NS41A-1109
Application of Geophysical Techniques to Identify and Map the Benthic Habitat and Sub- bottom Sediments of Delaware Bay
The Coastal Program of the Delaware Department of Natural Resources and Environmental Control is engaged in a bottom and sub-bottom imaging project to identify and map the benthic habitat and sub-bottom sediments of the Delaware Bay. The project, now in the third of its five year implementation, integrates the use of three types of acoustical systems: RoxAnn seabed classification, chirp sub-bottom profiling, and multi-beam bathymetric mapping. Verification of the acoustic data with bottom and sub-bottom sediments is performed through the collection of grab and core samples and underwater video images. All this information is being compiled into a GIS database to enable coastal decision makers to effectively manage this estuarine resource. The RoxAnn system measures bottom hardness and roughness along a ship's track, and when correlated with analyses of ground-truth grab samples and video, can be used to classify bottom sediments. Regional maps of sediment type based on the RoxAnn classification are constructed by interpolating between track lines. This project has identified the spatial extent and relative density of oyster shell on the bay bottom, while also determining the composition of regional surrounding sediments. Sub-bottom profiles, collected using an Edgetech X-STAR chirp sonar system, are being used to map potential, and past, offshore sand borrow sites that can be/have been used for beach replenishment and to study the deposition and/or erosion of sediments and the evolution of the Delaware Estuary over the past 10,000 years. In locations along Delaware Bay where beach replenishment is necessary, the chirp data has been integrated with additional information from sediment cores to develop maps of suitable sand deposits based upon location, thickness, overburden, and grain size. The sub-bottom data has been used to map the Holocene/Pre-Holocene boundary and to constrain the paleo-environmental setting of the recent transgressive sea-level sequence in the estuary. The morphology of the bay bottom is being mapped in selected areas using a Reson 8124 multibeam bathymetric system. The system has thus far been used to map selected high-interest areas of the bay including natural and artificial reefs, oyster beds, and critical fish habitat. The downstream reaches of several rivers that drain into the middle portion of estuary have also been mapped to assess the bathymetry and changes in channel morphology.
NS41A-1110
New Hybridized Surface Wave Approach for Geotechnical Modeling of Shear Wave Velocity at Strong Motion Recording Stations
Strong motion recording (SMR) networks often have little or no shear wave velocity measurements at stations where characterization of site amplification and site period effects is needed. Using the active Spectral Analysis of Surface Waves (SASW) method, and passive H/V microtremor method we have investigated nearly two hundred SMR sites in California, Alaska, Japan, Australia, China and Taiwan. We are conducting these studies, in part, to develop a new hybridized method of site characterization that utilizes a parallel array of harmonic-wave sources for active-source SASW, and a single long period seismometer for passive-source microtremor measurement. Surface wave methods excel in their ability to non-invasively and rapidly characterize the variation of ground stiffness properties with depth below the surface. These methods are lightweight, inexpensive to deploy, and time-efficient. They have been shown to produce accurate and deep soil stiffness profiles. By placing and wiring shakers in a large parallel circuit, either side-by-side on the ground or in a trailer-mounted array, a strong in- phase harmonic wave can be produced. The effect of arraying many sources in parallel is to increase the amplitude of waves received at far-away spaced seismometers at low frequencies so as to extend the longest wavelengths of the captured dispersion curve. The USGS system for profiling uses this concept by arraying between two and eight electro-mechanical harmonic-wave shakers. With large parallel arrays of vibrators, a dynamic force in excess of 1000 lb can be produced to vibrate the ground and produce surface waves. We adjust the harmonic wave through a swept-sine procedure to profile surface wave dispersion down to a frequency of 1 Hz and out to surface wave-wavelengths of 200-1000 meters, depending on the site stiffness. The parallel-array SASW procedure is augmented using H/V microtremor data collected with the active source turned off. Passive array microtremor data reveal the natural and resonance characteristics of the ground by capturing persistent natural vibrations. These microtremors are the result of the interaction of surface waves arriving from distant sources and the stiffness structure of the site under investigation. As such, these resonance effects are effective in constraining the layer thicknesses of the SASW shear wave velocity structure and aid in determining the depth of the deepest layer. Together, the hybridized SASW and H/V procedure provides a complete data set for modeling the geotechnical aspects of ground amplification of earthquake motions. Data from these investigations are available at http://walrus.wr.usgs.gov/geotech.
http://walrus.wr.usgs.gov/geotech.
NS41A-1111
Geophysical Exploration and Visualization of subsurface voids in urban Karst areas using the Multichannel Analysis of Surface Waves (MASW) technique
Geo-electrical geophysical techniques have been widely used to study Karst sinkholes in Puerto Rico, but these techniques are seriously affected by cultural noise in urban areas. There is an urgent need to assess different geophysical techniques that could be used in urban and developed regions to address problems related to geologic and engineering hazards in tropical regions. This investigation addresses the use of the Multi-channel Analysis of Surface waves (MASW) technique to study areas of limestone dissolution in urban areas covered with asphalt and concrete and shows to be a good assessment tool in areas with known/or suspected dissolution. We studied three sites with known subsurface dissolution with the MASW technique. At two sites we were able to correlate our results with soil boring information. One of the urban sites was a basketball court. The basketball court is a concrete slab constructed over the North Coast Karst Terrain of Puerto Rico. The concrete slab shows concentric cracks and the center of the slab has subsided about 2 inches with respect of the edges of the court. Our results show a bowl-shaped low velocity zone ( < 200m$/$sec) beneath the basketball court that we have interpreted as the zone of dissolution. Subsurface voids were identified at the other two sites and contrasted with soil borings.
