Seismic Inversion and Its Applications to Exploration and Global Earth Structures I Posters
Presiding: P D Anno, ConocoPhillips; P S Routh, Boise State University
S41A-01 0830h
Crust and Upper Mantle Structure Beneath Sichuan/Yunnan Provinces, SW China: Preliminary Results of a Joint MIT-CIGMR Broad-Band Seismometry Project
As part of a multi-year NSF Continental Dynamics project investigating the structure and geological evolution of the eastern part of the Tibetan plateau, we operated a 25 station array (3-component, broad band seismometers) in Sichuan and Yunnan provinces, from 09/2003-09/2004. A prime overall objective of the seismometry project is to establish the level of mechanical coupling between the (upper) crust and the mantle, and specific targets include (1) determination of crustal structure, (2) characterization of (shallow) mantle interfaces beneath SW China, (3) characterization of seismic anisotropy in crust and shallow mantle, and (4) determination of upper mantle structure through travel time and surface wave array tomography. Receiver function analysis has produced maps of Moho depth and evidence for a mid crustal low-velocity zone, which appears to thin eastwards from the high plateau to the lowlands of Yunnan and Sichuan. A two layer anisotropy model suggest a spatially coherent pattern of fast axes of olarization in the upper crust that is consistent with inferences from GPS studies but quite different from the direction of anisotropy at larger depth. This observation is strongly suggestive of mechanical decoupling of crust and mantle. Incorporating the travel times of phases recorded at the array allow us to constrain upper mantle structure with much greater resolution and confidence than before, and the preliminary models reveal intriguing variations in P-wavespeed in the upper mantle beneath the Red River fault zone. For example, some of the seismically slow structures beneath the eastern part of the plateau may connect to structure, and presumably, processes in the upper mantle beneath the South China Sea.
S41A-02 0830h
Interpretation of Lithospheric Images beneath China from P- and S-Wave Tomography
High-resolution images of the lithosphere beneath Chinese continent have been obtained through body-wave tomography exploiting the arrival times of both P and S waves. We have used high quality first arrivals extracted from the Annual Bulletin of Chinese Earthquakes (ABCE). Totally 345,000 P wave, and 230,000 shear wave arrivals were used. We first obtained 2338 five-layer 1-D models from a Monte-Carlo random search (Sun et al., 2004) with a 1° x 1° (latitude, longitude) interval. After combining and smoothing the 1-D models, we generated an equivalent 3-D model as the starting model for tomographic study. Finally we modified the tomographic method (Zhao, 1991) to perform the 3D P and S tomography with both the Conrad and the Moho discontinuities as well as sediment layer included. Both horizontal and vertical resolutions are highly controlled and smooth transitions among adjacent locations are guaranteed in the final models. The models have good correlation with tectonic features and the predicted travel times through the 3-D model matched well with the observed ones at local and regional distances. To validate both P and S velocity models we generated synthetic seismograms and test their agreement with observed ones. We use four events with M5.0 to M6.0, occurring between 2001 and 2003, in different areas of China. Each event is recorded by up to 47 broadband stations in the China Digital Seismic Network (CDSN). For each event, we select waveforms recorded in the stations at different azimuths and at distances ranging from 50 km to 1000 km. We use the discrete wavenumber method (Bouchon, 2003) and finite difference modeling to generate seismograms between the source and the station. Both body waves and surface waves fit well in eastern, northern and southeastern China. Our tomographic models provide new insights into the geological structure and tectonics of the region, such as lithological variations and large fault zones across the major geological terranes. Compared with previous tomographic studies, we have used a larger, higher quality data set and applied a tomographic method to take into account the effects of the complex Conrad and Moho geometry and to include the sediment layers in this region. Our results cast a new light over the complex structure and seismotectonics of China and surrounding areas.
