Detailed Velocity Information Research Articles

A new empirical method for obtaining horizontal site amplification factors with soil nonlinearity

AbstractCurrently, the theoretical prediction of soil amplification considering nonlinear soil behaviors can be implemented through nonlinear analysis of soil layers in time domains. However, in such theoretical methods, we need detailed velocity information down to the seismological bedrock and nonlinear properties of sediments, which are challenging to obtain in a wider area for a scenario‐type seismic risk evaluation. Recently, using diffuse‐field theory, we found that the horizontal‐to‐vertical spectral ratio of Earthquake (EHVR) is the ratio of the S‐wave horizontal transfer function with respect to the P‐wave vertical transfer function with the vertical incidence from the seismological bedrock with a coefficient for HVRs at the bedrock. Therefore, if there is no significant nonlinearity in the P‐wave vertical transfer function during strong shaking, we can regard soil nonlinearity emerging on the EHVR as the nonlinearity of the S‐wave horizontal amplification. In this study, we tried to distinguish the difference between the EHVRs in the linear and nonlinear cases, quantify the difference, and utilize a creative empirical approach to acquire the S‐wave horizontal amplification factors with soil nonlinearity to improve the quality of strong motion prediction in the near field in the future.

Predictive Equivalent Consumption Minimization Strategy With Segmented Traffic Information

This paper proposes a predictive equivalent consumption minimization strategy (P-ECMS) for a plug-in hybrid electric vehicle (PHEV), assuming the availability of two levels of traffic information. The two levels of traffic information include 1) segmented traffic information available from mobile mapping applications, and 2) detailed velocity information, possibly obtained by short-term speed forecasting. Battery state-of-charge (SOC) reference waypoints are obtained by a simplified speed profile constructed from that segmented traffic information. The proposed P-ECMS adjusts its co-state or equivalence factor based on the difference between the future SOC obtained from short-horizon prediction and a future reference SOC. The benefits of the proposed P-ECMS are evaluated through vehicle simulations on a specified trip, compared against an adaptive ECMS (A-ECMS) with the same control parameters minus prediction. Simulation results on the trip demonstrate that the proposed P-ECMS can achieve a 9.7% reduction in the corresponding fuel consumption on average, compared to the A-ECMS across different combinations of initial co-state and control gain. Moreover, its performance is robust to variations in the initial co-state, the co-state update rate, and the prediction horizon, as well as to inaccuracies in the velocity prediction. Compared to the A-ECMS, the standard deviation in fuel consumption with the P-ECMS can be reduced by 96% from 84.1 to 3.4 grams.

Particle image velocimetry investigation of steady flow over a backwardfacing step

The backward-facing step (BFS) is a heuristic example, allowing for complex phenomena to arise in a simple geometry. Particle Image Velocimetry (PIV) investigations of mean-velocity distributions of backward-facing step flow with steady inlet condition were carried out and good agreement was obtained between current and previously published results for 50 ≤ Re ≤ 400. This confirms that the current experimental capabilities can provide detailed and accurate velocity information. The flow behaviour downstream the step depends on the strength of separated shear layer, which the circulation depends on the bulk flow, recirculation zone length and vortex formation time. Since the vortex formation process is governed by the circulation flux convected along the wall layer from the step, for Re ≤ 400, all of the circulation contained in the shear layer is drawn into the recirculation region. Thus, in a case where the shear layer characteristics are modified (e.g. in higher Reynolds number and unsteady flows), the balance of circulation is modified that would result in shedding.

Use of proper orthogonal decomposition and linear stochastic estimation technique to investigate real-time detailed airflows for building ventilation

As energy efficiency concerns grow for building heating, ventilation, and air-conditioning systems, more advanced mathematical models and methods may be needed for implementing more comprehensive building environmental monitoring practices. This work investigates the methodology of applying a coupled proper orthogonal decomposition (POD) and linear stochastic estimation (LSE) technique to provide detailed real-time velocity information for a single-room test environment under either mechanical ventilation or wind-driven cross natural ventilation with the support of limited on-site monitoring measurements. In addition, an example application of the proposed POD–LSE methodology with limited measurements to obtain real-time fluctuating wind-driven ventilation rates using a POD–LSE reconstructed velocity profile across a window opening is presented and shows very good agreement with the measured ventilation rates.