http://geophysics.uprm.edu
NS41A-1112
Using Elastic Wave Seismic Data to Image an Ultra-shallow Buried Paleo- channel
Previously we conducted a set of compressional wave seismic investigations at a groundwater contamination site (Hill Air Force Base, Ogden, UT), in order to better characterize the subsurface environment determined from well data. The seismic target is the bottom of a paleo-channel eroded in a clay layer at a depth of 10-15 m that is overlain by a mix of sands, clays, and gravels with different compaction and water saturation. The channel acts as contaminant trap for dense non-aqueous phase liquids (DNAPLs), therefore a detailed map of its geometry and depth is crucial to the remediation effort. At the same site, we recently performed another experiment using elastic-wave sources, in order to determine subsurface elastic parameters, to complement the P-wave data already acquired and processed, and to exploit the lack of sensitivity of the shear modulus to fluid saturation. The seismic data were acquired along two profiles using 60 groups of three single-component 40 Hz geophones mounted in a Galperin configuration. The length of each profile is 29.5 m. The transverse component was extracted and processed in order to obtain SH-wave images of the subsurface. The SH-wave depth migrated sections show geometry of and depth to the clay layer that are remarkably similar to those derived from well data. Moreover, the sections can be compared to the velocity model previously obtained from P-wave waveform tomography: If one assumes that the 1100 m/s P-wave velocity contour the top of the clay in the waveform tomography, the difference between the channel imaged by the SH-wave data and waveform tomography is insignificant. We have also obtained P-wave and S-wave tomography velocity models along the two lines. By combining the Vp and Vs information, a map of the Vp/Vs ratio and the Poisson's ratio was reconstructed. These parameters can be related to lithology changes in the subsurface. Low Vp/Vs and Poisson's ratio values normally relate to stiff materials (i.e. sands), while high value are generally interpreted to be less rigid materials (i.e. saturated clays). Our study shows that it is possible to image and delineate the geometry of the channel, with fairly high resolution and accuracy, using shear wave sources, as well as the using compressional-wave sources. The images made using the different seismic methods are in good accordance with each other and the well data, and allow a cross- validation of the final results.
NS41A-1113
Near-Surface Seismic-Reflection Imaging of the 2001 Ms 8.1 Central Kunlun Earthquake Rupture Zone, Kunlun Pass Segment, Northwest China
The approximately east-west-striking, 1200-km-long strike-slip Kunlun fault in northwestern China (Qinghai-Tibet Plateau) accommodates a large part the eastward extrusion of Tibet from the Indian-Eurasian collision zone; consequently this results in relatively frequent, large to great earthquakes, as well as, a significant seismic hazard. Near-surface seismic (shear-wave) reflection profiles were collected over extension and compression jogs in the moletrack created along a segment of the fault, called the Kunlun Pass fault (KPF), during the 2001 Ms 8.1 Central Kunlun earthquake. Specifically, this study examined a strand of the KPF segment adjacent to its intersection with the Qinghai-Tibetan Railway. The objective was to define the site-specific, near-surface rupture width at the railway crossing for mitigation design purposes. The resultant stacked profiles show high-angle primary deformation across a width ranging between approximately 20 and 55 meters. Although surface observations across the moletrack transects indicate nearly 1 meter of relief, time displacement calculations show 4.5 m of subsurface relief across the deformation. This may result from extensive settlement in the rubble zone and/or out-of-plane effects. A more definitive answer will require higher-resolution analysis (i.e., trenching or drilling) of the site.
NS41A-1114
An Integrated Geophysical Study of Hidden Valley, Central McCullough Range, NV: Characterization of a Volcanotectonic Terrain
Hidden Valley is located in the north-central McCullough Range south of Las Vegas along the western edge of the Northern Colorado River extensional corridor (NCREC) in the central Basin and Range. The western portion of the NCREC is an area of highly extended crust, but the McCullough Range is a relatively unextended block. Hidden Valley is bound on all sides by volcanic rock of the central and northern McCullough Mountains. The evolution of Hidden Valley is still unresolved due to a lack inter-basin exposure. Four proposed models have been suggested for the formation of Hidden Valley. Models include a volcanic sag basin, half-graben, pull-apart basin, and antithetic synclinal accommodation zone. An east-west trending high-resolution seismic reflection/refraction profile and gravity measurements were collected in the valley to resolve the basin formation, determine the relationship between volcanism and extension in the area as well as to determine the basin structure. From the eastern edge of the basin moving west, a 2.18 km seismic profile was acquired in the summer of 2005 using a combination of alternating hammer source and 15-second linear vibroseis sweep. First arrivals were used to create a velocity model along this profile and integrated on the seismic reflection data. Gravity measurements were taken across the valley at 1 km spacing and at 250 m across the center of the valley during the winter of 2006. Gravity modeling was performed at the regional scale and local scale. Seismic reflection/refraction data were used for accurate gravity modeling and local well data was integrated into all data sets. Preliminary interpretations of the data sets show a thin basin fill on the eastern edge of the basin with basaltic basement rock on that side of the valley. The basin fill thickens to the middle of the valley, possibly associated with faulting. Results show little faulting and syn-tectonic volcanism, which would strengthen the evidence for volcanic sagging. Further refinement of the integrated data sets and modeling could reveal normal faulting, which alternatively would suggest a half-graben or accommodation zone. Faulting of Holocene sediments would indicate a possible seismic hazard for the growing population of the Las Vegas area.
