NS31B-0376
Role of Portable XRF Technology in the Recovery of Time Sensitive Geochemical Data from Core and Cuttings Materials
With world-wide increases in metal prices and the accompanying "boom" in mineral exploration activity, there has been an almost "frantic" search for verifiable geochemical data from past exploration and mining activities. Of particular importance is the acquisition and verification of assay and soil geochemical data that was gathered in the past. Much of this data is found in reports, geological\assay logs and assay sheets and has to be verified by either resampling or reassaying core, cuttings, rejects and pulp materials. A reliable, cost and time effective method for recovering this data is the use of Portable X-Ray Fluorescence (PXRF) equipment. Several case studies of the use of this equipment to recover time-sensitive data from core and cuttings materials will be presented.
NS31B-0377
Heat-pulse flowmeter test to characterize the seawater intrusion in fractured rock, western coast of Korea
Seawater intrusion occurs commonly along the western and southern coasts of Korea. Almost coastal area consists of a reclaimed land, and is affected by seawater intrusion through the fractured rocks connected the seaside within several kilometers of coasts. A combination of drilling, conventional geophysical well logging including caliper log, natural gamma log, fluid temperature/conductivity log etc., acoustic televiewer, flowmeter, hydrophysical logging, packer test, and freshwater injection test was performed to evaluate seawater intrusion through the fractured rock in Baeksu-eup, Youngkwang-gun, Korea. The geological structure of the survey area comprises mud, sand, and granite and andesite bedrock (below an approximate depth of 22 m). The test boreholes are located with the brackish area interpreted with surface geophysical survey and hydrogeochemcial survey. The depth of two test boreholes is 50m, and the diameter is 3 inch, the distance between boreholes is 10m. Although the core log showed the several fractures, we didn't identify the minor fractures using 3-arm caliper logs because of small aperture size of fractures. The electrical conductivity of the borehole fluid is seen to be more than 1000 μS/cm at depth of about 35 m, and the highest conductivity is about 5000 μS/cm. Several intervals shown the change of conductivity logs doesn't relate with fractures identified by 3-arm caliper logs. In order to verify the permeable fractures, heat-pulse flowmeter test was conducted within single hole and interpreted with Paillet inversion method. Five permeable fractures are detected and hydraulic properties are estimated. These results are compared with hydrophysical logging performed one borehole. After the replacement of borehole fluid with freshwater, the change of fluid conductivity shows at least seven fractures with different salinity. Main fractures with highest salinity detected acoustic televiewer show low dip angles. To define subsurface connection between discrete fractures cross-borehole flowmeter test performed. To define subsurface connection between discrete fractures cross-borehole flowmeter test performed. Open flow path between two boreholes is detected at the depth of about 47 meters and this result agrees with freshwater injection test using double packer system. Open fracture at the depth of about 47 m didn't find with 3-arm caliper log, and all permeable fractures but only one below casing bottom are identified using acoustic televiewer log having almost low dip angles. In small-scale survey area the boundaries between andesite and granite intruded into andesite show impermeable fractures. Above results indicate that there are many flow paths with different salinity and depth through fractured rocks connected with seaside.
NS31B-0378
A Laboratory Study of NMR Relaxation Times and Pore Coupling in Heterogeneous Porous Media
Nuclear magnetic resonance (NMR) relaxation measurements are utilized in geophysical applications to estimate internal pore geometry, but these estimates often fail in highly heterogeneous materials. Traditional interpretation of NMR relaxation data assumes a model of isolated pores in which each proton samples only one pore type, and the distribution of relaxation times is directly scaled to estimate a pore-size distribution. This model breaks down, however, for systems with strongly-coupled heterogeneous pores in which protons readily diffuse between multiple pores prior to relaxing. In this latter case, the meaning of the relaxation time distribution is not well understood. We have explored the link between NMR relaxation times and pore geometry in heterogeneous porous media though a series of laboratory experiments in which we have varied the degree of pore coupling. For these experiments, we analyzed water-saturated silica gels with a well-defined pore structure comprised of submicron intragranular pores (150 Å) and much larger intergranular pores (~30 μm). NMR measurements for pure samples exhibit strong pore coupling, and relaxation time distributions show a single broad peak resulting from diffusional averaging of the two pore types. When the surface relaxivity of the gels was increased by adsorbing paramagnetic Fe(III) to the pore surfaces, we found that pore coupling was reduced, yielding relaxation time distributions with two distinct peaks corresponding to the true pore size distribution. We attribute this decrease in pore coupling to accelerated surface relaxation and a subsequent reduction in the distance protons can diffuse before relaxing. One-dimensional analytical models supplement interpretation of our laboratory experiments and provide insight into the processes driving relaxation in each system. This study allows us to identify the conditions under which pore coupling affects the NMR measurement and provides a basis for further experiments to improve the interpretation of relaxation times in heterogeneous geologic materials.
NS31B-0379
A Novel Design for High-Speed Imaging in Laboratory Scale Electrical Resistivity Imaging
Electrical resistivity imaging (ERI) is seeing increased use for the study of near-surface processes. The quality of an image obtained with this method, however, is often degraded as a result of the spatial variability of the measurement resolution. Improving the accuracy of images obtained through ERI requires a more complete understanding of the information contained in measurements, especially with respect to the resolution and sampled volume. To this end we have developed a high-speed resistivity imaging system to study dynamic hydrologic processes at the laboratory scale. Drawing on technologies developed independently for medical and geophysical applications, we have created a system that incorporates the high sampling rates of medical devices with the flexible spatial and temporal sampling configuration which can be achieved with current geophysical instruments. The 48-electrode system can be programmed by the user to collect voltage data using a number of 3- or 4-electrode arrays, switching between arrays at frequencies on the order of kilohertz. A number of voltage measurements may be collected using many different electrode spacings in seconds, rather than minutes as required by instruments currently used for geophysical applications. Consequently, the system is able to gather a suite of measurements in a sufficiently short time so that imaged processes may be assumed to be static during data collection. This simplifies inversion of the data and minimizes temporal smearing. The noise of the data acquisition system is characterized using a Hele-Shaw cell filled with a saline solution to create a two- dimensional medium with homogeneous resistivity structure. A number of salinities and switching rates are tested to identify any resistivity-dependence and frequency-dependence in the measurement noise. With comprehensive characterization of the noise distribution, the system can be used to image hydrologic processes accurately at the laboratory scale, using a high sampling rate to avoid temporal smearing and realistic electrode arrays to quantify the resolution and support volume of resistivity measurements.
