Prestack Depth Migration Research Articles

Beam propagation of the 15‐degree equation and prestack depth migration in tilted transversely isotropic media using a ray‐centred coordinate system

ABSTRACTSeismic wave imaging in complex media requires an accurate wavefield simulation method that can accurately describethe wave propagation in realistic media. Reverse time‐depth migration is the preferred method for seismic wave imaging in complex media. Although it is relatively expensive, its imaging accuracy is usually better than migrations based on the ray method. Migration of primary reflection data requires a wave propagation simulation method that can accurately describe primary reflected/scattered wave energy and incorporate anisotropy. Accordingly, we propose the simulation of wave propagation in tilted transversely isotropic media using a 15° one‐way wave equation in a ray‐centred coordinate system, combining the flexibility of ray theory and the accuracy of wave theory. We use this equation to describe the propagation of body waves in a single ray tube, a ‘beam’. The wavefield along the beam, guided by its central raypath, has an angle limit defined only by the ray angle; therefore, wave propagation in complex and steeply dipping media can be simulated with a 15° one‐way wave equation. Numerical experiments show that the simulation results for beam propagation using the 15° equation in the ray‐centred coordinate system have good accuracy. For prestack depth migration in tilted transversely isotropic media, we built a beam imaging method using this propagator, and this migration method yielded accurate images with greater efficiency than reverse time‐depth migration.

Point‐spread function convolution to simulate prestack depth migrated images: A validation study

ABSTRACTSeismic migration commonly yields an incomplete reconstruction of the Earth model due to restricted survey aperture, band‐limited frequency content and propagation effects. This affects both illumination and resolution of the structures of interest. Through the application of spatial convolution operators commonly referred to as point‐spread functions, simulated prestack depth‐migrated images incorporating these effects may be obtained. Such simulated images are tailored for analysing distortion effects and enhance our understanding of seismic imaging and subsequent interpretation. Target‐oriented point‐spread functions may be obtained through a variety of waveform and ray‐based approaches. Waveform approaches are generally more robust, but the computational cost involved may be prohibitive. Ray‐based approaches, on the other hand, allow for efficient and flexible sensitivity studies at a low computational cost, but inherent limitations may lead to less accuracy. To yield more insight into the similarities and differences between point‐spread functions obtained via these two approaches, we first derive analytical expressions of both wave‐ and ray‐based point‐spread functions in homogeneous media. By considering single‐point scatterers embedded in a uniform velocity field, we demonstrate the conditions under which the derived equations diverge. The accuracy of wave‐based and ray‐based point‐spread functions is further assessed and validated at selected targets in a subsection of the complex BP Statics Benchmark model. We also compare our simulated prestack depth migrated images with the output obtained from an actual prestack depth migration (reverse time migration). Our results reveal that both the wave‐ and ray‐based approaches accurately model illumination, resolution and amplitude effects observed in the reverse time‐migrated image. Furthermore, although some minor deviations between the wave‐based and ray‐based approaches are observed, the overall results indicate that both approaches can be used also for complex models.

Eulerian partial-differential-equation methods for complex-valued eikonals in attenuating media

Seismic waves in earth media usually undergo attenuation, causing energy losses and phase distortions. In the regime of high-frequency asymptotics, a complex-valued eikonal is an essential ingredient for describing wave propagation in attenuating media, where the real and imaginary parts of the eikonal function capture dispersion effects and amplitude attenuation of seismic waves, respectively. Conventionally, such a complex-valued eikonal is mainly computed either by tracing rays exactly in complex space or by tracing rays approximately in real space so that the resulting eikonal is distributed irregularly in real space. However, seismic data processing methods, such as prestack depth migration and tomography, usually require uniformly distributed complex-valued eikonals. Therefore, we have developed a unified framework to Eulerianize several popular approximate real-space ray-tracing methods for complex-valued eikonals so that the real and imaginary parts of the eikonal function satisfy the classic real-space eikonal equation and a novel real-space advection equation, respectively, and we dub the resulting method the Eulerian partial-differential-equation method. We further develop highly efficient high-order methods to solve these two equations by using the factorization idea and the Lax-Friedrichs weighted essentially nonoscillatory schemes. Numerical examples demonstrate that our method yields highly accurate complex-valued eikonals, analogous to those from ray-tracing methods. Our methods can be useful for migration and tomography in attenuating media.

