Comment on essd-2022-258

In an ever more challenging context for the acquisition of seismic data in the Mediterranean Sea, reprocessing to improve the quality of legacy data has become increasingly important. This work presents the newly reprocessed, open access dataset SALTFLU acquired in the Algerian basin by the National Institute of Oceanography and Applied Geophysics (OGS) in 2012. We apply a ‘broadband’ reprocessing strategy adapted for short-offset, deep water airgun reflection seismic data and assess if the reprocessed images provide new geological insights on the Mediterranean sub-surface. The workflow relies on an integrated approach combining geophysics and geological interpretation to iteratively build the velocity model. In this way we aim to tackle some of the challenges linked to imaging complex geological structures containing high velocity contrasts with 2-D, short-offset seismic data. We first broaden the bandwidth of the data through multi-domain de-noising, deghosting and a source designature using an operator derived from the seabed reflection. We then perform iterative migration velocity analysis, pre-stack time migration and multiple attenuation in the Radon domain to obtain time-migrated images. The initial velocity model is derived from the resulting time migration velocities, and geologically driven model updates are generated using a combination of travel-time tomography, seismic interpretation of the major salt horizons and velocity gradient flooding. The gradient flooding aims to reproduce the large scale first-order velocity variations, while the travel-time tomography aims to resolve the smaller second-order velocity variations. The results improve our deep geological knowledge of the under-explored Algerian basin down to the base salt and the pre-salt. Fluid indicators are imaged within the Plio-Quaternary of the Algerian basin, which we interpret as thermogenic or biogenic gas sourced from either the Messinian Upper Unit or from the pre-salt, migrating through a hydro-fractured salt. The reprocessed data image lateral and vertical seismic facies variation within the Messinian units that could shed new light on the tectono-stratigraphic processes acting during the Messinian Salinity Crisis. It also reveals numerous previously unresolved volcanic structures within the Formentera basin.

The East Coast Marine Area (ECMA) of Trinidad and Tobago contains producing gas fields where the imaging of seismic data is known to be challenging. This difficulty has been attributed to the combination of the Dolphin Main Fault, with more than 1 km of throw and large velocity contrasts of up to 30% in magnitude across it, and shallow gas and gas clouds causing very high attenuation. These factors lead to difficulties producing reliable and consistent velocity models. Previous velocity models — generated using traveltime tomography — contained inaccuracies that led to poor imaging and structural positioning, resulting in uncertainty when planning exploration and development drilling programs. In an attempt to reduce these uncertainties, a full-waveform inversion (FWI) feasibility study was performed over the most problematic area, with an initial velocity model built from a combination of legacy prestack time and depth migration velocity models and additional first-break refraction tomography. The geologic complexity near the main fault and the associated poor seismic data quality meant this model gave a poor fit between the synthetic and observed data (from a cycle-skipping point of view) at the lowest usable frequency. This led to the idea of applying a diving-wave-driven, offset-stripping workflow across each frequency band in the FWI, going from 3.5 Hz to 7.5 Hz. After successful application on the test area, the workflow was applied to ∼1400 km2 of 3D seismic over the ECMA. The final velocity model is structurally consistent and highly resolved, containing detailed shallow geologic features such as mud volcanoes, well-defined faults, and potential gas anomalies that were not visible on the legacy tomographic model. A blind test of well-pressure data correlates strongly with velocity drops in the FWI model, showing that FWI models can potentially provide very good input to regional pore-pressure-prediction studies.

We investigate the impact of lossy seismic data compression on the amplitudes of a North Sea 2-D seismic data set processed conventionally. Processing includes attenuation of waterbottom multiples, true amplitude recovery, predictive deconvolution, attenuation of peg-leg multiples, prestack time migration, and stacking. Two sequences of the lossy compression method are investigated. In the first sequence the seismic data are compressed before predictive deconvolution (denoted predecon compression), while in the second sequence the seismic data are compressed after predictive deconvolution (denoted post-decon compression). Both prestack and poststack amplitude analysis are performed. For pre-decon and post-decon compression, compression ratios between 7.5:1-15:1 and 15:1-30:1, respectively, provide excellent reconstruction quality of the seismic data set. In general, migration and stacking reduce the effect of the coding noise at all compression levels, while for high compression ratios (i.e., much greater than 10:1) the coding noise has a destructive effect on the predictive deconvolution step and on the two applied multiple attenuation methods.

