Unmigrated Data Research Articles

Improving the resolution of impedance inversion in karst systems by incorporating diffraction information: A case study of Tarim Basin, China

Different scales of voids and cavities in karst systems demonstrate considerable potential as exploration targets in the Tarim Basin, northwest China. Numerous diffraction events exist in the seismic data above the karst reservoir in this area because of the strong impedance contrast and irregular shape of voids. The conventional impedance inversion method using migrated data as an input cannot easily estimate the location of voids and the impedance inside the voids. In this case, an alternative approach to impedance inversion that considers the diffractive component of the total wavefield and uses the unmigrated data as an input should be used. The inversion consists of a least-squares minimization of the misfit function between the observed and modeled data. Forward modeling incorporates a combination of reflection and diffraction wavefield components. The adopted method is applied to physical modeling and field data recorded above the karst reservoir. This study’s physical modeling test simulates observed field data and is performed using a high-resolution and high-fidelity 3D modeling system. Inversion results obtained by the proposed and conventional methods are compared. Physical modeling and a field data application show that the adopted impedance inversion method improves the karst location estimation and the acoustic impedance within the voids.

A comparison of diffraction imaging to incoherence and curvature

Detecting small subsurface features such as faults, fracture swarms, steep reef edges, and channel edges is a routine task in seismic interpretation. Common seismic attributes used for this task, such as curvature and incoherence, are derived from conventional images containing both reflections and diffractions. An emerging alternative approach is to interpret these edge features using the diffraction-imaging technique, which images diffracted energy separately. A diffraction-imaging volume is generated by separating specular reflections from diffractions in an unmigrated volume followed by migration of those diffractions. Different methods can be used to separate diffractions from reflections on unmigrated data. In a case history, they are separated with a plane-wave destruction (PWD) filter. An evaluation of diffraction-imaging performance is applied to a 3D seismic data set from the Cooper Basin of Australia. Diffraction-imaging horizon maps are compared with corresponding maps made from incoherence and curvature volumes. The resolution and the ability to detect faults, fractures, and channel-edge features are compared. The results show that diffraction imaging provides superior vertical and spatial resolution over conventional incoherence and curvature attributes for mapping faults and stratigraphy.

Internal Structure and Composition of a Rock Glacier in the Dry Andes, Inferred from Ground‐penetrating Radar Data and its Artefacts

AbstractUsing ground‐penetrating radar (GPR), we studied an entire 2.2 km long rock glacier (3780–4350 m asl) in the dry Andes of Chile with the aim of inferring its composition. In the high‐quality, unmigrated data, we identified the active layer base and the rock glacier floor. In between, hyperbolae generated by diffracting boulders were inventoried; the ones along the rock glacier floor (n = 51) allowed determination of the average electromagnetic (EM) velocity in the rock glacier, the latter being further used for migration. Within the rock glacier (16–39 m thick), the EM velocity varies between 0.076 and 0.167 m.ns‐1; the main stratigraphic features observed are upward‐dipping reflectors. The low EM velocities (<0.10 m.ns‐1) found at some locations suggest the presence of significant unfrozen water fractions. A strong (R2 = 0.77), inverse linear relationship was also found between the diffracting point density and the EM velocity, and was used to indicate the ice content in the rock glacier. The fraction of ice in the rock glacier was estimated to vary between 0.22 and 0.83, with an average of 0.66; these results were tested by recalculation of the EM velocity. The relationships between rock glacier development and glacial processes are questioned. Copyright © 2015 John Wiley & Sons, Ltd.

Modelling and Removing WAZ OBC Interbed Multiples

Interbed (or internal) multiples, caused by ringing between strong reflectors, can play havoc with the interpretation of primary reflections deeper within a seismic section, and have been historically difficult to suppress. True wide-azimuth, relatively sparsely acquired 3D data (like OBC) add to the complexity of this problem.For this case study, we present our testing and on-going developments on interbed multiple suppression. Two methods are shown. The first is an interpretive pattern- recognition technique applied after migration. The second, applied to unmigrated data, is a new version of wavefield modelling that attempts to predict interbed multiples without any knowledge of the actual multiple generators. A well-constrained adaptive subtraction methodology is required, and the testing and development of a new 3D subtraction algorithm is reviewed. Both methods have been shown to perform well.

3D seismic imaging of volcanogenic massive sulfide deposits in the Flin Flon mining camp, Canada: Part 1 — Seismic results

A [Formula: see text] 3D-3C seismic survey was conducted within the active Flin Flon mining camp located in Manitoba, Canada. The results for the vertical component data as obtained by conventional dip-moveout and prestack time-migration processing sequences and comparison of images from the 3D seismic volume with the subsurface location of known ore zones and the mine horizon generally showed a very good correlation. A well-defined diffraction response from the shallowest ore zone was observed in the unmigrated data with a corresponding phase reversal in the migrated data at the transition from intact ore to backfilled ore zone. The geometry of unmined and backfilled ore zones compared well with strong reflection amplitudes on corresponding cross sections to depths of [Formula: see text]. At greater depths, the ore zone had a weaker seismic signature due to a combination of effects, including imaging conditions, ore composition, and the increased presence of rhyolite within the mine horizon. In the case of the deeper ore zones that were characterized by low signal-to-noise levels, poststack migration was important in focusing weak ore-related reflections. The 3D data demonstrated the feasibility of detecting and accurately locating ore zones as small as a few million tons to depths of up to 1500 m.

