bluuebunny/crossref_metadata_2025 huggingface dataset:
Shallow to very shallow, high‐resolution reflection seismic using a portable vibrator system
Authors: Ghose Ranajit , Nijhof Vincent , Brouwer Jan , Matsubara Yoshikazu , Kaida Yasuhiro , Takahashi Toru
Published: October 2002

In shallow engineering‐geophysical applications, there is a lack of controlled, nondestructive, high‐resolution mapping tools, particularly for the target depth that ground‐penetrating radar cannot reach but which is too shallow for other conventional geophysical methods. For soft soil, this corresponds to a depth of 2 to 30 m. We have developed a portable, high‐frequency P-wave vibrator system that is capable of bridging this gap. As far as the important contribution of the seismic source is concerned, penetration and resolution can be individually controlled through easy modulation of the sweep signal generated by this electromagnetic vibrator. The feasibility of this system has been tested in shallow (10–50 m) to very shallow (0–10 m) applications. Seven field data sets representing varying geology, site conditions, and exploration targets are presented to illustrate the applicability. The first three examples show the potential of this portable vibrator source in shallow applications. Under favorable situations, a maximum resolution of about 20 cm for events located at 15–30 m depth could be achieved. Because high‐frequency seismic waves suffer from severe attenuation in the dry, unsaturated weathered zone, the penetration is relatively limited when the water table is deeper than 4–5 m. The fourth to seventh field examples illustrate very shallow applications at noisy, asphalt‐paved urban sites that are often encountered in civil, geotechnical, and environmental engineering projects. The prospecting targets were thin soil layers or small buried objects. On asphalt, the vibrator can produce high‐frequency energy easily. The fourth example shows high‐resolution delineation of very shallow soil structures. The last three examples present successful location of buried bodies—often small and closely spaced—in soft soil at depths of 0.5 to 5 m. We observe well‐defined reflection events of frequency exceeding 200 Hz. These results suggest that high‐frequency seismic reflection imaging using the portable vibrator system can indeed serve as a powerful, nondestructive technique for shallow to very shallow underground prospecting.
Comparison of high‐frequency seismic sources at the Grimsel test site, central Alps, Switzerland
Authors: Bühnemann Jan , Holliger Klaus
Published: October 2002

In August 1995, various high‐frequency seismic sources were tested at the Grimsel Test Site (GTS), located inside a crystalline rock body in the central Swiss Alps. These source tests were designed to facilitate future tomographic studies of potential radioactive waste disposal sites. The principal objective was to identify borehole and tunnel seismic sources capable of generating powerful high‐frequency signals such that frequencies up to 1000 Hz can be observed over distances of 1000 m in crystalline or consolidated sedimentary rocks. Seismic sources were situated in water‐filled boreholes (sparker, two piezoelectric sources, explosives) and at or near the tunnel wall (accelerated weight drop, minivibrator, bolt gun, buffalo gun, explosives). To evaluate and compare the source characteristics, the direct P-wave generated by the various seismic sources was investigated for the decay of its S/N and dominant frequency with offset and for the maximum distance at which first arrivals could be picked. Of the seismic sources tested, small explosive charges (5–100 g) had the most favorable S/N and frequency characteristics. At GTS, the target distance (∼1000 m) was reached with explosive charges of 50 g or more. None of the sources tested was capable of generating signals that sustained frequencies of 1000 Hz over distances in excess of 100 to 200 m. The unusually strong attenuation implied by this observation is likely due to the fact that the rocks at GTS underwent brittle deformation during the Alpine orogeny and therefore contain numerous fractures and shear zones.
Simultaneous inversion of airborne electromagnetic data for resistivity and magnetic permeability
Authors: Beard Les P. , Nyquist Jonathan E.
Published: October 2002

Where the magnetic permeability of rock or soil exceeds that of free space, the effect on airborne electromagnetic systems is to produce a frequency‐independent shift in the in‐phase response of the system while altering the quadrature response only slightly. The magnitude of the in‐phase shift increases as (1) the relative magnetic permeability is increased, (2) the amount of magnetic material is increased, and (3) the airborne sensor gets nearer the earth’s surface. Over resistive, magnetic ground, the shift may be evinced by negative in‐phase measurements at low frequencies; but over more conductive ground, the same shift may go unnoticed because of the large positive in‐phase response. If the airborne sensor is flown at low levels, the magnitude of the shift may be large enough to affect automatic inversion routines that do not take this shift into account, producing inaccurate estimated resistivities, usually overestimates. However, layered‐earth inversion algorithms that incorporate magnetic permeability as an additional inversion parameter may improve the resistivity estimates. We demonstrate this improvement using data collected over hazardous waste sites near Oak Ridge, Tennessee, USA. Using resistivity inversion without magnetic permeability, the waste sites are almost invisible to the sensors. When magnetic permeability is included as an inversion parameter, the sites are detected, both by improved resistivity estimates and by estimated magnetic permeability.
3-D symmetric sampling
Authors: Vermeer Gijs J. O.
Published: October 2002

