S11C-1747
Imaging the Shallow Crust in the Epicentral Area of the 1857 M7 Agri Valley Earthquake (Southern Italy) by Combined Traveltime and Full-Waveform Tomography
The Agri Valley is a Quaternary extensional basin located in the Southern Apennines range. This basin was struck by a M7 earthquake in 1857. In spite of extensive morphotectonic surveys and hydrocarbon exploration, major unsolved questions remain about the upper crustal structure, the recent tectonic evolution and seismotectonics of the area. Most authors consider a SW-dipping normal-fault system bordering the basin to the East as the major seismogenic source. Alternatively, some authors ascribe the high seismogenic potential of the region to NE-dipping normal faults identified by morphotectonic surveys along the ridge bounding the basin to the West. These uncertainties mainly derive from the poor performance of commercial reflection profiling that suffers from an extreme structural complexity and unfavorable near-surface conditions. To overcome these drawbacks, ENI and Shell Italia carried out a non-conventional wide-aperture survey with densely spaced sources (60 m) and receivers (90 m). The 18-km-long wide-aperture profile crosses the basin, yielding a unique opportunity to get new insights into the crustal structure by using advanced imaging techniques. Here, we apply a two-step imaging procedure. We start determining multi- scale Vp images down to 2.5 km depth by using a non-linear traveltime tomographic technique able to cope with strongly heterogeneous media. Assessment of an accurate reference Vp model is indeed crucial for the subsequent application of a frequency-domain full-waveform inversion aimed at improving spatial resolution of the velocity images. Frequency components of the data are then iteratively inverted from low to high frequency values in order to progressively incorporate smaller wavelength components into the model. Inversion results accurately image the shallow crust, yielding valuable constraints for a better understanding of the recent basin evolution and of the surrounding normal-fault systems.
S11C-1748
Active seismic imaging using microseismic events: results from the San-Andreas-Fault system at SAFOD
The analysis and interpretation of microseismic data sets are receiving increased attention, both in the exploration industry for the characterization of hydrocarbon and geothermal reservoirs as well as in academia for the general understanding of seismogenic processes at plate boundaries. The gain in data quality due to e.g. the deployment of borehole-receiver-arrays and the meanwhile common practice of recording the full waveform of the seismic events allows the processing of these data sets using modern seismic imaging and inversion algorithms. We have developed a passive seismic imaging approach which consists of two steps. Firstly, the hypocenter of the microseismic event is precisely located. Secondly, this event is treated as pseudo-active seismic source and we process the reflections within the recorded wavefield using a directional migration algorithm in order to construct a high-resolution image in the close vicinity of the located hypocenter. In this paper we describe this approach and demonstrate the application to several microseismic events recorded by a borehole array in the SAFOD (San-Andreas-Fault-Observatory-at-Depth) main hole. The results are high-resolution images of different fault branches related to the San-Andreas-Fault (SAF) system in the close vicinity of the borehole. The comparison of these findings with existing surface seismic reflection images as well as additional borehole information demonstrates some interesting features. In summary our results allow a spatial characterization of the complex internal structure of the SAF and can certainly be helpful for other studies which rely on this knowledge.
S11C-1749
Evidence of Shallow Faulting Across the Silver Creek Fault in San Jose, California
In June of 2007, we acquired an approximately 600-m-long, high-resolution, combined seismic refraction/reflection profile across the alluvial-covered northern extension of the Silver Creek fault, along Mission Street near downtown San Jose, California. To record the data, we used 120 active channels and 40- Hz vertical-component geophones, spaced at 5-m intervals. Seismic sources were generated by repeated accelerated-weight-drop (AWD) impacts at each geophone location. All shots were recorded on all channels, with shot-to-receiver offsets varying between 1 and 600 m. This shooting geometry allowed both tomographic refraction (velocity) and reflection (CDP stacks and migrated) images to be produced. The data were generally of good quality, with many shots propagating the entire 600-m length of the profile. We observe clear differences in the shot gathers across the Silver Creek fault, with disruptions in the first-arrival refractions and low-velocity zones confined to the northeastern end of the profile. Apparent velocities also vary across the fault zone, with velocities varying between 700 and 1400 m/s on the southwestern end of the profile, and between 300 and 1800 m/s near the northeastern end of the profile. The change in velocities and disruptions in first-arrivals coincide with a linear InSAR anomaly, and the relatively shallow depth of the disruptions and changes in velocity suggest that the fault is likely Holocene active.
S11C-1750
The Vallo di Diano Range-Bounding Fault System (Southern Italy): New Evidence of Recent Activity From High-Resolution Seismic Profiling
The Vallo di Diano is the largest intermountain basin in the Southern Apennines (Italy). The basin evolution was controlled by the Quaternary activity of a range-bounding, SW-dipping normal fault system located to the east (Vallo di Diano Fault System, VDFS). Geological and oil industry data define the sin-sedimentary activity of the VDFS up to the Middle Pleistocene. However, commercial profiles do not resolve the shallower, eastern portion of the basin, due to strong lateral heterogeneities and unfavourable surface conditions. Therefore, Late Pleistocene-Holocene activity of the VDFS and its seismogenic potential are still uncertain. To better constrain the shallow structure of the basin, we performed four high-resolution seismic surveys, along its eastern side, where slope breccias and fans cover the Mesozoic carbonate bedrock and bury the VDFS. We also investigated some NW-trending flexures affecting Late Pleistocene fans, that we had previously detected and dubitatively ascribed to recent faulting. Seismic data were acquired with a dense wide-aperture geometry. Two high-resolution (HR) NE-trending profiles, about 1.5 km long, were collected using respectively 5 m and 10 m spaced receivers and sources. Two very high-resolution (VHR) NE-trending profiles, 400 and 350 m long, with densely spaced sources (4 m) and receivers (2 m) were also collected. HR profiling was aimed at imaging alluvial fan thickness and morphology of the underlying carbonate bedrock. VHR surveys targeted the flexures and their possible origin. All lines were acquired with a HR vibroseis source, except for the shortest profile, where we used a buffalo-gun, better suited for very near-surface imaging (z < 50 m depth). Seismic imaging consists of reflectivity images obtained by CDP-processing of reflection data complemented by Vp images obtained by multi-scale seismic tomography. The stack sections illuminated the basin down to 0.4-0.5 s TWT and reveal an array of high-angle, generally SW-dipping faults dissecting the bedrock and the alluvial fans. Faulting created accommodation space in the hanging-wall and displaced the different fan generations. Clear reflection truncations in the stack-sections correspond to significant Vp lateral changes in the tomographic images. VHR tomography is well defined along the shortest line down to 40 m depth, where two steps within slope breccias are visible. Moreover, two low-velocity wedges (colluvial packages) are imaged in the near surface (5-20 m depth). These data support recent faulting consistently with surface geomorphic features. We interpret these fault structures as splays of the range bounding master fault. Comparison with commercial reflection profiles nearby reveals a great improvement in seismic imaging achieved by HR surveys, which allow a detailed seismostratigraphic analysis of the basin.