NS41A-1115
Imaging Shallow Aquitard Breaches with P waves: Results from a Walk-away test and a Reflection Survey at two Sites in Memphis, Tennessee, USA
We present the results of two seismic reflection experiments conducted in the Great Memphis area in April and July 2006. The two experiments consisted in a walk-away test and in the acquisition of a 1 km seismic reflection profile. The acquisition of the seismic data is part of a larger effort aimed at imaging the lateral continuity of the Upper Claiborne confining clay that separates the Memphis aquifer, the region's primary drinking water source, from the upper unconfined aquifer and protects the drinking aquifer from exposure to potential contamination. During the walk-away test, four P-wave sources, a 7.5 kg sledge hammer, a 20 kg weight drop, a 12-gauge Buffalo gun, and a Minivibe source were tested at two sites with the goal of selecting the best P-wave seismic source and acquisition parameters for shallow reflection surveys. Boreholes nearby both sites encountered the Upper Claiborne unit at a depth ranging from 10 m to 40 m. One site is located within a 100-meter length of road median that can be considered an urban environment. The second site is located at Shelby Farms within the City of Memphis yet reflects a rural setting with minimal noise and no subsurface infrastructure. Performing identical walk-away tests at both sites, the results indicate that the energy source selection is site dependent. At the urban site, the energy generated by the weight drop source is more coherent and can be interpreted with more confidence on the recorded data. However the Shelby Farms site the 12-gauge shotgun produced the strongest recorded energy, the highest dominant frequency and the broadest frequency band (6-110 Hz). Strong attenuations are observed at both sites with a much higher attenuation in the urban road median site, where the near surface materials consisted of gravels, sands, clays, and pebbles. For both sites, surface waves and refractions dominate the seismic recordings. Filtering and gain of the data revealed the presence of shallow reflections related to the targeted clay layer. Based on the results of the walk-away test and on additional supporting data such as water table measurements, neotectonic structural mapping and borehole data, the location for a 1 km long, north-south trending seismic reflection profile was chosen at Shelby Farms extending south to the Wolf River. Based on the walk-away testing the 12-gauge Buffalo gun was selected as the energy source, used to detonate a single 200 g black powder shell in a 0.6 m deep water filled hole with a 1 m source interval and a 0.25 m geophone interval. Preliminary analysis of the data indicates dominating surface waves and refractions. Upon filtering, consistent reflections can be observed. Correlation of reflections at the start of the seismic line to nearby boreholes evidenced the signature of the Upper Claiborne confining clay at a depth of 18 m.
NS41A-1116
Seismic Investigation of Faulting in the River Mountains, Henderson, Nevada
The River Mountains are located ~25 miles southeast of Las Vegas, NV and make up part of the boundary between the cities of Las Vegas and Henderson. The Itheca Road Fault, located in the River Mountains fault zone along the eastern base of the River Mountains, has been found to have the potential for 6.4-6.7 magnitude earthquakes. In January 2006, we conducted a seismic reflection/refraction experiment in an effort to characterize the Itheca Road Fault. These data were collected along a 300-m long profile that was laid out normal to the mapped surface expression of the fault. The seismic source (mini-vibroseis) and geophones were co-located at 5-m spacing along the profile. The data were recorded by a 60-channel seismograph without acquisition filters. Standard reflection processing was applied as well as a tomographic P-wave velocity model were produced using these data. The reflection and refraction results confirm the location of two fault strands at a depth of ~133 m that tie to the surface expressions. Two additional fault strands that lack surface expression are also interpreted in the reflection stack. Given the enormous rate of population growth and urban development in the area, the Itheca Road Fault could pose a significant seismic hazard and should be considered when developing in the area.
NS41A-1117
Imaging the Black Hills Fault, Clark County, Nevada Utilizing High-Resolution Seismic Reflection and Vibroseis
Historically, the location, geometries, and seismic potential of southern Nevada faults are poorly constrained. Collection of such data and seismic hazard characterization of the Black Hills fault (BHF) are important steps in better defining one of these faults. The BHF forms the northwestern structural boundary of the Eldorado Valley, which lies ~20 km southeast of Las Vegas, Nevada, between the growing communities of Henderson and Boulder City. Earthquake magnitude estimates based on surface rupture length (SRL) indicate an earthquake potential of Mw 5.7; however, estimates based on displacement values documented in a paleoseismic trench indicate a higher value of Mw 6.4-6.8. This implies that the subsurface rupture length is significantly greater than the length of the scarp. Although previous attempts to image the fault with a hammer source were inconclusive, gravity studies and local geology imply that the fault continues south of the scarp. Therefore, additional high-resolution seismic reflection and refraction data were acquired in SEG2 format along portions of a 1 km profile at 5 m station spacing utilizing a vibroseis source. At each shot point, a stack of four 30-160 Hz vibroseis sweeps of 15 s duration was recorded on a 60-channel system with 40 Hz geophones. A preliminary examination of these data indicates the existence of an eastward dipping structure, potentially confirming that the BHF continues in the subsurface south of the scarp.