Three‐dimensional seismic surveys have become accepted in the industry as a means of acquiring detailed information on the subsurface. Yet, the cost of 3-D seismic data acquisition is and will always be considerable, making it highly important to select the right 3-D acquisition geometry. Up till now, no really comprehensive theory existed to tell what constitutes a good 3-D geometry and how such a geometry can be designed. The theory of 3-D symmetric sampling proposed in this paper is intended to fill this gap and may serve as a sound basis for 3-D geometry design and analysis. Methods and theories for the design of 2-D surveys were developed in the 1980s. Anstey proposed the stack‐array approach, Ongkiehong and Askin the hands‐off acquisition technique, and Vermeer introduced symmetric sampling theory. In this paper, the theory of symmetric sampling for 2-D geometries is expanded to the most important 3-D geometries currently in use. Essential elements in 3-D symmetric sampling are the spatial properties of a geometry. Spatial aspects are important because most seismic processing programs operate in some spatial domain by combining neighboring traces into new output traces, and because it is the spatial behavior of the 3-D seismic volume that the interpreter has to translate into maps. Over time, various survey geometries have bee devised for the acquisition of 3-D seismic data. All geometries constitute some compromise with respect to full sampling of the 5-D prestack wavefield (four spatial coordinates describing shot and receiver position, and traveltime as fifth coordinate). It turns out that most geometries can be considered as a collection of 3-D subsets of the 5-D wavefield, each subset having only two varying spatial coordinates. The spatial attributes of the traces in each subset vary slowly and regularly, and this property provides spatial continuity to the 3-D survey. The spatial continuity can be exploited optimally if the subsets are properly sampled and if their extent is maximized. The 2-D symmetric sampling criteria—equal shot and receiver intervals, and equal shot and receiver patterns—apply also to 3-D symmetric sampling but have to be supplemented with additional criteria that are different for different geometries. The additional criterion for orthogonal geometry (geometry with parallel shotlines orthogonal to parallel receiver lines) is to ensure that the maximum cross‐line offset is equal to the maximum inline offset. Three‐dimensional symmetric sampling simplifies the design of 3-D acquisition geometries. A simple checklist of geophysical requirements (spatial continuity, resolution, mappability of shallow and deep objectives, and signal‐to‐noise ratio) limits the choice of survey parameters. In these consideration, offset and azimuth distributions are implicitly being taken care of. The implementation in the field requires careful planning to prevent loss of spatial continuity.
Nonhyperbolic reflection moveout for horizontal transverse isotropy
Authors: Al‐Dajani AbdulFattah , Tsvankin Ilya
Published: October 2002

The transversely isotropic model with a horizontal axis of symmetry (HTI) has been used extensively in studies of shear‐wave splitting to describe fractured formations with a single system of parallel vertical penny‐shaped cracks. Here, we present an analytic description of longspread reflection moveout in horizontally layered HTI media with arbitrary strength of anisotropy. The hyperbolic moveout equation parameterized by the exact normal‐moveout (NMO) velocity is sufficiently accurate for P-waves on conventional‐length spreads (close to the reflector depth), although the NMO velocity is not, in general, usable for converting time to depth. However, the influence of anisotropy leads to the deviation of the moveout curve from a hyperbola with increasing spread length, even in a single‐layer model. To account for nonhyperbolic moveout, we have derived an exact expression for the azimuthally dependent quartic term of the Taylor series traveltime expansion [t2(x2)] valid for any pure mode in an HTI layer. The quartic moveout coefficient and the NMO velocity are then substituted into the nonhyperbolic moveout equation of Tsvankin and Thomsen, originally designed for vertical transverse isotropy (VTI). Numerical examples for media with both moderate and uncommonly strong nonhyperbolic moveout show that this equation accurately describes azimuthally dependent P-wave reflection traveltimes in an HTI layer, even for spread lengths twice as large as the reflector depth. In multilayered HTI media, the NMO velocity and the quartic moveout coefficient reflect the influence of layering as well as azimuthal anisotropy. We show that the conventional Dix equation for NMO velocity remains entirely valid for any azimuth in HTI media if the group‐velocity vectors (rays) for data in a common‐midpoint (CMP) gather do not deviate from the vertical incidence plane. Although this condition is not exactly satisfied in the presence of azimuthal velocity variations, rms averaging of the interval NMO velocities represents a good approximation for models with moderate azimuthal anisotropy. Furthermore, the quartic moveout coefficient for multilayered HTI media can also be calculated with acceptable accuracy using the known averaging equations for vertical transverse isotropy. This allows us to extend the nonhyperbolic moveout equation to horizontally stratified media composed of any combination of isotropic, VTI, and HTI layers. In addition to providing analytic insight into the behavior of nonhyperbolic moveout, these results can be used in modeling and inversion of reflection traveltimes in azimuthally anisotropic media.
Time‐frequency coherency analysis of three‐component crosshole seismic data for arrival detection
Authors: Leśniak Andrzej , Niitsuma Hiroaki
Published: October 2002