S11C-1751
Seismic Images of Near-Surface Faulting Along the Northern Projection of the Silver Creek Fault, Eastern and Southern San Francisco Bay, California
We acquired high-resolution shallow-depth and lower-resolution crustal-scale images across the northern projection of the Silver Creek in the Eastern San Francisco Bay, California. On a regional seismic profile from the Pacific Ocean to the Livermore Valley, the Silver Creek fault approximately marks the boundary between high velocities beneath the San Francisco Bay and lower velocities to the east, suggesting that the Silver Creek fault represents a major structural boundary between the San Andreas and Hayward faults. Locally, we acquired a series of high-resolution seismic profiles across the alluvial-covered northern projection of the Silver Creek fault, as inferred from vertical offsets in the groundwater table and from InSAR images. In San Jose, we found evidence for near-surface faulting across the Silver Creek fault as reported in a companion abstract by Goldman et al. (this volume). Along the Fremont/Union City Border at Alameda Creek, we acquired an approximately 2-km-long high-resolution seismic reflection/refraction profile that shows vertical offsets of near-surface strata and the underlying bedrock, and farther north in San Lorenzo, we acquired an approximately 8-km-long high-resolution seismic reflection/refraction profile that also shows vertical offsets of near-surface strata and the underlying bedrock. Both profiles show the apparent faulting along the northward projection of the Silver Creek fault. Although the vast majority of seismic events recorded in the area can be attributed to the slip on the Hayward fault, the northern California seismic catalog shows that some events occur beneath the near-surface trace of the Silver Creek fault. Collectively, the available data indicate that the Silver Creek fault may be more than 80 km long and may be currently or recently active. Because of its proximity to high-population centers, more careful examination of this fault is warranted.
S11C-1752
Shallow seismic section in the central Kanto plain, to the north of Tokyo metropolitan area, Japan
Shallow seismic reflection surveys were carried out in the central part of the Kanto plain, 40 km northwest of Tokyo, Japan. The survey target ranges about from 50 m to 500 m in depth. The final purpose of the surveys is to reveal the relationship between the subsurface structure and the distribution and flow of the underground water in this area. The survey lines were divided into Kawagoe1(length of CMP line: 8km), Okegawa1(5.3km), Shobu1(8.7km) and Kazo1(6.3km). They line up southwest to northeast from the Iruma upland to the Kazo lowland via the Arakawa lowland and Omiya upland. The survey lines cross the Arakawa fault, the Ayasegawa fault and the Kuki fault. They are considered active, but only the Aysegawa fault has fault topography and is confirmed by subsurface structure. The total length of the lines is about 27km and there is a 2 km long opening between the Okegawa1 and the Shobu1, because of dense population and heavy traffic. Survey parameters are as follows. Seismic source: one EnviroVib or MiniVib or Mini Impactor, shot interval: 2.5m or 10m, sweep frequency: 15-120Hz, sweep length: 13s, receiver: UltraMark2, receiver interval: 10m, elements: 6 bunching, recording instrument: DAS-1, number of channel: 144, listening length: 3.3s, spread: shots from the edge to the 48th point of 144 fixed receiver points, maximum offset: between 1440m and 960m. The data were processed to make seismic sections for each survey line by conventional CMP method. Then the sections were cut and pasted into a series of seismic sections. Many continuous reflectors are imaged between several ten meters and 1 km in depth in the whole seismic section. Reflectors are discontinuous below 1 km, probably because of lack of source energy. In the Iruma upland and Arakawa lowland, the Pliocene and Pleistocene units thicken northeastward, indicating that sedimentation has been synchronous with northeastward tilting of the underlying basement rocks. Undulation and bending of reflectors suggest basement faults in the Arakawa lowland. Near the northern edge of the Omiya upland, deformation, vertical gap, disturbance and tilting of reflectors are perceived around the flexure scarp of the Ayasegawa fault. This indicates that displacement of the Ayasegawa fault in the basement has been deformed the sediment layers over 2 km in width crossing the flexure scarp. In the Kazo lowland, neither deformation nor gap of reflectors are perceived around the inferred Kuki fault in the seismic section.
S11C-1753
Geophysical Study of Cyclone Canyon Graben, Canyonlands National Park, Utah: Sediment Depth and Structural Implications
The grabens of Canyonlands National Park, Utah, are formed by the extension of a gently tilted Pennsylvanian to Permian sedimentary sequence accommodated by flow of the underlying Paradox evaporites. Sediment thickness on top of graben blocks was widely accepted to be 10-15 m, although poorly substantiated. Seismic and gravity measurements from field work in 1999 [Grosfils et al., 2003] indicated sediment depth in Devil's Lane graben to be 80-90 m shallowing toward the graben tips. In the summer of 2005, we conducted seismic and gravity surveys in Cyclone Canyon graben, a larger graben immediately west of Devil's Lane graben. Six P-wave seismic refraction lines were shot parallel to the graben axis using an accelerated weight-drop source. Forty-nine gravity stations, spaced at 100-m intervals along the graben axis were measured with a Lacoste-Romberg G meter with Aliod nulling. Relative locations of the gravity stations were optically surveyed, and the survey was georeferenced with WAAS corrected GPS. Gravity stations were also reoccupied on different days to provide consistency. Analysis of the seismic refraction lines revealed an approximate maximum sediment thickness of 75-m shallowing toward the graben tips, consistent with Devil's Lane graben. Parallel refraction lines in the center of Cyclone Canyon graben showed a 40-m sediment thickness difference, suggesting a "step-like" faulting in the graben block. Further examination of digital elevation models and bedrock exposures support this cross- sectional model. Our complete Bouguer gravity anomaly is over three times larger than observed in Devil's Lane graben. Assuming a plausible density contrast between sediments and bedrock, the anomaly suggests salt diapirism unaccountable in single block cross-sectional models, but accountable for a "step-like" cross- sectional model.
S11C-1754
High-resolution multicomponent seismic imaging for VMS deposits within the Paleoproterozoic Flin Flon Belt, Trans-Hudson Orogen, Canada
The Flin Flon-Glennie complex (Trans-Hudson Orogen) hosts the largest Paleoproterozoic volcanogenic massive sulphide (VMS) district in the world. The main deposits of the Flin Flon camp have mineral compositions of predominantly pyrite, pyrrhotite, sphalerite, and chalcopyrite. All of these minerals are characterised by high acoustic impedances relative to typical host rocks, thus making them excellent candidates for seismic exploration. In a concerted effort to support exploration for new ore deposits in the vicinity of Flin Flon and surrounding region, a program of seismic investigations has been implemented as part of the Targetted Geoscience Initiative-3 (TGI-3) Saskatchewan-Manitoba project. This project is a joint Federal-Provincial effort led by the Geological Survey of Canada with active participation by Hudson Bay Mining and Smelting Ltd. Rock property measurements, downhole geophysical logging and vertical seismic profiles acquired in advance of the main seismic survey demonstrated the expected reflectivity of the mining camp geology. The principle seismic survey was conducted during May-September, 2007 and comprised a total of 75 km of high- resolution 2D seismic profiles and a 3D survey covering approximately 10 km2. Seismic imaging in the Flin Flon area poses significant challenges due to the complex crystalline geology, the location of the imaging targets beneath an active town and operational mine site, and the highly variable terrain. Data were recoreded using IO System IV digital vector (3-component) accelerometers, spaced at 5 m intervals (for 2D survey) with recording times of 4 s. Seismic sources spaced at 20 m intervals included Vibroseis and dynamite sources on land, and an airgun for lake areas. The results of processing the vertical-component data for P-wave reflections reveal subhorizontal reflectivity associated mainly with the Missi metasedimentary complex and steeply dipping reflectivity associated with the polydeformed volcanic rocks, including the main rhyolite horizon which hosts the VMS deposits. Some important fault zones are also imaged.