S41A-03 0830h
Simultaneous Inversion of Fundamental Mode and Overtone Group and Phase Speed Maps
The lateral resolution of fundamental mode Rayleigh and Love wave dispersion maps has been improving steadily so that, at present, broadband tectonic-scale maps are available across much of the globe. As bandwidth has increased to include periods below 20 sec, the vertical resolution of shear wave speeds in the crust and upper mantle has also improved dramatically. Fundamental mode measurements, however, provide only relatively weak constraints on structures deeper than about 200 km. Observations of overtone phase speeds provide a useful complement to the fundamental mode observations, and push strong structural constraints to depths through the transition zone. We present the results of a simultaneous inversion of fundmental mode group and phase speeds with phase speed measurement from the first five overtone branches. The inversion is performed in two steps. In the first step, dispersion maps for each wave type are constructed on a grid of periods (e.g., ranging from 16 sec to 150 sec for the Rayleigh and Love wave group speeds). Finite frequency sensitivity kernels are used to construct all maps. In the second step, the dispersion maps are subjected to a Monte-Carlo inversion to estimate the range of seismic models of the crust, upper mantle, and transition zone consistent with the measurements. We discuss the power and limitations of this method, in particular the ability of the overtone measurements to reduce uncertainties in the resulting model.
S41A-04 0830h
A Scattering Inversion Experiment to Identify Fractures on a Granite Outcrop
A critical need in understanding open or fluid filled fractures in crystalline rock is the ability to identify these fractures, to characterize their source zones, to map flow paths within the fractures, and to assess residence time of water in the system. Fractures are an important component in the water resources of areas where crystalline rocks crop out at the surface. We have attempted to use scattered waves to identify and map fractures in a crystalline rock. Our research was conducted at the Panola Mountain Research Watershed (PMRW), GA. The scattering technique was tested in an area of exposed rock. We used 16 geophones, 100Hz each, placed in a circular array with diameter of 16m. A major effort was taken towards suppressing resonances in the geophones at high frequency. We used clay to better attach the geophones to the outcrop and sand bags to weight the instruments down to suppress the high frequency resonance. A small weight drop source was designed to generate high frequency input signal. The source provides signals in the 200 to 800 Hz range. The source was moved about the array in distance ranges of 5 to 60 meters. The recorded signals were highly filtered to pass only waves above 500 Hz. In processing the data, the distance is obtained from the travel time and direction to the scattering fracture is obtained by using apparent velocity across the array. The technique provides a theoretical way to map positions of varying scattering efficiency and hence the location of fractures.
S41A-05 0830h
Detailed Three-Dimensional P and S Wave Velocity Models for the New Madrid Seismic Zone
Detailed P and S wave velocity models and Vp/Vs values have been determined for the New Madrid Seismic Zone (NMSZ) based upon arrival times recorded by the New Madrid seismic network and by PANDA stations. The combined data set consists of 11,778 P wave arrivals and 8,579 S wave arrivals recorded by 97 stations. We used a modified version of the Benz et al., 1996 algorithm that inverts simultaneously for both P and S velocities and hypocentral locations (Tryggvason et al., 2002). The ability to specify independent P and S wave starting velocity models, rather than an S wave model linked to the P wave model via a constant Vp/Vs ratio, is particularly important for the NMSZ because the surface sediments have highly variable Vp/Vs ratios. Block size was reduced to 2x2x2 km, yielding the most detailed image of the NMSZ ever obtained. Inversion results were tested using chessboard models, spike tests and reconstruction techniques. Intrusions are imaged in great detail. Low P wave velocity and high S wave velocity appear to be correlated with earthquake occurrence; this results in distinct regions of low Vp/Vs values that correspond to the major arms of seismicity. The unusual low Vp/Vs values could be due to lithology variations but may be related to fluid saturation and porosity variations.
S41A-06 0830h
The Study Of 3D Vp, Vp/Vs and Qp Structures Beneath the Da-Tun Mt. Volcanic Area, Taiwan
The purpose of this research is to investigate 3D Vp, Vp/Vs and Qp structures beneath the Da-Tun Mt. volcanic area, Taiwan, by using damped least-square inversion method. The other intension is to study the interrelationship among velocity structure, fault systems and volcanic activity at the Tai-Tun Mt. area. Since Q structure is highly affected by anisotropy and heterogeneity, if we could incorporate with the knowledge of velocity structure, we can get better understanding the complex structure beneath the Ta-Tun Mt. volcanic area. We also calculate the ratios of Vp/Vs to investigate the relationship between the distribution of Vp/Vs ratios and the locations of fault systems. We find that the area beneath the Tai-Tun Mt., Sha-Mao Mt., and Qi-Xing Mt. toward to Hung-Zui and Zhu-Zi regions, there exists volcanic intrusion at depth around 10 km. In the southeastern direction of Chinshan fault, there exists a high velocity volcanic pipe structure. More importantly, between Chinshan fault and Kanchao fault, there has been a debate whether there exists a fault or not. From the velocity profile obtained in this study, we can clearly observe a fault with strike in southwestern direction between the above two faults. In conclusion, our results strongly suggest that by combining information of both velocity and Q structure, we can clearly observe the general pictures of volcanic activity, fault systems and structure evolution at Tai-Tun Mt. area.
