Wave Equation Datuming Research Articles

Integrated Geophysical Analyses of Shallow-Water Seismic Imaging With Scholte Wave Inversion: The Northern Adriatic Sea Case Study

The integrated analysis using different seismic wave types in a record is a very efficient approach for a comprehensive characterization of marine sediments, especially in shallow water conditions. The proposed integrated method to analyze seismic data in post-critical conditions consists of: 1) the inversion of Scholte waves to obtain a reliable Vs distribution of the near seafloor; 2) pre-processing of seismic data; 3) construction of the P-wave velocity field by using all available information, including available well data; and 4) the application of the wave equation datuming and post-processing, such as pre-stack time migration. We demonstrate how this approach could be successfully applied on seismic datasets characterized by post-critical conditions and the occurrence of the Scholte waves, which may be exploited to provide fundamental information instead of being only an unwanted effect. The integrated analysis of seismic events can thus help, together with data processing, by providing better seismic imaging, which is a priority for a reliable seismostratigraphic interpretation.

Elastic wave-equation datuming

Wave-equation datuming (WED) techniques have demonstrated superiority when waves occur on the acquisition surface nonvertically, and traditional static corrections based on the time shift become inaccurate. Meanwhile, as for multicomponent data, those scalar techniques can hardly maintain the vector characteristics for the following multicomponent data processing flows. Considering this, we have developed an elastic-wave datuming approach to handle the static corrections for multicomponent data. Different from those existing scalar WED techniques, the multicomponent data are first decomposed into multicomponent P- and S-wave data. Then, the decomposed data are transformed into the [Formula: see text]-[Formula: see text] domain, and they are extrapolated from the acquisition surface to the datum using the one-way elastic-wave continuation. Finally, the datumed multicomponent data are reconstructed at the output datum by adding up the datumed P- and S-wave data. This elastic WED can guarantee that the same wave modes on different components are equally datumed, and the data remain multicomponent so that they are still applicable to multicomponent-joint processing techniques. Finally, several test examples involved in this paper have proved our method’s effectiveness in multicomponent data datuming application.

Wave equation datuming applied to S-wave reflection seismic data

S-wave high-resolution reflection seismic data was processed using Wave Equation Datuming technique in order to improve signal/noise ratio, attenuating coherent noise, and seismic resolution and to solve static corrections problems. The application of this algorithm allowed obtaining a good image of the shallow subsurface geological features. Wave Equation Datuming moves shots and receivers from a surface to another datum (the datum plane), removing time shifts originated by elevation variation and/or velocity changes in the shallow subsoil. This algorithm has been developed and currently applied to P wave, but it reveals the capacity to highlight S-waves images when used to resolve thin layers in high-resolution prospecting. A good S-wave image facilitates correlation with well stratigraphies, optimizing cost/benefit ratio of any drilling. The application of Wave Equation Datuming requires a reliable velocity field, so refraction tomography was adopted. The new seismic image highlights the details of the subsoil reflectors and allows an easier integration with borehole information and geological surveys than the seismic section obtained by conventional CMP reflection processing.In conclusion, the analysis of S-wave let to characterize the shallow subsurface recognizing levels with limited thickness once we have clearly attenuated ground roll, wind and environmental noise.

Wave Equation Datuming Applied to Seismic Data in Shallow Water Environment and Post-Critical Water Bottom Reflection

Coastal areas are generally characterized by human manufacturing; thus, seismic data analysis is necessary to characterize the properties of the subsoil, the main purpose of which is to clarify risk situations. In the case of very shallow water environments, seismic multiple attenuation becomes a challenge when the reflection of the seafloor is post-critical, so it is not recorded because of the acquisition parameters. We propose an approach to attenuate the multiples by using wave equation datuming that does not require the detection of seafloor reflection and avoids the seafloor reflection prediction and related approximations in the post-critical conditions. Moreover, this approach allows for the enhancement of higher frequencies, and, consequently, an increase in resolution, demonstrating that it is a powerful tool to attenuate multiples and reverberations, especially where other approaches are found to be inefficient. An example of the application of seismic data acquired in the continental shelf of South Chile is reported.