S11C-1755
Shear wave reflection seismic surveying in the Trondheim harbour area - imaging of land slide processes
The harbour area of Trondheim, Norway, was build on man-made land fillings at the coast of the Trondheim Fjord in several expansions since the last 80 years. The whole area is located on the deltaic sediments of the river Nidelven, which are overlying marine sediments that reach the bed rock in nearly 150 m depth. Some submarine land slides at the border of the sediment body nearby the harbour area were reported during the last decades. Therefore, many geological and geophysical investigations were carried out in recent years to explore the structure of the sediment body and its stability onshore and offshore in detail. Whereas high-resolution marine seismic methods archieved excellent results in the offshore area, common seismic investigations for the mostly paved harbour area itself were a difficult challenge. Therefore, SH polarized shear wave reflection seismics using a land streamer combined with a newly developed shear wave vibrator buggy of 30 kN peak force was applied, because this method is advantageous for paved surfaces. Overall 4.2 km 2.5D profiling was carried out in the harbour area along roads and parking places after optimizing of the field procedure. The whole operation was done at night in time slots from midnight to 5 am by road closures due to savety reasons and to minimize the noise from surrounding heavy traffic of trains, trucks and other heavy equipment. The field measurements achieved high resolution results of the sediment body structure, clear detection of the bedrock, and probably deeper structures within the bedrock. Due to the clear and continuous reflection events, also the shear wave velocity could be calculated at least down to the bedrock to indicate the stiffness of the sediment layers. The results of these onshore seismic profiles will be integrated in a combined onshore-offshore seismic profile grid for structural interpretation. Furthermore, the derived shear wave velocities will be combined with cone penetrometer testings and borehole results to get a comprehensive database about the geotechnical parameters of the sediment body.
S11C-1756
Midget Seismic in Sandbox Models
Analog sandbox simulation has been applied to study geological processes to provide qualitative and quantitative insights into specific geological problems. In nature, the structures, which are simulated in those sandbox models, are often inferred from seismic data. With the study introduced here, we want to combine the analog sandbox simulation techniques with seismic physical modeling of those sandbox models. The long-term objectives of this approach are (1) imaging of seismic and seismological events of actively deforming and static 3D analogue models, and (2) assessment of the transferability of the model data to field data in order to improve field data acquisition and interpretation according to the addressed geological problem. To achieve this objective, a new midget-seismic facility for laboratory use was designed and developed, comprising a seismic tank, a PC control unit including piezo-electric transducers, and a positioning system. The first experiments are aimed at studying the wave field properties of the piezo- transducers in order to investigate their feasibility for seismic profiling. The properties investigated are their directionality and the change of waveform due to their size (5-12 mm) compared to the wavelengths (< 1.5 mm). The best quality signals and least directionality and waveform change are achieved when the center source frequency is between 350-500 kHz, and the offset is less than 8 cm for a reflector depth of 10 cm. With respect to the technical hardware reflection processing on such a small scale is feasible as long as the offset does not exceed a certain value, which is dependent on the reflector depth and frequency. The next steps will include a study of material properties and the effects of wave propagation in an-/isotropic media by physical studies, before we finally start using different seismic imaging and processing techniques on static and actively deforming 3D analog models.
S11C-1757
Mapping the Stratigraphy of Booming Sand Dunes
Booming dunes emit a loud rumbling sound after a man-made or natural sand avalanche is generated on the
slip face of a large desert dune. The sound consist of one dominant frequency (70 - 105 Hz) with several
higher harmonics. A recent publication (Vriend et al., 2007) presented a model of an internal, natural
waveguide that propagates the booming emission, amplifies the sound, and sets the booming frequency. The
mapping of the subsurface layering, which is necessary for the existence of a waveguide, prompted
additional work on the dune structure and stratigraphy.
The current work highlights geophysical measurements at Eureka Dunes in Death Valley National Park, CA
and Dumont Dunes in the Mojave Desert, CA. Seismic refraction studies indicate strong layering with large
velocity jumps across the interfaces. Ground Penetrating Radar (GPR) profiles, at frequencies of 100 MHz
and 200 MHz, map out the stratigraphic structure of the dunes. Variations in the near surface layering are
able to predict the seasonal variability in booming frequency both quantitatively and qualitatively. The
Kirchhoff migrated GPR profiles are superimposed on the local topography obtained with a laser rangefinder.
The complex dune structure is resolved to a depth of over 30 meters for the 100 MHz antenna.
The GPR profiles of the longitudinal Eureka dune display complex internal structures from old dune crests.
Both slopes have slip faces at 30 degrees with parallel layering (< 2m) at the near surface. At the
transverse Dumont dune the GPR profile exhibits strong parallel layering on the booming leeward slipface
only. The shallower windward face features a remarkable tilted repetitive layering that cuts through the
surface. At Dumont Dunes the layering on the leeward face explains the change in booming frequency
between 70 - 95 Hertz in the period 2005 - 2008. The tilted layering structure of the shallow windward face
prevents the formation of a waveguide and is never able to sustain the booming sound.
The Dumont dune progresses slowly, estimated at ~ 1 m/year from correlating satellite images, by
forming new slip faces on the leeward face over time. Large precipitation events may cause a new layer to
form. Sand sampling provides a quantitative measure on the chemical composition and water content of the
layering.
http://www.its.caltech.edu/~nmvriend/research/
S11C-1758
Imaging With Secondary Scattered Waves
From the point of view of wave propagation and time-reverse back propagation, multiples and secondary scattered waves, which are usually abandoned or treated as noises during traditional one-way wave- equation-based migration and imaging, can contribute to the final image if they are properly handled during migration and imaging. We propose an approach to migration and imaging with secondary scattered waves. Secondary scattered waves are extracted by a back propagation plus cross-correlation approach. The surface shot gathers are thus redatumed to subsurface shot gathers with sources located at some subsurface scatterers (gathers of scattering sources). Numerical examples confirm the validity of this Green's function retrieval approach. Migration and imaging from these redatumed shot gathers improves the final image. The proposed approach has a potential application to migration and imaging in 'shadow zones' of primaries such as in some subsalt region.