NS31B-0380
Application of resistivity image profiling method and laser scanning to monitor landside
Taiwan, located at the Western Pacific, bears the threats from both typhoons and earthquakes. The 921 earthquake resulted in the unstable ground so that typhoons or torrential rain could cause serious disasters. Currently, the hazard mitigation is the target of the authorities. However, using the traditional approaches not only wastes a lot of time but also is ineffectively. The deformation of slope terrain is detected by physical measured instrument traditionally. Collecting deformation data of tiltmeter and piezometer inner well drilling. A few of wells are limited by terrain and cost. The wells can not cover all landslide area. The results of analysis data are debated. The collapse slope once has more probabilistic collapse. Particularly designing engineering against natural calamities with debated conclusion and the rebuilt slope will be instable. This paper demonstrates the possibility of using resistivity image profiling method (RIP) and 3d laser scanning to monitor a landside at No.3 freeway of 85 km in Taiwan. The slope was rebuilt by grid-beam engineering, spraying concrete engineering and cut-slope engineering. The first, exploring underground substance by resistivity image profiling method . To obtain the destructive sliding position of slope, the range of slope with the same sliding position and groundwater data. Time-lapse RIP sections for different time period also indicate that the obvious resistivity change zones were limited under a strongest topography change surface.Fitting 3d laser scanner on suitable site for scanning research target. To analyze overlay scanning data by correcting the cloud-points with the different periods. The study utilizes the 3d scanning technology and geoelectric resistivity technology to obtain analysis information of slope probability and terrain deformation trend. The way of detecting deformation of rebuilt slope that combines RIP and 3D laser scanner is overall technology. The RIP detects the sensitive area of underground and the 3D laser scanner detects terrain surface of slope. Especial collecting the data by period observation. The data can provide the trend of rebuilt slope creeping and the policy decision of remaking engineering. Keyword¡Gresistivity image profiling ,3d laser scanning,landslide
NS31B-0381
Geophysical Investigation of the Arbuckle-Simpson Aquifer, Oklahoma, to Determine the Influence of Subsurface Structure on Groundwater Flow
We investigate an area of the Arbuckle-Simpson aquifer in southern Oklahoma by employing near-surface geophysical surveying. This predominantly carbonate aquifer encompasses the Simpson, Arbuckle, and Timbered Hills Groups, which range in age from Upper Cambrian to Middle Ordovician. The aquifer serves as a principle water source for the surrounding area and feeds several major springs and creeks, including Pennington Creek, around which a number of rare species dwell. Due to the high amount of fracturing and faulting present in the aquifer system, the possible effects of large-scale groundwater withdrawal are poorly understood and have spurred increased interest in a comprehensive investigation of the aquifer. We seek to understand the subsurface structure of a small area of the aquifer, focused around Pilot Springs, by identifying and interpreting faults and their interaction with the groundwater hydrology. To this end, we have carried out electrical resistivity soundings using Wenner and Schlumberger array spreads. Analysis of these data is being performed to identify faults and estimate depths to the water table. Modeling of horizontal layers is carried out by means of the IPI2win Resistivity Sounding Interpretation algorithm from Moscow State University. This modeling approach allows us to obtain for each sounding point an optimized subsurface model that specifies the number of layers present and the apparent resistivity and depths to each layer. We are able to refine this model based on our prior understanding of the geology of the region. At least one sounding appears to reveal the presence of one of the major north bounding faults in the area. Further surveying is planned, including additional resistivity soundings as well as resistivity profiling and ground-penetrating radar.