Crustal structure of Central Sicily

We processed crustal seismic profile SIRIPRO, acquired across Central Sicily. To improve the seismic image we utilized the wave equation datuming technique, a process of upward or downward continuation of the wave-field between two arbitrarily shaped surfaces. Wave equation datuming was applied to move shots and receivers to a given datum plane, removing time shifts related to topography and to near-surface velocity variations. The datuming procedure largely contributed to attenuate ground roll, enhance higher frequencies, increase resolution and improve the signal/noise ratio. Processed data allow recognizing geometries of crust structures differentiating seismic facies and offering a direct image of ongoing tectonic setting within variable lithologies characterizing the crust of Central Sicily. Migrated sections underline distinctive features of Hyblean Plateau foreland and above all a crustal thinning towards the Caltanissetta trough, to the contact with a likely deep Permo-Triassic rifted basin or rather a zone of a continent to oceanic transition. Inhomogeneity and fragmentation of Sicily crust, with a distinct separation of Central Sicily basin from western and eastern blocks, appear to have guided the tectonic transport inside the Caltanissetta crustal scale syncline and the accumulation of allochthonous terrains with south and north-verging thrusts. Major tectonic stack operated on the construction of a wide anticline of the Maghrebian chain in northern Sicily. Sequential south-verging imbrications of deep elements forming the anticline core denote a crust wedge indenting foreland structures. Deformation processes involved multiple detachment planes down to decoupling levels located near crust/mantle transition, supporting a presence of high-density lenses beneath the chain, interrelated to a southwards push of Tyrrhenian mantle and asthenosphere.

Reflection seismic sections across the Geothermal Province of Tuscany from reprocessing CROP profiles

We processed the seismic lines CROP-18 and the Tuscany segment of CROP-03, acquired in Southern Tuscany. The analyzed transects were originally designed to highlight deep crustal structures between the major reflector characterizing the geothermal fields (the K-horizon) and the Moho discontinuity. Our main target was to better depict the upper crustal structures, reprocessing the first 4–6s reflection times with the application of advanced techniques (wave equation datuming), improving signal/noise ratio and extracting information previously hidden by approximate static corrections and surface noise. We choose the wave equation datuming technique, a process of upward or downward continuation of the wave-field between two arbitrarily shaped surfaces. The wave equation datuming was applied to move shots and receivers to a given datum plane, removing time shifts related to topography and to near-surface velocity variations. The datuming procedure largely contributed to attenuate ground roll, enhance higher frequencies, increase resolution and improve the signal/noise ratio.The time migrated output sections show new evidence of regional continuity of high amplitude horizons, a better fitting with the surface geological setting and with well data resembling the local industry investigations. The interpretation confirms the fluids role, the contribution to the crustal reflectivity mostly by thermo-metamorphic processes and defines the tectonic setting.

Reverse time migration from irregular surface by flattening surface topography

Abstract Prestack reverse time migration (RTM) is a powerful tool to provide high resolution structure images of crust and upper mantle in wide-angle seismic experiments. As a standard migration algorithm assumes that data are recorded on a flat surface, RTM meets severe challenge on the issue of complicated topography where many seismic surveys are carried out. Up till now, there are two popular techniques for handling elevation changes, i.e., elevation-static correction (or time shift) and wave equation datuming. Numerous studies demonstrate that the elevation-static correction is inaccurate for non-vertically traveling energy and subsequently may migrate reflectors to incorrect positions. Wave equation datuming is more accurate but more sophisticated and too computationally demanding for routine use. Here, we present an alternative scheme to deal with topography in prestack RTM that can migrate seismic data acquired in areas with irregular topography to the reflector positions directly without datuming or elevation static corrections. In this scheme, surface topography is flattened by a transformation of coordinates from Cartesian to curvilinear. Such a transformation is equivalent to map a rectangular grid onto a curved one, and is implemented by the introduction of boundary conforming grid. Then we use an explicit finite-difference scheme with second-order accuracy to discretize the elastic wave equations and perform the RTM in the curvilinear coordinate system. Moreover, a flux-corrected transport (FCT) algorithm, adapted from hydrodynamics, is incorporated in the RTM implementation, which can not only overcome problems of numerical dispersion that arise in finite difference algorithms, but also offer the opportunity to use a coarse grid to obtain the same accuracy as conventional finite difference methods on a dense grid. The FCTRTM method presented here has no limitations on surface irregularity, complexity of the subsurface structure, or velocity function. Extensive numerical examples demonstrate the feasibility and effectiveness of our method.