NS41A-1118
Subsurface imaging of the eastern Ramon fault, Wadi Neqarot, south Israel
The Ramon fault is a major element within the Central Sinai-Negev shear zone, extending across the Negev and central Sinai, from the Dead Sea Transform (DST) to the Gulf of Suez. Right lateral strike-slip motion, dated as post early Miocene, has been recorded along several elements of this system. Some evidence of earlier activity has been noticed along several faults belonging to this system, but only in few cases the nature of this activity is known. The eastern part of the Ramon fault from Wadi Geled to the DST is known from surface geology as a high angle reverse fault. A vertical separation of almost 200 meters is based on lower Cretaceous sandstone abutting upper Cretaceous units. Recent seismic lines carried out across the eastern segment of the Ramon fault reveal new information from subsurface concerning the geometry and history of the Ramon fault zone. We suggest two alternative models to image the subsurface, both leaning on interpretation of seismic lines across the fault. No other subsurface stratigraphic information apart from a distant borehole was available to this research. The first model show three phases of deformation: I. Post Triassic tensional phase, forming normal fault. This fault changes its inclination from a sub vertical angle at surface to a sub horizontal within few hundred meters. II. Compressional phase, during the late Cretaceous, which recorded by a thicker section of units at south of the fault compared to northern part. III. Deformation which took place after early Miocene, and based on surface geology. The second model based on structural considerations and consists of two phases of deformation I. Post Triassic normal fault, which form a listric fault within few hundred meters. II. Post Miocene reverse faulting, which is a reactivation phase along same fault plane that is also responsible for the antithetic reverse faults.
NS41A-1119
High Resolution Images of the Granitic Plutons Along the Iberseis Deep Seismic Reflection Transect: Southwestern Iberia
IBERSEIS is a 303 Km long deep seismic profile in the S-W of Iberian Peninsula. The parameters used for the acquisition allow for a high resolution imaging of the shallow subsurface. The deep seismic transect goes across several characteristic granitic plutons. Detailed imaging of these outcropping granites and the neighboring geologic structures has been attempted. The trace of the profile followed roads and paths, resulting in a irregular acquisition geometry. The quality of the final image is improved considerably by using crooked line techniques which took into account the irregular distribution of sources and receivers. The rugged topography which can reach more than 300 m height and the highly heterogeneous surface geology required carefully estimated static corrections. Reliable shallow velocity models were obtained by first arrival travel time tomographic inversions. These velocity models were also used for pre-stack depth migration imaging. The reprocessing improved the seismic reflection images allowing for a better geological interpretation and, in some cases, provide a direct correlation between the surface geology and the imaged features. The imaged structures suggest possible emplacement mechanisms.
NS41A-1120
Geophysical investigation of Al Jaww plain, eastern Abu Dhabi: Implications for structure and evolution of the frontal fold belts of Oman Mountains
An integrated geophysical investigation was carried out in Al Jaww plain, southeast of Al Ain. This plain is bounded by the Oman Mountains in the east and Jebal Hafit to the west, and occupies an area of approximately 550 km2 which is mainly covered by gravel materials transported from the surrounding mountains. The data for the investigation includes seismic reflection profiles, gravity and magnetic anomalies, outcrop and well data. The aim of the investigation was to determine the structure and evolution of Al Jaww plain and its surrounding areas. The plain is of tectonic significance because it can be used as a model for the Late Cretaceous and Tertiary structures which are developed in both the autochthonous and allochthonous rocks which outcrop along the flanks of the plain. The study has revealed two major phases of tectonic events in the area. The first, a Late Cretaceous phase in the eastern part which is associated with both high magnetic (> 200 nT) and bouguer gravity (> -58 mGal) anomalies. These effects have resulted from the emplacement of the obduction-related allochthonous thrust sheets of the Oman Mountains. The second, a Tertiary deformation phase which is developed in most parts of the plain and produces a series of southerly plunging asymmetrical anticlines and synclines trending in a NNW-SSE direction. Associated high angle reverse faults striking parallel to the fold axes and dipping east close to the eastern boundary of the plain are observed. The most visible of these folds is a syncline whose fold axis runs parallel and approximately 7 km east of the Jebal Hafit anticline. It is associated with a sequence of post-Eocene carbonate sediments with a minimum thickness of 2km, and a low gravity anomaly of <-74 mGal. This Tertiary folding and thrusting is formed as a result of regional compressive deformation due to the rejuvenation of the Late Cretaceous thrust faults during post-Middle Eocene times, possibly in a complicated superposition of transpressional tectonics along deep seated faults.
