subsurface cavity imaging by crosshole borehole radar measurements travel time tomography uses direct waves of crosshole radar data a great deal of information concerning the media is contained in later arrivals such as reflections and diffractions more reliable results can be obtained by incorporating both direct waves and later arrivals in this paper the combination of travel time tomography and migration is introduced a smoothed velocity model obtained by travel time tomography is used as the basis of migration then a reverse time migration technique is applied to direct wave suppressed crosshole data the result of tomography indicates that there is an abnormal area and the result of migration gives a more accurate position of the abnormal area which is a cavity the real example shows that the cooperation of tomography and migration is an effective technique for subsurface cavity imaging the synthesis of all information including parallel crosshole measurement and forward simulation assists in the interpretation of the results of tomography and migration

application of vertical radar profiling technique to sendai castle the vertical radar profiling vrp technique is able to explore much deeper than conventional surface ground penetrating radar gpr because it uses boreholes it has been successfully applied at the sendai castle site in japan to investigate the extent of an old stone wall and strata buried by a more recent stone wall the transmitter of a polarimetric radar system was moved within a borehole and the receiver was fixed on the ground surface several meters away from the borehole head cross and copolarization data were measured at a receiver position with a different orientation to the receiver ten data sets were acquired by placing the receiver in five directions the depolarization is strong indicating the subsurface contains a great amount of gravel to get clear and intuitive images of the subsurface we applied data processing techniques including the separation of direct and reflected waves of raw vrp data using f k filtering approach and kirchhoff migration of separated reflected waves by com

reverse time migration a prospect of seismic imaging methodology abstract reverse time migration rtm is a seismic imaging method to map the subsurface reflectivity using recorded seismic waveforms the practice in exploration seismology has long established a two fold approach of seismic imaging using velocity modeling building to establish the long wavelength reference velocity models and using seismic migration to map the short wavelength reflectivity structures among various seismic migration methods for different situations rtm is the only method that is capable to use all seismic wave types that can be computed numerically being initiated in early 1980 s rtm seeks an image of the subsurface reflectivity as the best match in an image space between the extrapolation of time reversed waveform data and the prediction based on estimated velocity model and source parameters judging the image quality in the same space of forming the images is more advantageous than the approaches of modeling and inversion which seek the solution in the model space but judge its fitness in data space considering that most seismic migration applications today still use primary reflection as the only signal the capability of rtm to use all computable wave types is unique and helpful reducing the imaging artifacts due to mistaking non primary waves as primary reflections hence we refer to those rtm algorithms using only primary reflections as the first generation rtm methods and the rtm algorithms making a full use of primary reflections multiple reflections and other non primary waveform data as the second generation rtm methods this paper reviews the development history of the rtm along with its major challenges current solutions and future perspectives

wave field processing of data from a large aperture seismic experiment in southwestern oklahoma data from a wide aperture survey performed in january 1985 in southwestern oklahoma have been processed using prestack imaging and the images are interpreted to reveal a new picture of the crustal structure in the wichita uplift and the anadarko basin as the data were in recording apertures of approximately 90 km it was necessary to develop new processing concepts and software a complete new imaging system based on common source gathers was conceived implemented tested and applied to these data the heart of the system consists of velocity estimation by slant stacking and acoustic prestack reverse time finite difference migration the migration produces reflectivity images in regions of 50 km 100 km has no dip restrictions and operates in an arbitrarily complicated two dimensional velocity distribution for comparison the data were also processed using standard common midpoint processing but this is a less valid approach for wide aperture data the main ambiguities and limitations in processing and interpretation are a consequence of the small number of shots processing and interpretation of the data provide a new picture of the subsurface structure of southwestern oklahoma the environment is clearly compressive with thrust faulting as the main accommodating mechanism in the upper and central crust beneath a possible detachment zone at 30 km depth the lower crust has responded to the compression by thickening by 50 the moho is interpreted to be 45 km in depth beneath the thrust zone shallowing to 40 km beneath the wichita uplift and the anadarko basin