Wave-equation global datuming based on the double square root operator

Current schemes for removing near-surface effects in seismic data processing use either static corrections or wave-equation datuming (WED). In the presence of rough topography and strong lateral velocity variations in the near surface, the WED scheme is the only option available. However, the traditional procedure of WED downward continues the sources and receivers in different domains. A new wave-equation global-datuming method is based on the double-square-root operator, implementing the wavefield continuation in a single domain following the survey sinking concept. This method has fewer approximations and therefore is more robust and convenient than the traditional WED methods. This method is compared with the traditional methods using a synthetic data example.

Wave equation datuming applied to marine OBS data and to land high resolution seismic profiling

One key step in seismic data processing flows is the computation of static corrections, which relocate shots and receivers at the same datum plane and remove near surface weathering effects. We applied a standard static correction and a wave equation datuming and compared the obtained results in two case studies: 1) a sparse ocean bottom seismometers dataset for deep crustal prospecting; 2) a high resolution land reflection dataset for hydrogeological investigation. In both cases, a detailed velocity field , obtained by tomographic inversion of the first breaks, was adopted to relocate shots and receivers to the datum plane. The results emphasize the importance of wave equation datuming to properly handle complex near surface conditions. In the first dataset, the deployed ocean bottom seismometers were relocated to the sea level (shot positions) and a standard processing sequence was subsequently applied to the output. In the second dataset, the application of wave equation datuming allowed us to remove the coherent noise, such as ground roll, and to improve the image quality with respect to the application of static correction. The comparison of the two approaches evidences that the main reflecting markers are better resolved when the wave equation datuming procedure is adopted. ► Application of wave equation datuming to OBS and land high resolution seismic data. ► Comparison between conventional methods and wave equation datuming. ► Translation of OBS to an ocean bottom cable data and standard processing.

Upward continuation with topographic datuming operator: the integrated wave equation datuming scheme revised

ABSTRACTThe integrated wave equation datuming scheme is an integrated datuming method to implement the wave equation velocity replacement with a one‐way extrapolator for overthrust data. The integrated wave equation datuming scheme is accomplished by downward continuing the data from the topography to a non‐planar base of the weathering layer, followed by an upward continuation from the base of the weathering layer to a final planar datum. Both the downward and the upward continuation are accomplished by a one‐way extrapolator by a classical ‘two‐step’ implementation. The topographic datuming operator is a distinctive technique by which the shots and receivers are downward continued simultaneously. Although its inadaptability to non‐planar datum and the strong lateral variation of the near‐surface structure constrains its application to the downward continuation of the overthrust data, we found that the ‘one‐step’ feature of the topographic datuming operator is extremely suitable for the upward continuation. In this paper, the topographic datuming operator is incorporated into the integrated wave equation datuming scheme to perform upward continuation instead of a one‐way extrapolator – thus a revised integrated wave equation datuming scheme is constructed. Compared with the previous scheme, the computatiol cost of the revised integrated wave equation datuming scheme is greatly reduced, thus the applicability of the integrated wave equation datuming scheme is further improved. The synthetic and real data examples demonstrate its effectiveness and efficiency.

Sub-basalt seismic imaging using optical-to-acoustic model building and wave equation datuming processing

The presence of highly heterogeneous basalt layers has a detrimental effect on seismic imaging of underlying structures. Due to the prevalence of basalt, the challenge of imaging below basalts impinges on a broad spectrum of geological problems, from hydrocarbon exploration to volcano seismology. Both internal basalt characteristics and rough interfaces can degrade the sub-basalt image quality. In this paper we investigate the effects of both internal and surface heterogeneity on the wavefield. Using highly heterogeneous models of basalts derived from optical imagery and borehole sonic logs, combined with wave simulations to generate synthetic data, we explore the causes of poor sub-basalt imagery. We establish the significant role that surface roughness plays in the imagery problem and demonstrate, through a novel application of wave equation datuming, how to suppress its effect. The methodology demonstrated herein may be applicable in any environment where heterogeneous surface structures/interfaces severely degrade seismic image quality.