The pre-stack time migration and velocity analysis methods on common scatter-point (CSP) gathers are based on pre-stack Kirchhoff time migration. The concept of equivalent offset provides a new idea for the research of seismic pre-stack migration and velocity estimation. In this paper, according to the theory of the equivalent offset, we derive the travel-time equation of CSP gathers, introduce the related pre-stack time migration method and discuss velocity analysis methods based on the CSP gathers, and point out the advantages of CSP methods over conventional methods. Processing of a model example and field seismic data show the validity of CSP pre-stack time migration and the velocity analysis methods based on it.

Prestack time migration (PreSTM) still represents the majority of seismic imaging activities in the industry. The reason for this is the speed and robustness of time imaging and its ability to focus seismic events for most geological settings. However, well defined migration velocities are necessary. When we speak about migration velocities and migration operators we implicitly assume time migration velocities and time migration operators. Usually, the migration velocities are not known and, therefore, approximated by stacking velocities. The stacking velocity is attached to the stationary point of the migration operator, however the migration velocity is defined at the apex of the migration operator. Due to this discrepancy, the stacking velocity may deviate from the migration velocity, e.g., in the case of dipping reflectors, and the final time image may be unfocused. In order to correct for the velocity difference and to enhance the time imaging, the velocity model needs to be refined. There are several techniques to provide an updated velocity model. However, all the techniques are based on an iterative approach requiring a strong manual interaction. We propose a new workflow for the time imaging which includes an automatic update of time migration velocities. The workflow is based on an automatic Common‐Reflection‐Surface (CRS) stack of Common Scatter Point (CSP) gathers and provides an improved time migration velocity model and, hence, a highly focused time image.

Anisotropic depth migration (ADM) has become more commonplace over the past four years. The data-processing examples detailed here illustrate the robustness of the method in a variety of structured settings in the Alberta Foothills. The public-domain structural line, the Husky/Talisman dataset, illustrates subtle improvements in imaging with a dramatic improvement in accuracy of horizon depths when ADM is applied to these data. A 3D survey in a difficult imaging area, Nordegg/Chungo, shows significant healing of broken basement reflectors when we correct for anisotropy in the complex-dipping clastic overburden. Finally, in the Blackstone area where we observe intense folding in the near surface, comparisons between poststack time migration, prestack time migration, prestack depth migration, and prestack anisotropic depth migration show the similar step-change improvements as we imrove the technology of our algorithm. Introduction Traditional depth migration corrects only for lateral velocity heterogeneity. Current anisotropic depth migration algorithms also correct for velocity changing with direction in the dipping clastic overburden. Seismic anisotropy is a well-quantified physical characteristic of clastic rocks. Studies of anisotropic behaviour in clastic rocks include Thomsen (1986), Vernik and Liu (1997), Leslie et al (1997). Several papers over the past seven years illustrate the improvements in imaging when we stop ignoring anisotropy and begin to correct for it (Ball, 1995; Vestrum and Muenzer, 1997; Vestrum, et al., 1998; Ferguson and Margrave, 1998; Schmid et al., 1998; Dai et al., 1998; Fei et al., 1998; Vestrum et al., 1999, Kirtland Grech et al, 2001; etc.). The seismic data processing case histories presented here outline the improvements in seismic imaging and structural positioning when we correct for seismic anisotropy in depth migration. Benjamin Creek The Husky/Talisman dataset is the focus of our first processing case history. In this example, I illustrate the improvement in correlation between well depths and seismic depths. Figure 1 shows this improvement. Isotropic depth migration for this line (Figure 1a) shows a typical result (Shultz and Canales, 1997; Etris et al., 2001) of isotropic depth migration where the seismic depths are deeper than the well depths. When we ignore anisotropy, we need to use a velocity faster than the vertical velocity if we are to flatten the image gathers and obtain a quality image. The anisotroic migration (Figure 1b), models the velocity behavour changing with direction, so it produces a clear image and correlates to the well depths. One can see in Figure 1b that there is still some uncertainty in the correlation, but we have removed a systematic error from the depth correlation. We could now go back and adjust our imaging velocities so that our final depth migration matched perfectly with well depths, without needing a separate depth-conversion step after the depth migration. We can now consider well depths as a constraint on our imaging velocity model. There are also several imaging improvements on the anisotropic depth migration over the isotropic depth migration. Figure 1: (a) isotropic depth migration and (b) anisotropic depth migration and overlain well depths to top and bottom of the carbonate sheet. Nordegg/Chungo The Nordegg/Chungo 3D is a Time Seismic data volume acquired in the Alberta Foothills. A more difficult imaging area than the Benjamin Creek area, this volume shows imaging problems that arise when the anisotropic overburden has dramatically changing dip. Figure 2 shows the comparison between isotropic and anisotropic depth migrations for a line out of this volume. Note the improvement in basement continuity beneath the steeply dipping near-surface when we use anisotropic depth migration (Figure 2b). Several other potential hydrocarbon-target events show similar improvement. Note that the target event, as indicated by the red arrow, shows more consistent imaging over the range of dips from the back limb over the crest of the structure and into the forelimb. The anisotropic velocity model more accurately represents the velocity structure of the overburden. With more accurate raytracing over a larger range of reflector angles, the migration is better able to image the range of reflector dips on this structure. Blackstone This Talisman dataset is a very dramatic anisotropic imaging example. The extensive folding of the clastic overburden creates difficulties imaging structures below. Figure 3 shows a comparison between poststack time migration (Figure 3a), prestack time migration (Figure 3a), prestack depth migration (Figure 3c), and prestack anisotropic depth migration (Figure 3d). Each step forward in complexity of the migration