S41A-07 0830h
Structures of Taipei basin from waveform inversion
The purpose of this research is to investigate 3D Vp, Vp/Vs structures beneath the Taipei basin area, Taiwan. We have performed the nonlinear waveform inversion for the velocity structures beneath the Taipei basin. The wave propagation modeling is carried out using a 3D precisely spectral element simulations, and the inversion is implemented by a neighbourhood algorithm, minimizing the misfit between synthetic and observed seismograms. Good results are obtained with both synthetic and observed data from our inversion. And we also obtain the high resolution of velocity structure especially in the shallow layer and get better understanding the complex structure beneath the Taipei basin area. These performs are highly parallel and easily be distributed across pc clusters. Finally, the results also shown how the computation time and memory requirements scale with the dimension of the parameter space and size of the ensemble.
S41A-08 0830h
Crustal and Upper Mantle Structure Beneath the Western Ghats in the Deccan Volcanic Province
Receiver function studies were carried out to image the crustal and sub crustal structure beneath the Western ghats in the western segment of the Deccan Volcanic Province of India. About 500 teleseismic events recorded by a seismic network comprising of 5 stations including 2 broadband and 3 short period stations deployed in an area of 300 km2 in the Western ghats, about 200km NE of Mumbai were used. Strong P-to-s conversions are observed at all stations from the base of the Deccan Traps and the Moho. The crustal thickness in this region is observed to average 34km.The crustal composition as inferred from the Poisson's ratio of 0.26 is felsic to intermediate. A prominent feature observed in the RFs from the Broadband station is the pronounced negative peak at 6s immediately following the Pms conversion from the Moho. Absence of intracrustal layers suggests that this phase is not an artifact and possibly reflects a shallow sub-Moho LVL. Synthetics were generated to model the negative phase which revealed a 20km thick low velocity zone with 4-5 percent lower velocities in the shallow upper mantle beneath the Western ghats. A similar LVL is observed even in the coastal plains adjoining the Western ghats near Mumbai. The regional presence of the shallow sub-Moho LVL along the west coast suggests that it may represent signatures of rift related volcanism associated with the seperation of Seychelles from India.
S41A-09 0830h
Seismic Reflection/Refraction-Imaging at the San Andreas Fault
A refraction/reflection seismic survey at the SAFOD drill site near Parkfield carried out in 2003 provides a detailed characterization of the subsurface structures. Three-component stations at a spacing of 50m were deployed on a 46 km long line perpendicular to the surface trace of the SAF. From the first arrival times of 62 explosive shots, a refraction p-wave velocity model was obtained. The most prominent feature of the model is a strong lateral contrast near SAF between Salinian granitic rocks and the Franciscan sedimentary melange. This model was used to perform a prestack migration of the reflected wavefield focusing on steep dips. The resulting images show a bright reflector down to at least 2 km depth at the position of the Coast Range Fault (CRF), which separates the Great Valley sequence from the Franciscan. Its near-vertical dip suggests a strike slip rather than a thrust contact. The reflection is interpreted to arise from fault-zone structure rather than from an impedance contrast across the fault. Despite a subtle p-velocity gradient at that position, neither p- nor s-wave velocities indicate a significant contrast. The high reflection strength may indicate the presence of fluids. The SAF, which was previously imaged at the same location by a shallower survey at a few hundred meters depth, is not a strong feature in the prestack migration result. This finding may be related to some extent to image quality, which is affected by large shot spacing, rugged topography and statics, and a complex direct arrival wavefield over shallow basement. We are currently working on a full-wavefield inversion to improve the velocity model, and thereby the migration image.