Stochastic determination of entrainment risk in uniformly sized sediment beds at low transport stages: 2. Experiments

Fluvial sediment transport is caused by a complex interaction of interdependent grain and fluid processes many of which are stochastic in nature and cannot be adequately represented by deterministic equations. Random variable analysis has been used previously but limited data are available to describe the variability of grain resistance combined with particle arrangements, and thus validate such analysis. In this study low to medium bed load transport tests were carried out in a flume where sediment movement was monitored using a three‐camera 3D PIV system. Simultaneous grain motion and flow velocity measurements were made on a plane located slightly above and parallel to the sediment bed. Detailed statistical velocity information was acquired to model the velocity distribution at the bed level. This was combined with the joint probabilistic distribution of particle exposures and grain resistance to motion, which were obtained from discrete particle modeling (DPM) simulations. DPM simulations were used to provide a stochastic mathematical description of the risk that a stationary particle is entrained by the flow. Predictions from the stochastic model equations replicated the observed pulsation in sediment transport. This demonstrates that it is possible to simulate sediment entrainment and transport at a high resolution by adequately modeling all the sub‐processes. A number of flow patterns were identified that caused large fluctuations of the entrainment rate. These all exhibit high velocity flow structures, but they selectively cause the dislodgement of individual particles located at different positions. This selective behavior follows from the variability of the interaction between the near‐bed flow and the particles having different exposure.

InSAR Permanent Scatterer analysis reveals fault re-activation during inflation and deflation episodes at Campi Flegrei caldera

Permanent Scatterers Synthetic Aperture Radar Interferometry (PSInSAR) and Global Position System (GPS) are applied to investigate the most recent surface deformation of the Campi Flegrei caldera. The PSInSAR analysis, based on SAR data acquired by ERS-1/2 sensors during the 1992–2001 time interval and by the Radarsat sensor during 2003–2007, identifies displacement patterns over wide areas with high spatial resolution. GPS data acquired by the Neapolitan Volcanic Continuous GPS network provide detailed ground velocity information of specific sites. The satellite-derived data allow us to characterize the deformation pattern that affected the Campi Flegrei caldera during two recent subsidence (1992–1999) and uplift (2005–2006) phases. PSInSAR results show the re-activation of the caldera ring-faults, intra-caldera faults, and eruptive fissures. We discuss the results in the light of the available volcanological, structural and geophysical data and propose a relationship between the structures activated during the recent unrest episodes and those responsible for the recent (<3.8–4ka) volcanism. The combined interpretation of the collected data show that (a) the caldera consists of two sectors separated by a N–S striking faulting zone and (b) the intra-caldera NW–SE faults and eruptive fissures in the central-eastern sector re-activated during the studied unrest episodes and represent possible pathways for the ascent of magma and/or gas to the surface. In this sector, maximum horizontal strain, recent volcanism (3.8–4ka), active degassing and seismicity concentrate. The fault re-activation is related to the dynamics of the caldera and not to tectonic stress. The deformation fields of the uplift and subsidence episodes are consistent with hydrothermal processes and degassing from a magmatic reservoir that is significantly smaller than the large (∼40km3) magma chamber responsible for the caldera formation. We provide evidence that the monitoring of the horizontal and vertical components of deformation improves the identification of active, aseismic faults. Accordingly, we suggest that future ground deformation models should include the re-activation of the detected structures.