suppressing unwanted internal reflections in prestack reverse time migration prestack reverse time migration a wave equation technique using two way propagation correctly handles multiarrivals and enables imaging of overturned reflections however image artifacts occur when backscattered waves cross correlate these artifacts are particularly strong where high velocity contrasts occur a method for removing unwanted internal reflections during propagation of both the source and receiver wavefields is presented this method applies a directional damping term to the wave equation in areas of the velocity model where unwanted reflections occur tests on synthetic data show good suppression of image artifacts

multicomponent common receiver gather migration of single level walk away seismic profiles seismic data are usually separated into p waves and s waves before being put through a scalar acoustic migration the relationship between polarization and moveout is exploited to design filters that extract the desired wavetype while these filters can always be applied to shot records they can only be applied to a triaxial common receiver gather in special cases since the moveout of scattered energy on the receiver gather relates to path differences between the surface shots and the scatterer while the polarization is determined by the path from scatterer to downhole geophone without the ability to separate wavefields before migration a vector scalar or an elastic migration becomes a necessity in this paper the propagation of the elastic wavefield for a given mode e g p s is approximated by two scalar acoustic propagation steps in a vector scalar migration vector in that multicomponent data is migrated and scalar in that each propagation step is based on a scalar wave equation for the appropriate mode it is assumed that interaction between the wavefields occurs only once in the far field of both the source and receiver extraction of the p sv and sh wavefields can be achieved within the depth migration if one assumes isotropymore in the neighborhood of the downhole receiver by a projection onto the polarization for the desired mode since the polarization of scattered energy is only a function of scatterer position and receiver position and not source position the projection may be taken outside the migration integral in the special case of the depth migration of a common receiver gather the extraction of the desired mode is then performed for each depth migration bin after the separate scalar migration of each receiver gather component less

elastic reverse time migration elastic prestack reverse time finite difference migration of two component seismic surface data requires data extrapolation and application of an imaging condition data extrapolation involves synchronous driving of the vertical component and horizontal component finite difference meshes with the time reverse of the recorded vertical and horizontal traces respectively extrapolation uses the coupled elastic wave equation for variable velocity solved with a second order explicit finite difference scheme the imaging condition at any point in the grid is the one way traveltime from the source to that point elastic migrations of both synthetic test data and real two component common source gathers produce simpler images than acoustic migrations because of the coalescing of double reflections compressional waves and shear waves into single loci

imaging of offset vsp data acquired in complex areas with modified reverse time migration a modified reverse time migration algorithm for offset vertical seismic profiling data is proposed this algorithm performs depth imaging of target areas in the borehole vicinity without taking into account the overburden originally recorded seismograms are used reliable results can be obtained using only the velocity profile obtained along the well the downgoing wavefield emitted from a surface source is approximated in the target area using the transmitted p wave recorded by the receivers deployed in the well this is achieved through a reverse time extrapolation of the direct transmitted p wave into the target area after its separation in offset vertical seismic profiling seismograms generated using a finite difference scheme for the solution of the scalar wave equation the proposed approach produces kinematically reliable images from reflected pp and ps waves and furthermore can be applied as a salt proximity tool for salt body flank imaging based on the transmitted ps waves our experiments on synthetic data demonstrate that the modified reverse time migration provides reliable depth images based on offset vertical seismic profiling data even if only the velocity profile obtained along the borehole is used

waveform tomography imaging of a megasplay fault system in the seismogenic nankai subduction zone abstract we apply frequency domain waveform tomography to form quantitative high resolution p wave velocity images of a megasplay fault system within the central nankai subduction zone offshore of southwest japan using controlled source ocean bottom seismometer obs data originally acquired in 2004 the waveform tomography method exploits recorded seismic waveforms beyond their first arrivals and thus achieves a much higher resolution of the order of a wavelength than that of conventional traveltime tomography methods frequency domain waveform tomography facilitates a multi scale approach to stabilize the inversion in which initial traveltime tomography results are sequentially improved on by first fitting low frequency components of the seismic records starting at 2 25 hz higher frequency components up to 8 5 hz are then introduced progressively our final waveform tomography image allows velocity anomalies as small as 700 m horizontally and 350 m vertically to be discerned and interpreted as confirmed by checkerboard modeling tests the improved explanatory power of the final images is verified by observing that synthetic waveforms calculated from the final results yield much better fit to the observed waveforms than those estimated from the original traveltime tomography image apparent lithological boundaries from waveform tomography agree well with corresponding reflections on seismic migration images providing further confidence in the validity of the results the megasplay fault is evident on the final waveform tomography image as a sharp velocity discontinuity delineating the upper surface of a velocity reduction of nearly 1 km s with respect to the regional 1d velocity trend the megasplay fault can be traced continuously throughout the entire image from a nearly horizontal section at the landward extent moving seaward through to a steeper angle penetrating the old accretionary prisms with several additional splays appearing to branch in the shallow subsurface the location of the low velocity zones imaged by our waveform tomography method coincides with two previously identified low velocity zones the image reveals a low velocity zone that is continuous from deeper to shallower portions of the subsurface suggesting that pore fluids may be transported from the inner wedge to the transition zone and to the surface through fluid conduits associated with the megasplay fault system