Application of an integrated wave-equation datuming scheme to overthrust data: A case history from the Chinese foothills

An integrated wave-equation datuming scheme improves the imaging quality of seismic data from overthrust areas. It can be regarded as integrated because upward-layer replacement is included. In this scheme, data are downward continued to a nonplanar datum (such as the base of the weathering layer), followed by upward continuation from the nonplanar datum to a final planar datum using a one-way extrapolator. When compared with a Kirchhoff integral, this method can deal better with the strong lateral velocity variation within the near surface. After a test on synthetic data, the scheme is applied successfully to real 2D overthrust data acquired in the Qi-Lian foothills, western China. Compared with results using static corrections, integrated wave-equation datuming results lead to better reconstruction of the diffractions and reflections, more reliable migration-velocity analyses, and stronger stack and final depth images.

A data-mapping approach to 3-D wavefield redatuming

SUMMARY In case of a complex overburden, the seismic data can be greatly improved by applying a full wavefield redatuming procedure. In practice, the application of the redatuming process to 3-D data acquired by conventional acquisition designs is non-trivial. Because of the large amount of data involved in the 3-D redatuming process and because of the sparseness of these data, it is impossible to apply conventional wave equation datuming directly. We present a data mapping approach to redatuming (DMR), which follows the concept of Kirchhoff data mapping. A simplified background medium where no ray bending occurs is assumed for the medium below the datum in order to map an input data set referenced to the acquisition surface to an output data set referenced to the new datum level. The DMR method can be interpreted as a simplified version of the Kirchhoff summation redatuming (KSR) method, where one of the 2-D integrals over the acquisition coordinates can be solved analytically. Consequently, in this approach fewer traces are involved in the computation of one time sample (a 2-D integral is computed instead of a 4-D integral), which makes it particularly attractive for the application to 3-D data sets. In this paper the theory underlying data mapping redatuming is discussed and the proposed approach is tested on fully sampled 2-D and 3-D synthetic data from models with both simple and complex velocity distributions in the subsurface. The tests clearly show that the objective of producing results that are comparable to the conventional KSR has been achieved. The redatumed traces are dynamically and kinematically correct. Furthermore, these results confirm that the dependency of the new approach on the assumed medium below the datum level is, indeed, weak because the assumption of a velocity medium where no ray bending occurs is already sufficient to produce correct results.

Velocity and AVO analysis for the investigation of gas hydrate along a profile in the western continental margin of India

The occurrence of gas hydrate has been inferred from the presence of Bottom-Simulating Reflectors (BSRs) along the western continental margin of India. In this paper, we assess the spatial and vertical distribution of gas hydrates by analyzing the interval velocities and Amplitude Versus Offset (AVO) responses obtained from multi-channel seismics (MCSs). The hydrate cements the grains of the host sediment, thereby increasing its velocity, whereas the free gas below the base of hydrate stability zone decreases the interval velocity. Conventionally, velocities are obtained from the semblance analysis on the Common Mid-Point (CMP) gathers. Here, we used wave-equation datuming to remove the effect of the water column before the velocity analysis. We show that the interval velocities obtained in this fashion are more stable than those computed from the conventional semblance analysis. The initial velocity model thus obtained is updated using the tomographic velocity analysis to account for lateral heterogeneity. The resultant interval velocity model shows large lateral velocity variations in the hydrate layer and some low velocity zones associated with free gas at the location of structural traps. The reflection from the base of the gas layer is also visible in the stacked seismic data. Vertical variation in hydrate distribution is assessed by analyzing the AVO response at selected locations. AVO analysis is carried out after applying true amplitude processing. The average amplitudes of BSRs are almost constant with offset, suggesting a fluid expulsion model for hydrate formation. In such a model, the hydrate concentrations are gradational with maxima occurring at the base of hydrate stability zone.

A Study on the Application of the 3‐D Wave‐Equation‐Datuming

AbstractThe wave‐equation‐datuming (WED) technique is a necessary alternative to the conventional static corrections when the surface‐consistent assumption is invalid. Much pervious work had been done on the 2‐D cases which demonstrated its applicability potential to be extended to 3‐D. In this paper, a “step‐by‐step” recursive 3‐D frequency‐space WED scheme is presented to deal with real data. The processing results revealed that 3‐D WED is a more reasonable datum‐correction method thereby the accuracy of velocity analysis and the imaging quality are improved significantly.