A processing method for three‐component wide‐band crosshole seismic data is presented. It is based on the observed phenomena of a decrease in the coherency between the coda waves recorded by closely spaced sensors or excited by closely spaced seismic sources. We transfer signals to the time‐frequency domain and compute the instantaneous coherency at each moment of time between signals propagated on adjacent raypaths. As a result, we obtain the distribution of the high‐coherency time‐frequency areas connected with coherent components of the signal. We can then see different modes of the recorded seismic signal and identify them as direct P-and S-waves, reflected waves, and converted tube waves. A reliable method of evaluation of the onset’s time for different modes of the seismic signal is also proposed. The technique has good resolution and allows separation of even very closely spaced wavefield components.
Interwell seismic imaging at the Savannah River Site, South Carolina
Authors: Parra Jorge O. , Price Van , Addington Carl , Zook Brian J. , Cumbest Randolph J.
Published: October 2002

Crosswell and continuity logging seismic measurements were made beneath a large tank (27 m diameter) used for processing radioactive waste at the Department of Energy (DOE) Savannah River Site in the Atlantic Coastal Plain of South Carolina. We used the data to delineate a low‐velocity zone (soft materials) and image the connectivity of a clay unit between wells. The low‐velocity zone depicted on the crosswell seismic tomogram integrated with data from cores and well logs revealed soft materials in the region between 150 and 180 ft (46–55 m). The bottom boundary of this low‐velocity zone correlates with a reflection observed in the crosswell seismic image at a depth of 180 ft (55 m). This reflection corresponds to the impedance contrast between the soft materials and the more rigid Tinker Formation. The low‐velocity zone of soft materials indicates a dissolution margin of a carbonate unit (which is part of the Utley limestone) and the presence of loose sands of the Griffins Landing Member. Ray tracing and common source seismograms show that the rigid part of the Utley limestone extends horizontally about 12.5 ft (4 m) west of the receiver well. The continuity logging data showed leaky and normal modes in the region between 140 and 150 ft (43–46 m). The computed group velocity contours of leaky and normal modes are consistent with waveguide models based on well logs and crosswell seismic data. This indicates that the low‐velocity tan clay (confining unit) within the Griffins Landing Member is connected between wells.
Magnetotelluric tensor decomposition: Part II, Examples of a basic procedure
Authors: Lilley F. E. M. (Ted)
Published: October 2002

The decomposition procedure suggested for magnetotelluric (MT) data in a companion paper is illustrated by examples. Adjacent pairs of stations are examined to test for common features in their MT response. The method analyzes the real and quadrature parts of an impedance tensor separately, and the results for the real and quadrature parts are compared, and in some cases combined, at the end. Values are obtained which are interpreted directly in terms of nearest 2-D principal impedances, local geologic strike, and regional geologic strike. The requirement for valid data derived in the companion paper, that [Formula: see text] together with [Formula: see text], is shown to be a useful criterion in the winnowing of erroneous data. The examples presented typically show the direction interpreted as local geologic strike to be well determined, period independent, and consistent between real and quadrature MT matrices. In these examples, however, the direction interpreted as regional geologic strike is period dependent and not in good agreement between real and quadrature MT matrices. These results suggest caution is desirable in the use of period‐independent galvanic distortion models, which should be checked first with regional strike determinations.
Comments on the electromagnetic “smoke ring” concept
Authors: Reid James E. , Macnae James C.
Published: October 2002

The electromagnetic (EM) fields in a one‐dimensional (1-D) earth due to a dipole or loop transmitter have been studied by a number of authors, including Lewis and Lee (1978), Pridmore (1978), Nabighian (1979), and Hoversten and Morrison (1982). Nabighian (1979) aptly described the time‐domain‐induced current system in a homogeneous half‐space as resembling a “smoke ring” blown by the transmitter, which moves outwards and downwards and diminishes in amplitude with increasing time after the transmitter is turned off. In a homogeneous half‐space, the physical electric field maximum moves outward from the transmitter loop edge at an angle of approximately 30° with the surface. Hoversten and Morrison (1982) show how the direction of propagation of the time‐domain electric field maximum is affected by conductivity structure. In the case of a highly conductive overburden over a resistive basement, the electric field maximum travels essentially horizontally away from the transmitter, and is effectively trapped in the upper layer.
Modeling ground‐penetrating radar wave propagation and reflection with the Jonscher parameterization
Authors: Hollender Fabrice , Tillard Sylvie
Published: October 2002

