S13D-01
New Seismic Images of the San-Andreas-Fault System at SAFOD
We have processed active and passive seismic data sets acquired in the vicinity of the San-Andreas-Fault at SAFOD in order to derive a detailed structural image of the fault system. On one hand we have complemented our existing active seismic images obtained from the vertical component of the SAFOD2003 reflection seismic data set by processing also the horizontal components. Due to the subvertical orientation of most fault branches and the low emergence angles at the surface the horizontal components contain a significant part of those reflections. The imaging approach involves the separation of P- and S-waves from shot gathers and the subsequent application of our Fresnel-Volume- Migration technique which is particularly suited to image steeply dipping structures. The results are in good agreement with the existing images derived from the vertical component and furthermore provide some new and interesting constraints on the internal structure of the fault system as well as the deeper structure (5-10 km) on both sides below the main surface trace of the SAF. On the other hand we have further exploited the microseismic wavefield recorded by an 80-level-3C-receiver array in the SAFOD main hole. We used our prior location of some of the events and treated them as pseudo-active sources with the hypocenter as the source point. This allows to apply the same active seismic migration approaches to the corresponding passive seismic wavefield. Hence we have obtained high-resolution images of some faults branches in the vicinity of the borehole and the hypocenter. We have combined our results also with additional available information from other geophysical disciplines (borehole, MT, etc.). This yields some new interesting insights into the structure and the dynamics of this mega-shear zone on different scales and in particular in the vicinity of the SAFOD borehole.
S13D-02 INVITED
Understanding the Long-Term Deformation in the Mississippi Embayment: the Mississippi River Seismic Survey
The Central US hosts one of the most active intraplate seismic areas in the world, the New Madrid seismic
zone (NMSZ). Here the high level of historic and instrumental seismicity clashes with the subdued topography
of the Mississippi embayment, minimal geodetic vectors and a puzzling lack of substantial deformation in the
post Late-Cretaceous sediments. To explain this apparent paradox it has been proposed that the seismicity
in the NMSZ is either 1) very young (at least in its present form), 2) episodic, or 3) migrates throughout a
broad region.
In order to test these hypotheses and to understand how the deformation is partitioned within the Mississippi
embayment, we collected a 300 km-long high-resolution seismic reflection profile along the Mississippi river,
from Helena, Arkansas to Caruthersville, Missouri. The profile images a portion of the embayment outside the
area of influence of the NMSZ in a region where evidence has been mounting of a seismic source, predating
the NMSZ, for which no corresponding structure has yet been identified.
The seismic survey exploited the advantages of marine acqui9sition (time effective, low cost) using a 245/245
cm3 (15/15 in3) mini-GI airgun fired at 13.790MPa (2000 psi), a 24-channel 75 m-long active
streamer, with 3.125 m group and 12 m nominal shot interval. The high quality data image the Cretaceous
and younger sedimentary section, from the top of the Paleozoic unconformity to the Quaternary deposits.
Preliminary interpretation of the dataset confirms the general deepening of the Paleozoic basement from
~800 ms at Caruthersville, to ~1 s at the southern end of Crowley's Ridge. In addition, the data reveal
prominent recent deformation coincident with the Blytheville arch, the Eastern Reelfoot Rift margin and the
White river Fault zone, accommodated by folding and faulting that extend from the top of the Paleozoic
through the sedimentary section, and that involves the Quaternary deposits.
http://www.ceri.memphis.edu/people/mmagnani/missriver/miss_river.html
S13D-03
Near-Vertical Moho Reflections Under the Hanoi Basin, Vietnam
Surface expression of the Red River fault, a major strike-slip fault originating from the India-Eurasian collision, terminates as a large pull-apart basin in northern Vietnam. The onshore part is called the Hanoi basin and is an extension of the larger offshore Song Hong basin. Rifting in these basins began early Eocene with inversion in the late Miocene and continued subsidence today. Gravity studies indicate crustal thinning under the Hanoi basin, however both the Hanoi and Song Hong basins are in near isostatic balance resulting in relatively small gravity anomalies from large crustal-scale features. Hence, seismic methods would seem to be a more appropriate method for studying crustal structure in this situation. In January 2008 we shot the first deep crustal seismic lines in Southeast Asia in and near the Hanoi basin. Crustal seismic experiments in densely populated areas are challenging because of the high cultural noise levels and the lack of available space for shotpoints. This experiment however produced an unusually strong near-vertical reflection from the Moho under the Hanoi basin as well as a number of other arrivals. Analyses of these arrivals indicate the crust is 27 km thick, thinner than estimated from gravity data. They also show the Moho is a complex reflector with a high impedance contrast.