NS41A-1121
An Assessment of the Value of Full Tensor Gradient Gravity Data for Determining 3-D Structure in an Integrated Geophysical Interpretation of the Styldrift Region, Bushveld Complex, South Africa
The Bushveld Complex (2 060 � 2 054 Ma) is the largest known layered mafic intrusion in the world, at 7-9 km thick and covering approximately 65 000 km2, and is mined for its high grades of PGEs and chromium. Styldrift lies in a structurally complex region (due to the intrusion of the Pilanesburg, approximately 1 300 Ma) where dykes, faults, potholes and Iron-Rich Ultramafic Pegmatoids (IRUPs) present a problem to mining activities. Interpretation of 3-D seismic data, constrained by drill-holes, has produced a 3-D geological model in gOcad, which will assist in mine design and planning. A 1 km2 grid over the 3-D geological model has had high resolution ground gravity and ground magnetic data collected over it. Values of the vertical gravitational component were used to calculate the Full Tensor Gradient (FTG) gravity components, by first constructing the equivalent layer. Airborne FTG gravity data have been flown over the area, which may be compared to the calculated ground data, to test the accuracy of the FTG calculation. Aeromagnetic data over the region may also be compared to the ground data. The calculated FTG gravity data and magnetic data were used to run inversions (steepest descent and UBC algorithms) on the 3-D geological model. Highly reliable inversions of the FTG gravity data adjusted the lithological contacts of the 3-D geological model, constrained by seismic and borehole data, as well as densities of norites and anorthosites in the model, constrained by down-hole density measurements. A second 1 km2 grid, in close proximity to the first grid but with no corresponding seismic data, also had gravity and magnetic data (both ground and airborne) collected over it. A simple 3-D geological model was constructed, with lithological contacts and densities constrained by borehole data. Inversions of the calculated FTG gravity and magnetic data, and extending geological trends of the first geological model, lead to improvements in this geological model.
NS41A-1122
Shallow Seismic Reflection Study of Recently Active Fault Scarps, Mina Deflection, Western Nevada
During the spring and summer of 2006 University of Kansas geophysics students and faculty acquired shallow, high resolution seismic reflection data over actively deforming alluvial fans developing across the Emmigrant Peak (in Fish Lake Valley) and Queen Valley Faults in western Nevada. These normal faults represent a portion of the transition from the right-lateral deformation associated with the Walker Lane/Eastern California Shear Zone to the normal and left-lateral faulting of the Mina Deflection. Data were gathered over areas of recent high resolution geological mapping and limited trenching by KU students. An extensive GPR data grid was also acquired. The GPR results are reported in Christie, et al., 2006. The seismic data gathered in the spring included both walkaway tests and a short CMP test line. These data indicated that a very near-surface P-wave to S-wave conversion was taking place and that very high quality S-wave reflections were probably dominating shot records to over one second in time. CMP lines acquired during the summer utilized a 144 channel networked Geode system, single 28 hz geophones, and a 30.06 downhole rifle source. Receiver spacing was 0.5 m, source spacing 1.0m and CMP bin spacings were 0.25m for all lines. Surveying was performed using an RTK system which was also used to develop a concurrent high resolution DEM. A dip line of over 400m and a strike line over 100m in length were shot across the active fan scarp in Fish Lake Valley. Data processing is still underway. However, preliminary interpretation of common-offset gathers and brute stacks indicates very complex faulting and detailed stratigraphic information to depths of over 125m. Depth of information was actually limited by the 1024ms recording time. Several west-dipping normal faults downstep towards the basin. East-dipping antithetic normal faulting is extensive. Several distinctive stratigraphic packages are bound by the faults and apparent unconformitites. A CMP dip line was also run across a large active scarp in Queen Valley near Boundary Peak. Due to slope steepness and extensive boulder armoring shot and receiver locations had to be skipped within several meters of the actual scarp location. Initial structural and stratigraphic interpretations are similar to those in the Fish Lake Valley location. Overall the data prove that the actively deforming fans can be imaged in detail sufficient to perform structural and possibly seismic stratigraphic analysis within the upper one hundred meters of the fans, if not deeper.
NS41A-1123
Ground Penetrating Radar Imaging of the Emigrant Peak Fault Zone and Alluvial Fan
Near-surface geophysical studies at the University of Kansas are investigating active faulting in the Eastern California Shear Zone. The Emigrant Peak Fault, in Fish Lake Valley, Nevada, is a normal fault that aids in the transfer of right-lateral deformation associated with the Furnace Creek/Fish Lake/Death Valley fault system of the Walker Lane Belt/Eastern California Shear Zone. During the spring and summer of 2006 we collected ground penetrating radar (GPR) across the deformed alluvial fan associated with the Emigrant Peak Fault. The GPR study is conducted in conjunction with high resolution shallow seismic and geologic investigations underway to more fully characterize the fault zone. The GPR data crosses the surface expression of the Emigrant Peak Fault and it is comprised of a 50 MHz 3-D grid and 25 MHz 2-D lines. The 3-D grid covers an area of 115m X 500m at 1m trace spacing, 5m in-line spacing and intersecting cross-lines at 50, 100, 150, 250, and 450m across the in- lines. 2-D GPR lines were acquired at coincident locations with the shallow seismic data and along a 1500m regional line over the fault and alluvial fan deposits. Depth of imaging ranged between 17m for the 50 MHz data and 25m for the 25 MHz data. GPR imaging aids in the characterization of the fault zone structurally as well as characterizing alluvial fan stratigraphy. Data shows stratigraphic reflectors on a 1m scale. Reflector geometries are quite complex, showing continuous coherent events, as well as areas that are less coherent which appear to signal a change to more boulder/cobble-rich deposition, a common characteristic in debris-flow dominated alluvial fans. The reflectors are also heavily influenced by the structural components that are imaged. The GPR shows a number of west-dipping faults that seem to migrate towards the basin. The faults are not imaged merely as interrupted reflectors, but the fault surfaces are actually imaged. Stratigraphic reflectors truncate at the faults in many instances. Some of the reflectors do not truncate, but instead roll-over into steeply dipping reflectors. The GPR data shows that not only is it useful for determining the near-surface stratigraphy of alluvial fans, but is very useful in determining the shallow structures associated with normal fault deformation in the alluvial fan as well as imaging the fault surface.