reverse time migration from rugged topography using irregular unstructured mesh we developed a reverse time migration scheme that can image regions with rugged topography without requiring any approximations by adopting an irregular unstructured grid modelling scheme this grid which can accurately describe surface topography and interfaces between high velocity contrast regions is generated by delaunay triangulation combined with the centroidal voronoi tessellation method the grid sizes vary according to the migration velocities resulting in significant reduction of the number of discretized nodes compared with the number of nodes in the conventional regular grid scheme particularly in the case wherein high near surface velocities exist moreover the time sampling rate can be reduced substantially the grid method together with the irregular perfectly matched layer absorbing boundary condition enables the proposed scheme to image regions of interest using curved artificial boundaries with fewer discretized nodes we tested the proposed scheme using the 2d seg foothill synthetic dataset

reverse time migration of offset vertical seismic profiling data using the excitation time imaging condition to apply reverse time migration to prestack finite offset data from variable velocity media the standard time zero imaging condition must be generalized because each point in the image space has a different image time or times this generalization is the excitation time imaging condition in which each point is imaged at the one way traveltime from the source to that point reverse time migration with the excitation time imaging condition consists of three elements 1 computation of the imaging condition 2 extrapolation of the recorder wave field and 3 application of the imaging condition computation of the imaging condition for each point in the image is done by ray tracing from the source point this is equivalent to extrapolation of the source wave field through the medium extrapolation of the recorded wave field is done by an acoustic finite difference algorithm imaging is performed at each step of the finite difference extrapolation by extracting from the propagating wave field the

depth imaging of offset vertical seismic profile data1 amundsen l arntsen b and mittet r 1993 depth imaging of offset vertical seismic profile data geophysical prospecting 41 1009 1031 depth migration consists of two different steps wavefield extrapolation and imaging the wave propagation is firmly founded on a mathematical frame work and is simulated by solving different types of wave equations dependent on the physical model under investigation in contrast the imaging part of migration is usually based on ad hoc principles rather than on a physical model with an associated mathematical expression the imaging is usually performed using the u d concept of claerbout 1971 which states that reflectors exist at points in the subsurface where the first arrival of the downgoing wave is time coincident with the upgoing wave inversion can as with migration be divided into the two steps of wavefield extrapolation and imaging in contrast to the imaging principle in migration imaging in inversion follows from the mathematical formulation of the problem the image with respect to the bulk modulus or velocity perturbations is proportional to the correlation between the time derivatives of a forward propagated field and a backward propagated residual field lailly 1984 tarantola 1984 we assume a physical model in which the wave propagation is governed by the 2d acoustic wave equation the wave equation is solved numerically using an efficient finitedifference scheme making simulations in realistically sized models feasible the two imaging concepts of migration and inversion are tested and compared in depth imaging from a synthetic offset vertical seismic profile section in order to test the velocity sensitivity of the algorithms two erroneous input velocity models are tested we find that the algorithm founded on inverse theory is less sensitive to velocity errors than depth migration using the more ad hoc u d imaging principle