The Guando field is in an area of rough topography and complex geology. In order to better site wells in such fields it is of value to have a sound understanding of this geological complexity. Obtaining a satisfactory seismic image to enable such interpretation requires pre-stack imaging. The geological complexity is such that post stack imaging fails due to the failure of the common mid point assumption. Is it sufficient to perform pre-stack time migration or is it necessary to perform pre-stack depth migration? In this instance both have been performed on this field and results for each of them will be shown. The basic theory of both pre-stack time and pre-stack depth migration will be described without using mathematics. Model data will then be used to illustrate the shortcomings of pre-stack time migration using a model of similar geological complexity to the Guando field. Both pre-stack migrations require a velocity field. These differ and the manner of their derivation also differs. Velocity model building for both techniques will be described. Particular emphasis will be placed on the use of tomographic and scanning techniques for pre-stack depth migration. Examples will be shown of the velocity models from both migrations. It will be shown that the pre-stack depth migration velocity model is also a valuable interpretation asset. Data from both migrations will be shown. These will demonstrate the superiority of the pre-stack depth migrated data and the reasons why it is superior.

In seismic data processing removal of very low frequency is the conventional approach but the preservation of low frequency become important in highly absorptive media, where we are interested in extracting the geological information associated with low frequencies. The reflection coefficient is the function of frequency in fluid saturated media and anomalously high amplitude at low frequency can indicate presence of hydrocarbon which is normally not observed in conventional seismic data. Continuous wavelet transform (CWT) is used for extraction of low frequency from the seismic data, which is a spectral decomposition method. The application of spectral decomposition includes layer thickness determination, stratigraphic visualization and direct hydrocarbon detection (DHI), which have a great significance in hydrocarbon exploration. The concept of reflectivity of low frequency band of seismic energy related to presence of hydrocarbons is established theoretically for quite some time. Now efforts are made to establish this in the industry for detection of hydrocarbons. The present study is the case study from Western offshore basin, India where data is processed with preservation of low frequencies and taking utmost care while selecting input parameters like band pass filter, wavelet matching and amplitude matching. Broadly, the processing steps included standard signal processing techniques without affecting the relative amplitudes and preserving low end of frequency bandwidth. To examine low frequency, input data has been analyzed for the maximum frequency present in the study area. The input data for low frequency analysis was taken as Pre Stack Time Migration (PSTM) volume. Using in-house developed program for complex continuous wavelet transform (CCWT) of seismic traces low frequency volumes from input PSTM data were extracted. We have analyzed where the peak amplitude has occurred with respect to scale in low frequency volume. This peak amplitude value with respect to low frequency extracted from the volume may signify the indication of hydrocarbon bearing zones. In a properly processed low frequency section/volume the amplitude brightening may be used as direct hydrocarbon indicator. The frequency dependent reflectivity which is supposed to be higher at lower frequency range for a fluid saturated interface should produce higher amplitude on a low frequency section compared to the surrounding (with no fluid saturation). From the seismic data suitable low frequency volume is generated using continuous wavelet transform (CWT) method. The present study reveals that how low frequency based seismic data processing can help to identify the high amplitude at low frequency in fluid saturated zones, which is not observed in conventional processed data. This methodology is significantly useful for generation of locales for exploration/exploitation in complex lithological environment where other approaches fail.