S11C-1759
A New Look at the Lithospheric Structure of the Southern Rocky Mountains and the Cheyenne Belt Suture
We have used the Southern Rocky Mountains and the Cheyenne belt suture as a test bed for integrating tectonic scale controlled- and passive-source seismic datasets. The CD-ROM 1999 experiment in the western U.S. was an example of a multi-discipline geoscientific experiment, including a 1000 km long controlled-source seismic line that extended from central Wyoming to central New Mexico. In addition, two passive source seismic transacts focusing on the Cheyenne belt and the Jemez lineament were deployed for one year along the controlled-source seismic (CSS) line. For the large-scale refraction/wide-angle reflection seismic dataset, we applied a new picking strategy and forward-modeled and inverted the resulting seismic picks for a 2-D velocity and interface model of the area. Furthermore, we identified and picked the S-wave phases that were present, and established an independent S-wave velocity model, which allowed us to construct the Vp/Vs and Poisson's ratios. We calculated teleseismic receiver functions for the area based on northern passive seismic transect, which targeted the Cheyenne belt. We then applied a slant stacking technique to determine crustal thickness and Vp/Vs ratios, as well as common conversion point (CCP) stacking and migration techniques, which provide us with additional two-dimensional images of the target area. Only the joint interpretation of both the CSS and receiver function results enabled us to undertake a detailed interpretation of the Cheyenne belt area, which constitutes the transition zone between the 2.7 Ga Archean Wyoming craton to the north and the Proterozoic terranes to the south. The crustal structure is distinctively different between these two areas. A strong mid-crustal layer underneath the Wyoming craton was confirmed, which was identified in the earlier Deep Probe seismic experiment. Furthermore, this layer terminates at depth ~100 km north of the Cheyenne belt, which represents the surface expression of the suture. Our tectonic synthesis shows that the crustal and uppermost mantle structure has the appearance of a crocodile structure, which often forms as a result of the growth of old continents by accretion of younger continental and oceanic units. Our interpretation is consistent with the results of this and earlier studies, and illustrates the advantages of an integrated analysis and interpretation of a variety of datasets.
S11C-1760
Controlled-Source Seismic Imaging of Rift Processes and Earthquake Hazards in the Salton Trough
The NSF MARGINS program, the NSF EarthScope program, and the U.S. Geological Survey have funded a
large seismic refraction and reflection survey of the Salton Trough in southern California and northern
Mexico, including the Coachella, Imperial, and Mexicali Valleys. The purpose of this presentation is to
communicate plans for the seismic project and encourage synergy with piggyback and complementary
studies. Fieldwork is tentatively scheduled for January 2010. The goals of the project include both rifting
processes at the northern end of the Gulf of California extensional province and earthquake hazards at the
southern end of the San Andreas Fault system. In the central Salton Trough, North American lithosphere
appears to have been rifted completely apart. The 20-22 km thick crust is apparently composed entirely of
new crust added by magmatism from below and sedimentation from above. The seismic survey will
investigate the style of continental breakup, the role and mode of magmatism, the effects of rapid Colorado
River sedimentation upon extension and magmatism, and the partitioning of oblique extension. The
southernmost San Andreas Fault is considered at high risk of producing a large damaging earthquake, yet
structure of the fault and adjacent basins are not currently well constrained. To improve hazard models, the
seismic survey will image the structure of the San Andreas and Imperial Faults, structure of sedimentary
basins in the Salton Trough, and three-dimensional seismic velocity of the crust and uppermost mantle.
http://www.geophys.geos.vt.edu/hole/salton/
S11C-1761
Seismic Constraints on the Structure of the Cape Verde Swell
The Cape Verde archipelago, located 600 km off the coast of Senegal, sits on top of oceanic crust accreted during rifting of Africa from the Americas during the Berriasian (early Cretaceous, ~150 Ma). The volcanic islands are superimposed on a large topographic swell ~2500 km in diameter, and reach elevations of ~2 km higher than would be expected from thermal subsidence curves for the plate age. The swell also has a positive geoid anomaly (up to +8 m) and an elevated heat flow (+16 mW m-2), when compared to that of "normal" oceanic lithosphere of similar age. The islands, and their associated submarine edifices, are thought to have formed during the Neogene (<20 Ma), but the age of the regional swell remains uncertain. Existing geophysical data are limited to several 2-D multichannel seismic (MCS) reflection profiles between the islands, and a dated refraction survey with poor coverage and resolution. In September 2004, the first detailed deep seismic experiment in the region was conducted using 40 ocean-bottom seismographs (OBSs) to record wide-angle refraction data along a 500 km transect running north-south between the islands, intersecting the crest of the regional mid-plate swell. This study aims to determine the crustal structure across the swell. The resulting velocity-density-depth model will be used, together with the gravity field, to model various swell support mechanisms and determine the relative importance of buoyancy originating in the lithosphere and the asthenosphere, and will inform a discussion about the possible origin and age of formation of the Cape Verdes. A comparison with other similar features observed globally on plates over a wide range of ages will also be undertaken. Currently 2-D travel-time modelling is being conducted to produce a velocity-depth model of the oceanic crust, uppermost mantle and overlying sediments. This model will be tested for uniqueness using modeller-independent travel- time inversions before moving forward to 3-D flexural and gravity modelling in an attempt to recreate the observed swell signature.
S11C-1762
Wide-angle seismic reflection constraints on the lithosphere of the Variscan Belt on SW- Iberia.
Two wide-angle seismic transects (A and B) were acquired across the Variscan Belt on SW-Iberia. They run across three major tectonic units in the area: South Portuguese Zone, Ossa-Morena Zone, and Central Iberian Zone. Transect A is approximately 300 km long and coincides with the course of the IBERSEIS deep seismic reflection profile. Transect B is 220 km long and is located to the SE of Transect A. The close station spacing along the transects (400 m on Transect A and 150 m on Transect B) allows to identify well defined arrivals within the upper, middle and lower crust as well as in the upper mantle. Resulting velocity models were obtained by forward modeling. The most remarkable features on these models are high velocity areas at mid crustal depths (15-20 km) with velocities in the range of 6.8-7.1 km/s. The Moho discontinuity is located at 31-33 km depth, characterized by a velocity jump from 7.1 km/s to 8.2 km/s. Shot gathers show also a sharp mantle reflection at offsets larger than 180 km which has been modeled as a fairly continuous feature with a velocity increase from 8.2 km/s to 8.4 km/s at 65-67 km depth. The nature of this boundary in still uncertain but it likely reflects a lithological change with subtle velocity/density contrasts, only visible at relatively high incidence angles. The velocity depth function for the crust in the area does not fit any of the standard average crustal velocity models due to the anomalous mid crustal velocities.