wave field processing of data from the 1986 program for array seismic studies of the continental lithosphere ouachita experiment in may 1986 a wide aperture seismic field experiment was conducted in southwestern arkansas and northwestern louisiana this experiment was sponsored by the incorporated research institutions for seismology as part of the program for array seismic studies of the continental lithosphere the survey line runs from the south edge of the ouachita mountains out into the gulf coastal plain the experiment involved 21 shot points and two major deployments each of 400 recorders with an effective aperture of 200 km the goal of the experiment was to attempt to provide constraints on the diverse development sequences that have been proposed to explain the structure of the area the data were processed using a previously developed prestack common source acoustic finite difference migration to produce images of reflectivity in a slice 200 km long and 40 km deep this migration algorithm has no dip restrictions and uses a two dimensional variable velocity distribution for wave extrapolation interpretation of the results of the prestack processing together with the results of complementary two dimensional ray trace modeling not shown here of the data provides an enhanced picture of the structure of the paleozoic continental margin that is consistent with the observations in most previous geophysical and geological studies the main physical and temporal features of the model are crustal thinning during precambrian rifting a continent ocean boundary sedimentary wedge and carbonate bank thrusting of deepwater sediments during the ouachita orogeny and triassic rifting

migration of wide angle seismic reflection data from the grenville front in lake huron under the auspices of the great lakes international multidisciplinary program on crustal evolution glimpce coincident steep and wide angle seismic reflection data sets were acquired in lake huron the wide angle data were recorded at stationary onshore receivers during the shooting and recording of the conventional marine steep angle survey to extract information from the wide angle data a variety of processing techniques including automatic trace editing amplitude balancing f k filtering static corrections and predictive deconvolution were applied to the wide aperture common receiver gather before migration the application of a slowness weighted depth migration algorithm which was first tested on synthetic data and which included a slowness filter resulted in an image of the grenville front tectonic zone gftz remarkably similar to that observed in the multichannel steep angle section the migrated wide angle data suggest that prominent bands of east dipping seismic reflections observed across the gftz extend unattenuated to depths of at least 45 km

vertical seismic profile migration using the kirchhoff integral wave equation migration can form an accurate image of the subsurface from suitable vsp data the image s extent and resolution are determined by the receiver array dimensions and the source location s experiments with synthetic and real data show that the region of reliable image extent is defined by the specular zone of illumination migration is achieved through wave field extrapolation subject to an imaging procedure wave field extrapolation is based upon the scalar wave equation and for vsp data is conveniently handled by the kirchhoff integral the migration of vsp data calls for imaging very close to the borehole as well as imaging in the far field this dual requirement is met by retaining the near field term of the integral the complete integral solution is readily controlled by various weighting devices and processing strategies whose worth is demonstrated on real and synthetic data

inverse theory applied to multi source cross hole tomography part 1 acoustic wave equation method1 frequency domain methods are well suited to the imaging of wide aperture cross hole data however although the combination of the frequency domain with the wavenumber domain has facilitated the development of rapid algorithms such as diffraction tomography this has also required linearization with respect to homogeneous reference media this restriction and association restrictions on source receiver geometries are overcome by applying inverse techniques that operate in the frequency space domain in order to incorporate the rigorous modelling technique of finite differences into the inverse procedure a nonlinear approach is used to reduce computational costs the method of finite differences is applied directly to the frequency domain wave equation the use of high speed high capacity vector computers allow the resultant finite difference equations to be factored in place in this way wavefields can be computed for additional source positions at minimal extra cost allowing inversions to be generated using data from a very large number of source positions synthetic studies show that where weak scatter approximations are valid diffraction tomography performs slightly better than a single iteration of non linear inversion however if the background velocities increase systematically with depth diffraction tomography is ineffective whereas non linear inversion yields useful images from one frequency component of the data after a single iteration further synthetic studies indicate the efficacy of the method in the time lapse monitoring of injection fluids in tertiary hydrocarbon recovery projects

full wavefield inversion of wide aperture sh and love wave data summary reverse time linearized inversion is implemented for synthetic wide aperture sh and love wave data including multiple reflections to estimate the s wave velocity in a two dimensionally inhomogeneous medium complete wide aperture wavefields in which triplications pre and post critical reflections and surface waves love waves are present can be imaged by reverse time inversion to estimate the sh velocity distribution the algorithm operates on common source gathers and involves cross correlation of the source and recorded wavefields to define the steepest descent direction to update the velocity at each point on a finite difference grid effects of noise can be reduced by stacking information from different sources