As in seismic surveys, forward modeling is essential to the geological interpretation of georadar data. Because a radar signal is broad band, modeling radar waves in a realistic medium requires knowing the frequency dependence of the effective dielectric permittivity of rocks. Various models can be found in the literature. With the support of laboratory measurements carried out on various rock samples, the complex effective dielectric permittivity describing polarization and conduction effects can be modeled by the Jonscher law. In the megahertz range, the Jonscher parameterization involves only three real, constant parameters that are characteristic of the investigated media. Radar plane waves propagating in 1-D models are generated and compared to field data recorded in transmission mode for different frequencies, distances, and geological formations. The signal distortion from propagation is emphasized. Quality factors are calculated and found to be compatible with the propagation in real media. Reflection coefficients are written using the Jonscher relation. Two cases are illustrated: reflection from a halfspace and reflection from a thin layer. From the study of the influence of frequency, reflection modes, electric properties of rocks, and layer thickness, it can be concluded that reflection phenomena, like propagation phenomena, lead to phase shifts and frequency content variations. Thus, processing based on the assumption of stationary wavelets is not suitable for georadar signals, even in low‐loss media.
Analysis of potential field anomalies in Lavrion mining area, Greece
Authors: Tsokas Gregory N. , Stampolidis Alexandros , Angelopoulos Antonis D. , Kilias Stefanos
Published: October 2002

Mining activities in Lavrion began during the first millennium B.C. after the decline of ancient Athens and then restarted more deliberately during the nineteenth century. Aeromagnetic data from a 1967 survey of the mining area was recompiled, processed, and interpreted for the present study. The original flight lines were digitized and leveled, and the international geomagnetic reference field (IGRF) was removed. The data were inverted by means of a terracing technique that defines separate domains of uniform distribution of physical properties that cause the magnetic anomalies. The log power spectrum was computed; along with the results of terracing, it suggested the existence of two sources of the magnetic anomaly. The long‐wavelength anomaly reflects a large, concealed body that is most probably a granitic intrusion, consistent with local geological evidence. The source of the short‐wavelength anomaly is a strongly magnetized body attributed to the net effect of various thin, magnetite‐bearing sulfide zones. The anomalies were then separated in the wavenumber domain. Magnetic susceptibility measurements were made in situ on the exposed parts of the local formations. Three‐dimensional models whose effect simulates the observed anomalies were calculated. Results of the modeling show that the large magnetic body is buried at 0.68 km depth. The small, relatively shallow body is about 0.035 km thick and buried at 0.6 km depth. The bodies do not show any corresponding gravity anomaly on the regional Bouguer gravity anomaly map.
Estimation ofQfrom surface seismic reflection data
Authors: Dasgupta Rahul , Clark Roger A.
Published: October 2002

Reliable estimates of the anelastic attenuation factor, Q, are desirable for improved resolution through inverse Q deconvolution and to facilitate amplitude analysis. Q is a useful petrophysical parameter itself, yet Q is rarely measured. Estimates must currently be made from borehole seismology. This paper presents a simple technique for determining Q from conventional surface seismic common midpoint (CMP) gathers. It is essentially the classic spectral ratio method applied on a trace‐by‐trace basis to a designatured and NMO stretch‐corrected CMP gather. The variation of apparent Q versus offset (QVO) is extrapolated to give a zero‐offset Q estimate. Studies on synthetics suggest that, for reasonable data quality (S/N ratios better than 3:1, shallow (<5°) dips, and stacking velocity accuracy <5%), source‐to‐reflector average Q is recoverable to within some 3% and Q for a specific interval (depending on its two‐way time thickness and depth) is recoverable to 15–20%. Three case studies are reported. First, Q versus offset and vertical seismic profiling (VSP) Q estimates for a southern North Sea line were in close agreement, validating the method. For Chalk, Mushelkalk‐Keuper, and Bunter‐Zechstein, Q was estimated as 130 ± 15, 47 ± 8, and 156 ± 18, respectively. Next, two alternative lithological interpretations of a structure seen in a frontier area were discriminated between when Q estimates of 680 to 820 were obtained (compared to some 130–170 in the overlying units), favoring a metamorphic/crystalline lithology rather than (prospective) sediments. This was later confirmed by drilling. Third, a profile of Q estimates along a 200-ms-thick interval, known to include a gas reservoir, showed a clear and systematic reduction in Q to a low of 50 ± 11, coincident with the maximum reservoir thickness, from some 90–105 outside the reservoir. Q for the reservoir interval itself was estimated at 17 ± 7.
The 4-D microgravity method for waterflood surveillance: A model study for the Prudhoe Bay reservoir, Alaska
Authors: Hare Jennifer L. , Ferguson John F. , Aiken Carlos L. V. , Brady Jerry L.
Published: October 2002