NS31B-0382
Imaging Nuclear Waste Plumes at the Hanford Site using Large Domain 3D High Resolution Resistivity Methods and the New Parallel-Processing EarthImager3DCL Program
High Resolution Resistivity data is currently used by hydroGEOPHYSICS, Inc to detect and characterize the distribution of suspected contaminant plumes beneath leaking tanks and disposal sites within the U.S. Department of Energy Hanford Site, in Eastern Washington State. The success of the characterization effort has led to resistivity data acquisition in extremely large survey areas exceeding 0.6 km2 and containing over 6,000 electrodes. Optimal data processing results are achieved by utilizing 105 data points within a single finite difference or finite element model domain. The large number of measurements and electrodes and high resolution of the modeling domain requires a model mesh of over 106 nodes. Existing commercially available resistivity inversion software could not support the domain size due to software and hardware limitations. hydroGEOPHYSICS, Inc teamed with Advanced Geosciences, Inc to advance the existing EarthImager3D inversion software to allow for parallel-processing and large memory support under a 64 bit operating system. The basis for the selection of EarthImager3D is demonstrated with a series of verification tests and benchmark comparisons using synthetic test models, field scale experiments and 6 months of intensive modeling using an array of multi-processor servers. The results of benchmark testing show equivalence to other industry standard inversion codes that perform the same function on significantly smaller domain models. hydroGEOPHYSICS, Inc included the use of 214 steel-cased monitoring wells as "long electrodes", 6000 surface electrodes and 8 buried point source electrodes. Advanced Geosciences, Inc. implemented a long electrode modeling function to support the Hanford Site well casing data. This utility is unique to commercial resistivity inversion software, and was evaluated through a series of laboratory and field scale tests using engineered subsurface plumes. The Hanford site is an ideal proving ground for these methods due to the large contrast in resistivity between contaminant plume (10 to 100 Ohm-m) and background soil (1000 to 1500 Ohm-m). The resulting improvements in hardware and inversion software radically increased the size of imaging domain and decreased processing time while allowing for a higher resolution three-dimensional visualization of possible subsurface contaminant plumes. http://www.hgiworld.com
NS31B-0383
Electrical Resistivity Imaging of Subterranean Void Space for Assessment of Endangered Species Habitat
The challenge of identifying and delineating subterranean habitat for endangered species in karst environments has been addressed through the application of near-surface geophysical techniques. Electrical resistivity imaging (ERI) in both galvanic DC and capacitance-coupled modes has been applied to the problem of imaging subsurface voids, potentially conducive to karst invertebrate habitat, in two distinctly different geologic, geophysical, and environmental settings. Surveys were conducted in extrusive volcanic terrain on the south shore of Kauai, Hawaii, a site known for lava tube formation, and in limestone karst terrain in central Texas. The two study sites were distinctly different in their geophysical settings in terms of surface layer and subsurface background resistivities, values at the Kauai site ranging from 1000 - 5000 ohm-meters and at the Texas site 100 - 800 ohm-meters, values reflecting differing lithology, porosity, and pore fluid content. An Advanced Geosciences Inc. (AGI) Supersting R8 DC resistivity system was the primary instrumentation utilized for both surveys, with a capacitance-coupled Geometrics Inc. OhmMapper TR-2 system utilized on the Kauai site for reconnaissance profiles. Opportunities existed for direct comparisons of Supersting and OhmMapper pseudo- section profiles. Supersting lines were acquired with a mixed array combining the horizontal resolution sensitivity of the dipole-dipole array with the vertical resolution sensitivity of the Inverse Schlumberger array. At both sites, surveys were conducted over known and mapped cave passage for validation of the techniques. Forward simulation modeling was conducted to verify resistivity anomaly signatures of known void spaces. Results were highly encouraging and serve to reinforce the karst-imaging capabilities of electrical resistivity, especially when mixed array types are utilized.
NS31B-0384
Diffusion of electromagnetic eddy currents in unconsolidated alluvium
Recent studies on the diffusive transport of electromagnetic eddy currents in complex geologic materials have invited speculation on novel theoretical frameworks to encapsulate the macroscopic effects of multi-scale geologic complexity. These ideas have been based on a limited amount of data from a restricted suite of geologic environments. To test the veracity of the various diffusion theories proposed in the literature, a new data set is presented here which consists of transient electromagnetic data collected over an alluvial fill aquifer in the Estancia Basin of central New Mexico. Geologic mapping and correlation of historical well-log data show the experiment site to consist of mainly unconsolidated felsic alluvium with intermittent cobble horizons. Analysis of the electromagnetic data in terms of its diagnostic move-out behavior of the source wavelet shows some similarity to previously obtained results in floodplain environments. Further analysis of the data shows that this behavior is dominated by the alluvial texture and not influenced by the underlying basement rock.
NS31B-0385
Subsurface Imaging Based on Frequency Dispersion
It has been known that different rocks has different responses (absorption, reflection and so forth) on different frequency electromagnetic (EM) waves and, sometimes, seismic waves. Change of the wave field dependent on frequencies is the called frequency dispersion. Seismic wave and EM wave used in geophysical exploration is ordinarily excited by pulse source, and so they are broadband wave. The measured response in the geophysical survey contains the contribution of the frequency dispersion. If we can determine quantificationally the dispersion relation, we can image the subsurface rock by inversion of the dispersion field. We can also obtain dispersion images by calculation the difference between the images obtained from different single frequency data. Imaging based on frequency dispersion let us can ¡°see" the subsurface earth medium from a new angle of view. The frequency dispersion of the wave can be divided into two kinds. One is that the dispersion is dependent only on the frequency of the wave, and another is that the dispersion is dependent on both the frequency of the wave and the medium physical parameters. In the first case, the physical parameters of the medium are independent of the frequency of the wave. In the second case, the physical parameters of the medium are dependent on the frequency of the wave, in other words, the physical parameters of the medium are function of the frequency of the wave. In the second case, we can obtain the frequency-dependent information of the rocks. In most cases, it is of logo quality to the rocks. We have developed two subsurface imaging techniques based on frequency dispersion. One is for broadband electromagnetic wave field inversion, and another is for seismic wave field imaging. The application cases show that both can give us new information of the subsurface medium. The dispersion imaging for broadband electromagnetic wave is a tomographic inversion of the frequency dispersion field, which is defined as the difference of the field values measured at two different frequencies. In the deduction of the tomographic inversion equations, a supposition that the electromagnetic parameters are independent of the frequency is introduced. The dispersion imaging for seismic wave field is implemented by calculation the difference of two single frequency images, which are obtained by time-frequency analysis. The work was supported by NSFC under grant No. 40574032.
NS31B-0386
Geothermal Reservoir Boundary Characterization Using Detailed Shallow Geophysical Methods at the Redfield Campus in Steamboat Hills, NV.