S41A-10 0830h
National Archive of Marine Seismic Surveys (NAMSS): Status Report on U.S. Geological Survey Program Providing Access to Proprietary Data
During the last four decades, hundreds of thousands of line kilometers of 2D marine seismic reflection data have been collected by the hydrocarbon exploration industry within the United States Exclusive Economic Zone. The commercial value of much of these data has decreased significantly because of drilling moratoria and new technology such as 3D acquisition. However, these data still have tremendous value for scientific research and education purposes. The U.S. Geological Survey has recently made agreements with two commercial owners of large data holdings to transfer to the public domain over 250,000 line kilometers of marine data from off the eastern, western, and Alaskan coasts of the United States. In order to provide access to the data, the USGS has developed the National Archive of Marine Seismic Surveys (NAMSS) program. For a small fraction of the money that would be required to collect new data, work is underway to organize and recover digital data currently stored on tens of thousands of 9-track tapes. Even where new data collection efforts could be funded, current environmental restrictions on marine seismic exploration could preclude operations. The NAMSS web site at http://walrus.wr.usgs.gov/NAMSS/ has trackline maps of surveys that are now or will soon be available for downloading in SEG-Y format. As more owners and users become aware of this new data resource, it is hoped that additional partners in will join this data rescue effort.
http://walrus.wr.usgs.gov/NAMSS/
S41A-11 0830h
Converted-wave PS arrivals in tilted TI media: Analysis of ray-paths and amplitudes
Converted-waves, i.e., downgoing P-waves that convert upon reflection to S-waves, excited by a compressional source and recorded by horizontal geophones provide a measure of media properties that relate to rock type and fluid saturation. Other uses that have arisen from measuring, analyzing and interpreting PS-waves, include improved structural imaging in formations with occurrence of gas and analysis of seismic anisotropy for estimating fracture density and orientation. Addressing anisotropy in the seismic processing flow is paramount due to the strong influence of this phenomenon in the wave path. Furthermore, analyses of both traveltimes and amplitudes that include anisotropic parameters provide a better picture of the subsurface. Anisotropy types used to describe common geologic scenarios include vertical and horizontal transversely isotropy (VTI and HTI, respectively). Presence of horizontal shale formations is a common cause of the VTI type, while a single vertical fracture system is a typical example of the HTI. Another degree of complexity is to consider a tilted symmetry axis corresponding to a formation with thin layers that are dipping, or the presence of non-vertical fractures. These media, described as tilted TI, have important observable effects on the converted-wave arrivals: i) asymmetry of the wave-path; ii) shear-wave splitting; and iii) non-zero amplitudes at zero offset. Based on numerical modeling, effects of the tilt in the simmetry axis on arrival times, travel path, and amplitudes of the converted waves are analyzed. Ultimate objectives of the work include quantifying these effects on converted-wave data processing, and investigating the reflected wavefield signature for retrieving tilt angle in situations where the stress field induces thin layering or fractures with a tilted symmetry axis.
S41A-12 0830h
Free gas and gas hydrate saturation from multi-component seismic reflection data from Hydrate Ridge
A seismic experiment comprising streamer, OBS and VSP surveys was carried out in summer 2002 at the Hydrate Ridge to map gas hydrate distribution within the hydrate stability zone. We have carried out an interactive velocity analysis of P- and converted S-waves in the tau-p domain. Our approach to multicomponent data analysis comprises three steps: 1) P-wave analysis, 2) PP to PS event correlation, and 3) S-wave analysis. PP to PS correlation is performed using synthetic seismograms. Once the P- and S-wave velocities are estimated, they are matched with modeled velocities based on an appropriate rock-physics model to estimate the gas hydrate saturation. Here we modeled the P- and S-wave velocities using an equation based on a modification of Wood equation that makes use of a rock physics model and an empirical relation. Our analysis of the multi-component ocean bottom seismometer data recorded at Hydrate Ridge, offshore Oregon reveals that the P-wave velocity is more sensitive to the saturation of gas hydrates and free gas than the S-wave velocity. The gas hydrate saturation is estimated to be up to 7 percent of the rock volume. The S-wave velocity does not show an anomalous increase in the hydrate-bearing sediments. It is more likely that the hydrates form within the pore space within our survey area of southern Hydrate Ridge.