Multi-Level Detachment Deformation of the West Segment of the South Dabashan Fold-and-Thrust Belt, South China: Insights From Seismic-Reflection Profiling

The South Dabashan arcuate tectonic belt located at the northern margin of the Yangtze Block in South China, which primarily comprises a series of northwestern (NW)-trending foreland fold-and-thrust belts (FTBs), is useful for determining the intracontinental orogeny processes of the Yangtze Block. In this study, we integrated the latest pre-stack depth migration of three- and two-dimensional seismic profiles, drill hole, and outcrop data to explore the structural geometric and kinematic features of the west segment of the South Dabashan FTB. This belt is characterized by multi-level detachment structures due to the presence of three predominant sets of weak layers: the Lower Triassic Jialingjiang Formation gypsum interval, Silurian mudstone beds, and Cambrian shale beds. The belt is accordingly subdivided vertically into three structural deformation systems. The upper system appears above the Jialingjiang Formation gypsum layer and exhibits Jura-type folds, which were formed by alternating anticlines and synclines that are parallel to each other. The middle system comprises Silurian shale as the base and Jialingjiang Formation gypsum interval as the passive roof and exhibits NW-striking imbricate thrusts. The lower system is bounded by Cambrian and Silurian detachment layers, forming a duplex structure. The Sinian and Proterozoic basements below the Cambrian were not involved in deformation. The west segment of the South Dabashan FTB underwent four periods of tectonic evolution: Late Jurassic to Early Cretaceous, Late Cretaceous, Paleogene, and Neogene to Quaternary. The deformation was propagated southward in imbricate style, resulting in the passive uplifting of the overlying strata. Based on the magnetotelluric and deep seismic profile, the tectonic processes of the west segment of the South Dabashan FTB are inferred to be primarily controlled by the Yangtze Block northward subduction under the Qinling Orogenic Belt and the pro-wedge multi-level thrusting during the Late Jurassic to Cretaceous.

Full-Waveform Inversion for Imaging Faulted Structures: A Case Study from the Japan Trench Forearc Slope

Full-waveform inversion (FWI) of limited-offset marine seismic data is a challenging task due to the lack of refracted energy and diving waves from the shallow sediments, which are fundamentally required to update the long-wavelength background velocity model in a tomographic fashion. When these events are absent, a reliable initial velocity model is necessary to ensure that the observed and simulated waveforms kinematically fit within an error of less than half a wavelength to protect the FWI iterative local optimization scheme from cycle skipping. We use a migration-based velocity analysis (MVA) method, including a combination of the layer-stripping approach and iterations of Kirchhoff prestack depth migration (KPSDM), to build an accurate initial velocity model for the FWI application on 2D seismic data with a maximum offset of 5.8 km. The data are acquired in the Japan Trench subduction zone, and we focus on the area where the shallow sediments overlying a highly reflective basement on top of the Cretaceous erosional unconformity are severely faulted and deformed. Despite the limited offsets available in the seismic data, our carefully designed workflow for data preconditioning, initial model building, and waveform inversion provides a velocity model that could improve the depth images down to almost 3.5 km. We present several quality control measures to assess the reliability of the resulting FWI model, including ray path illuminations, sensitivity kernels, reverse time migration (RTM) images, and KPSDM common image gathers. A direct comparison between the FWI and MVA velocity profiles reveals a sharp boundary at the Cretaceous basement interface, a feature that could not be observed in the MVA velocity model. The normal faults caused by the basal erosion of the upper plate in the study area reach the seafloor with evident subsidence of the shallow strata, implying that the faults are active.

A comprehensive model of seismic velocities for the Bay of Mecklenburg (Baltic Sea) at the North German Basin margin: implications for basin development