Evidence for fluid circulation, overpressure and tectonic style along the Southern Chilean margin

The southern Chilean convergent margin, between 50° and 57° S, is shaped by the interaction of the three main plates: Antarctic, South America and Scotia. North of 53° S, the convergence between Antarctic and South America plates is close to orthogonal to the continental margin strike. Here, the deformational style of the accretionary prism is mainly characterized by seaward-verging thrusts and locally by normal faults and fractures, a very limited lateral extension of prism, a very shallow dip (∼ 6°) décollement, and subduction of a thick and relatively undeformed trench sedimentary sequence. South of 53° S, convergence is oblique to the margin, locally, the trench sediments are proto-deformed by double vergence thrusting and the front of the prism grows through landward-verging thrusting. The décollement is sub-horizontal and deep, involving most of the sediment over the oceanic crust in the accretionary process, building a comparatively wide and thicker prism. A Bottom Simulating Reflector is present across the whole prism to the abyssal plan, suggesting the presence of gas in the sediments. The analysis of P- and S-wave velocity reflectivity sections, derived by amplitude versus offset technique (AVO), detailed velocity information and the velocity-derived sediment porosity have been integrated with the structural analysis of the accretionary prism of two selected pre-stack depth migrated seismic lines, aiming to explain the relation between fluid circulation and tectonics. Accretion along double vergence thrust faults may be associated with the presence of overpressured fluid, which decreases the effective shear stress coefficient along the main décollement and within the sediments, and modify the rheolgical properties of rocks. The presence of an adequate drainage network, represented by interconnected faults and fractures affecting the entire sedimentary sequence, can favour the escape of pore fluid toward the sea bottom, while, less permeable and not faulted sediments can favour fluid accumulations. Gravitational and tectonic dewatering, and stratigraphy could control the consolidation and the pore overpressure of sediments involved in subduction along the trench. The results of our analysis suggest the existence of a feedback between tectonic style and fluid circulation.

Time-resolved particle image velocimetry and laser doppler anemometry study of the turbulent flow field of bileaflet mechanical mitral prostheses

Dynamic particle image velocimetry (PIV) was applied to the study of the flow field associated with prosthetic heart valves. The results were compared with those of laser Doppler anemometry (LDA). Anatomically and antianatomically oriented Jyros (JR) and St. Jude Medical (SJM) valves were compared in the mitral position to study the effects of valve design on the downstream flow field. The experimental program used a dynamic PIV system utilizing high-speed, high-resolution video to map the true time-resolved velocity field inside the simulated ventricle. This system was complemented by a study using the more traditional LDA system for comparison. Based on the experimental data, the following general conclusions can be made. High-resolution dynamic PIV can capture true chronological changes in the velocity and turbulence fields. It also produces very detailed velocity and turbulence information comparable to the LDA results. In the vertical measuring plane that passes both the center of the aortic and mitral valves (A-A section), the two valves (the SJM and the JR) show distinct circulatory flow patterns when the valve is installed in the antianatomical orientation. Small differences in valve design can generate noticeable differences, particularly during the accelerating flow phase. The SJM valve maintains a relatively high velocity through the central orifice; the curved leaflets of the JR valve generate higher velocities with a divergent flow during the accelerating and peak flow phases. In the velocity field directly below the mitral valve and normal to the previous measuring plane (B-B section), where characteristic differences in valve design will be visible, symmetrical twin circulations were observed because of the divergent nature of the flow generated by the two inclined half-disks installed in the antianatomical orientation. The SJM valve, with a central downward flow near the valve, is contrasted with the JR valve, which has a peripheral downward circulation with higher, turbulent stresses.