The University of Nevada, Reno's new Redfield campus in south Reno, Nevada directly overlies the Northwestern edge of the Steamboat Hills geothermal reservoir. Ormat Technologies currently generates over 90 MW of electric power from this significant geothermal complex. Although the geothermal field has been the subject of numerous geophysical studies (gravity, 3-D seismic, airborne magnetics and electromagnetics) over the past 70 years, the work has typically been conducted at large station spacing's, providing a only low resolution picture of the reservoir's bounding structures. Largely spaced drill hole isotherm data and InSAR observations of subsidence from reservoir cooling provide strong evidence for a sharp N60E, linear control on the Northwestern reservoir boundary, which projects through the campus. The proximity of the underlying geothermal resource and the prospect of future geothermal space heating at the campus motivated a program of integrated shallow geophysical surveys to investigate the subsurface and characterize the reservoir boundary under the campus. Completed surveys include closely spaced gravity measurements, ground magnetics, and electric resistivity. The reduced geophysical data have been integrated with surface geology and surface heat flow in a geographic information system. Interpretive results to date include a gravity defined fault running approximately N60E across the campus with ~50 meters of basement offset with the down block to the northwest, this fault correlates with a zone of low magnetic susceptibility, which is consistent with hydrothermal alteration. Resistivity data shows conductivities indicative of clay or hydrothermal fluids more than 80 meters thick extending within 10 to 20 meters of the surface, although no clear resistivity offset was apparent across the gravity and magnetically defined structure. A seismic reflection line is also being collected across the possible fault to constrain its spatial attributes. Although this group of shallow surveys examine only the Redfield campus, an area of approximately 500 by 430 m, covering a small fraction of the larger Steamboat Hills Geothermal field (approximately 3.2 by 2.8 km) the insights grained are proving useful in understanding the nature of the northwestern boundary and other parts of the geothermal field.
NS31B-0387
An Interactive GIS Procedure for Building and Basement Corrections in Urban Microgravity Surveys
Construction of a new underground railway in Lausanne, a highly-urbanized city in Switzerland, was an opportunity to test the feasibility and reliability of microgravity surveys in urban environments. The goal of our microgravity survey was to determine the depth-to-bedrock along the project corridor. Available drilling information allowed us verify the density model obtained. The geophysical results also provided spatially exhaustive subsurface information that could not be obtained with drilling methods alone. Gravimetry is one of the rare geophysical methods that can be used in noisy urban environments. An inevitable constraint of this method is terrain correction. It is not easy to obtain a simple and accurate digital elevation model (DEM) of an urban environment considering that buildings and basements are not included. However, these structures significantly influence gravity measurements. We calculate, with software that we have developed, the influence of buildings and basements in order to correct our gravity data. Our procedure permits the integration of gravity measurements, cadastral information (building typology and geometry) and basement geometry in an Access database that allows interactive determination of the Bouguer anomaly. A geographic information system (GIS) is used to extract building geometries based on cadastral information and to correct the influence of each building using a simplified architectural style. Basement voids are then introduced in the final DEM using building outlines given by cadastral maps. The depths and altitudes of the basements are measured by visiting them, and then linking the results to a regional topographic map. All of these corrections can be calculated before the gravity acquisition has begun in order to optimize the design of the survey. The surveys are executed late at night so as to minimize the effects of traffic noise. 160 gravity measurements were carried out before and after digging of the underground tunnel. The difference between gravimetric values of both surveys permitted validation of our modelling code.
NS31B-0388
Ship Rock Diatreme: is it a Classical Volcano? New Evidence on Magma Ascent and Emplacement Within the Navajo Volcanic Field
The Navajo Volcanic Field (NVF) is an area of late-Tertiary volcanism along the New Mexico-Arizona border near the Four Corners region of the American Southwest. Among the roughly 80 exhumed diatremes that comprise the NVF, Ship Rock and The Thumb are two diatremes that present an interesting problem concerning magma ascent and emplacement within the NVF. Are the diatremes remnants of classical volcanoes with underlying magma chambers, or are the diatremes formed from buds off of upward propagating dike swarms? The 2006 Keck Consortium Geophysics Project collected non-invasive gravity and magnetic data to image the subsurface of Ship Rock and The Thumb to suggest constraints concerning the formation of these diatremes within the Navajo Volcanic Field. At Ship Rock, we collected over 120 gravity points spaced 500 m apart along 10 lines. We also collected about 65,000 magnetic points that cover an area of 1,570,000 square meters surrounding Ship Rock. The gravity data reveal gravity lows several kilometers away from Ship Rock, probably as a result of thick sedimentary units close to the surface. A steep gradient of 5 mGal/km separates the gravity lows from a strong gravity high immediately to the southwest of Ship Rock. We interpret this gravity high to be uneven basement topography or a magma chamber at depth; further studies are required to determine which of the interpretations is more likely. The Ship Rock magnetic data show the prominent west and northeast dikes extend well beyond their surface outcrops while the southern dike extends only to its visible termination. The magnetic data we collected at The Thumb along ~18 km of lines reveal a linear northeast-southwest trending magnetic anomaly about 105 to 360 nT in amplitude that crosses the diatreme. We interpret the anomaly to be a dike beneath The Thumb. Models of the total field magnetic data suggest a dike at shallow depths of about 0.1 to 4.8 m and widths of about 0.25 to 1.5 m with a steep dip to the northwest. The analytic signal of the dike also reveals a steep dip to the northwest, or possibly en echelon behavior, which is consistent with the en echelon segmentation of the northeast dike of Ship Rock. The presence of a dike at The Thumb suggests that the diatremes in the NVF may have been formed by upward propagating dike swarms. The subsurface geology determined by our geophysical modeling can be compared to other diatremes in the NVF, as well as other maar- diatreme fields to better comprehend diatreme formation.