The geometry of sedimentary basins is normally described by the interpretation of seismic reflectors. In addition to that, rock properties of the sedimentary successions between these reflectors give further insight into the subsurface geology. Here, we present a model for the Bay of Mecklenburg, situated at the northeastern margin of the North German Basin. The model consists of eight layers; it covers seismic velocities of sediments from the Neogene down to the base of the Permian Zechstein. We use eight seismic profiles for model building and apply seismic migration velocity analysis in combination with pre-stack depth migration. The results are interval velocities down to a depth of 5000 m. A further aim of the study is to investigate the sensitivity of these indirectly deduced velocities in comparison to direct measurements within drill holes. The velocities from this study are in good agreement with earlier results from vertical seismic profiling at a nearby well. Cenozoic and Mesozoic strata within the Bay of Mecklenburg show clear depth-dependent velocity trends. A comparison of these trends with predicted compaction trends shows that burial anomalies within Lower Triassic units are significantly higher than in Upper Cretaceous units. This finding could be explained by a greater amount of erosion during Upper Jurassic/Lower Cretaceous times than during Cenozoic times. The Zechstein layer shows a decreasing interval velocity with increasing thickness. Our study demonstrates that seismic velocities deduced from surface-based measurements are of high value in areas with sparse drilling coverage.

Broadband Q-Factor Imaging for Geofluid Detection in the Gulf of Trieste (Northern Adriatic Sea)

Seismic surveys allow estimating lithological parameters, as P-wave velocity and anelastic absorption, which can detect the presence of fracture and fluids in the geological formations. Recently, a new method has been proposed for high-resolution imaging of anelastic absorption, which combines a macro-model from seismic tomography with a micro-model obtained by the pre-stack depth migration of a seismic attribute, i.e., the instantaneous frequency. As a result, we can get a broadband image that provides clues about the presence of saturating fluids. When the saturation changes sharply, as for gas reservoirs with an impermeable caprock, the acoustic impedance contrast produces “bright spots” because of the resulting high reflectivity at its top. When the fluid content changes smoothly, the anelastic absorption becomes a good detector, as fluid-filled formations absorb more seismic energy than hard rocks. We apply this method for imaging the anelastic absorption in a regional seismic survey acquired by OGS in the Gulf of Trieste (northern Adriatic Sea, Italy).

3D geometry and kinematics of the Niudong Fault, Baxian Sag, Bohai Bay Basin, Eastern China—Insights from high-resolution seismic data

Baxian sag is a typical half-graben-like depression whose controlling boundary is a normal fault, the Niudong Fault. We studied the three-dimensional (3D) geometry and kinematics of this fault based on high-resolution 3D pre-stack depth migration seismic data and drilling data from the Baxian Sag. By analysing the geometries and structural deformation of the hanging wall units of the Niudong Fault, we defined the spatial variation and kinematical characteristics of the fault. The Niudong Fault is marked by a major listric normal fault that soles into a detachment surface at depth. Based on fault strikes and dips, we divided the Niudong Fault into three sections and seventeen zones. Geometrically, the Niudong Fault seems to be a single fault. While kinematically, it is not a single fault in the strict sense as various blocks of the fault have their own growth activities dominated by unidirectional and lateral growth. Map-view restoration of the Niudong Fault shows that the hanging wall was dominated by anticlockwise rotation; while cross-section restoration demonstrates that, the hanging wall was deformed by inclined shear.

A reliable velocity estimation in a complex deep-water environment using downward continued long offset multi-channel seismic (MCS) data

The estimation of a reliable velocity–depth model from towed streamer marine seismic data recorded in deep water, especially with a complex seafloor environment, is challenging. The determination of interval velocities from the normal move-out (NMO) of the reflected seismic signals for shallow reflectors (<1 km below the seafloor) is compromised by the combination of a long wave path in the water column and the complex ray paths due to topography, leading to small move-out differences between reflectors. Furthermore, low sediment velocities and deep water produce refraction arrivals only at limited far offsets that contain information about deeper structures. Here, we present an innovative method where towed streamer seismic data are downward continued to the seafloor leading to the collapse of the seafloor reflection and the emergence of refraction events as first arrivals close to zero offset, which are used to determine a high-resolution near surface velocity–depth model using an efficient tomographic method. These velocities are then used to perform pre-stack depth migration. We found that the velocity–depth model derived from tomography of downward continued towed streamer data provides a far superior pre-stack depth migrated image than those produced from velocity–depth models derived from conventional velocity estimation techniques.