comparison of kirchhoff and reverse time migration methods with applications to prestack depth imaging of complex structures the performance of two popular migration methods the kirchhoff integral and reverse time migrations is evaluated through applications to imaging complex structures using prestack shot records the migration results from the marmousi model data demonstrate that reverse time migration is more accurate than kirchhoff migration in imaging the steeply dipping faults however the improved accuracy of reverse time migration requires higher computational costs in the application to the alberta foothills data where a good estimate of the velocity model is available however both the kirchhoff and the reverse time migration methods produce almost identical results this implies that in the real world of exploration seismology it will be relatively difficult to identify which method performs better because we never know the exact answer of the subsurface

prestack depth migration of an alberta foothills data set the husky experience we produce depth images for an alberta foothills line by iteratively using a number of migration and velocity analysis techniques in imaging steeply dipping layers of a foothills data set it is apparent that thrust belt geology can violate the conventional assumptions of elevation datum corrections and common midpoint cmp stacking to circumvent these problems we use migration from topography in which we perform prestack depth migration on the data using correct source and receiver elevations migration from topography produces enhanced images of steep shallow reflectors when compared to conventional processing in addition to migration from topography we couple prestack depth migration with the continuous adjustment of velocity depth models a number of criteria are used in doing this these criteria require that our velocity estimates produce a focused image and that migrated depths in common image gathers be independent of source receiver offset velocity models are estimated by a series of iterative and interpretive steps involving prestack migration velocity analysis and structural interpretation overlays of velocity models on depth migrations should generally show consistency between velocity boundaries and reflection depths our preferred seismic depth section has been produced by using prestack reverse time depth migration coupled with careful geological interpretation

variation of sea floor depth with age a test of models based on drilling results previous analyses of the variation of depth with age in oceanic basins have demonstrated a systematic square root of age dependence in crust younger than 80 ma these studies used depths determined from marine seismic data which have uncertainties due to the assumed sediment velocity as an alternative we compiled the basement depths from all dsdp and odp drill sites that sampled normal ocean crust in the atlantic pacific and indian oceans up to recent drilling leg 136 we applied further criteria to the data rejecting any site not formed at a mid ocean ridge or that did not penetrate extrusive basalts of morb composition the remaining high quality data set is small 77 sites but adequate to test models of the thermal structure of the upper mantle our new results also show that the square root of age t variation incorrectly estimates the observed depth for crustal ages older than 90 ma however our data suggest a plate thickness of 104 9 km and a basal temperature of 1400 140 c and agree at all ages with the recent inversion by stein and stein 1992

3d acoustic reverse time migration1 acoustic reverse time finite difference migration for zero offset data is extended from two to three dimensional media the formulation is based on the full three dimensional acoustic wave equation and so has no dip restrictions and it involves extrapolation in a velocity distribution variable in three dimensions the algorithm is demonstrated by successful migration of synthetic data sets for three models a point diffractor an oblique pinch out and a dome overlying a planar reflector

imaging reflection blind areas using transmitted ps waves we illustrate the use of mode converted transmitted e g ps or sp waves in vertical seismic profiling vsp data for imaging areas above receivers where reflected waves cannot illuminate three depth domain imaging techniques move out correction common depth point cdp mapping and prestack migration are described and used for imag ing the transmitted waves moveout correction converts an offset vsp trace into a zero offset trace cdp mapping maps each sample on an input trace to the location where the mode conversion occurs for complex media prestack migration e g reverse time migration is used by using both synthetic and field vsp data we demonstrate that images derived from transmissions complement those from reflections as an important application we show that transmitted waves can illuminate zones above highly de viated or horizontal wells a region not imaged by reflection data because all of these benefits are obtained without extra data acquisition cost we believe transmissi

sub basalt decoupled walkaway vsp imaging walkaway vertical seismic profile vsp data acquired in basalt covered areas can be used to improve knowledge of the sub basalt structure a synthetic example and a case study from the north atlantic uk show that elastic two way downward continuation migration combined with the stationary phase principle is well suited to the processing of vsp data vector data are processed using decoupled elastic migration algorithms in both isotropic and anisotropic media to illustrate the value of decoupled imaging equations conventional pp imaging is carried out on the enhanced vsp data and compared with the decoupled scheme decoupled vector migration operates directly on the displacement vector and uses various wave modes downgoing waves are migrated to image basalt lava flows and measure their anisotropy upgoing waves are used for high resolution sub basalt imaging