NS41A-1124
Seismic Evidence for a Resonance Layer in Booming Sand Dunes
``Booming sand dunes'' are large desert dunes that emit a loud droning, low-frequency sound during an avalanche of sand on the leeward face of a dune. The monotone sound (70-105 Hz) may continue for up to a minute after initiation, even after all visible motion has ceased. The source of the booming sound has long been a mystery and no accepted scientific explanation has been proposed yet. Seismic refraction experiments conducted with a closely-spaced 48 channel system show a shallow ( < 10 m) subsurface layering inside the dune with significant velocity contrasts between the individual layers. The seismic body wave velocities in the top three layers of sand (240 m/s, 360 m/s and 460 m/s) are very close to the acoustic velocity in air (355 m/s) while at the same time the surface waves are highly attenuated. The seismic velocity changes cannot be explained by increasing confined pressure but must be provided by a seasonally changing physical structure. From the seismic survey it is further noted that the layering narrows towards the foothill. During the sustained boom, the frequency rises slightly (from 85 Hertz to 95 Hertz) as the source due to the avalanche proceeds downwards. This observation was deduced from analyzing the booming emission on all 48 geophones in conjunction with a high-quality air microphone. The multi-layer internal structure of the dune provides a resonance cavity that amplifies particular frequencies and creates the loud booming sound. The resonance and its interaction with the air is modeled with finite-difference simulations.
http://www.its.caltech.edu/~nmvriend/research
NS41A-1125
Using Ground Penetrating Radar to Image Terrestrial Analogs of Martian Aeolian Deposition: The Coral Pink Sand Dunes, Kanab, Utah
On Mars, wind is the dominant active landscape forming process. The Martian surface contains abundant duneform and other depositional features related to aeolian processes. To investigate the effectiveness of GPR for Martian imaging, we tested GPR on terrestrial analogs for the Martian surface. We collected two, perpendicular, 250 and 500 MHz GPR lines across the active dunefield at the Coral Pink Sand Dunes State Park, Kanab, Utah. The transects crossed dunefield features including barchanoid dunes, interdune swales, the shallow water table, and interdune bedrock exposures. These features provide analogs for a range of potential targets on Mars. The collected data provide detailed images of internal features of the dune such as crossbedding, the water table, and the sand/bedrock contact. Shallow ( < 4 m) hand-augered borings, coincident with the GPR transects, provide ground truth for the depth to the water table and shallow bedrock. These measured boundaries coincide with strong, continuous reflections in the GPR data. As expected, the 250 MHz GPR images penetrate more deeply into the dunes, but the 500 MHz images are more detailed. Shallow features (< 10 m) are evident at both frequencies.
NS41A-1126
The Wigner-Ville Transform, An Approach to Interpret GPR Data: Outlining a Rik Zone
In this investigation, a time-frequency analysis is performed, based in the decomposition of the GPR signal in high- and low-frequencies. This process is combined with a statistical approach to detect signal changes in time and position simultaneously. The spectral analysis is carried out through the Wigner-Ville distribution (WVD). A cross-correlation can be computed between the original signal and the time-frequency components to obtain structural anomalies in the GPR observations, and to perform a correlation with the available geology. An example of this methodology is presented, where a series of traces where analyzed from a GPR profile surveyed in an eastern area of Mexico City. This is a heavily urbanized region built on the bottom of an ancient lake. The sediments are poorly consolidated and the extraction water rate has increased the areas of subsidence. Nowadays, most of family homes and public buildings, mainly schools have started to suffer heavy damages. The geophysical study carried out in the area permitted to detect areas of high risk. The data analysis combined with previous geological studies, which included stratigraphic columns allowed to identify the geophysical characteristics of the area, which will allow to the authorities to plan the future development of the area.
NS41A-1127
Detecting Underground Mine Voids Using Complex Geophysical Techniques
In July 2006, the National Energy Technology Laboratory in collaboration with Department of Geology and Planetary Science, University of Pittsburgh conducted complex ground geophysical surveys of an area known to be underlain by shallow coal mines. Geophysical methods including electromagnetic induction, DC resistivity and seismic reflection were conducted. The purpose of these surveys was to: 1) verify underground mine voids based on a century-old mine map that showed subsurface mine workings georeferenced to match with present location of geophysical test-site located on the territory of Bruceton research center in Pittsburgh, PA, 2) deliniate mine workings that may be potentially filled with electrically conductive water filtrate emerging from adjacent groundwater collectors and 3) establish an equipment calibration site for geophysical instruments. Data from electromagnetic and resistivity surveys were further processed and inverted using EM1DFM, EMIGMA or Earthimager 2D capablilities in order to generate conductivity/depth images. Anomaly maps were generated, that revealed the locations of potential mine openings.