Crustal structure of northwestern Svalbard and the adjacent Yermak Plateau: evidence for Oligocene detachment tectonics and non-volcanic breakup

SUMMARY In 1999 new seismic refraction data were collected off northwestern Svalbard and the adjacent Yermak Plateau. A 260 km long profile provides detailed velocity information for the northeastern edge of the Eurasian Continent and the adjacent Yermak Plateau. North of Forlandsundet Graben the depth of the Moho varies between 23 and 28 km, and remains at this depth to the northern edge of the profile at 81°N. The crustal lithology off western Svalbard can be related to the basement province west of the Raudfjorden Fault Zone. Off the northern shoreline of Svalbard the structure of the Tertiary Danskoya Basin is mapped. Below this, a Late Silurian/Early Devonian basin, with seismic velocities between 5.1 and 5.8 km s−1 and a thickness of up to 8 km is present. A Palaeozoic sequence of up to 6 km thickness is expected below the Tertiary cover north of the Danskoya Basin. An earlier suggestion, that Oligocene rift processes affected the southern Yermak Plateau, is confirmed. A detachment structure is situated below the Palaeozoic Basin below Danskoya Basin, which is probably a consequence of simple shear tectonics. The middle crust exhibits low seismic velocities above the detachment fault. The lowermost crust beneath is slightly contaminated by mantle-derived melts, which is deduced by the slightly elevated velocities of the lowermost crust. These melts can be attributed to decompressive melting caused by modest uplift of the Moho during stretching. The velocity–depth model provides no evidence for large magmatic activity, which implies a non-volcanic rifted margin history. This leads to the assumption that the proposed Yermak Hotspot during the breakup of Svalbard from northern Greenland did not exist.

Hagedoorn's plus‐minus method: the beauty of simplicity

Hagedoorn developed the plus‐minus method with the objective of providing the exploration world with a simple and rapid approximation of Thornburgh's wavefront reconstruction method. Straightforward adding and subtracting produces depths and velocities at all geophones where the refracted waves from a set of reciprocal shots are first arrivals. Even though seismic refraction has not kept up with the revolutionary advance of reflection technology during the past decades, the method still has a lot to offer, especially in shallow engineering, and environmental, groundwater and sea‐bed surveying. Its strong points are the ease of application, the low costs, the effectiveness in the shallow zone, the unique ability to provide detailed velocity information on the deepest refractor and the capability for producing parameters for layer and rock characterization.Because of its simplicity, the plus‐minus method is ideally suited for real‐time processing of refraction data in the field, thus monitoring the data quality and the optimal shot configuration. Today, such field processing is easily performed with a laptop and dedicated software. matlab® is a software package that enables tailor‐made processing, offering a combination of a programming language, visualization tools and a large library of ready‐made functions. This paper presents a plus‐minus program developed in matlab and illustrates its application with a case study in Yemen, where seismic refraction was used in a regional groundwater study. Here, refraction provided not only a detailed section across the recharge area of a coastal plain but also the additional information needed to reduce ambiguity in the interpretation of the vertical electrical soundings made in the area.

Jet eccentricity: A misleading source of agreement between Doppler/catheter pressure gradients in aortic stenosis

Characterization of the severity of aortic stenosis relies on accurate measurement of the pressure gradient across the valve and the valve area. Pressure gradients measured by Doppler ultrasound based on the clinical form of the Bernoulli equation often overestimate pressure gradients by catheter as the result of pressure recovery. Doppler techniques measure the velocity of the vena contracta of the stenotic jet. This corresponds to the maximal pressure gradient and the minimal effective valve area. Pressure recovery can be characterized by analysis of the spread of the stenotic jet downstream of the valve as it fills the aorta and should be influenced by the shape of the velocity profile of the decaying jet. In this study, we addressed the hypothesis that the site of complete pressure recovery (the point at which the jet fully expands to the size of the aorta), the effective valve area, and the maximal pressure gradient are affected by jet eccentricity. To accomplish this, we developed a computational model of aortic stenosis that provides detailed velocity and pressure information in the vicinity of the valve. The results show that the width of the eccentric wall jet decreased and maximal velocity increased with greater jet eccentricity. Furthermore, for a constant anatomic area, the effective valve area decreased, the distance to complete pressure recovery increased, and the maximal pressure gradient increased with the degree of eccentricity. Failure to take this into account could fortuitously drive Doppler and catheter measurements toward agreement because the distal pressure sensor will not record the fully recovered pressure. Therefore the pressure gradient across a stenotic valve depends on jet eccentricity. The spread of the wall jet after attachment must be characterized to develop a robust method for the prediction of pressure recovery. (J Am Soc Echocardiogr 2001;14:853-62.)