Structure and present-day compression in the offshore area between Alicante and Ibiza Island (Eastern Iberian Margin)

Low S/N ratios, interfering diffractions, and dip‐related problems (e.g., reflector point dispersal, dip‐dependent NMO, and reflection angle) make reliable amplitude versus offset (AVO) analysis a difficult task. Prestack time migration (PSTM) collapses diffractions, increases the S/N ratio, and reduces dip‐related problems. Therefore, PSTM is usually required before offset dependent information can be extracted from seismic data, and PSTM is mandatory before comparing real seismic data with 1-D earth model synthetic data. We present a 2-D frequency‐wavenumber common‐offset prestack time migration algorithm. To treat the amplitudes correctly, a 3-D to 2-D transform of the data is required before doing the migration. This is done by correcting the data for out‐of‐plane geometrical spreading. Migration artifacts are attenuated, exploiting the fact that the maximum dip to be migrated decreases with increasing traveltime and offset. The final processing steps before further processing are 2-D geometrical spreading correction and removal of the implicit NMO correction inherent in the migration. Two marine data examples show improved data quality after prestack time migration, making subsequent amplitude analysis more reliable.

The hyper-extended Eastern Sardinian margin is due to the eastward migration of the Appennine-Calabria subduction zone, creating the Neogene back-arc Tyrrhenian Basin. This area was affected by strong erosion during the Messinian Salinity Crisis (MSC, 5.97 - 5.33 Ma) on the continental shelf and slope leading to a major discontinuity, known as the Messinian Erosion Surface (MES), constituting, therefore, a remarkable stratigraphic marker. It is also a powerful paleo-topographic marker of the MSC times and can be used as a marker of the deformation during Plio-Quaternary times. The end of the rifting phase in the Eastern Sardinian margin is dated during the Tortonian (11.63 - 7.25 Ma) attested by the occurrence of a relatively thick syn- and post-rift sequence pre-dating the MES.The “METYSS 4” cruise led to the acquisition of more than 2,000 km of very high-resolution (VHR) seismic reflection data, following a dense grid, on the Eastern Sardinian continental shelf and slope, which has been little explored until now. Seismic interpretation allowed for mapping the major erosion surface, the MES, across the continental shelf and slope. At the base of the PQ sequence, the MSC paleo-topography highlights a hydrographic paleo-network identical to the current one and a general progradation of the shelf-break toward the east during the Plio-Quaternary. In the southern part of the study area, several east-dipping normal faults, oriented N-S, significantly shift the MES (between 5 and 55 m; assuming sound wave velocity of 1700 m/s in Plio-Quaternary sediments). The MES is tilted toward the fault and is covered by Plio-Quaternary deposits, which display a fan-shaped geometry (eg. 50 m thick on the hanging wall). These NS-trend faults are cross-cut by E-W trending messinian canyon and this fault pattern is also observed on the other flank of the canyon. The along-strike geomorphological analysis of canyons reveals the occurrence of knickpoints (slope breaks) coinciding with the front of the two fault patterns. Moreover, the shifts in water depth of most knickpoints are at the same order of amplitude than fault offsets (ie. 10 to 50 m). These geomorphologic markers reinforce the hypothesis that the fault activity is recent (ie. less than 5 Ma). We interpret these observations as markers of a recent reactivation of the structures inherited from the rift in the western part of the Tyrrhenian Sea.