S11C-1763
Detailed P- and S-wave Velocity Models Along the LARSE II Transect, Southern California
Structural details of the crust determined from P-wave velocity models can be improved with S-wave velocity models, and S-wave velocities are needed for model-based predictions of strong ground motion in southern California. We picked P- and S-wave travel times for refracted phases from explosive-source shots gathers of the Los Angeles Region Seismic Experiment, Phase II (LARSE II), and we developed refraction velocity models from these picks using two different inversion algorithms. Vp/Vs ratios were calculated from the resulting P- and S-wave models where both models are constrained by ray coverage. The two P-wave velocity models are compared to each other and to results from forward modeling. Generally, the P-wave inverse and forward models agree well for velocities lower than 5.0 km/s but only broadly agree with each other for velocities above 5.0 km/s. Similarly, the S-wave inverse models agree well with each other for velocities lower than 2.5 km/s but only broadly agree for velocities higher than 2.5 km/s. The most prominent structures in our S-wave models are two north-dipping low-velocity zones in the Central Transverse Ranges that we interpret as faults. These low-velocity zones differ somewhat between the two inversion models, but the Vp/Vs models (one model for each technique) show these features to be remarkably similar. Interestingly, both Vp/Vs models have several features that are not visible in either the P- or S-wave models alone. Two of these features (relatively high Vp/Vs ratios) occur in the vicinity of wells that bottom in "granitic" rocks and we interpret these high Vp/Vs ratios to indicate that the granitic rocks are highly fractured or even brecciated. Finally, to evaluate the Southern California Earthquake Center (SCEC) Community Velocity Model (CVM), which predicts Vs based on the Vp model, we compare data from our Vp and Vp/Vs models to empirical formulas that relate P- to S-wave velocities (see Brocher, 2005). These empirical curves provide an adequate average relationship between Vp and Vs, but our model Vs varies as much as ± 20 % for Vp ≤ 5.0 km/s and as much as ± 35 % for Vp ≥ 5.0 km/s. This large variation in the predicted S- wave velocity demonstrates the value of determining Vs independently from Vp.
S11C-1764
Combined tomographic forward and inverse modeling of active seismic refraction profiling data
We present a new code for combined forward and inverse tomographic modeling based on first-arrival travel times of active seismic refraction profiling data (PROFIT – Profile Forward and Inverse Tomographic modeling). The main features of the algorithm involve the original version of bending ray tracing, parameterization based on nodes, variable grid size definition determined by the ray density, and regularization of the inversion. The key purpose of applying the PROFIT code is rather not in solely producing the tomographic image of a continuous velocity field, but in creating a geologically reasonable synthetic model. This model then includes first-order velocity changes representing petrophysical boundaries and is thus better suited for a geological-tectonic interpretation than its smoothed tomographic counterpart. After performing forward and inverse modeling, the synthetic model will reproduce a congeneric model to the tomographic inversion result of the observed data. We demonstrate the working ability of the code using two marine datasets acquired in the Musicians Seamount Province (Pacific Ocean). The results of the tomographic inversion clearly resolve the dominating extrusive volcanism. In addition, the combined forward and inverse approach tests a large variety of synthetic models to fit the observed data tomography. Along both profiles, the preferred structural model includes a strong positive velocity anomaly extending into the seamount edifice. We suggest that this anomaly pattern represents secondary intrusive processes, which are only revealed by the combined tomographic forward and inverse modeling and could not be resolved by exclusively applying a tomographic inversion. In addition, we present examples of imaging salt domes in the Precaspian oil province as well as a higher-resolution field study that was conducted as a preinvestigative study for tunnel construction to demonstrate the capability of the code in different regimes and on different scales.
S11C-1765
Combining the PASSCAL and EarthScope Texan instrument pools for 3D and 3C imaging of the High Lava Plains, Oregon
In early September 2008, 67 scientists, students, and volunteers deployed 2612 Texan short-period seismic
recorders and 120 RT-130 recorders from the PASSCAL and EarthScope instrument pools, and fired 15
seismic sources spaced across the High Lava Plains (HLP) of eastern Oregon and adjacent parts of Idaho
and Nevada. This massive effort is the largest number of instruments deployed in an on-land controlled-
source seismic experiment. This army of helpers includes 42 students from 12 different universities, mainly
the University of Oklahoma, Oregon State, Arizona State, MIT, Stanford, Miami-Ohio, University of Texas at
Dallas, and Rhode Island, ably assisted by 6 staff members from the PASSCAL/EarthScope Instrument
Center. This deployment takes advantage of 100 broadband seismometers in the existing HLP array placed
during the past three years by Carnegie Institution and Arizona State University. The University of Oregon,
Michigan Tech, and the U. S. Geological Survey also deployed an array in the Newberry volcano area to
record earthquakes and the seismic source. Together, these efforts will provide a deep and three-
dimensional image of the structure of this region. New instrumentation built by PASSCAL allowed us to carry
out 3C recording using the Texan facility to study crustal anisotropy. The seismometers were located to
provide high-resolution images of the mantle and crust directly beneath the path of volcanism that dotted the
High Lava Plains during the past 16 Ma. In addition to the seismic component, the overarching project,
funded by the National Science Foundation's Continental Dynamics program, includes field geologists,
petrologists, and geodynamicists interested in resolving the origin of the sudden massive outpouring of
basalt volcanism 16 million years ago and a puzzling trend of age-progressive rhyolite domes that reaches
west toward Newberry volcano, the youngest complex in the trend.
http://www.dtm.ciw.edu/research/CEO
S11C-1766
2D Seismic Imaging of Elastic Parameters by Frequency Domain Full Waveform Inversion
Thanks to recent advances in parallel computing, full waveform inversion is today a tractable seismic imaging method to reconstruct physical parameters of the earth interior at different scales ranging from the near- surface to the deep crust. We present a massively parallel 2D frequency-domain full-waveform algorithm for imaging visco-elastic media from multi-component seismic data. The forward problem (i.e. the resolution of the frequency-domain 2D PSV elastodynamics equations) is based on low-order Discontinuous Galerkin (DG) method (P0 and/or P1 interpolations). Thanks to triangular unstructured meshes, the DG method allows accurate modeling of both body waves and surface waves in case of complex topography for a discretization of 10 to 15 cells per shear wavelength. The frequency-domain DG system is solved efficiently for multiple sources with the parallel direct solver MUMPS. The local inversion procedure (i.e. minimization of residuals between observed and computed data) is based on the adjoint-state method which allows to efficiently compute the gradient of the objective function. Applying the inversion hierarchically from the low frequencies to the higher ones defines a multiresolution imaging strategy which helps convergence towards the global minimum. In place of expensive Newton algorithm, the combined use of the diagonal terms of the approximate Hessian matrix and optimization algorithms based on quasi-Newton methods (Conjugate Gradient, LBFGS, ...) allows to improve the convergence of the iterative inversion. The distribution of forward problem solutions over processors driven by a mesh partitioning performed by METIS allows to apply most of the inversion in parallel. We shall present the main features of the parallel modeling/inversion algorithm, assess its scalability and illustrate its performances with realistic synthetic case studies.