S13D-04 INVITED
Faulting at the Epicenter of the 1886 Charleston, South Carolina Earthquake Imaged by Seismic Reflection Profiling
The 1886 Charleston, South Carolina earthquake was one of the best-documented earthquakes of the 19th century. However, many basic questions remain concerning the geologic nature of the seismic source. Reprocessing of several seismic-reflection profiles collected almost thirty years ago in the epicentral area near Summerville is shedding new light on this problem. The study area is within the Atlantic Coastal Plain and is underlain by approximately 800 meters of Cretaceous and Cenozoic sediments. The basement is a Mesozoic terrane comprised of clastic sedimentary and volcanic rocks. The top of basement throughout the area is marked by a strong reflection due to the unconformable contact between Cretaceous sediments and a lower Jurassic basalt. The thickness of the Mesozoic section is unknown. Our initial work involved seismic reflection line VT-3b collected in 1981 by the Virginia Tech Regional Geophysics Laboratory, in collaboration with the U.S. Geological Survey, along the Ashley River southeast of Summerville. VT-3b shows clear evidence of a down-to-the-east, steeply-dipping normal fault with approximately 200 m of vertical offset, displacing horizontally layered Lower Mesozoic sedimentary and volcanic rocks. The overlying Cretaceous and Tertiary sediments show associated reverse displacement, resolved by the data to within 100 meters of the ground surface. This fault is associated with very intense diffracted energy, which led to its discovery. Two other near-vertical faults with down-to-the east offset of Lower Mesozoic units were imaged on VT-3b immediately to the northwest of the major fault. The location coincides with the epicenters of modern seismic activity, and maximum intensity mapped in 1886. The results of our more recent work with the remaining seismic profiles collected by Virginia Tech, USGS and COCORP in the Summerville area suggest that the faulting imaged on VT-3b is in the central portion of a graben within the Triassic-Jurassic basement rocks. Four seismic profiles a few kilometers to the west and north of Summerville image a feature that may mark the western boundary of this graben. This apparent boundary fault is northeast-trending, near-vertical and both the top of basement and overlying sedimentary sequence exhibit a temporal offset of approximately 30 msec, down-to-the-east. This apparent fault coincides with a strong magnetic gradient separating low magnetic intensity to the west-northwest from high magnetic intensity to the east-southeast. To the southeast of this feature, the basement reflectors both deepen and brighten. This suggests the presence of basalt/diabase interbedded with the lower Mesozoic clastic sedimentary rocks of the graben, and may be the source of the high-intensity magnetic anomaly. The center of the graben imaged by the reflection data is southeast of Summerville, in the vicinity of the intense faulting imaged on profile VT-3b, where the down-to-the-east normal faults within the basement have been reactivated with reverse displacement affecting the Cenozoic section. The basement reflectors shallow further to the southeast and the southeastern margin of the graben is approximately 5 km west of the Charleston Peninsula, based on the one seismic profile in that area.
S13D-05
Seismic Waveform Tomography Using Multi-component Data at a Shallow Groundwater Contamination Site
Seismology is becoming a more commonly used technique for characterizing contaminated groundwater aquifers and other near-surface problems. Currently, active-source seismic studies typically focus on the P- wave field, however SH-wave data may offer benefits such as having higher intrinsic resolution at a given frequency and being relatively independent of pore fluid, potentially allowing for a less ambiguous determination of lithology. We have acquired a 2D, nine-component seismic dataset over a shallow (~10 m depth) contaminated aquifer at Hill Air Force Base, Utah. To date, work has focused on two components: a vertical source and receiver (P-wave dataset), and a horizontal transverse source and receiver (SH-wave dataset). Previously, the P- and SH-wave datasets have been processed using post-stack depth migration and traveltime inversion to produce a structural image and velocity model. These previous results indicate that the quality of the SH-wave and P-wave datasets are comparable. Although noisy, both datasets are sufficient to produce a useful image of the subsurface. In the current study we use full waveform tomography to produce high-resolution velocity models of the shallow subsurface for interpretation. Full waveform tomography (FWT) has so far been developed primarily under the acoustic approximation, and several previous studies have used acoustic FWT to image single-component surface seismic data. To our knowledge there have been no previous attempts to use FWT on an SH-wave surface seismic dataset. We have developed an alternate version of the acoustic FWT method based on the SH-wave equation. Acoustic and SH-wave FWT are applied to the P- and SH-wave datasets respectively. The quality of the resulting images will be compared to determine what benefits are obtained using an SH dataset for FWT. In particular, we expect SH-wave FWT to provide better resolution immediately below the water table where the S-wavelength is much shorter than the P-wavelength. Furthermore, the P- and S-wave velocity models may be jointly interpreted to further constrain elastic properties in the subsurface beyond the velocity information provided by acoustic tomography alone. This should enable us to enhance our understanding of near- surface structure and lithology at the contamination site and provide valuable counsel for efficient targeting of remediation efforts.