We conduct the seismic signal analysis on vintage and recently collected multichannel seismic reflection profiles from the Ionian Basin to characterize the deep basin Messinian evaporites. These evaporites were deposited in deep and marginal Mediterranean sedimentary basins as a consequence of the “salinity crisis” between 5.97 and 5.33 Ma, a basin‐wide oceanographic and ecological crisis whose origin remains poorly understood. The seismic markers of the Messinian evaporites in the deep Mediterranean basins can be divided in two end‐members, one of which is the typical “trilogy” of gypsum and clastics (Lower Unit – LU), halite (Mobile Unit – MU) and upper anhydrite and marl layers (Upper Unit – UU) traced in the Western Mediterranean Basins. The other end‐member is a single MU unit subdivided in seven sub‐units by clastic interlayers located in the Levant Basin. The causes of these different seismic expressions of the Messinian salinity crisis (MSC) appear to be related to a morphological separation between the two basins by the structural regional sill of the Sicily Channel. With the aid of velocity analyses and seismic imaging via prestack migration in time and depth domains, we define for the first time the seismic signature of the Messinian evaporites in the deep Ionian Basin, which differs from the known end‐members. In addition, we identify different evaporitic depositional settings suggesting a laterally discontinuous deposition. With the information gathered we quantify the volume of evaporitic deposits in the deep Ionian Basin as 500,000 km3 ± 10%. This figure allows us to speculate that the total volume of salts in the Mediterranean basin is larger than commonly assumed. Different depositional units in the Ionian Basin suggest that during the MSC it was separated from the Western Mediterranean by physical thresholds, from the Po Plain/Northern Adriatic Basin, and the Levant Basin, likely reflecting different hydrological and climatic conditions. Finally, the evidence of erosional surfaces and V‐shaped valleys at the top of the MSC unit, together with sharp evaporites pinch out on evaporite‐free pre‐Messinian structural highs, suggest an extreme Messinian Stage 3 base level draw down in the Ionian Basin. Such evidence should be carefully evaluated in the light of Messinian and post‐Messinian vertical crustal movements in the area. The results of this study demonstrates the importance of extracting from seismic data the Messinian paleotopography, the paleomorphology and the detailed stratal architecture in the in order to advance in the understanding of the deep basins Messinian depositional environments.

Seismic processing is expected to deliver a reliable product for seismic structure interpretation and reservoir characterization to support subsequent reservoir modeling and drilling operation. 3D OBC wide-azimuth (WAZ) seismic data has a significant advantage in this respect, since it contains large offset and wide azimuth information, which can be utilized for amplitude versus offset (AVO) and amplitude versus azimuth (AVAz) analysis respectively. However, these advanced studies require area-specific data processing efforts in this region for strong linear noise, multiple noise, designature uncertainty etc. Seismic data re-processing was newly conducted in pre-stack time migration (PrSTM) sequence to overcome such technical challenges. Surface-wave noise attenuation was achieved by 2D and 3D model-based dispersive energy attenuation and multi-pass structure oriented filtering in pre-stack and post-stack stages. Source-side deghosting highlighted a low frequency residual bubble and was followed by another pass of de-bubbling based on a smooth spectrum assumption. Shallow water reverberation was minimized by 3D deconvolution and high-resolution radon demultiple. 5D regularization and interpolation with trace densification provided evenly populated traces for each angle and azimuth sectors. Intensive signal preservation protected diffraction energy and resulted in better fault imaging than the vintage processing outcome. Both the vintage and the re-processed seismic data were evaluated in quantitative manners to compare with each other. Angle stacks from the re-processing showed more reasonable response for AVO analysis. Azimuth stacks showed stable S/N ratio leading to higher confidence in azimuthal anisotropy analysis. All these seismic deliverables enable accurate elastic property estimation and detail fault interpreation at target reservoir intervals. The novelty of the proposed workflow is to overcome common technical problems with targeted noise and multiple reduction focusing on angle / azimuth stacks. It unlocks advanced geophysical study for carbonate reservoir in shallow water areas. Processing deliverables will be fully utilized in fault interpretation, AVO and AVAz analysis and will support the forthcoming field development activity.