NS31B-0389
Ground-penetrating radar survey use for documentation of a historic cemetery in Sand Brook Township, NJ
Much of the early history of the United States can be traced through old cemetery sites scattered throughout the Northeast. Today, many historic burial sites are in less-than-ideal condition, as upkeep of old grave locations is neglected and written records of the details of cemeteries are lost through time. In addition, tombstones can be rendered invisible at the surface through erosion and soil movement processes, both natural and anthropogenic in origin. We document the ground-penetrating radar (GPR) survey of a graveyard dating back to the colonial era, and still in use today. A control survey of locations with known burials shows that disruption of strata at grave sites is clearly visible using 200 and 400 MHz antennas; another control survey in an area where graves are not expected demonstrates the undisturbed stratigraphy of the slump deposit comprising the hill where the graveyard is located. Subsequent survey of areas within and near the cemetery without marked graves has allowed us to document the positions of individual burials and, in at least one case, even rediscover an original tombstone beneath ~5 cm of modern soil. This study demonstrates that ground-penetrating radar serves as an effective non- invasive method for documentation and restoration of historical-era burials.
NS31B-0390
Seismic refraction and GPR measurements of depth to bedrock: A case study from Randolph College, Virginia
Randolph College is located in Lynchburg, VA, in the eastern edge of the Blue Ridge Mountains. Lynchburg city lies in the James River Synclinorium and consists of metasedimentary and metaigneous rocks. As part of College's plan to expand, a new soccer field will be build. For that purpose, part of a hill has to be excavated. Information was needed on the depth to the bedrock at the site. We conducted a seismic refraction experiment as part of an eight week summer research program for undergraduate students. We used 24 vertical geophones, spaced at 1.5 m interval. Our recording device was a 12 channel Geometrics geode (ES 3000). The source was an 8 pound sledge hummer. Source positions were chosen to be at 5, 10, 15 and 20 m on both sides of the array. We collected data along a tree line (in two segments) and across a hockey field. The data collected from the hockey field had very low signal to noise ratio and clear refraction arrivals. The other two acquisition lines were much noisier and difficult to interpret. Our results are consistent with data from seven bore holes in close proximity to the field site. We interpreted depth to bedrock to be between 4 and 12 m. The bedrock velocities are consistent with weathered gneiss. To improve the interpretation of the tree line records, we conducted a GPR survey. The preliminary radar images are showing highly heterogeneous subsurface with multiple point reflectors.
NS31B-0391
Measurements of ice thicknesses using georadar in alpine caves
Several caves in high elevated alpine regions host massive ice fillings and underground glaciers. The age of the ice may exceed several hundred or thousand years and the ice bodies possibly have recorded paleoclimatic information. Despite their scientific value, the knowledge on ice caves is relatively sparse, and even the genesis and evolution of the ice fillings is not well understood. The project AUSTRO*ICE*CAVES*2100 is a pilot study funded by the Austrian Academy of Sciences and aims at providing basic information on the formation and composition of underground ice bodies. In the presented study, we focus on the results of ground penetrating radar (GPR) which was used to determine the ice thickness in three ice caves in the Northern Calcareous Alps of Austria (Eisriesenwelt, province of Salzburg; Dachstein- Mammuthoehle and Dachstein-Rieseneishoehle, province of Upper Austria). It could be shown that shielded antennas with relatively high frequencies (500 MHz) are sufficient to penetrate the ice up to 15 m depth. 3D layouts (crossing profiles) were necessary to delineate the strongly curved subsurface in detail and to verify that certain reflections in the radargramm sections originate from the subsurface. In almost all radargramm sections, the lower boundary of the ice body is identified by the onset of strong and sharp reflections. We attribute this to either increased humidity at the ice - rock contact (due to melting) or to a sedimentary layer between ice and rock. Pronounced layering of the ice body itself is clearly seen at some locations, which may results from alternating air content. The maximum thickness is 7.5 m in Eisriesenwelt, 6 m in the Dachstein- Mammuthoehle and 15 m in the Dachstein-Rieseneishoehle. The propagation velocity of the ice bodies (0.165 m/s) is close to the velocity of temperate glaciers. To conclude, GPR with shielded high-frequency antennas proves to be a very effective tool to investigate the extent and internal structure of underground glaciers.
NS31B-0392
3-D ADI-FDTD modeling of GPR backscatter from complex targets for the training of artificial neural networks
Artificial neural networks can provide approximate solutions to ground-penetrating radar (GPR) problems in cases where real time performance is needed. Examples include discrimination of landmines or UXO's, and in circumstances that require a high number of successive forward problems, for example inversion or imaging. The training of neural networks to work within even a limited range of targets and electromagnetic properties requires a large set of successive examples generated from numerical methods such as finite difference time domain (FDTD). The traditional FDTD technique suffers from numerical dispersion unless time steps are kept below the Courant stability limit. The accurate modeling of electromagnetic scattering by complex targets require a refined grid, subgrids, or conformal grids that can significantly increase computation time, making neural network training inefficient. A relatively recent FDTD technique, ADI-FDTD, uses implicit equations that help to cancel numerical dispersion and allow for unconditionally stable modeling of EM propagation and therefore is not bound by the Courant stability limit. The technique is especially efficient for the accurate modeling of complex targets. Our ADI-FDTD code includes the ability to refine the model grid and to implement a conformal gridding to improve model accuracy without effecting the overall computation time. We will explore the tradeoff in computation time and accuracy in modeling the GPR backscatter of various targets using both the ADI-FDTD technique and the traditional FDTD technique for the purpose of neural network training.