Forward and inverse gravity modeling is carried out on a suite of reservoir simulations of a proposed water injection in the Prudhoe Bay reservoir, Alaska. A novel surveillance technique is developed in which surface gravity observations are used to monitor the progress of a gas cap waterflood in the reservoir at 8200-ft (2500-m) depth. This cost‐effective method requires that high‐precision gravity surveys be repeated over periods of years. Differences in the gravity field with time reflect changes in the reservoir fluid densities. Preliminary field tests at Prudhoe Bay indicates survey accuracy of 5–10 μGal can be achieved for gravity data using a modified Lacoste & Romberg “G” type meter or Scintrex CG-3M combined with the NAVSTAR Global Positioning System (GPS). Forward gravity modeling predicts variations in surface measurements of 100 μGal after 5 years of water injection, and 180–250 μGal after 15 years. We use a constrained least‐squares method to invert synthetic gravity data for subsurface density distributions. The modeling procedure has been formulated and coded to allow testing of the models for sensitivity to gravity sampling patterns, noise types, and various constraints on model parameters such as density, total mass, and moment of inertia. Horizontal‐feature resolution of the waterflood is about 5000 ft (1520 m) for constrained inverse models from synthetic gravity with 5 μGal standard deviation (SD) noise. The inversion method can account for total mass of injected water to within a few percent. Worst‐case scenarios result from inversion of gravity data which are contaminated by high levels (greater than 10–15 μGal SD) of spatially correlated noise, in which case the total mass estimate from inverse models may over or underestimate the mass by 10–20%. The results of the modeling indicate that inversion of time‐lapse gravity data is a viable technique for the monitoring of reservoir gas cap waterfloods.
Revealing stratigraphy in ground‐penetrating radar data using domain filtering
Authors: Young Roger A. , Sun Jingsheng
Published: October 2002

A common‐offset ground‐penetrating radar profile was collected from a mid‐continent fluvial environment characterized by sand with discontinuous clay layers. Signal was separated from coherent noise by a domain filter that exploits local differences in dip between signal and noise. This filter proved effective in separating reflections from the direct air wave and from system ringing. The processed section locates the base of a Pleistocene river channel incised into bedrock and shows a change in a recent position of the river on the basis of radar character. The water table, which lies at a depth of approximately 2 m, is characterized by a continuous reflection. An earlier reflection is interpreted as a perched lens of water near the ground surface. The predominant frequency of both reflections varies greatly with position along the profile. Shallow cores taken along the profile provide evidence that grain size in the transition zone from saturated to unsaturated sediments may determine the predominant frequency of the associated radar reflection. The domain filter is also successful in separating transition zone reflections from saturated layer boundary reflections that are coincident in arrival time. It is possible, therefore, to determine both grain size in the transition zone and the dip of sediments immediately beneath it even if the reflections from the two are superimposed.
Using even terms of the scattering series for deghosting and multiple attenuation of ocean‐bottom cable data
Authors: Ikelle Luc T.
Published: October 2002

Inverse scattering multiple attenuation (ISMA) is a method of removing free‐surface multiple energy while preserving primary energy. The other key feature of ISMA is that no knowledge of the subsurface is required in its application. I have adapted this method to multicomponent ocean‐bottom cable data (i.e., to arrays of sea‐floor geophones and hydrophones) by selecting a subseries made of even terms of the current scattering series used in the free‐surface multiple attenuation of conventional marine surface seismic data (streamer data). This subseries approach allows me to remove receiver ghosts (receiver‐side reverberations) and free‐surface multiples (source‐side reverberations) in multicomponent OBC data. I have processed each component separately. As for the streamer case, my OBC version of ISMA preserves primary energy and does not require any knowledge of the subsurface. Moreover, the preprocessing steps of muting for the direct wave and interpolating for missing near offsets are no longer needed. Knowledge of the source signature is still required. The existing ways of satisfying this requirement for streamer data can be used for OBC data without modification. This method differs from the present dual‐field deghosting method used in OBC data processing in that it does not assume a horizontally flat sea floor; nor does it require the knowledge of the acoustic impedance below the sea floor. Furthermore, it attenuates all free‐surface multiples, including receiver ghosts and source‐side reverberations.
On: “Unambiguous signal recovery above the Nyquist using random‐sample‐interval imaging,” (R.D. Wisecup, GEOPHYSICS, 63, 763–771)
Authors: Vermeer Gijs J. O.
Published: October 2002

Wisecup (1997) claims that “a simple, exact sample‐maping methodology, random‐sample‐interval imaging, can be used to overcome aliasing in many of the processes currently used for the imaging of seismic data.” Other statements are that “increased equipment costs are incurred due to the presumed (italics by GJOV) requirement for antialias filter circuitry in the recording instruments,” and “many antialias strategies currently in use may be inappropriate,” and many more such remarks. The question comes up whether this random‐sample‐interval imaging (RSI2) is really so wonderful as it is purported to be.
Differentiating anisotropy and lateral effects using azimuthal resistivity offset Wenner soundings
Authors: Watson Kathryn A. , Barker Ron D.
Published: October 2002