S13D-06
Combined Seismic Refraction Inversion, Reflection, and Electrical Resistivity Tomography Imaging of a Glacially Buried Valley
Buried valleys are common in the regions of the Northern Hemisphere covered by ice sheets during the last glaciation. The valleys are filled by a variety of glacio-lacustrine and glacio-fluvial sedimentation. These valleys are important sources of fresh water, aggregates, and even shallow methane deposits. The surface expressions of the buried valleys, however, are often not apparent and they are often only serendipitously found during drilling for water or petroleum. Geophysical investigations of one deep (~ 350 m) buried valley in northern Alberta that had been located on the basis of geophysical logs were carried out. This buried valley was somewhat unique in that shallow (~ 30 m) methane deposits, providing a significant hazard to drilling, exist. A 10 km high resolution seismic profile (vibrator sweep 14 Hz to 250 Hz, 40 Hz geophone singles at 4 m spacing) was obtained. Significant differences in the raw shot records were apparent across the array due to the lateral differences in compressional wave velocity between the untouched bedrock and the valley fill sediments. Travel time inversion of first arrivals and deeper reflections further quantifies this lateral variation showing that the valley fill and bedrock velocities differ by more than 50% ranging from about 1700 m/s to nearly 3000 m/s, respectively. The reflection seismic image agrees well with the refraction inversion. The gross structure of the steep-sided valley is apparent. The internal architecture, however, shows a variety of clino-form dipping reflectors at the edge of the valley that are possibly related to subglacial sedimentation, a strong dipping reflector that is unconformable with the others and may be representative of recurrent discharge events, and numerous flat lying reflectors that are likely related to lacustrine sedimentation. Co-incident electrical resistivity tomography, too, is largely in agreement with the gross structure. The clay rich bedrock shales are substantially more conductive than the valley fill materials. Large variations of electrical conductivity within the fill are associated with fresh water pockets and the dry, gas saturated zone. This combination of both the seismic profiliing and the ERT conductivity mapping provides complementary information leading to better interpretation.
S13D-07 INVITED
A Look at the Future of Controlled-Source Seismology
Facilities like EarthScope and IRIS/PASSCAL offer a framework in which to re-assess the role of our highest- resolution geophysical tool, controlled-source seismology. This tool is effective in near surface studies that focus on the upper 100 m of the crust to studies that focus on Moho structure and the lithospheric mantle. IRIS has now existed for over two decades and has transformed the way in which passive-source seismology in particular is carried out. Progress over these two decades has led to major discoveries about continental architecture and evolution through the development of three-dimensional images of the upper mantle and lithosphere. Simultaneously the hydrocarbon exploration industry has mapped increasingly large fractions of our sedimentary basins in three-dimensions and at unprecedented resolution and fidelity. Thanks to the additional instruments in the EarthScope facility, a clear scientific need and opportunity exists to map, at similar resolution, all of the crust - the igneous/metamorphic basement, the non-petroliferous basins that contain the record of continental evolution, and the seismogenic faults and active volcanoes that are the principal natural hazards we face. Controlled-source seismology remains the fundamental technology behind exploration for all fossil fuels and many water resources, and as such is a multi-billion-dollar industry centered in the USA. Academic scientists are leaders in developing the algorithms to process the most advanced industry data, but lack the academic data sets to which to apply this technology. University and government controlled-source seismologists, and their students who will populate the exploration industry, are increasingly divorced from that industry by their reliance on sparse spatial recording of usually only a single-component of the wavefield, generated by even sparser seismic sources. However, if we can find the resources, the technology now exists to provide seismic images of immense scientific and societal value that play a key role in fulfilling the ambitious missions of EarthScope and other NSF programs, as well as, those of agencies such as the U. S. Geological Survey and Department of Energy. US controlled-source community has self-organized to form an IASPEI U.S. National Committee on Controlled-Source Seismology to facilitate communication and to present and pursue the fundamental needs to sustain this scientific community as a resource for all earth scientists.
S13D-08
High-Resolution, Shallow Seismic Imaging of the Imperial Fault, Imperial County, California
During April 2008, we acquired high-resolution, P- and S-wave seismic reflection and refraction data across the Imperial fault, north of its junction with the Brawley fault at Harris Road, Imperial Valley, California, to determine the dip, possible structural complexities, and seismic velocities associated with the Imperial fault. Our 600-m-long P-wave profile was recorded on 120 active channels, with geophones spaced at 5-m intervals. P-wave sources were generated by multiple accelerated weight-drop impacts or by Betsy-Seisgun 'shots.' S-wave sources were generated along the central 300 m of the P-wave profile using multiple sledgehammer impacts on a metal block. We developed a tomographic seismic velocity image and stacked and migrated reflection images from the P-wave data. Preliminary interpretation of data suggests that the Imperial fault at Harris Road is much more structurally complex than previously recognized, with at least six near-surface fault strands along the profile; the main trace, two fault strands to the east, and three fault strands to the west. In the past 60 years, repeated measurements of primary tectonic slips, afterslip, and triggered surface slips have been made at this location following significant earthquakes, but only a single fault trace has ruptured the surface at the location of our seismic profile. However, on the east side of the main fault, two surface breaks from previous earthquakes project toward our seismic profile, suggesting slip occurred below the surface on the other seismically imaged fault strands. Such 3-D complexity of the fault zone needs to be understood to fully evaluate the slip rate and history of the Imperial fault.