NS31B-0393
Determining the Empirical Relationship Between Surface Ground Penetrating Radar (GPR) Reflection Amplitudes and Sub-wavelength "Thin-layer" Fracture Aperture Under No-flow Conditions
Current methods of collecting data for modeling groundwater flow in fractured media (e.g., fractured bedrock) involve expensive and invasive procedures that typically yield poorly-constrained results due to highly spatially variable fracture apertures and the resulting channelization. Surface ground penetrating radar (GPR) surveys present an attractive alternative because the full two-dimensional distribution of fracture aperture may be determined. Typical fractures have sub-wavelength apertures (i.e., are considered "thin layers") and the fluid flow through the fractures is governed by the cubic law; therefore, precise aperture estimates are critical. In practice, researchers have observed a qualitative change in reflection amplitudes and presumed this change correlates with fracture aperture at the sub-wavelength scale. Several researchers have attempted to determine the relationship theoretically. One method involves a simplification of the thin-layer problem by reducing the scope of the investigation to the first interface of the layer. This simplification reduces the thin-layer to a boundary, thereby effectively removing the noise generated by the constructive or destructive interference generated by the second interface. The resulting theoretical relationships describe the relationship between the reflected amplitude and thickness of the layer (e.g., fracture aperture). We have created a physical model consisting of 2 large ultra-high molecular weight polyethylene (UHMW-PE) blocks that have electromagnetic properties of real earth materials, separated by thin (~0.1 mm) inserts to create a range of aperture sizes. To change the aperture, we started by adding 1 insert for the first 51 surveys (0-5 mm) followed by 2 inserts for the next 25 surveys (5.2-10 mm), followed by 5 inserts for the next 20 surveys (10.5-20 mm) and lastly 1 additional survey taken at 300 inserts (30 mm). At each aperture increment, a GPR survey using 1000 MHz antennae was run at the center of the block. By directly obtaining the reflected amplitudes in an idealized fracture media, we then tested the various theoretical equations for modeling fracture aperture. Our preliminary results are not in agreement with the model and warrant a new empirical equation.
NS31B-0394
Investigating the acoustic properties of gassy marine sediments: Results of Laboratory experiments
The acoustic properties of marine sediment containing free gas bubbles are considerably more complex, and therefore more poorly understood, than the acoustic properties of saturated sediment. A more detailed understanding of gassy sediments would therefore be advantageous to a wide range of marine users, ranging from marine surveyors to those responsible for homelands defence. To provide a simplified system through which the acoustic properties of gassy sediments can be investigated, laboratory experiments have been performed. Samples of both gassy mud and a synthetic bubbly gel mimic were examined. Results for the compressional wave velocity and attenuation coefficient are presented from 30 to 110 kHz. These are compared to the predictions of linear theoretical models for both sediment and water based bubble distributions. In addition, the results of combination frequency techniques are presented. These exploit the non linear interaction between the acoustic field the bubbles, therefore allowing bubble size distributions to be inferred though the analysis of the sum and difference frequency scattered fields. All of the results are compared to bubble size distributions measured independently using a X-Ray CT scanner, which is able to resolve bubbles with radii greater than 20 micrometers.
NS31B-0395
The applicability and limitation of aperture size estimation from acoustic televiewer using 3 inches physical borehole model
Acoustic televiewer (ATV) has been used to detect the fractures properties which have been intersected by the drilling, and it gives very useful information for the characterization of the fractures such as azimuth, dip, opening, apparent aperture, and so on. Acoustic televiewer, however, tends to interpret fractures larger than real aperture size as well known already. For identifying the applicability and limitation of acoustic televiewer for the characterization of the fractures, 3 inches physical aluminum borehole model has been specially manufactured. The total length of model is 24 centimeters: the intervals of fractures at the dip angle of 0° and 90° extend to 10 centimeters respectively, and the remainder consists of fractures with the different orientation at the dip angle of 30° and 60°. The aperture size is 1, 2, 3, and 5 millimeters, and the depth is the same as aperture size. ATV sonde with main frequency of 1.5 MHz has been used for the data acquisition. And the logging velocity is from 0.1 to 0.9 m/min, the sampling rate per revolution is 360, and number of revolution per second is 20. In case of zero dip angle: There is no vertical resolution in spite of very small logging velocity of 0.1 m/min. The amplitude and travel time data represent 6 millimeters or more regardless the aperture sizes. The vertical resolution in the case of the logging velocity of 0.9 m/min seems to be good compared with 0.1 m/min. But, the vertical resolution for quantitatively estimating the aperture size of fracture with the dip angle of zero is thoroughly very low considering the principle of the measurement. In case of vertical dip angle: The travel time data tends to make it larger 20 percents than the aperture size of model under the velocity of 0.2 m/min and 360 scan, and the amplitude data enlarges over 50 - 60 percents. In case of dip angle 30°: Though the aperture size has been detected larger than model under 3 - 5 millimeters, the estimation of aperture size is possible. But the case of 1 and 2 millimeters is not. In case of dip angle 60°: Though the aperture size has been detected larger than model under 2, 3, 5 millimeters, the estimation of aperture size is possible. This result which the resolution for estimating fracture aperture size is low when dip angle is low more, is very good correlation with the results of ATV application for the 3 inches boreholes located in seawater intrusion zone of Yeonggwang- gun, Jeonnam, South Korea.
NS31B-0396
Spatial Variability of Seafloor and Shallow Sub-surface Acoustic Properties, Shallow Water Gulf of Mexico
In June 2007 we carried out the first of two ONR field programs aboard the R/V ‘Pelican' designed to evaluate variations in seafloor and shallow sub-surface acoustic properties at three sites in the northern Gulf of Mexico. The follow-up field program is scheduled for January, 2008, and will use the same boat and same systems to survey the same sites to determine whether there are any seasonal variations in the datasets as well spatial variability between the sites. Data acquisition includes multibeam bathymetry, multibeam backscatter, side scan sonar, and sub-bottom profiling. In addition to temporal (seasonal) and spatial variations, we are examining the differences in acoustic signature of the seafloor at different frequencies. The three areas of interest in this two-year program are: (1) off the coast of Louisiana, west of Atchafalaya Bay, at the site of the ONR multi-year research program ‘Mechanism of Fluid-Mud Interactions Under Waves' (MURI), (2) off Panama city, Florida in an area of known sub-surface features and (3) off Louisiana's Southwest Pass in an area of where mudflows triggered by Hurricanes Ivan and Katrina destroyed or damaged platforms and pipelines (West Delta). Water depths in the areas of interest range from 3.5 to 96 m. Preliminary analysis of the data indicates spatial differences between and within the sites. Within individual sites there are differences in the data as a function of the frequencies used to map the seafloor. Off Southwest Pass we find differences in mudflow character associated with a known dredge site suggesting a possible anthropogenic component to mudflow sourcing and level of activity.