Imaging low-velocity anomalies with the aid of seismic tomography

Theoretical considerations (Snell's law) suggest that low-velocity fanomalies are undersampled and therefore should be poorly resolved by inversion schemes based on ray-tracing methods. A synthetic study considering a 40×20 m low-velocity anomaly (300 m/s) placed at the center of a 400×50 m block with gradient background velocity model (from 3000 m/s at the surface to 4000 m/s at the base) indicates that the low ray density in ray-tracing coverage diagrams of tomographic inversion studies can be used as evidence for the existence of low-velocity anomalies. Combined normal incidence seismic reflection images and the velocity models obtained by tomographic inversions of first-arrival travel times form an efficient scheme to resolve low-velocity anomalies such as fracture zones. Furthermore, the velocity models derived from tomographic inversions are used in a wave equation datuming algorithm to account for statics caused by a strongly laterally variable shallow surface (weathering) layer and provide seismic reflection images of fracture zones (low-velocity anomaly) within a granitic pluton.

Interface scattering versus body scattering in subbasalt imaging and application of prestack wave equation datuming

The presence of high‐velocity, highly heterogeneous layers such as basalt have a detrimental effect on imaging below those structures. Internal characteristics of the basalt itself can also degrade the subbasalt image quality. In many cases the boundary surfaces of a basalt sequence are not smooth but contain some roughness at a scale similar to the seismic wavelength. Interface scattering resulting from interface roughness can be a serious problem. In this paper, body and interface scattering problems are addressed through acoustic finite‐difference modeling. Interface scattering seems to have the most detrimental effect on imaging at depth. A wave equation prestack datuming procedure is applied to synthetic acoustic data to remove the effects of rough interfaces. This computational process transforms seismic data to a new datum plane in the subsurface, eliminating propagation effects between the surface and the new datum plane. The technique leads to a much improved image of below the basalt, with the great advantage that accurate knowledge of the velocity field for the overburden only is required as opposed to the entire velocity‐depth model (e.g., for prestack depth migration).

Technical article: Application of pre-stack wave equation datuming to remove interface scattering in sub-basalt imaging

Technical article: Application of pre-stack wave equation datuming to remove interface scattering in sub-basalt imaging

Mathematics of Multidimensional Seismic Imaging, Migration, and Inversion. Interdisciplinary Applied Mathematics, Vol 13

<i>Mathematics of Multidimensional Seismic Imaging, Migration, and Inversion. Interdisciplinary Applied Mathematics, Vol 13</i>

Seismic exploration in complex terrains: A processing experience in the Southern Apennines

We discuss a data‐processing sequence adopted to reprocess a seismic line that crosses the Italian southern Apennines from the Tyrrhenian Sea to the Adriatic margin and investigate both the overthrust and foreland areas. We first determine the main causes of the very low S/N ratio in the field data and then propose a processing sequence aimed at exploiting the signal content, also making use of a priori geological knowledge of this area. Our work indicates a combination of causes for the very low quality of the seismic data. These include length of the spread (about 20 km) that is unfavorable because of the rapid variation in the near‐surface geology, tectonic complexity, crooked‐line acquisition, and the rough topography associated with outcropping rocks characterized by highly variable velocities. Based on the outcome of this data analysis, we present a processing sequence driven by knowledge of the regional tectonic setting and by knowledge of the shallow subsurface geology. The main effort is in removing the large, near‐surface related noise components. The low S/N ratio makes it impossible or nearly impossible to successfully apply highly sophisticated techniques such as depth migration or wave equation datuming. Thus, we used robust techniques specifically designed to solve each problem that degraded data quality. The most relevant of these techniques were the removal of bad traces where unacceptably low quality was detected by energy and frequency decay criteria; estimation and correction for static time shifts attributable to near‐surface conditions; optimization of common midpoint (CMP) sorting to attenuate the deleterious effects of the crooked‐line acquisition; application of a weighted stacking technique to maximize stack power and application of prestack f-x deconvolution to attenuate uncorrelated noise. The outcome of this processing sequence is compared with the result of a more standard sequence that was previously applied to the same data and is also discussed in terms of the possible geological model it might evidence. The realization of a seismic section showing rather continuous and structured events down to 8 s which, depending on the interpretation, may be related to Moho discontinuity or to very deep sedimentary layers supports the efficacy of the processing approach we propose.