The impact of common‐offset migration on porosity estimation by AVO inversion

The impact of prestack time migration on porosity estimation has been tested on a 2-D seismic line from the Valhall/Hod area in the North Sea. Porosity is estimated in the Cretaceous chalk section in a two‐step procedure. First, P-wave and S-wave velocity and density are estimated by amplitude variation with offset (AVO) inversion. These parameters are then linked to porosity through a petrophysical rock data base based on core plug analysis. The porosity is estimated both from unmigrated and prestack migrated seismic data. For the migrated data set, a standard prestack Kirchhoff time migration is used, followed by simple angle and amplitude corrections. Compared to modern high‐cost, true amplitude migration methods, this approach is faster and more practical. The test line is structurally fairly simple, with a maximum dip of 5°; but the results differ significantly, depending on whether migration is applied prior to the inversion. The maximum difference in estimated porosity is of the order of 10% (about 50% relative change). High‐porosity zones estimated from the unmigrated data were not present on the porosity section estimated from the migrated data.

Interpretive imaging of seismic data

An interval velocity field for depth migration derived from Dix conversion of an rms velocity field often is judged to be inaccurate and, consequently, can require an excessive number of iterations to attain a final velocity-depth model with acceptable accuracy. The most likely reason for failing to derive an accurate interval velocity field from Dix conversion is that the input rms velocity field usually is based on velocities estimated from unmigrated data. For instance, a common approach to derive an rms velocity field is to take a set of stacking or DMO velocity functions picked at analysis locations, edit for any outliers, and interpolate them with some lateral and vertical smoothing. The key to an accurate interval velocity field from Dix conversion, however, is to estimate the rms velocity field from prestack time-migrated data—not from unmigrated data. By doing a better initial estimate, you minimize the number of iterations required to derive a final velocity-depth model suitable for accurate ima...

Theory of 2-D complex seismic trace analysis

The ideas of 1-D complex seismic trace analysis extend readily to two dimensions. Two‐dimensional instantaneous amplitude and phase are scalars, and 2-D instantaneous frequency and bandwidth are vectors perpendicular to local wavefronts, each defined by a magnitude and a dip angle. The two independent measures of instantaneous dip correspond to instantaneous apparent phase velocity and group velocity. Instantaneous phase dips are aliased for steep reflection dips following the same rule that governs the aliasing of 2-D sinusoids in f-k space. Two‐dimensional frequency and bandwidth are appropriate for migrated data, whereas 1-D frequency and bandwidth are appropriate for unmigrated data. The 2-D Hilbert transform and 2-D complex trace attributes can be efficiently computed with little more effort than their 1-D counterparts. In three dimensions, amplitude and phase remain scalars, but frequency and bandwidth are 3-D vectors with magnitude, dip angle, and azimuth.

Three-dimensional seismic crustal structure of the Monashee hinterland core complex on the Lithoprobe Southern Canadian Cordillera Transect

Three-dimensional seismic coverage in an approximately 12 km × 12 km area of the southern Monashee metamorphic complex in the south-central portion of the Canadian Cordillera reveals a complex geometry to the Mesozoic–early Tertiary contractional Monashee décollement. Data were acquired as part of the Lithoprobe Southern Canadian Cordillera Transect where two approximately perpendicular lines intersected on the south flank of the Monashee mountains in the hinterland of the Cordillera. Stacks of traces within 100 m × 100 m bins are nominally 6-fold, but range from zero to 108-fold due to the crooked nature of the lines. Both migrated and unmigrated data have been examined for interpretation, but the highly variable data quality and discontinuous reflectivity cause excessive added noise during the migration process.

Fast model building using demigration and single‐step ray migration

This work provides explorationists with simple procedures to perform depth conversion more accurately than can be achieved with simple vertical layer cake depth conversion. The use of image rays, which are inadequate in structurally complex areas, is avoided. Migrated time interpretations are still used and are "demigrated" using the Kirchhoff time migration equations. This backs out the effect of the time migration prior to a ray depth migration and enables the lateral shifts between the time migrated image and a depth migrated image to be quantified. These shifts can be separated into a mismigration component and a refraction component. The relative size of the components define whether time or depth migration is required and may be used to justify a remigration of the seismic image. Furthermore, the tedious layer by layer approach to ray depth migration may be avoided by using the velocity depth model from the vertical layer cake depth conversion of the time‐migrated data for ray depth migration of the unmigrated data for all horizons in a single step. A satisfactory result is usually achieved without the need to iterate. These methods are illustrated with both a synthetic example and a real 3-D data set from the Norwegian North Sea.