NS31B-0397
Overview of a High Resolution VSP Survey in the International Continental Drilling Program Outokumpu Borehole, Finland: Progress Towards an Anisotropic Velocity Model
A high resolution borehole seismic survey was conducted in the ICDP Outokumpu, Finland scientific borehole to 1) assist in the interpretation of some earlier seismic profiles and 2) provide traveltime information that can be used to construct an anisotropic velocity model for use in future induced micro-seismic studies. The uppermost 1.3 km consists of a nearly flat-lying biotite schist expected to be highly intrinsically anisotropic; and the surveys further provide opportunity to study ansistropy from core to geological formation scales. The survey consisted of several parts, including a series of multi-azimuth multi-depth walk-away VSPs, a zero-offset VSP, and a surface seismic component. The zero-offset VSP was acquired at 2 m depth increments from 2500 m to 50 m, while the walk away VSPs were acquired with the receiver at the static depths of 1000 m, 1750 m and 2500 m. At the surface, the mica schist is overlain by a thin layer of glacial till. This stratification was confirmed by our VSP surveys. The effects of near surface heterogeneity were reduced using static corrections calculated from a velocity model developed using refraction tomography. The walk away seismic traces were then passed through a polarization filter and rotated into the direction of the P- and S-wave arrivals to increase the signal to noise ratio. Finally, a plane-wave decomposition technique was applied in order to determine the seismic velocity as a function of angle of incidence. Preliminary results from the 1000 m walk away VSP indicate that the P-wave velocity increases from 5250 m/s for vertical propagation to ~5620 m/s at 60° from vertical. Further work includes extending the anisotropy measurements to the 1750 m and 2500 m walk away, comparing the results to sonic logs, televiewer image logs, and laboratory anisotropy measurements on candidate core samples.
NS31B-0398
Offset Dependence of Effective Q (Scattering and Intrinsic) Attenuation: Measurements from Permanent Down-hole Geophones
Attenuation (Q) profiles were determined from a VSP study of a well in the Lawrence Livermore National Lab facility using permanent down-hole geophones and a vertical elastic generator source. We used the zero-offset data to determine the interval velocity profile, and combined gamma ray, and other log information to model the density and P- and S- wave velocity profile. From these profiles we generated synthetic offset data using a 2- dimensional elastic finite difference model assuming laterally homogeneous layers and compared the attenuation measured from the synthetic data with the attenuation measured from the real data. The attenuation of the zero-offset synthetic data is slightly larger than the attenuation predicted by the O"Doherty and Anstey formula for 1-D scattering, whereas the attenuation of the zero-offset real data is substantially greater than the attenuation of the synthetic data, and the amplitudes predicted by the ODA approach. We modeled the angle-dependent scattering Q with an empirical function that captures the behavior of the offset synthetic data, and adjusted the amplitudes of the real data to examine whether the intrinsic Q is laterally homogeneous. We then generated additional viscoelastic models for a Voigt and Standard Linear Solid Q to match the amplitude and phase components in the real data. We computed horizontal space average Q factors at different depth intervals and concluded that the primary source of attenuation in our location is intrinsic.
NS31B-0399
The use of High Resolution Multichannel Seismic Data and Geotechnical Records to Investigate Near Surface Seabed Structures in the North Sea
Due to highly variable seabed characteristics within very small areas in the North Sea, the near surface investigation becomes a dominating aspect in order to ensure an economic design of offshore structures, e.g. for the foundation design of windmills. The aim of a research project at the Research Center Ocean Margins (RCOM) at the University of Bremen, Germany, is the geophysical seabed characterization and its correlation with geotechnical data. In the course of the project a new shallow-water multi-channel seismic system was developed to collect high-resolution multichannel seismic data and a new dynamic triaxial testing unit was designed to simulate the effects of cyclic loads on the sedimentary strata. During two cruises in 2005 and 2007, which were carried out in a cooperation between the RCOM and the BGR (Hannover, Germany) as well as the Senkenberg Institute (Wilhelmshaven, Germany), respectively, two designated wind farm areas in the shallow North Sea were visited to collect high resolution multichannel seismic data, very high resolution acoustic data as well as swath sounder bathymetry data. Additionally, cores for geotechnical measurements were drilled with a vibrocore system. As seismic source, a Mini GI Gun of 0.2 L primary chamber volume was used, the seismic signals were recorded with a 50 meter long streamer of 48- channels, which are realised by single hydrophones evenly distributed at a spacing of 1 m. In both study areas the seismic data reveal a complex and heterogonous subsurface. The main structural features are filled channels of different dimensions, depths, and filling characteristics. These channels are often cut into each other. The reliefs of the channels vary between just a few meters and up to 50 m, their widths reach values between 200 m and 2000 m. In one of the study areas at least two stages of channel development could be identified. Ongoing work on the seismic data and cores should establish in the future a correlation between the seismic stratigraphy and the geotechnical parameters.