Seismic images of the Northern Chilean subduction zone at 19°40′S, prior to the 2014 Iquique earthquake

SUMMARY The Northern Chilean subduction zone is characterized by long-term subduction erosion with very little sediment input at the trench and the lack of an accretionary prism. Here, multichannel seismic reflection (MCS) data were acquired as part of the CINCA (Crustal Investigations off- and onshore Nazca Plate/Central Andes) project in 1995. These lines cover among others the central part of the MW 8.1 Iquique earthquake rupture zone before the earthquake occurred on 1 April 2014. We have re-processed one of the lines crossing the updip parts of this earthquake at 19°40′S, close to its hypocentre. After careful data processing and data enhancement, we applied a coherency-based pre-stack depth migration algorithm, yielding a detailed depth image. The resulting depth image shows the subduction interface prior to the Iquique megathrust earthquake down to a depth of approximately 16 km and gives detailed insight into the characteristics of the seismogenic coupling zone. We found significantly varying interplate reflectivity along the plate interface which we interpret to be caused by the comparably strong reflectivity of subducted fluid-rich sediments within the grabens and half-grabens that are predominant in this area due to the subduction-related bending of the oceanic plate. No evidence was found for a subducted seamount associated to the Iquique Ridge along the slab interface at this latitude as interpreted earlier from the same data set. By comparing relocated fore- and aftershock seismicity of the Iquique earthquake with the resulting depth image, we can divide the continental wedge into two domains. First, a frontal unit beneath the lower slope with several eastward dipping back-rotated splay faults but no seismicity in the upper plate as well as along the plate interface. Secondly, a landward unit beneath the middle slope with differing reflectivity that shows significant seismicity in the upper plate as well as along the plate interface. Both units are separated by a large eastward dipping mega splay fault, the root zone of which shows diffuse seismicity, both in the upper plate and at the interface. The identification of a well-defined nearly aseismic frontal unit sheds new light on the interplate locking beneath the lower continental slope and its controls.

Combination of the common reflection surface-based prestack data regularization and reverse time migration: Application to real land data

The application of reverse time migration (RTM) to land seismic data is still a great challenge due to its low quality, low signal-to-noise ratio, irregular spatial sampling, acquisition gaps, and missing traces. Therefore, prior to the application of this kind of depth migration, the input prestack data must be judiciously preconditioned; that is, it must be interpolated, regularized, and enhanced. There are several Fourier-based methods for seismic data preconditioning, but for 2D real land data, as we will show, the regularization of prestack data based on the common reflection surface (CRS) method provides high-quality enhanced preconditioned data, which is suitable for prestack depth migration and velocity model building. We have determined the successful application of RTM to onshore seismic data, revealing the great potential of combining RTM and CRS-based prestack data regularization. We apply this processing combination to real land data with low quality and irregular spatial sampling, from geologically complex areas with the presence of diabase sills and steeply dipping reflectors. Normally, it is very difficult to extract the wavelet of the seismic source from this type of low-quality onshore data because the reflection events are hidden or mixed with noise, and, because of this, RTM migration is often applied using artificial sources (e.g., a Ricker wavelet), which can introduce ringy side lobes in the reflectors. Therefore, we develop a practical algorithm for wavelet determination from the power spectrum of the CRS regularized prestack data, and we apply it successfully in the RTM migration. We develop applications of our prestack data preconditioning based on CRS followed by RTM for two 2D onshore seismic lines from the Tacutu and Parnaiba basins, Brazil. Comparisons with standard Kirchhoff depth migration reveal that the combination of CRS followed by wavelet extraction and then RTM improves the quality and resolution of the final migrated images.

The Design of Payload Lunar Regolith Penetrating Radar on Chang'E-5 Lander

Sampling return in Chang'E-5 (CE-5) mission is the third phase of three strategic steps of China's lunar program. The three strategic steps are orbiting, landing, and returning. The sampling and drilling device is mounted on the Lander. One task of the CE-5 Lunar Regolith Penetrating Radar (LRPR) is to provide information support for the drilling and sampling device. This article introduces the scientific objectives of CE-5 LRPR and the difference with Chang'E-3 lunar penetrating radar. The system design, working principle, imaging, and calibration methods of LRPR are the highlight. Considering that the Lander is stationary, and its bottom is about 90 cm from the lunar surface, an antenna array is adopted to probe the subsurface structure and the regolith thickness for supporting the drilling. The design uses a pre-amplifier circuit to increase the dynamic range of the whole system, which enhances the detection effect of shallow lunar soil structure. For verifying the probing ability of LRPR, the marble slabs, metal balls, and stones are buried in the volcanic ash pool in advance for detection and verification. The pre-stack Kirchhoff depth migration imaging method is adopted to identify the burial depth and thickness of the target. The verification results meet the task requirements.