The Fresnel zone for P-SV waves

Mode‐converted (P-SV) reflections in field data contain information about an area on the reflecting boundary, rather than a single point. For unmigrated data, the total area contributing to the observed reflection amplitude is approximated by the Fresnel zone. We derive formulas for the P-SV Fresnel‐zone radius for both surface and VSP geometries. In the surface case, the Fresnel radius can be expressed in a form similar to that of the P-P case by substitution of the P-SV single‐layer migration velocity. Numerical modeling shows that the relative changes in size and shape of the P-SV Fresnel zone as a function of offset are not large and are comparable to the P-P case. For a given depth and frequency, the P-SV Fresnel radius is smaller than the P-P Fresnel radius by a factor of about 0.8; hence the lateral resolution of converted‐wave (P-SV) data may be somewhat better than the P-P data. Two sample calculations using field data are presented. For the VSP example, the P-SV Fresnel radius (184 m) is smaller than the corresponding P-P Fresnel radius (231 m), but for the surface example the two are similar (315 m and 295 m, respectively), because of differences in the frequency contents of the reflected P and SV waveforms.

Extended FK domain migration ? An efficient variable velocity algorithm

Constant velocity FK domain migration, as developed by Stolt in 1978, has significant performance advantages over all other commercially available migration algorithms.However, even with Stolt's pre-migration time stretching (or psuedo-depth conversion), this algorithm suffers serious accuracy problems in the presence of any significant velocity variations. This paper develops a method of extending the accuracy of FK domain migration to steep dips in arbitrary velocity fields. The basis of the new method is the partitioning of the unmigrated data set into dip ranges and computing unique time stretching factors for each. Stolt's original time stretching is shown to be the zero-dip special case of this more general algorithm. Following the separate migration of the partitions, the data is merged to form a correctly migrated data set. As few as four partitions is shown to produce very significant improvements. Even though Extended FK Domain Migration is a few times slower than conventional FK domain migration, it can still produce accuracy comparable to phase shift migration at a fraction of the cost.

Migration - why doesn't it work for deep continental data?

Summary. Conventional migration of deep seismic reflection data often produces disappointingly poor results even when the original unmigrated data are of high quality and are relatively noise free. Surprisingly, deep data often subjectively appear to be best migrated at velocities which are up to 50% less than appropriate interval velocities derived from crustal refraction experiments or directly from stacking velocities. The explanation for this behaviour is that near surface features distort and attenuate the seismic wave field and produce apparent discontinuities in deep reflections. Since these discontinuities are spurious they are not associated with the appropriate large diffractions which real discontinuities at depth would produce. During the process of migration reflections are invented in order to cancel out the missing diffractions thereby producing a “smiley” section which appears overmigrated. Since the lateral extent of individual smiles increases with increasing migration velocity, increasing two-way-time, and increasing seismic wavelength, the effect” is almost unnoticeable on conventional shallow seismic data but is overwhelming for deep crustal data.

Migration of seismic data from inhomogeneous media

Because conventional time‐migration algorithms are founded on the implicit assumption of locally lateral homogeneity, they leave events mispositioned when overburden velocity varies laterally. The ray‐theoretical depth migration procedure of Hubral often can provide adequate first‐order corrections for such position errors. Complex geologic structure, however, can so severely distort wavefronts that resulting time‐migrated sections may be barely interpretable and thus not readily correctable. A more accurate, wave‐theoretical approach to depth migration then becomes essential to image the subsurface properly. This approach, which transforms an unmigrated time section directly into migrated depth, more completely honors the wave equation for a medium in which variations in interval velocity and details of structural shape govern wave propagation. Where geologic structure is complicated, however, we usually lack an accurate velocity model. It is important, therefore, to understand the sensitivity of depth migration to velocity errors and, in particular, to assess whether it is justified to go to the added effort of doing depth migration. We show a synthetic data example in which the wave‐theoretical approach to depth migration properly images deep reflections that are poorly resolved and left distorted by either time migration or ray‐theoretical depth migration. These imaging results are, moreover, surprisingly insensitive to errors introduced into the velocity model. Application to one field data example demonstrates the superior treatment of amplitude and waveform by wave‐theoretical depth migration. In a second data example, deep reflections are so influenced by anomalous overburden structure that the only valid alternative to performing wave‐theoretical depth migration is simply to convert the unmigrated data to depth. When the overburden is laterally variable, conventional time migration of unstacked data can be as destructive to steeply dipping reflections as is CDP stacking prior to migration. A schematic example illustrates that when migration of unstacked data is judged necessary, it should normally be performed as a depth migration.