Laser-based particle measurements of spherical and nonspherical particles

Experiments were performed to collect a detailed set of particle information in a co-axial jet and a swirling flow to assess the influence of particle shape on the particle dynamics. Spherical glass beads and a ground glass powder of the same material and similar size distribution were used for the experiments. The experiments presented also provide a comprehensive data set of particle information in a well-defined laboratory environment suitable for model validation. Results presented were measured with a two-component phase-Doppler particle analyzer (PDPA) and contain detailed particle velocity information for both spherical and nonspherical particles. Significant differences between spherical and nonspherical particles were observed for both co-axial jet and swirling flows in regions where velocity gradients are high. In general, nonspherical particles in the 30–70 μm range were able to follow the gas-phase velocity much better than the spherical particles in the same size range. Size-resolved velocity information was also collected directly below the inlet to measure the different velocity characteristics of different size classes within the overall distribution.

Ultrashallow seismic reflection monitoring of seasonal fluctuations in the water table

Abstract Ultrashallow seismic-reflection data were collected at a test site in Great Bend, Kansas. The purpose of the experiment was to image seasonal submeter-scale fluctuations in the water table over a period of one year to identify the factors important in monitoring the water table when using seismic-reflection techniques. The study indicates that detailed velocity information must be used when interpreting water-table levels. Using detailed velocity information as a control when depth-converting the seismic profiles yielded correct positioning of the water table within + or -12 cm at the test site.

Methanol Masers as Tracers of Circumstellar Disks

We show that in many methanol maser sources the masers are located in lines, with a velocity gradient along them which suggests that the masers are situated in edge-on circumstellar, or protoplanetary, disks. We present VLBI observations of the methanol maser source G309.92+0.48, in the 12.2 GHz transition, which confirm previous observations that the masers in this source lie along a line. We show that such sources are not only linear in space but, in many cases, also have a linear velocity gradient. We then model these and other data in both the 6.7 GHz and the 12.2 GHz transition from a number of star formation regions, and show that the observed spatial and velocity distribution of methanol masers, and the derived Keplerian masses, are consistent with a circumstellar disk rotating around an OB star. We consider this and other hypotheses, and conclude that about half of these methanol masers are probably located in edge-on circumstellar disks around young stars. This is of particular significance for studies of circumstellar disks because of the detailed velocity information available from the masers.

The equivalent offset method of prestack time migration

A prestack time migration is presented that is simple, efficient, and provides detailed velocity information. It is based on Kirchhoff prestack time migration and can be applied to both 2-D and 3-D data. The method is divided into two steps: the first is a gathering process that forms common scatterpoint (CSP) gathers; the second is a focusing process that applies a simplified Kirchhoff migration on the CSP gathers, and consists of scaling, filtering, normal moveout (NMO) correction, and stacking. A key concept of the method is a reformulation of the double square‐root equation (of source‐scatterpoint‐receiver traveltimes) into a single square root. The single square root uses an equivalent offset that is the surface distance from the scatterpoint to a colocated source and receiver. Input samples are mapped into offset bins of a CSP gather, without time shifting, to an offset defined by the equivalent offset. The single square‐root reformulation gathers scattered energy to hyperbolic paths on the appropriate CSP gathers. A CSP gather is similar to a common midpoint (CMP) gather as both are focused by NMO and stacking. However, the CSP stack is a complete Kirchhoff prestack migrated section, whereas the CMP stack still requires poststack migration. In addition, the CSP gather has higher fold in the offset bins and a much larger offset range due to the gathering of all input traces within the migration aperture. The new method gains computational efficiency by delaying the Kirchhoff computations until after the CSP gather has been formed. The high fold and large offsets of the CSP gather enables precise focusing of the velocity semblance and accurate velocity analysis. Our algorithm is formulated in the space‐time domain, which enables prestack migration velocity analysis to be performed at selected locations and permits prestack migration of a 3-D volume into an arbitrarily located 2-D line.

