunnel axis and appear to intersect older boreholes which are nearly perpendicular to the axis of the FEBEX gallery.
NS33A-07
Delineating the Rattlesnake Springs, New Mexico Watershed Using Shallow Subsurface Geophysical Techniques and Geologic Mapping
Rattlesnake Springs serves as the sole water source for Carlsbad Caverns National Park. The recent development of oil and gas leases and agricultural lands surrounding the springs has led to concern about contamination of the karst aquifer. We have used geophysical techniques, combined with geologic mapping, to delineate possible fracture systems in the gypsum and carbonate bedrock that feed the spring system. Our initial work has focused on a 700 m by 700 m region surrounding the springs. We conducted a series of ground conductivity surveys with follow-up DC resistivity surveys (Wenner array vertical electrical soundings and a pole- pole survey) to determine variations in soil grain size and moisture content. Surface geologic mapping was used to identify a series of Holocene terraces and valleys that incise the terraces. Our combined results suggest that northwest-southeast and north-south trending fractures and dissolution features control regional water flow. Relict spring valleys are found to the west of the present springs. A pole-pole survey conducted around the perimeter of the springs suggests main water flow into the springs occurs from the northwest. We plan to complete a precision gravity survey in September and October 2007 to map bedrock topography and determine its relation to structural and dissolution features. Ground penetrating radar data will be collected on the northwestern side of the springs in an attempt to better delineate structures controlling inflow into the springs.
NS33A-08
Ground-Penetrating Radar Studies of Late Pleistocene-Holocene Lacustrine Deltas in Lower Taylor Valley, Antarctica: Implications for Lake History
We conducted multiple-frequency (100 to 900 MHz) ground-penetrating radar surveys of sandy terraces along streams in lower Taylor Valley, Antarctica, to determine their origin. These terraces are presently identified as relict deltas formed by glacial meltwater streams entering Glacial Lake Washburn which was dammed by a lobe of the Ross Sea component of the West Antarctic Ice Sheet (WAIS) during the Last Glacial Maximum (LGM) and subsequent deglacial period. The maximum elevation of the lake surface during the LGM, 350 m asl, and its subsequent history are controversial. Previous evidence for paleo lake levels comes largely from the deltas. We collected radar profiles on four sandy terraces along two streams in the Lake Fryxell basin, Delta and Crescent Streams, up to 120 m asl. Reflector geometry is interpreted as primarily representing sedimentary bedding. The GPR images show deltaic sediments composed of topset and foreset beds with thicknesses of 2 to 3 m draped over crudely stratified, most likely alluvial sediments. The deltaic sediments are commonly less than about 50 percent of the volume of the morphologic features that exhibit 4 to 6 m of relief. Thus, most terraces are delta- capped alluvial deposits. The deltaic sediments are commonly composed of multiple sets of topset-foreset units separated by erosion surfaces. The architecture of the units as well as the trajectories of topset-foreset contacts within the units indicate that, as the deltas prograded into the lake, lake-levels both fell and rose a few meters. Overall, we infer that the glacial lake was shallower at its margin than previously inferred. Further, meter-scale lake-level fluctuations were superimposed on the long-term, deglacial lake-level fall of at least 100 m.