NS41A-1128
Integrated and Portable Electric & Magnetic Field Sensing System for Geosciences
We have developed a lightweight, low power, man-portable electromagnetic sensing system for geosciences applications. The system integrates a high sensitivity, 3 axis electric field and 3 axis magnetic field sensing system into an easily deployable package capable of probing the conductivity of the earth's crust to several kilometers in depth. The system is equipped with onboard 24 bit continuous, and 16 bit triggered burst capture data acquisition hardware, GPS synchronization, and Ethernet output. System performance will be demonstrated through the results of a field tests in the Snake River Plain area of southeastern Idaho. Magnetotelluric inversions from field data compare favorably with previous studies of subsurface structure.
NS41A-1129
Transient Electromagnetic Study of the Geologic Framework, Upper San Pedro Basin, Mexico
The San Pedro River flows north from near Cananea, Mexico into the U.S. and ends at its confluence with the Gila River in Arizona. The river supports a riparian habitat that is important for its biological diversity and is the most significant flyway for migrating birds in southwest North America. Nearby communities in Arizona and Mexico are pumping groundwater from basin fill in the San Pedro valley and there is much concern about the effects of this pumping on the baseflow of the San Pedro River. Local, State, and National agencies have been studying this issue from a wide variety of perspectives. To help address the problem, the U.S. Geological Survey has undertaken an integrative research effort utilizing geologic mapping, multiple geophysical techniques, and multi-spectral imaging to better understand the geology, geometry, structure and basin fill sediments in the San Pedro basin in Mexico. An existing aeromagnetic survey coupled with a natural source audio frequency magnetotelluric study indicates that the basin is less than 1km deep in most places and contains at least two sub-basins. These sub-basins are separated by a subsurface bedrock high that is well below the present water table. Also, several vertical electrical soundings, acquired by Grupo Mexico in the region, indicate that the central portion of the basin may contain several, potentially thick, sequences of clay. The transient electromagnetic (TEM) survey was designed to provide more information on the nature of the basin fill sediments and to provide a better estimate of the depth of the bedrock structural high. The TEM data was acquired with a Zonge ZeroTEM instrument at 104 locations with 150m per side square loops using approximately 3.6 amps of current at a base frequency of 16 Hz. The locations of the TEM stations, generally spaced 0.5km apart, were designed to obtain a conductivity-depth profile along approximately 30km of the San Pedro River in Mexico and at three locations perpendicular to the river. At each station location one reading was taken in the center of the loop and two were taken outside the loop at a distance of 150m from the loop center. The data was inverted for subsurface resistivity layers using EMIGMA software with most inversions being constrained to depths of 500m or less. The outside readings are used to give an indication if there are three- dimensional geologic effects present that may influence the one-dimensional inversion routine. The results indicate that the central portion of the basin along the path of the San Pedro River contains two to three clay sequences that are over 100m thick in some locations. The clay sequences diminish in the basin margins to the east and west of the river. The bedrock structural high between the two sub-basins is not shallower than about 250m and likely does not significantly influence groundwater flow. The location and thickness of the clay sediments appear to be the important factor in the basin groundwater hydrology in the upper 300m of the basin fill. The TEM data also indicates that the basin bedrock geometry may be much more complicated than two simple sub-basins separated by a bedrock high.
NS41A-1130
Constraints on formation, structure, and environmental effects of the Odessa (TX) meteorite impact based on a near-surface applied geophysical investigation
Meteorite impacts are a common occurrence throughout Earth's geological history. Many of the geological signatures of large ancient impacts have been subsequently erased by weathering and erosion processes. It is therefore important to recognize and study preserved meteorite impacts in order to better understand this natural hazard which has been increasingly linked to rapid climate change and mass extinctions. The ca.60 ka Odessa meteorite main crater located in Ector Co. TX. is ca.150 m in diameter. Geological mapping and detailed numerical simulations indicate that the crater was caused by a relatively small, oblique impactor. The crater rim is remarkably well- exposed. Much of the ejecta blanket is present, although deeply eroded. There has been considerable site disturbance due to drilling, shaft excavation, trenching, construction of museum and trails, and oil/gas activity in the surrounding fields. We have performed time-domain electromagnetic and magnetic gradiometer geophysical surveys in order to constrain the subsurface portions of the steeply dipping Cretaceous fossiliferous limestone, marly limestone, and maroon shale formations that comprise the rim. Our data clearly respond to the large-scale deformation and thrusting of these beds in response to the impact. Our geophysical imagery provides a useful constraint on numerical simulations of the impact and its immediate, regional-scale environmental effects.
NS41A-1131
An Iterative Method for Potential Field Downward Continuation
A new iterative method for potential field downward continuation to a horizontal plane A either from a horizontal plane B or from an undulating surface B is proposed. The iterative procedure of the plane-to-plane downward continuation is: (1) choose the initial potential field value at each point on the plane A as the observed field value on the plane B at the point with the same horizontal coordinates; (2) upward continue the potential field values on the plane A and obtain the potential field values on the plane B using the Fast Fourier Transform (FFT) technique; (3) the new potential field values on A are then obtained by adding the initial values and the weighted difference between the observed and the calculated potential field values on B; (4) repeat steps (2) and (3) until the difference becomes negligible. The iterative procedure of the downward continuation from the undulating surface B to the horizontal plane A that passes the lowest point of B is: (1) choose the initial potential field value at each point on the plane A as the observed field value on the undulating surface B at the point which is vertically projected from the plane A; (2) intersect the surface B with several planes and obtain the field values on these planes by upward continuing the fields on the plane A using the FFT technique, then the fields on the surface B can be interpolated from the fields on these planes; steps (3) and (4) are the same as those in the procedure of the plane-to-plane downward continuation. The fields on any other plane can be calculated from the fields on the plane A that passes the lowest point of B. The new iterative downward continuation method can stably downward continue data to a much deeper depth than downward continuation using standard FFT techniques, up a depth equal to more than 10 times the grid interval. Both synthetic and real data continuation tests are very good.