Experimental investigation of two-stream mixing flows with streamwise and normal vorticity

This study provides detailed velocity information in the mixing region of two-stream mixing flows with streamwise and normal vorticity for the computational fluid dynamics (CFD) methods validation. Three trailing-edge configurations for the dividing mechanism had been considered; namely, a convoluted plate, a lobed forced mixer, and a scalloped forced mixer. The first two have been examined previously by visualization tests (Manning 1991). A velocity ratio of 0.6 was generated with initial turbulent boundary layers and was nominally two-dimensional (2-D). Velocity measurements were made on fine cross-plane grids across the wake region and at several streamwise locations using a two-component laser-Doppler anemometer (LDA). The streamwise vortex development for the lobe mixer and the scalloped mixer underwent a three-step process by which it was formed, intensified, and quickly dissipated. The shear layer growth rates were rapid for the scalloped mixer and lobed mixer cases from the trailing edge to a point where the streamwise vorticity was almost dissipated. Consequently, the growth rate reduced and became lower than that of the convoluted plate. In the absence of the streamwise vorticity, the growth rate for the convoluted plate relied only on the 2-D roll-up of the normal vorticity.

Seismic imaging of oceanic layer 2A between 9°30′N and 10°N on the East Pacific Rise from two‐ship wide‐aperture profiles

We analyze two‐ship multichannel seismic wide aperture profiles (WAP) acquired on the East Pacific Rise (EPR) between 9°30′C and 10°N in May and June 1985. For offsets between 2 and 5 km, most of the WAP common depth point (CDP) gathers exhibit a strong retrograde, reflection‐like seismic arrival that corresponds to rays turned within a very strong velocity gradient zone underlying the uppermost crustal layer 2A. Stacking of this postcritical arrival for hundreds of CDP gathers along the WAP lines yields remarkably clear, and virtually continuous, images of the bottom of layer 2A for several tens of kilometers along and across the EPR. An innovative combination of this image with others of the seafloor (SF) and of the axial magma chamber (AMC) provides an unusually clear composite picture of young upper crust. We combine these images with detailed seismic velocity information, also obtained from the CDP gathers, to achieve an accurate determination of the layer 2A thickness in the area. Along the EPR axis between 9°30°N and 10°N, layer 2A shows a relatively constant thickness between 100 and 150 m (80–120 ms two‐way travel time). Across the axis, however, a line near 9°30′N indicates that layer 2A thickness rapidly increases by approximately a factor of 2 within 2–4 km of the axis and remains nearly constant afterwards. Thus the final thickness of layer 2A is attained before the development of large‐scale tectonic faulting, strongly suggesting that the thickening of layer 2A away from the axis is due to successive episodes of volcanic activity increasing the thickness of the uppermost crustal section from the top, rather than tectonic fracturing lowering the uppermost crustal seismic velocities and pushing the bottom of layer 2A downwards as the crust moves away from the axis.

Angle‐dependent reflectivity by means of prestack migration

Most present day seismic migration schemes determine only the zero‐offset reflection coefficient for each grid point (depth point) in the subsurface. In matrix notation, the zero‐offset reflection coefficient is found on the diagonal of a reflectivity matrix operator that transforms the illuminating source‐wave field into a reflected‐wave field. However, angle dependent reflectivity information is contained in the full reflectivity matrix. Our objective is to obtain angle‐dependent reflection coefficients from seismic data by means of prestack migration (multisource, multioffset). After downward extrapolation of source and reflected wave fields to one depth level, the rows of the reflectivity matrix (representing angle‐dependent reflectivity information for each grid point at that depth level) are recovered by deconvolving the reflected wave fields with the related source wave fields. This process is carried out in the space‐frequency domain. In order to preserve the angle‐dependent reflectivity in the imaging we must not only add all frequency contributions but we should extend the imaging principle by adding along lines of constant angle in the wavenumber‐frequency domain. This procedure is carried out for each grid point. The resulting amplitude information provides a rigorous approach to amplitude‐versus‐offset related methods. The new imaging technique has been tested on media with horizontal layers. However, with our shot‐record oriented algorithm it is possible to handle any subsurface geometry. The first tests show excellent results up to high angles, both in the acoustic and in the elastic case. With angle‐dependent reflectivity information it becomes feasible to derive detailed velocity and density information in a subsequent stratigraphic inversion step.