S11C-1767
JAMSTEC R/V Kairei; New marine active-source seismic system (for high resolution deep seismic image)
Active source seismic imaging is a powerful way to reveal subsurface structure with reflection profiles and velocity models. From the reflection images and velocity models, we can understand the structural characteristics as deformation of faultings and foldings, and velocity inhomogenity of seismogenic zones. To understand the seismogenic process at the subduction zone and arc evolution, Japan Marine Science and Technology (JAMSTEC) has been conducted marine seismic experiments using R/V Kairei since 1997 with a non-tuned 12000 cu. in. airgun array and a 204-ch streamer with 25 m group interval and 5-km length. To improve the seismic imaging, JAMSTEC installed new seismic system to the R/V Kairei in March 2008, which consisted of a tuned airgun array of 7800 cu. in. and a 444-ch streamer with 12.5 m group interval and 6-km length. The new seismic system is aimed not only for shallow imaging with near offset signals for multichannel reflections (MCS) but also deep imaging with those of long offset for ocean bottom seismometers (OBS). Sea trials were successfully finished with smooth operations and good quality data with varied acquisition parameters. The MCS data shows the bubble-free dense stratified reflections and sharp layer boundaries. Source comparisons were also conducted for OBS using the tuned airgun array of R/V Kairei and a non- tuned 12000 cu. in. airgun array of R/V Kaiyo. We will present these MCS and OBS data using the tuned and non-tuned sources for the comparison and analysis.
S11C-1768
Integrated seismic imaging of active and passive data for the delineation of active faults and crustal structure in the Kitakami Lowland, Northeast Japan
The deep geometry of active faults and the mid-crustal detachment at the base of seismogenic layer is important for understanding active tectonic process and accessing the risk of destructive earthquakes. To investigate the deeper extension of active faults within the seismogenic layer, we conducted a seismic reflection profiling experiment across the western marginal faults of Kitakami lowland, northeast Japan, in 2006 and 2007. The combination of telemetry and independent recording system has provided the deployment of wide-angle survey line with dense seismic array. The simultaneous data acquisition of regional refraction, low-fold wide-angle reflection and dense reflection survey has been optimized by the integration of vibrator source focused on effective low-frequency bandwidth of sweep signal and the three-component digital accelerometers with broader frequency responses. The seismic reflection profile shows that the deeper extension of the western marginal faults of Kitakami lowland converges on the mid-crustal detachment. Along the reflection survey line, from September to December, 2007, a dense seismic array with the combination of short-period seismometers and digital accelerometers was deployed for teleseismic and regional-earthquake observation. We utilized multimode prestack migration for receiver function and interferometric seismic imaging for back scattered phases to investigate the heterogeneous crustal structure and Moho boundary. The Kitakami lowland is located 15km northeast of the source region of 2008 Iwate- Miyagi Nairiku earthquake (Mj7.2). We further discuss the relation between aftershock distribution of this earthquake and crustal structure estimated from the joint imaging of active and passive seismic data.
S11C-1769
Processing of high resolution seismic reflection data of Outokumpu, Finland
The Outokumpu area, located in eastern Finland, is well known for its unconventional Precambrian sulphide deposits. In 2004-2005 a 2,5 km deep research borehole of ICDP (International Continental Scientific Drilling Program) was drilled on the south-east side of the main ore belt. The main lithologies observed in Outokumpu deep drill hole were mica schist with biotite-gneiss layers (upper 2 km) underlain by pegmatic granite. The ophiolite-related Outokumpu-assemblage rocks were observed at depth range of 1,3-1,5 km. In May 2006 high resolution seismic soundings were done near the drill hole in two crooked lines to further refine the geological model of the area. Vibrator source with linear upsweep from 15 to 250 Hz was used in 20 m interval. In reflection/refraction survey 14 Hz geophones were spaced in 4 m apart. During VSP measurements 3C downhole receiver was positioned at depths of 1000, 1750 and 2500 m. Processing of Outokumpu high resolution seismic reflection data included amplitude and gain corrections, band-bass filtering, careful velocity analysis and static corrections. In Outokumpu substantial topographical variation and significant velocity contrast between the glacially deposited overburden and the bedrock caused a severe travel time variations in near surface. Static corrections were done by using standard refraction method and tomographic approach. Tomographic model of near surface layers was done using traveltime inversions of critically refracted P-wave arrivals of refraction data collected in May 2006. Quality of unmigrated stack was clearly better when tomographic model was used for static corrections. In Outokumpu both sonic log data and velocity model derived from VSP-measurements were used to improve the quality of velocity analysis. Processing of the reflection seismic data revealed a good correlation between the seismic section and the lithologies observed in deep drill hole. Sonic and density logs were used to calculate acoustic impedances and reflection coefficient between adjacent rock units.
S11C-1770
Multi-azimuth Anisotropic Velocity Measurements in Fractured Crystalline Rock From the International Continental Drilling Program Outokumpu Borehole, Finland
A high resolution seismic survey consisting of a multi-depth multi-azimuth VSP, a zero-offset VSP and a reflection/refraction survey was conducting in May, 2006, near the town of Outokumpu, Finland, using the International Continental Scientific Drilling Program 2.5 km deep fully cored scientific borehole. The survey was undertaken in order to create an anisotropic velocity model for future micro-seism studies as well as to provide a higher resolution reflection profile through the area than was previously available. The seismic survey high frequency seismic vibrator as a source, employing 8 s linear taper sweeps from 15-250 Hz at 20 m shot spacing. Receivers were 14 Hz single component geophones on the surface and a three component geophone downhole. The walk-away VSP included measurements over two azimuths with the receiver at depths of 1000, 1750 and 2500 m, while the zero-offset VSP used a 2 m depth increment. Surface geophones were located along the same seismic lines as employed in the walk-away VSP and were nominally 4 m apart. The survey area is located on the Fennoscandian shield, and the glacial history of the area required significant static corrections to account for the variable overburden overlying the mica-rich schist and pegmatitic granite composing the bedrock. These were calculated using travel-time inversion of the refraction data and were applied to the walk-away VSP and reflection profiles, significantly improving the quality of both. Anisotropic velocity analysis was performed using a plane-wave decomposition of the processed walk-away VSP. The maximum anisotropy was observed in the walk-away VSPs along the Southeastern azimuth, with the P-wave phase velocity ranging from 5330-5950 m/s between 50-1000 m in depth, and up to 6150 m/s between 1000-1750 m in depth. Shear wave splitting was observed in the Northeastern direction. Preliminary analysis of the zero-offset VSP has revealed shown good agreement with the relevant portions of the anisotropic velocity measurements and the reflection profile.