Effective and careful noise attenuations are crucial for any seismic processing project to control and assure the product quality. The robustness of the data processing can be often identified and quantified by existing well data in production oil and gas fields during the processing. However, if we drill new infill and/or appraisal wells on the flank of the structure based on the processed seismic data, we always need to address the thorny issue of unforeseen mismatching between predicted and actual geological formation tops. In this paper, we present a recent seismic re-processing study from an offshore an Abu Dhabi oil field utilizing relevant seismic processing techniques to overcome these problems as a case study. The original data processed in 2006-2007 exhibits several technical issues: wavelet instability, poor signal-to-noise ratio, limited resolution and offset range, remnant acquisition footprint and multiples. This imperfect dataset led to significant depth discrepancies on a recent appraisal well that was planned using this seismic dataset. The recent processing techniques such as adaptive subtraction of ground roll, 3D deconvolution, 3D Radon-demultiple, 3D projection filtering in the OVT (offset vector tile) domain were carefully applied to address these issues. In addition, we demonstrated that PreSTM (prestack time migration) is an effective de-noise tool while unnecessary and harmful noise attenuations could not be applied to the data because of the potential damage to diffractions and signals before PreSTM. These noise attenuation efforts allowed longer offset ranges into the stack, which proved to be the crucial factor in improving the seismic data quality. Our recent structural interpretation results based on the reprocessing have successfully demonstrated more accurate depth matching on key geological formation tops at this appraisal well location. In addition, blind test utilizing nearly all wells in the field has reinforced our observations. Since subtle reservoir depth variation at each well location will make significant changes to oil production and recovery factor in this particular field, this cast history brought added value to the field development plan particularly for planning horizontal hole drilling and optimizing appraisal well locations.

<p>The Ordos block is located on the west side of the North China Carton, adjacent to the northeastern part of the Tibetan Plateau. Affected by two tectonic movements, Ordos block internal structure remains relatively stable structure, but surrounded by active tectonic belts. With the development of the second and third part of the “China Seismological Science Array”, the distribution of seismic observation stations in Ordos region has been greatly improved. This study will use the new seismic observation data of "Array III", and combined with the phase observation data of "Array II" to form a more complete seismic phase travel time data set. The regional seismic body-wave travel time tomography will figure out a more reliable three-dimensional velocity structure of P waves in Ordos.</p><p>Our study area spans from 32°N to 42°N and 108°E to 114°E , which includes the Ordos block and its adjacent structures . The seismic data we used for inversion were recorded by 1244 stations including: 198 permanent stations and 1043 temporary stations (ChinArray II and III), from November 2013 to August 2017. After manual labeled the seismic phase, we select events with more than ten phase records of individual seismic events. The epicentral distance is less than 200km. Finally, we obtained about 22,500 phase records of 1882 local seismic events.</p><p>The preliminary results are consistent with previous studies and surface structures of a wide range of velocity distributions. However, in the middle-upper crust under the Liupan Mountain west, the low-speed anomaly extending downward is shown, which may be caused by the shallow crustal damage caused with the continuous eastward compression of asthenosphere in the northeastern margin of the Qinghai-Tibet Plateau during the Cenozoic. It is worth noting that there is an EW-trending low-velocity zone under the Dingbian-Suide fault beneath the Ordos Basin, with a depth form lower crust to 50 km in upper mantel. This low-velocity anomaly divides the high-speed disturbance in the Ordos block into two parts，indicate the depth of the fault can reach the upper mantel. In the Taihang Mountains in the west of the study area, low-velocity anomalies extending to the upper layer of the mantle are shown. We initially believe that this anomaly is related to the volcanic thermal motion that once existed on the area.</p>