Using MT data to find velocity traps in middle and shallow layers

In the complex piedmont oil and gas exploration, due to the large change of lithology in the middle and shallow strata, the vertical and horizontal changes of velocity in the middle and shallow strata are more intense. It is difficult to identify some strata with obvious changes in velocity on the seismic section, which leads to the inaccuracy of velocity model or obvious velocity trap in the seismic prestack depth migration processing. This will further cause the incorrect deep structure and target layer solution. MT method are mainly characterized by deep sounding depth, high sensitivity to high conductivity geological bodies in high resistivity layers. This makes it very effective to study the deep structures of basements and lithology distribution. We here analyze the technical advantages of MT method in tracing in middle and shallow layers by MT modeling geoelectric model of mountainous areas, and demonstrate that MT method can play an important role in oil and gas exploration in complex areas, especially in finding velocity traps for seismic processing by analyzing some cases.

Extracting Q Anomalies From Marine Reflection Seismic Data Using Deep Learning

Anelasticity of the earth subsurface medium, which is quantified by the quality factor <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$Q$ </tex-math></inline-formula> , causes the dissipation of seismic energy. Strong attenuation effect resulting from geology such as gas clouds (gas-filled sandstone) is a challenging problem for high-resolution imaging. To compensate the attenuation effect, first we need to accurately estimate the attenuation parameter. However, it is difficult to directly derive a heterogeneous attenuation Q model. This research letter proposes a method to derive a Q model corresponding to strong attenuative media from marine reflection seismic data using convolutional neural network (CNN), a popular deep learning framework. We treat Q anomaly detection problem as a semantic segmentation task and train a network to perform a pixel-by-pixel prediction to invert a pixel group that belongs to the strong level of attenuation probability. The proposed method uses a volume of marine 3-D reflection seismic data for network training and validation, which needs only a small part of real data as the training set due to the feature of U-Net. In the final stage, to evaluate the attenuation model, we validate the predicted heterogeneous Q model using deabsorption prestack depth migration (Q-PSDM), a high-resolution imaging result in depth domain with appropriate compensation is obtained.

The CarbonNet Pelican 3D seismic survey – A first for Australia’s offshore greenhouse gas storage

The CarbonNet Pelican 3D seismic survey – A first for Australia’s offshore greenhouse gas storage

Refraction wavefield migration

Refraction methods are often applied to model and image near-surface velocity structures. However, near-surface imaging is very challenging, and no single method can resolve all of the land seismic problems across the world. In addition, deep interfaces are difficult to image from land reflection data due to the associated low signal-to-noise ratio. Following previous research, we have developed a refraction wavefield migration method for imaging shallow and deep interfaces via interferometry. Our method includes two steps: converting refractions into virtual reflection gathers and then applying a prestack depth migration method to produce interface images from the virtual reflection gathers. With a regular recording offset of approximately 3 km, this approach produces an image of a shallow interface within the top 1 km. If the recording offset is very long, the refractions may follow a deep path, and the result may reveal a deep interface. We determine several factors that affect the imaging results using synthetics. We also apply the novel method to one data set with regular recording offsets and another with far offsets; both cases produce sharp images, which are further verified by conventional reflection imaging. This method can be applied as a promising imaging tool when handling practical cases involving data with excessively weak or missing reflections but available refractions.

SEMBLANCE RESIDUAL MOVEOUT ANALYSIS TO FIND ERORRS AND UPDATING THE INTERVAL VELOCITY MODEL MODE IN THE HORIZON BASED TOMOGRAPHY METHOD

The tomography method requires an excellent initial velocity model. On the horizon based tomography, it will correct the travel time error of seismic waves along the horizon which is analysed using input results from the analysis of residual depth moveout. In this study, a semblance residual moveout analysis will be conducted after the interval velocity model has applied to the SBI field seismic data (CDP Gathers and RMS velocity). Based on the imaging results generated by the PSDM running process, an aperture value of 550 for inline and 800 for crossline is selected. PSDM generated from the initial interval velocity model has an acoustic impedance value between 1000 kg/m2s to 14339.2 kg/m2s. The PSDM process, residual moveout analysis, and horizon-based tomography are carried out iteratively until the error in the interval velocity model approaches zero. In this study, five iterations were performed. The resulting residual moveout is increasingly oscillating around zero after the 5th iteration, which indicates that the error in the interval velocity model is getting smaller. There are two types of residual moveout, namely residuals moveout positively and residuals moveout negatively. Residual moveout positive indicates that the velocity used is too high, while the residual moveout negative indicates that the velocity used is too low. The identification of interval velocity model errors with analysis of residual moveout semblance is calculated from depth gathers. The semblance residual moveout analysis is used for the Pre Stack Depth Migration (PSDM) depth image analysis stage along with the marker (well data). .