NS41A-1132
Dipole Logging in Fractured Formation via TBEM
In this paper a simulation of the 3D wave propagation in a fractured medium, penetrated by an acoustic log, excited by a sonic tool, is described. The fractures are assumed to be empty with thickness tending towards zero. To overcome the thin-body modelling problems, a Traction Boundary Element Method (TBEM) is used together with a conventional direct Boundary Element Method (BEM) in a frequency dependent formulation. The influence of the length of the fracture, its orientation and position in relation to the acoustic well in the wave field is assessed. Three different fracture orientations have been modelled: vertical, horizontal and inclined (forming an angle of $45\deg$ with the horizontal direction). As expected, the time and frequency domain responses show that the influence of the fracture lessens as its length decreases and as the distance between the fracture and the borehole increases. The horizontal and the inclined fracture have a slight influence on the responses when compared with the homogeneous case. When the vertical fracture is close to the borehole it gives rise to significant changes in the response, owing to the multireflections between the fracture and the borehole wall. The 3D problem is solved using a 2.5D formulation since the geometry of the cross-section of the borehole and the crack is assumed to be constant along the borehole axis.
NS41A-1133
Hydraulic Fracture Stimulation in a Fluid-filled Borehole via a TBEM Approach
In this paper, the Traction Boundary Element Method (TBEM) and the Boundary Element Method (BEM) are combined so as to evaluate the 3D wave field generated by 2D fluid-filled boreholes with a connecting hydraulic fracture. The problem is formulated in the frequency domain. The hydraulic fracture may exhibit arbitrary orientation and may present a very small thickness. The proposed formulation avoids the thin-body difficulty posed by the classical BEM formulation. All singular and hypersingular integrals are evaluated analytically, which overcomes one of the drawbacks of this formulation. Different source types such as 3D monopoles and dipoles loads may excite the borehole. The proposed model is first verified using a cylindrical circular fluid-filled borehole, for which analytical responses are known. Complex frequencies are used to avoid the aliasing phenomena in the time domain. The numerical results are analysed and a selection of spectra and animation plots from different hydraulic fracture configurations are presented. This allows the illustration of the significant scattered wave field modifications generated by a hydraulic fracture connected to the borehole.
NS41A-1134 INVITED
Multi-dimensional surface NMR imaging and characterization of selected aquifers in the Western US.
This work outlines the development of a multi-channel surface NMR instrument and its application to 2-D and 3-D imaging and characterization of various aquifers in the western US. The multi-channel surface NMR instrumentation and the mathematical foundations for multi-dimensional surface NMR are described. Experimental results, including 2-D and 3-D estimates of porosity and T2* (a measured NMR signal parameter empirically related to permeability), are presented from field tests conducted over a variety of aquifer types: an alluvial aquifer system in western Nebraska, an alluvial and fractured bedrock environment in central Iowa, a Karst environment in southeast Minnesota, and a basaltic aquifer system near the Columbia River in south- central Washington.
NS41A-1135
Application use of the Proton Nuclear Magnetic Resonance Method for Ground Water Investigations in Various Geological Environments
The Proton Nuclear Magnetic Resonance method, also called the Magnetic Resonance Sounding method (MRS), after having been in a research tool during a long maturation period, is in the way of being more and more applied in the groundwater surveys for complementing the traditional geophysical methods. Its capacity to give quantitative information for characterising the water layers (depth and thickness, porosity, permeability after calibration) give it a special place in the range of geophysical tool for hydrogeologists. Due to the low levels of the signals which are measured in Magnetic Resonance field surveys, to make the method efficient, one must take special care of the accuracy of the Larmor frequency used in relation with the local Earth Magnetic field and of the filtering of the natural and industrial electromagnetic noises. The shape of the wire loop used to energise the ground and to receive the relaxation signals (square loop, eight-square loop, compensated square loop, �) from which the initial amplitudes and the time constants are determined is also a matter of importance as it shares the control of the depth of penetration together with the transmitter power and it directly acts on the way the local noise is primarily filtered. This study will report on the advantages and the limitations of these various shape loops with respect to these two parameters, also the basic magnetic properties of the materials which permit a measurable expression of the relaxation effects. Besides, to optimise the use of a new geophysical method in groundwater surveys, it is important to have a clear view of its real output within the various backgrounds which can be met in such surveys. Field results coming from different types of aquifer, depths, geological conditions, and countries in the Western World, Asia and Africa show the data quality and the field efficiency which can be expected with the present state of the art of the technology used. Also, the correlation with hydrogeological borehole parameters, such as the yield or the transmissivity, when available, opens the way for characterizing the geological materials from surface geophysical readings.