Colour Doppler Flow Mapping: Providing an Insight into Cardiac Hydrodynamics

Colour Doppler flow mapping can now provide spatial velocity information in relation to surrounding structural detail as imaged by conventional echocardiography and, as such, represents a major advance in non-invasive cardiac imaging. This article describes the basic principles of colour Doppler flow mapping, details how the Doppler information is processed and displayed to arrive at the real-time two-dimensional flow-enhanced image. Since colour Doppler flow mapping is really the first available technique which allows detailed real-time velocity information to be displayed in vivo and in relation to intracardiac structures, it can allow important insight into the haemodynamics of intracardiac flow. With the application of digital computer analysis techniques it may also be possible to extract valuable quantitative information from colour Doppler flow mapping.

VSP Fundamentals that Improve CDP Data Interpretation: ABSTRACT

This paper emphasizes the fundamentals of the vertical seismic profile (VSP) that improve the interpreter's confidence and understanding in its use as an exploration tool. The areas covered are VSP acquisition, critical points of the processing sequence, and applications to borehole information in depth and CDP data in time. Correlating reflection character to the stratigraphic section is done typically with synthetic seismograms computed from sonic logs and check shot traveltimes. Problems often make this task difficult, even in areas where the synthetic or surface seismic data are of good quality. The VSP provides an alternative approach and a valuable link between the synthetic and CDP data. A VSP extends check shot surveys from first break traveltime analysis to reflection analysis by recording the complete seismogram at a fine depth interval in the borehole (at least two depths per seismic wavelength). The unique feature of a VSP is the simultaneous measurement of downgoing and upgoing seismic waves as they propagate at depth. A repeatable source such as Vibroseis and the fine depth sampling make it possible to separate the VSP downgoing waves End_Page 465------------------------------ from the upgoing reflections by f-k filtering. The downgoing wave field can be used as signatures to deconvolve the upgoing wave field as the best estimate of the primary reflections in the vicinity of the borehole. VSP processing preserves the polarity and amplitude of reflections, and after deconvolution, a trough corresponds to a positive reflection coefficient. These reflections can be shifted to their two-way traveltime and stacked to produce a VSP extracted trace (VET), which is used to correlate to the CDP data. An important application of a VSP is to correlate the reflection character to depth and the stratigraphy observed in the borehole. The direct downgoing arrival contains the two-way time to depth relationship and the enhanced primary reflections contain the reflection character. Any formation top in the VSP survey can be correlated to time using these features. VSP reflection character indicates the significant features in the sonic log velocities that produce the reflections observed in the surface seismic data. Sonic logs and VSPs attempt to measure the same velocities and reflectivity near the borehole but with significantly different resolution. The sonic log only penetrates a short distance into the formation but provides detailed velocity information vertically. A VSP, however, has poor vertical resolution (50 to 100 ft [15 to 30 m] intervals) but samples a large area around the borehole similar to CDP data. Therefore, the VSP correlates well with CDP data because they have the same resolution. It also correlates well with the synthetic seismogram because they both contain primary reflections without multiples. Finally, multiples in the CDP data are easily identified utilizing the VET and synthetic seismogram. In addition, the depth of origin and periodicity of multiples in the processed VSP can be observed directly because they are generated by downgoing multiples and not by the direct arrival. End_of_Article - Last_Page 466------------