S11C-1771
Structure of the Periadriatic Fault in the Eastern Alps from reflection seismic imaging
The Periadriatic Fault (PF) is the most important fault system of the Alps, separating the Southern Alps over a length of ~ 700 km from the European plate. Despite its prominence, the fault is seismically inactive, and its tectonic role and deep structure are a contentious matter, particularly in the Eastern Alps, where the European margin is overlain by thick thrust sheets of Apulian origin. We used Vibroseis recordings from the deep seismic TRANSALP survey to directly image the steeply dipping PF. Reflection images from these data published by Lueschen et al. (2004) are based on CMP-stacking methods, and can not provide direct evidence of the steeply dipping PF. Instead, Lueschen et al. used circumstantial evidence (dip and termination of a series of nearby reflectors) to propose two possible dip directions for the PF: 45¡ã towards the South, or 60¡ã towards the North. Based on wide-angle observations of Vibroseis data, Bleibinhaus and Gebrande (2006) argued against these interpretations, and they presented some evidence, albeit weak, for a vertically dipping PF. In order to decide this controversy, we reprocessed some of the Vibroseis reflection-data with methods appropriate for steep-dip imaging. We used a Kirchhoff-prestack-depth-migration algorithm and the tomographic p-wave velocity model of Bleibinhaus and Gebrande (2006) for imaging the upper and middle crustal structure in a N-S-profile at 12¡ãE between the Tauern Window and the Valsugana Thrust Belt. The resulting images show the PF dipping almost vertically in the upper 3 km, with a minor branch offset ~ 2 km to the South. At greater depth the PF increasingly bends towards the South from vertical to ~75¡ã at 8 km to ~ 60¡ã at 16 km depth. Reflection amplitudes gradually fade out near 20 km depth, where the PF appears to cut the reflectors that have been related to the Valsugana thrust. If the bending continues, the PF would become horizontal near 25 km depth. Obviously, tectonic models that require a N-dipping or a shallow PF are inconsistent with our images. The intrusion of a wedge of European basement into Apulian crust could explain the new observations. The PF could have acted as a thrust ramp, before it steepened during the ascent of the Tauern Window. Its structure at depth suggests a relation to the mid-crustal shear horizon imaged by Kummerow et al. (2004) and Lueschen et al. (2004).
S11C-1772
Imaging the Socorro Magma Body Using Free Above-Ground Sources
The Socorro Magma Body (SMB) is located within the Rio Grande Rift and is intersected by the Precambrian Socorro Fracture Zone near Socorro, NM. The SMB seems to be the source of a 5,000 km2 area of elevated seismic region known as the Socorro Seismic Anomaly. The first evidence of a subsurface reflector was from microearthquake studies. A COCORP seismic reflection profile provided further evidence for an essentially flat magmatic sill-like intrusion approximately 19 km below the surface, with less than a 1° slope and a lateral area of about 3400 km2 with an estimated thickness of about 100 m. A fundamental question regarding the SMB is related to the nature of its activity. The uplift associated with the SMB coupled with the presence of shallow earthquake swarms in the area is typically associated with the movement of magma, which may be indicative of active magmatic emplacement. As a pilot test to obtain P-wave velocity data, we used free explosive sources from the Energetic Materials Research and Testing Center (EMRTC) at New Mexico Tech in Socorro, NM. Our goals were to determine how much seismic energy is necessary to receive a decent signal back on the recorders and also to develop a preliminary refraction velocity model over the SMB. For this refraction experiment, 59 single-channel recorders (Texans - RT125a) were deployed over a distance of 125 km for a 1-week period centered at the EMRTC blast site. Over that time period, EMRTC set off six ~9,000 lb (4,082 kg) ANFO shots above ground. Although much of this energy went into the air, we were able to recover a small amount of this energy to build preliminary velocity models. The energy created by the blasts propagated about halfway through the array. These data have been used to produce a couple of 1-D models and a preliminary 2-D model of apparent velocity. We plan to use these results to develop a proposal to conduct a full controlled and passive-source experiment over the SMB in the near future.
S11C-1773
Characteristics of Moho transition zone: MCS reflection records and petrological aspects and physical properties
The Moho is defined as the seismological discontinuity at the crust and mantle boundary. Its global depth, thickness of transition zone, and velocity structure has not been studied well. It is also poorly known whether the Moho has the same petrological and seismological properties in the continent and in the ocean, or not. Previous studies propose several petrological models for the Moho: 1) phase transition boundary from basalt to eclogite, and 2) material boundary of mafic and ultramafic rocks. By the petrological observation in the Oman ophiolite, the oceanic crust is modeled as 3) diabase-homogeneous gabbro - layered gabbro – Moho transition layer - harzburgite. The thickness of Moho transition zone (MTZ), at the boundary between Earth's crust and the subjacent mantle, has significant effect on the seismic responses from the Moho. We examined seismic characteristics of Moho reflection (hereafter PmP) using MCS (Multi Channel Seismic) reflection records obtained by high quality seismic experiments in the western Pacific by JOGMEC (Japan Oil, Gas and MEtals national Corporation). The MCS records show clear reflections at ~ 6-10 km in depth from the ocean bottom in the north and south of Ogasawara Plateau. However, considering horizontal variation in the PmP intensity, the nature of the MTZ varies from place to place. In the seismic profile D00-D, across Ogasawara Plateau in the N-S direction, the PmP abruptly disappears far from the nearby seamount where the overlain sedimentary section has less change. In another case, shown in D00-C that is located 130km west of D00-D, the PmP clearly shows high-amplitude continuous reflection near the seamount's flank. Data acquisition is relatively constant for the Ogasawara MCS reflection lines; therefore, the difference in the PmP intensity between D00- D and D00-C may relate to the nature of the Moho. The comparison of reflection records and synthetic waveforms calculated by Tsuruga et al.(this meeting) shows that if the gradient of the Moho transition zone is greater than 1 (km/s)/km, PmP can be observed by the current MCS survey equipments. If the dominant frequency of the MCS reflection survey is ~15 Hz, penetrating down to the Moho depth, then the thickness of the Moho to identify the PmP should be less than a few hundred meters. The MCS reflection records in the western Pacific and the western Philippine Sea Basin suggest that the thickness of MTZ varies from ~100 m to over a few kilometers. This is consistent with the petrological observation in Oman ophiolite, sections of oceanic crust and possible mantle rock, showing the thickness of the mafic crust to ultra-mafic mantle transition varies from the order of meters to a few kilometers.
S11C-1774
Joint Interpretation of Multi-parameter Tomographic Models (e.g., Seismic P and S Velocity, Anisotropy, Attenuation): A Neural Network Approach
Seismic tomography can provide a set of models which represent different properties of the same target region. A typical example is the development of coincident P and S velocity cross sections from travel time tomography. Other applications may include additional determination of attenuation and anisotropy. Self-organizing maps (SOM) are powerful neural network techniques to classify and interpret multi-attribute data sets. The coincident tomographic images are translated to a set of data vectors in order to train a Kohonen layer. The total gradient of the model vectors is determined for the trained SOM and a watershed segmentation algorithm is used to visualize and map the lithological clusters with well-defined seismic signatures. The principal working flow is demonstrated for a synthetic data set. Further examples include P and S velocity tomography across a sub-volcanic ring complex in Namibia, and combination of velocity, anisotropy, and attenuation tomography to characterize gas hydrate bearing sediments in the Mackenzie Delta, NW Canada.
S11C-1775
Characteristics of the Moho Transition zone: Seismological interpretation by synthetic seismograms
MCS reflection records in the western Pacific Ocean show wide variations of seismological characteristics of
the Moho from no reflections to strong and continuous reflections (Kasahara et al., this meeting). In order to
find the relation of such wide variation of characteristics and the nature of the Moho Transition Zone (MTZ),
we examined the characteristics of the Moho reflections using seismic waveform simulation. Our results
indicated that some particular characteristics give us a petrological prospect for the MTZ.