A complex gas hydrate system imaged jointly using MCS and OBS data from the northern South China Sea

Much of our knowledge of gas hydrate in the Dongsha area of the South China Sea has been gained through intensive geophysical surveys and drilling. However, many factors remain unclear, such as the co-existence of shallow gas hydrate and deeper gas hydrate. This lack of clarity is partially due to the lack of a depth-domain velocity model and accurate imagery of the gas hydrate-bearing sediments. In this study, pre-stack depth migration (PSDM) is used to produce subsurface images using a depth-domain velocity model from OBS tomography as the migration velocity. A simple initial velocity model was built using the seafloor depth information derived from multi-channel seismic (MCS) data. This simple model was used to build a more accurate velocity model by using first arrival time tomography from OBS data. Three different approaches were used to assess the reliability of the velocity model. Resolution tests and uncertainties analysis were used to further evaluate the model. The final PSDM image revealed some features more clearly than they were seen on the time migration image. These features help better explain the complex gas hydrate system. The depth of a bottom-simulating reflector (BSR) was picked directly from the PSDM image. This BSR is very consistent with the drilling result. Our work highlights the importance of PSDM in the study of gas hydrate in complex settings and highlights the potential and value of using OBS refraction in building shallow velocity models.

Case study: joint seismic reflection and CSAMT data interpretation for mineral explorations in Fujian, China

The Southwestern area of Fujian Province in China contains a major metallogenic belt. There have been three major tectonic movements since the Late Paleozoic, and the thrusts here are the main mineral deposits. These occurred between the Early Carboniferous and Late Permian eras. Due to dramatic undulations in the surface and associated complex underground structures, a single geophysical method cannot provide reliable imaging results. This is largely due to difficulties in data acquisition and processing. Seismic exploration provides one example. Using this method, raw shot gathers have a low signal-to-noise ratio because of the undulations in the topography. Furthermore, strong lateral velocity variation makes the migration process exceedingly difficult. This means that interpretations of the resulting reflection seismic profile give rise to uncertainty. For this reason, two-dimensional reflection seismology and controlled source audio-frequency magnetotellurics (CSAMT) electromagnetic sounding techniques were performed. After initially completing conventional processing on the seismic reflection data, subsequent pre-stack depth migration (PSDM) over the rugged topography yielded a much better image. It was able to identify thrust faults and magma intrusions. Then, using CSAMT, inversion was conducted with the same topography as the seismic reflection. By combining the migrated seismic profile, inverted resistivity profile, and borehole data, a vertical geological model was constructed. This proved the existence of an overlapped thrust, and it also indicated the possible presence of deeper mineralization.

Maintaining line of sight in oil and gas exploration: A case study from Mahakam Delta, Indonesia

In all exploration processes, the evaluation of basins, permits, and individual prospects changes over time with incremental availability and quality of data, technical effort expended, and knowledge gained. The NU prospect, located in the Mahakam Hilir PSC (East Kalimantan), is an example in which geologic chance of success (GCOS) predictions can change over time with increasing acquisition and availability of geophysical and geologic data and the studies done on them. We show how studies done on any one prospect or group of prospects can progressively increase/decrease the chance of at least one success in an exploration campaign of several wells. After a series of four wells was drilled in the PSC, which did not deliver commercial success, a change in approach was required to continue exploration. This included the acquisition of airborne gravity gradiometry data, initial trial prestack depth migration (PSDM) reprocessing of two key 1989 vintage 2D lines, acquisition of vintage well data from four Sambutan Field wells, acquisition of nine vintage 2D seismic lines over the field, and PSDM reprocessing of the nine 2D seismic lines. All data were then integrated to build a new geologic model. As a result, the NU prospect GCOS progressively moved from less than 10% to nearly 40%.