Azimuthal resistivity surveys increasingly are being used by hydrogeologists in the identification and characterization of fractured rocks. In these investigations, electrical resistivity is measured as a function of azimuth about a fixed central point. In most recent published examples, any observed change in apparent resistivity with azimuth is interpreted as being indicative of fracture anisotropy. However, interpretation of rotational sounding data is actually more complicated, as azimuthal variations in apparent resistivity are also produced by the presence of dipping stratigraphy and other lateral changes in formation resistivity. Such effects are generally overlooked because the field techniques normally employed are incapable of detecting them. Consequently, it is quite probable that the results of many published surveys have been wrongly interpreted. An alternative field procedure and interpretation methodology has been developed to differentiate anisotropy, dipping layers, and lateral effects. This approach makes use of the offset Wenner technique and examines the different responses of the individual Wenner resistances above different geological structures. Analysis of data obtained using the azimuthal offset Wenner technique from sites in Britain and Ireland have successfully identified subsurface structures and determined the anisotropy where it is present.
AVD—An emerging new marine technology for reservoir characterization: Acquisition and application
Authors: MacBeth Colin , Li Xiang‐Yang
Published: October 2002

Several significant developments in marine technologies in the past few years have resulted in the creation of acquisition techniques suited to azimuthal anisotropy analysis in the offshore environment. The developments have parallelled the evolution in the theory underlying the use of P-P and P-S amplitude versus direction (AVD) for seismic anisotropy estimation. The demands of such AVD methods for a wide azimuthal coverage have only recently been met. To guide future work, the AVD method has been assessed using data from intersecting streamer lines. Application of the method in this example permits an identification of the strike direction of hydrocarbon‐filled fractures within a chalk formation in the central North Sea. The results of this study provide confidence that the method is sufficiently sensitive to fractures and can help guide future analyses. The new generation of vertical cables, seabed seismic sensors, and walk‐away (and/or 3-D) vertical seismic profiles will eventually lead to high‐resolution anisotropy estimation in the offshore environment using this approach.
Image mispositioning due to dipping TI media: A physical seismic modeling study
Authors: Isaac J. Helen , Lawton Don C.
Published: October 2002

A scaled physical model was constructed to investigate the magnitudes of imaging errors incurred by the use of isotropic processing code when there is seismic velocity anisotropy present in the dipping overburden. The model consists of a block of transversely isotropic (TI) phenolic material with the TI axis of symmetry dipping at an angle of 45°. Its scaled thickness is 1500 m, and it is intended to simulate the dipping clastic sequences found in many fold‐thrust belts. A piece of isotropic Plexiglas, affixed to the underside of the anisotropic block, has a step function in it to simulate a target reef edge or fault. The anisotropy parameters of the material are δ = 0.1 and ε = 0.24. On zero‐offset data the imaged position of the target is shifted laterally 320 m in the updip direction of the beds, whereas on time‐ and depth‐migrated multichannel sections the shift is 300 m. The lateral shift is offset dependent, with the amount of shift in any common‐midpoint gather decreasing from 320 m on the near offsets to 280 m on the far offsets. Prestack depth‐migration velocity analysis based upon obtaining consistent depth images in the common‐offset domain results in the base of the anisotropic section being imaged 50 m (about 3%) too deep.
Imaging structures below dipping TI media
Authors: Vestrum Robert W. , Lawton Don C. , Schmid Ron
Published: October 2002

Seismic anisotropy in dipping shales causes imaging and positioning problems for underlying structures. We developed an anisotropic depth‐migration approach for P-wave seismic data in transversely isotropic (TI) media with a tilted axis of symmetry normal to bedding. We added anisotropic and dip parameters to the depth‐imaging velocity model and used prestack depth‐migrated image gathers in a diagnostic manner to refine the anisotropic velocity model. The apparent position of structures below dipping anisotropic overburden changes considerably between isotropic and anisotropic migrations. The ray‐tracing algorithm used in a 2-D prestack Kirchhoff depth migration was modified to calculate traveltimes in the presence of TI media with a tilted symmetry axis. The resulting anisotropic depth‐migration algorithm was applied to physical‐model seismic data and field seismic data from the Canadian Rocky Mountain Thrust and Fold Belt. The anisotropic depth migrations offer significant improvements in positioning and reflector continuity over those obtained using isotropic algorithms.
Detection and analysis of naturally fractured gas reservoirs: Multiazimuth seismic surveys in the Wind River basin, Wyoming
Authors: Grimm Robert E. , Lynn Heloise B. , Bates C. R. , Phillips D. R. , Simon K. M. , Beckham Wallace E.
Published: October 2002