NS31B-0400
Use of Seismic Reflection Data and Traveltime Tomography to Image the Near Surface Velocity Structure in the Mississippi Embayment
The Memphis aquifer represents one of the highest quality reservoirs of drinking water in the nation and it is separated from the shallow unconfined aquifer by the Upper Claiborne clay. Recent studies show that the confining unit might be discontinuous over the greater Memphis area exposing the Memphis aquifer to potential contamination. We present the results of a seismic reflection profile collected near Memphis, TN with the goal of imaging the structures and potential breaches in the Upper Claiborne confining clay. The imaged area is characterized by a highly heterogeneous shallow velocity structure and low P wave velocities in the ultrashallow unconsolidated materials. The data were collected using a shotgun source and a 1 m source spacing, 0.25 m receiver spacing and a 168-geophone spread for a max offset of 42 m. Raw seismic data show several reflected arrivals in the first 200ms, widespread ground roll, and air wave energy as well as consistent refracted phases across the 1 km - long profile. In addition to the reflection profile we present the preliminary results of first arrival travel time tomography performed along the profile to constrain the velocity field in the shallow portion of the profile. The velocity was then used to remove the effect of the near surface velocity variations. The main data processing steps included elevation statics and frequency and FK filtering. First arrival travel time modeling started with an initial estimate of the 2-layer velocity model using the slope/intercept method. We then modeled first-arrival picks on 1095 shot gathers using the Geo TOMO+ package. The algorithm computes travel times by tracing turning rays and is also able to handle raypaths through low-velocity zones (blind zones). The final resolution is estimated through a ray-information density map, which shows the cumulative contribution of the ray segments traversing different areas of the model. Synthetic models were generated and tested for the tomography inversion, which indicates that the inversion is reliable.
NS31B-0401
Seismic prediction ahead of tunnel constructions
To increase safety and efficiency of tunnel constructions, online seismic exploration ahead of a tunnel can become a valuable tool. Within the \it OnSite project founded by the BMBF (German Ministry of Education and Research) within \it GeoTechnologien a new forward looking seismic imaging technique is developed to e.g. determine weak and water bearing zones ahead of the constructions. Our approach is based on the excitation and registration of \it tunnel surface waves\rm. These waves are excited at the tunnel face behind the cutter head of a tunnel boring machine and travel into drilling direction. Arriving at the front face they generate body waves (mainly S-waves) propagating further ahead. Reflected S-waves are back- converted into tunnel surface waves. For a theoretical description of the conversion process and for finding optimal acquisition geometries it is of importance to study the propagation characteristics of tunnel surface waves. 3D seismic finite difference modeling and analytic solutions of the wave equation in cylindric coordinates revealed that at higher frequencies, i.e. if the tunnel diameter is significantly larger than the wavelength of S-waves, these surface waves can be regarded as Rayleigh-waves circulating the tunnel. For smaller frequencies, i.e. when the S-wavelength approaches the tunnel diameter, the propagation characteristics of these surface waves are then similar to S- waves. Field measurements performed by the GeoForschungsZentrum Potsdam, Germany at the Gotthard Base Tunnel (Switzerland) show both effects, i.e. the propagation of Rayleigh- and body-wave like waves along the tunnel. To enhance our understanding of the excitation and propagation characteristics of tunnel surface waves the transition of Rayleigh to tube-waves waves is investigated both analytically and by numerical simulations.
NS31B-0402
Comparison of Acoustic Full Waveform Tomography in the Time and Frequency Domain
For better parameter estimation, both in active source and earthquakes seismology, we need to exploit the richness of full seismic waveforms. Full waveform tomography (FWT) is a powerful method to reach this goal. Although first implementations in the 1980's were conducted in the time-domain (Tarantola (1984)), the frequency domain version of FWT developed in the 1990s by G. Pratt and co- workers (Pratt (1999)) has now emerged as an efficient imaging tool. The main advantage of the frequency-domain approach is the possibility of starting the inversion at low frequencies (large scale structures) and then moving to higher frequency compounds (smaller scale structures), thereby realizing a multi-scale approach. The main advantage of the time-domain method is the efficient parallelisation by domain decomposition leading to a significant speedup on parallel computers (Bohlen (2002)). In this paper we present a new approach that combines both features (frequency selection and parallel computing) by applying iterative wavelength filtering in the time-domain FWT. We compare the results of the new multi-scale time-domain and frequency domain methods using a cross-hole configuration. {References} BOHLEN, T. 2002 Parallel 3-d viscoelastic finite-difference seismic modelling. \textsl{Computers and Geosciences}, {28}, 887-899. PRATT, R. 1999 Seismic waveform inversion in the frequency domain, part 1: Theory and verification in a physical scale model. \textsl{Geophysics} {64}, 888-901. TARANTOLA, A. 1984 Inversion of seismic reflection data in the acoustic approximation. \textsl{Geophysics} {49}, 1259-1266.
NS31B-0403
Fast Seismic Volume Rendering in a Multi-resolution Framework
Accurate, fast, and convenient visualization of large data sets is a recurring need in seismic data processing and analysis. Modern data viewers should exploit state-of-the-art volume rendering technologies to effectively generate images with short time delays and maintain viewer interactivity. A new data viewer called Vis3D is presented which uses a wavelet-based multi-resolution framework to achieve fast volume rendering. Each data set is converted into a multi-resolution pyramid, and the resolution most appropriate for the available computing resources is chosen. Multi-resolution representation enables large data sets to be visualized on even low-end computers, supports progressive refinement of low-resolution images to higher-resolution images, and accomplishes level-of-detail selection so that only a necessary level of detail is rendered. The data viewer also includes viewing of slices along arbitrary directions, customization of color maps and histogram-based color scaling, and changing of directional lighting. Vis3D can be a convenient tool for geophysicists faced with the challenge of visualizing increasingly larger data sets. http://www.stanford.edu/~dmchen/Vis3D/