Narrow Azimuth Research Articles

Wide azimuth seismic data processing technology and application: a case study of tight gas reservoirs in western China

As the difficulty of oil and gas field exploration and development increases both domestically and internationally, onshore exploration targets have gradually shifted from the shallow to the deep and from conventional oil and gas reservoirs to unconventional ones. Particularly in the exploration and development of unconventional oil and gas horizontal wells, there is an increasing demand for higher precision and quality of seismic data to better identify formation lithology, rock fractures, and improve the characterization of reservoirs, reservoir positioning, and connectivity. Wide-azimuth seismic exploration possesses significant technical advantages in addressing exploration challenges such as lithologic exploration, small fault imaging, and detailed characterization of oil and gas reservoirs. Wide azimuth seismic data reduces blind spots in seismic acquisition and improves the imaging accuracy of small faults. Notably, there exist distinct anisotropic characteristics in fault areas and fractured reservoirs. Wide azimuth seismic data is particularly advantageous for studying amplitude variation with variations in amplitude with offset (AVO), incident angle (AVA), or azimuth (AVAZ), as well as velocity with azimuth (VVA). These variations aid in identifying faults, fractures, and changes in formation lithology. As the focus of oil and gas exploration gradually shifts to complex lithological reservoirs and unconventional oil and gas reservoirs, narrow azimuth seismic exploration has been gradually replaced by wide azimuth exploration. However, as observation azimuth increases, challenges related to velocity variations with azimuth, azimuth-related traveltime differences, and azimuth-related anisotropy arise. Based on wide-azimuth seismic data from tight gas reservoirs in western China, this study conducted wide-azimuth anisotropic velocity analysis, OVT domain data regularization processing, OVT domain prestack time/depth migration, and horizontally transverse isotropy (HTI) azimuth anisotropy correction techniques. After applying specialized processing to the wide-azimuth seismic data, significant improvements were observed in the S/N and resolution of the target layer. The delineation of fractures related to hydrocarbon sources also became more distinct. These advancements not only provided high-quality results for high-fidelity, high-resolution imaging of tight gas reservoirs but also provided azimuth volume corresponding to fast and slow wave velocities for seismic data interpretation, facilitating velocity variation with azimuth (VVAZ) fracture detection and AVO analysis research.

RETRACTED ARTICLE: Improvement of seismic data quality and recognition of fault discontinuities through seismic data conditioning applications: a case study of Issaran oil field, Gulf of Suez, Egypt

Obtaining high-quality seismic imaging in shallow heterogeneous carbonate reservoirs with complicated structural regimes, such as the Issaran field, is difficult. Issaran field is a heavy oil shallow heterogeneous fractured carbonate reservoirs located in the Gulf of Suez of Egypt. It has many geological factors that affect image quality and pose numerous challenges. In addition, the seismic data was acquired more than 12 years ago, with narrow azimuth and short offsets. As a result, the fault zones are not sharply defined. Furthermore, the seismic data was processed about 10 years ago. The signal-to-noise ratio is relatively poor due to the random and coherent noises. A robust data conditioning workflow for noise suppression and fault discontinuity sharpening is used to improve the post-stack seismic data quality. In this context, a steering dataset was generated, and a dip-steered median filter (DSMF), a dip-steered diffusion filter, and a fault enhancement filter (FEF) were applied to sharpen the discontinuities. Structural attributes were evaluated, to investigate how the newly applied data conditioning affects the clarity of fault patterns. Furthermore, multiple physical attributes were extracted, including instantaneous phase, instantaneous frequency, and RMS amplitude to better understand the reservoir stratigraphic heterogeneity. The application of DSMF is useful in removing the residual random noises. The FEF-similarity attribute revealed small-scale faults with a 50-foot vertical throw. The physical attributes proved that the Issaran carbonate facies is controlled by structure. Moreover, the RMS amplitude attribute helped in distinguishing between porous and non-porous dolomite facies. As imaging quality has significantly improved, the applied seismic data conditioning workflow is beneficial for the field development in Issaran field. It is also suggested that this data conditioning workflow be applied in other heterogeneous carbonate reservoirs with complex structures around the world.

Estimating SHmax azimuth with P sources and vertical geophones: Use P-P reflection amplitudes or use SV-P reflection times?

We compared two methods for extracting the azimuth of maximum horizontal stress (SHmax) from 3D land-based seismic data generated by a P source and recorded with vertical geophones. In the first method, we used the direct-SV mode that is produced by all land-based P sources. P sources generate SV illumination that radiates in all azimuth directions from a source station and creates SV-P reflections that are recorded by vertical geophones. Unless stratigraphy has steep dip, SV-P raypaths recorded by vertical geophones are the reverse of P-SV raypaths recorded by horizontal geophones. Thus, SV-P data provide the same S-wave sensitivity to stress fields as popular P-SV data do. In the second method, we retrieved P-P reflections and then performed an amplitude-variation-with-azimuth (AVA) analysis of the amplitude-gradient behavior of P-P reflection wavelets. We did this analysis in narrow azimuth corridors to determine the gradient of reflection-wavelet amplitudes as a function of azimuth. This P-P AVA amplitude-gradient method has been of great interest in the reflection seismology community since it was introduced in the late 1990s. Each of these methods, AVA analysis of the gradient of P-P reflection amplitudes and azimuth-dependent arrival times of SV-P reflections, can be used to determine the azimuth of SHmax stress. We compare the results of the two methods with ground truth measurements of SHmax azimuth at a CO2 sequestration site in the Michigan Basin. SHmax azimuths were determined from P-P and SV-P data at three major boundaries at depths of approximately 3500 ft (1067 m), 5500 ft (1676 m), and 7500 ft (2286 m). Two estimates of SHmax azimuth (one using SV-P data and one using P-P data) were made at each stacking bin inside a 24 mi2 (62 km2) image space. The result was approximately 98,000 estimates of SHmax azimuth across each of these three boundaries for each of these two prediction strategies. Histogram displays of PP AVA gradient estimates had peaks at correct azimuths of SHmax at all three depths, but the spread of the distributions widened with depth and split into two peaks at the deepest boundary. In contrast, each histogram of SHmax azimuth predicted by azimuth-dependent SV-P traveltimes had a single, definitive peak that was positioned at the correct SHmax azimuth at all three boundary depths.