Multiazimuth binning of 3-D P-wave reflection data is a relatively simple but robust way of characterizing the spatial distribution of gas‐producing natural fractures. In our survey, data were divided into two volumes by ray azimuth (approximately perpendicular and parallel (±45° to the dominant fracture strike) and separately processed. Azimuthal differences or ratios of attributes provided a rough measure of anisotropy. Improved imaging was also attained in the more coherent fracture‐parallel volume. A neural network using azimuthally dependent velocity, reflectivity, and frequency attributes identified commercial gas wells with greater than 85% success. Furthermore, we were able to interpret the physical mechanisms of most of these correlations and so better generalize the approach. The apparent velocity anisotropy was compared to that derived from other P- and S-wave methods in an inset three‐component survey. Prestack determination of the azimuthal moveout ellipse will best quantify velocity anisotropy, but simple two‐ or four‐azimuth poststack analysis can adequately identify regions of high fracture density and gas yield.
Permeability prediction based on fractal pore‐space geometry
Authors: Pape Hansgeorg , Clauser Christoph , Iffland Joachim
Published: October 2002

Estimating permeability from grain‐size distributions or from well logs is attractive but difficult. In this paper we present a new, generally applicable, and relatively inexpensive approach which yields permeability information on the scale of core samples and boreholes. The approach is theoretically based on a fractal model for the internal structure of a porous medium. It yields a general and petrophysically justified relation linking porosity to permeability, which may be calculated either from porosity or from the pore‐radius distribution. This general relation can be tuned to the entire spectrum of sandstones, ranging from clean to shaly. The resulting expressions for the different rock types are calibrated to a comprehensive data set of petrophysical and petrographical rock properties measured on 640 sandstone core samples of the Rotliegend Series (Lower Permian) in northeastern Germany. With few modifications, this new straightforward and petrophysically motivated approach can also be applied to metamorphic and igneous rocks. Permeability calculated with this procedure from industry porosity logs compares very well with permeability measured on sedimentary and metamorphic rock samples.
Manual seismic reflection tomography
Authors: Murphy Gary E. , Gray Samuel H.
Published: October 2002

Prestack depth migration needs a good velocity model to produce a good image; in fact, finding the velocity model is one of the goals of prestack depth migration. Migration velocity analysis uses information produced by the migration to update the current velocity model for use in the next migration iteration. Several techniques are currently used to estimate migration velocities, ranging from trial and error to automatic methods like reflection tomography. Here, we present a method that combines aspects of some of the more accurate methods into an interactive procedure for viewing the effects of residual normal moveout corrections on migrated common reflection point (CRP) gathers. The residual corrections are performed by computing traveltimes along raypaths through both the current velocity model and the velocity model plus suggested model perturbations. The differences between those sets of traveltimes are related to differences in depth, allowing the user to preview the approximate effects of a velocity change on the CRP gathers without remigrating the data. As with automatic tomography, the computed depth differences are essentially backprojected along raypaths through the model, yielding a velocity update that flattens the gathers. Unlike automatic tomography, in which an algebraic inverse problem is solved by the computer for all geologic layers simultaneously, our method estimates shallow velocities before proceeding deeper and requires substantial user intervention, both in flattening individual CRP gathers and in deciding the appropriateness of the suggested velocity updates in individual geologic units.
The effect of 3-D prestack seismic migration on seismic coherence and amplitude variability
Authors: Rietveld Walter E. , Kommedal Jan H. , Marfurt Kurt J.
Published: October 2002

We compare the effect of 3-D poststack versus 3-D prestack imaging on seismic coherence, seismic amplitude, and seismic amplitude variation. We find that the improved resolution and amplitude preservation of the prestack imaging result in more sharply defined terminations and hence better delineation by coherence and amplitude gradients even though the (macro) velocity models used in both imaging approaches are laterally invariant [v(z)].
Parallel Butterworth and Chebyshev dip filters with applications to 3-D seismic migration
Authors: Zhou Hongbo , McMechan George A.
Published: October 2002

Dip filtering is a necessary part of accurate frequency‐space domain migration, so design and application of reliable and efficient filters are of practical as well as theoretical importance. Frequency‐space domain dip filters are implemented using Butterworth and Chebyshev algorithms. By transforming the product terms of the filter transfer function into summations, a cascaded (serial) Butterworth or Chebyshev dip filter can be made parallel, which improves computational efficiency. For a given order of filter, the cost of the Butterworth and Chebyshev filters is the same. However, the Chebyshev filter has a sharper transition zone than that of a Butterworth filter with the same order, which makes it more effective for phase compensation than a Butterworth filter, but at the expense of some wavenumber‐dependent amplitude ripples. Both implementations have been incorporated into 3-D one‐way frequency‐space depth migration for evanescent energy removal and for phase compensation of splitting errors; a single filter achieves both goals.
The footsteps of the receiver ghost in the f-k domain
Authors: Aytun Katibe
Published: October 2002