We calculated the pressure waveforms by using 2-D finite difference method code (Larsen, 2000), with
assuming Vp, Vs, density, and Q values. We used 4-Hz 0-phase Ricker wavelet as explosive sources near
sea surface. The travel times of P-waves were calculated by graph method (Kubota et al., 2005). Crustal
structure model consists of four major horizontal layers beneath the water depth of 6 km: sedimentary layer
(Vp=1.8-2.2 km/s), basaltic upper crust (Vp=2.5-6.5 km/s), lower crust, (MTZ) and upper mantle. Based on
several previous studies (e.g., Penrose model), we examined four velocity models as follows: (A) Sharp Vp
boundary at the Moho discontinuity: Vp jumps from Vp=7.0 km/s to Vp=7.6, 8.0 or 8.8 km/s at the boundary;
(B) MTZ beneath the lower crust: Vp increases from 7.0 to 8.0 km/s in a MTZ as a mixing layer of crustal and
mantle materials; (C) MTZ beneath a sharp Vp boundary: Vp jumps from 7.0 to 7.6 km/s and increases to 8.2
km/s within MTZ; (D) Lack of large Vp contrast within the MTZ as a large amount of serpentinized peridotite:
Vp increases from 6.8 to 8.2 km/s through the lower curst and the upper-most mantle.
We obtained the following characteristics of reflection phases from the MTZ observed at smaller offset
distance of 5 km on synthetic seismograms: (A) PmP (reflected P-wave from the Moho) has large amplitude
at small offset distance. Its amplitude increases when a velocity gap is large at the Moho. PmPfs amplitude
at 0-offset is 1/5-1/3 of the amplitude of the reflections from the seafloor and sediment/hard-rock boundary.
(B) Reflected phases (Px1P and Px2P) from the top and the bottom of MTZ, and refracted phase in the MTZ
occur. Travel time of Px2P changes with the thickness of MTZ. Amplitude of Px1P is 1/50 of the reflected P-
wave at the seafloor and soft-sediment/hard-rock boundary, but Px2Pfs amplitude is generally small. (C)
Amplitudes of Px1P for all cases are similar to PmP in the case of Model-A associated with Vp=7.6 km/s.
Reflection Px2P from the bottom of the MTZ is extremely small. (D) PmP has no significant amplitude in the
synthetic seismograms. It is difficult to discriminate the feature of PmP from Model-D to Models-B or -C.
These results shows that the resolution of reflection phase from the MTZ observed at a small offset distance
depends on the limited aperture of MCS reflection survey as well as the nature of the MTZ (e.g., velocity and
its gradient). In other words, if the wide-angle reflection and refraction records with far offsets (i.e., > 10
km) are also used, we conclude that some particular seismic characteristics may give us the possible
petrological models for the MTZ.
S11C-1776
Using Pseudo 3-D P-wave Seismic Reflection Data for Developing a Robust Geologic Conceptual Model in Site Characterization
P-area at the Savannah River Site is located in the upper Atlantic Coastal Plain of South Carolina. The site consists of approximately 350 m of interspersed, unconsolidated sand, clay, and gravel deposits. At P-area there is evidence for a contaminant plume of dissolved phase trichloroethylene (TCE) located in the Eocene age sand. The geometry of the plume, based on initial site characterization, appears to be confined to a narrow corridor within the sand overlying a clay unit approximately 25 meters below land surface. As part of a multi-scale hydrogeophysical and modeling study, a pseudo 3-D seismic reflection survey was conducted over the plume area to enhance the existing geologic model by resolving uncertainty in the litho-stratigraphic sequence. The survey area was 34 by 170 m, and the data were processed as a 3-D data volume instead of a series of closely spaced 2-D lines, allowing for better interpretation of the target horizons. The results show that there are two unexpected sand channel complexes that were interpreted on the seismic volume. These sand channels were not present in the initial conceptual model, and the middle and lower clays were found not to be continuous as previously thought. The geometry of the primary sand channel has been transposed over the plume to investigate any potential correlation between the shape of the plume and the presence of the channel complex. Based on this analysis, it is clear that the sand channel controls the plume shape. We also calculated the seismic attributes to correlate with the other hydrogeophysical data to be used in the modeling portion of the project .The outcome was the production of realistic horizon surfaces maps. Calibrating the seismic data with existing borehole geophysical logs, core data, as well as vertical seismic profiling (VSP) data at the site, allowed the seismic data to be inverted from two-way travel time to depth, thereby facilitating full integration of the seismic data into a solid earth model that is the fundamental part of a site conceptual model.
S11C-1777
Spectral-element simulations of wave propagation in complex exploration-industry models: Mesh generation and forward simulations
Seismic-wave propagation in exploration-industry settings has seen major research and development efforts for decades, yet large-scale applications have often been limited to 2D or 3D finite-difference, (visco- )acoustic wave propagation due to computational limitations. We explore the possibility of including all relevant physical signatures in the wavefield using the spectral- element method (SPECFEM3D, SPECFEM2D), thereby accounting for acoustic, (visco-)elastic, poroelastic, anisotropic wave propagation in meshes which honor all crucial discontinuities. Mesh design is the crux of the problem, and we use CUBIT (Sandia Laboratories) to generate unstructured quadrilateral 2D and hexahedral 3D meshes for these complex background models. While general hexahedral mesh generation is an unresolved problem, we are able to accommodate most of the relevant settings (e.g., layer-cake models, salt bodies, overthrusting faults, and strong topography) with respectively tailored workflows. 2D simulations show localized, characteristic wave effects due to these features that shall be helpful in designing survey acquisition geometries in a relatively economic fashion. We address some of the fundamental issues this comprehensive modeling approach faces regarding its feasibility: Assessing geological structures in terms of the necessity to honor the major structural units, appropriate velocity model interpolation, quality control of the resultant mesh, and computational cost for realistic settings up to frequencies of 40 Hz. The solution to this forward problem forms the basis for subsequent 2D and 3D adjoint tomography within this context, which is the subject of a companion paper.
S11C-1778
Spectral-element simulations of wave propagation in complex exploration-industry models: Imaging and adjoint tomography
Seismic imaging in the exploration industry is often based upon ray-theoretical migration techniques (e.g., Kirchhoff) or other ideas which neglect some fraction of the seismic wavefield (e.g., wavefield continuation for acoustic-wave first arrivals) in the inversion process. In a companion paper we discuss the possibility of solving the full physical forward problem (i.e., including visco- and poroelastic, anisotropic media) using the spectral-element method. With such a tool at hand, we can readily apply the adjoint method to tomographic inversions, i.e., iteratively improving an initial 3D background model to fit the data. In the context of this inversion process, we draw connections between kernels in adjoint tomography and basic imaging principles in migration. We show that the images obtained by migration are nothing but particular kinds of adjoint kernels (mainly density kernels). Migration is basically a first step in the iterative inversion process of adjoint tomography. We apply the approach to basic 2D problems involving layered structures, overthrusting faults, topography, salt domes, and poroelastic regions.