http://www.ig.utexas.eddu
S41A-13 0830h
Sharp variation of low velocity zone atop the 410 seismic discontinuity beneath the edge of western Basin and Range
Song et al (nature, 2004) model reginal S wave triplication in the northwestern US and find low velocity zone atop the 410 seismic discontinuity. Here we present detail modeling of waveforms travelling along the westernmost tectonic North America and recorded by the TriNet broadband seismic array. We adopt regional tomographic model to constrain the shallow upper mantle. 2-D finite difference method interfacing Kirchhoff difffraction operator (DiFD) is implemented to account for the response off the great circle path. Strong azimuthal variations in the interferences of S wave triplication are clearly observed over less than 100 km. To reconcile such rapid change in waveform interferences, we model this array data with DiFD and find a low velocity zone with a sharp western edge atop the 410 can explain the whole dataset quite well. Though the geometry of the LVZ is not unique due to limited data analyzed, the sharp edge of the LVZ is more robustly constrained with the array data and DiFD modeling. Low velocity zone with a sharp edge atop the 410 has strong implications on its origin. It is unlikely that a thermal anomaly could explain such a sharp edge. However, a neutrally buoyant melt layer due to dehydration melting across the 410 might be able to explain the sharp edge.
S41A-14 0830h
Tomostatics: Long-offset Deformable Layer Tomography
Static corrections aim to remove the contamination of near surface anomalies on seismic data. It is therefore very important to seismic imaging of subsurface. The near surface is characterized with large lateral and vertical velocity variations, usually low velocities, but sometimes high velocities such as in permafrost or karst areas. The manifestation of near-surface static problem is in time distortions of reflection events that will cause poor image. A popular approach to correct for static distortions is to use tomographically determined near-surface velocity models. Following this approach, we are testing a new multi-scale deformable layer tomography that will robustly determine both interface geometry and layer velocity values using first arrivals. We try to address several issues, such as how much picking error is acceptable for tomostatics, and how to better use the tomographic velocity model for static corrections. The main assumption used in conventional static corrections is that raypaths are vertical in near surface, and the most common method is datuming at some depth level. We are evaluating the vertical raypath approximation, and assessing the effect of using different datum depths. We use waveform-generated data from a synthetic model to test and calibrate our new tomostatics algorithm. In each test, the corrected data based on the tomographic velocity model and different datum depths will be stacked or migrated, and the image sections will be analyzed for various factors affecting statics and image quality. We plan to formulate a best practice procedure for the tomostatics correction, and apply our methodology to a field seismic data that has severe statics problem.
S41A-15 0830h
Long-offset and multi-fold ocean bottom seismographic survey for imaging lithospheric scale structures in plate convergent margins
Recent availability of a large number of ocean bottom seismographs (OBSs), a large volume of air-gun array and a long streamer cable for academics provide several new findings of lithospheric scale structures in plate convergent margins. Japan Agency for Marine-Earth Science and Technology (JAMSTEC) has acquired long-offset seismic data using a super-densely deploy OBS (i.e. 1 - 5 km spacing OBSs along 100 - 500 km long profiles) since 1999. Long-offset multichannel seismic (MCS) data by a two-ship experiment, as well as conventional 2D MCS data, have been also acquired at a part of the profiles. Some of those profiles have been designed as combined onshore - offshore profiles for imaging a land-ocean transition zone. In a plate convergent margin, an oceanic plate subducts deep into a lithosphere and an island arc crust grows due to an accretion of melts welling up from subducted materials. Our obtained long-offset and multi-fold seismic data successfully provide fine images of subducting and overriding plates, which had not been imaged by conventional type of wide-angle seismic survey, in those complicated tectonic setting. One of the most striking findings is an image of several scales of subducted seamounts/ridges in the Nankai trough seismogenic zone, the SW Japan. We detected the subducted seamount/ridges, which are 50 - 100 km wide, distributing from near trough axis to ~ 40 km deep beneath the Japanese island. An important aspect, from a point of view of a geodynamic process, those structures are strongly correlated with slip zones of magnitude 8-class earthquakes, i.e.; subducted seamounts/ridge control the rupture propagations. We have also acquired very long offset seismic data (more than 500 km long) along and across the Izu-Bonin-Mariana (IBM) subduction zone in which ongoing crustal accretion process is proposed. Although the data is still processing, we expect, from our data, new and important information for the crustal accretion process at the IBM, because the crustal accretion is mainly proceed at the middle to lower crust, which have never been imaged without long-offset seismic data