Waves traveling upward from subsurface reflective strata continue propagating after they are recorded by receivers as primary reflections. When a sharp velocity discontinuity exists above the receivers, the waves are then reflected back, and are once more recorded by the same receivers but as downgoing waves. This phenomenon is known as the receiver ghost. Based on a thorough study of the f-k response of the receiver ghost in a record, this paper shows that the null frequencies are caused by the time differences between the primary and the ghost arrivals, and that they vary with the angle of incidence. It is further shown that the loci of the null frequencies of each harmonic is a member of a family of hyperbolas with a common pair of asymptotes in the f-k domain.
Origin of Gassmann’s equations
Authors: Berryman James G.
Published: October 2002

Gassmann’s relations are receiving more attention as seismic data are increasingly used for reservoir monitoring. Correct interpretation of underground fluid migration from seismic data requires a quantitative understanding of the relationships among the velocity data and fluid properties in the form of fluid substitution formulas, and these formulas are very commonly based on Gassmann’s equations. Nevertheless, confusion persists about the basic assumptions and the derivation of Gassmann’s (1951) well‐known equation in poroelasticity relating dry or drained bulk elastic constants to those for fluid‐saturated and undrained conditions. It is frequently stated, for example, but quite incorrect to say that Gassmann assumes the shear modulus is constant (i.e., mechanically independent of the presence of the saturating fluid). This note clarifies the situation by presenting an unusually brief derivation of Gassmann’s relations that emphasizes the true origin of the constant shear modulus result, while also clarifying the role played by the shear modulus in the derivation of the better understood result for the bulk modulus.
Concepts of normal and dip moveout
Authors: Liner Christopher L.
Published: October 2002

Both normal moveout and dip moveout are key elements of modern seismic data processing. A nonmathematical discussion of the nature and action of these processes helps to clarify their importance. Special emphasis is given to their history and geometry, as well as the effect of dip, velocity variation, and anisotropy. The relationship between dip moveout and prestack migration is also developed.
2-D gravity modeling with analytically defined geometry and quadratic polynomial density functions
Authors: Martín‐Atienza Beatriz , García‐Abdeslem Juan
Published: October 2002

New methods for 2-D modeling of gravity anomaly data are developed following an approach that uses both analytic and numerical methods of integration. The forward‐model solution developed here is suitable to calculate the gravity effect caused by a 2-D source body bounded either laterally or vertically by continuous functions. In our models, the density contrast is defined by a second‐order polynomial function of depth and distance along the profile. We present several examples to show that our models are capable of accommodating a broad variety of geologic structures.
Staggered mesh for the anisotropic and viscoelastic wave equation
Authors: Carcione José M.
Published: October 2002

Computation of the spatial derivatives with nonlocal differential operators, such as the Fourier pseudospectral method, may cause strong numerical artifacts in the form of noncausal ringing. This situation happens when regular grids are used. The problem is attacked by using a staggered pseudospectral technique, with a different scheme for each rheological relation. The nature and causes of acausal ringing in regular grid methods and the reasons why staggered‐grid methods eliminate this problem are explained in papers by Fornberg (1990) and Özdenvar and McMechan (1996). Thus, the objective here is not to propose a new method but to develop the algorithm for the viscoelastic and transversely isotropic (VTI) wave equation, for which the technique can be implemented without interpolation. The algorithm is illustrated for one physical situation that requires very high accuracy, such as a fluid‐solid interface, where very large contrasts in material properties occur. The staggered‐grid solution is noise free in the dynamic range where regular grids generate artifacts that may have amplitudes similar to those of physical arrivals.
Shape and depth solutions from magnetic data using a parametric relationship
Authors: Abdelrahman El‐Sayed M. , Hassanein Hamdy I.
Published: October 2002

We have developed a simple method to simultaneously determine the shape (shape factor) and the depth of a buried structure from magnetic data. The method is similar to Euler deconvolution, but it solves for shape and depth independently. The method involves using a relationship between the shape factor, the depth to the source, and a combination of observations at symmetric points with respect to the coordinate of the source center with a free parameter (graticule spacing). The relationship represents a parametric family of curves. For a fixed free parameter, the depth is determined for each shape factor. The computed depths are plotted against the shape factors representing a continuous monotonically increasing curve. The solution for the shape and depth of the buried structure is read at the common intersection of the parametric curves. The parametric curves method is applied to two theoretical magnetic anomaly profiles due to a thin dike and a horizontal cylinder. The method is also tested on a field example from Brazil. In all cases, the shape and depth solutions obtained are in good agreement with the actual ones.