Learning From Reliable Unlabeled Samples for Semi-Supervised SAR ATR

Synthetic aperture radar automatic target recognition (SAR ATR) has been suffering from the insufficient labeled samples as the annotation of SAR data is time-consuming. Thus, adding unlabeled samples into training has attracted the attention of researchers. In this letter, a semi-supervised method based on consistency criterion, domain adaptation and Top-k loss is proposed to alleviate the need for labeled samples. According to consistency criterion that samples generated by the weak and strong augmentations from the same sample belong to the same category, we use the weak and strong augmented unlabeled samples to predict pseudo labels and train the model respectively. Then, to overcome the issue caused by the domain discrepancy between labeled and unlabeled samples especially when labeled samples concentrate on a narrow azimuth range, a domain adaptation component is designed to reduce their discrepancy. Besides, considering the incorrect pseudo labels will hamper the model training, the Top-k loss is adopted for unlabeled samples to mitigate the negative effects. The experimental results on Moving and Stationary Target Acquisition and Recognition (MSTAR) dataset demonstrate the superiority of our method in semi-supervised SAR ATR. Specifically, we achieve about a 14.29% improvement in recognition accuracy compared to the state-of-the-art when the labeled samples concentrate on a narrow azimuth range.

The Influence of the Azimuth RCS Pattern of Calibrator on SAR Absolute Calibration

The realization of radiometric calibration relies on the known artificial point targets as the reference, such as corner reflectors. One of the criteria of the calibrator is that the 3 dB-beamwidth of the azimuth radar cross section (RCS) pattern needs to be larger than the synthetic aperture radar (SAR) antenna 3-dB beamwidth to ensure the SAR antenna, which has a narrow azimuth beamwidth, can align to the calibrator while also making the nominal RCS value of the calibrator approximately constant during the radiometric calibration. However, the RCS of the calibrator is actually inconstant within a synthetic aperture period, especially when the SAR antenna beamwidth is wide. The simplified treatment will introduce errors to calibration. In this letter, the azimuth RCS pattern is taken into account. The error caused by the constant RCS is modeled. Based on the error model, an improved criterion of the calibrator is discussed. The experiments use the known SAR system parameters for numerical simulation based on dihedral corner reflectors (DCRs) with different sizes. The results verify the error model and indicate that when the SAR beamwidth is larger than the beamwidth of the calibrator, and the introduced error cannot be ignored. The relationship between the size of the calibrator and the SAR beamwidth is further analyzed through the experiments.

Day after day variations of arrival angles and polarisation parameters of Q bursts recorded at Antarctic station “Akademik Vernadsky”

We present variations in the source bearings and polarization of the horizontal magnetic field of Q bursts recorded at Antarctic station “Akademik Vernadsky” (65.2458° S, 64.2572° W). We analyzed the parameters of the “polarization ellipse” inscribed in the initial impulse of the Q-burst, which is formed by a direct wave propagating from lightning to the observer along a short path in the Earth-ionosphere cavity. Three days, from March 31 to April 2, 2019, a significant part of the Q burst flux was stably observed in a narrow northerly azimuth sector associated with the direction to lightnings in South America. During this period, regular deviations of the measured azimuths from the northern direction were observed up to 8–10°, to the west when the morning terminator passed and to the east at the evening terminator. Duration of these deviations was about 4 h. The azimuth deviations were accompanied by characteristic oscillations in the “ellipticity” value of the direct wave field, N-shaped during the passage of the evening terminator, and a mirror-symmetric И-shaped oscillation on the morning terminator. We found that, on average, the polarization of the direct wave field is close to linear when the paths from the sources to the observer were on the day side, while at night side its “ellipticity” increases to 0.1. The observed azimuth deviations were qualitatively interpreted using the concept of geometric optics of the reflection of radio waves from the day-night non-uniformity in the Earth-ionosphere cavity. The measurement period was close to the day of the vernal equinox, when the propagation path from the sources and the lines of the solar terminator run almost parallel and radio waves are incident on the day-night interface at “sliding angles”, which enhances their “reflection".

Application of common reflection surface (CRS) to velocity variation with azimuth (VVAz) inversion of the relatively narrow azimuth 3D seismic land data

Abstract The fracture direction and its intensity are critical properties related to hydrocarbon characterization and identification. Both these properties have an essential role in identifying the direction of hydrocarbon migration, determining the sweet spot area, and optimizing the drilling design. The velocity variation with azimuth (VVAz) is a well-known method to estimate the fracture direction and its intensity. This method is of widespread interest because it predicts the properties based on seismic data without any practical constraints. Despite this interest, the technique requires rich azimuth 3D seismic data in our case, which is rare. This study aims to apply regularization and interpolation by including the wave front attributes based on the Common Reflection Surface (CRS) method before the VVAz inversion. The motivation of using the CRS method is to enrich the current azimuth of the 3D seismic data and improve the S/N ratio. The synthetic and the real 3D seismic data are evaluated to examine the interpolation scheme of the proposed CRS method’s performance. Based on the evaluation of the 3D seismic data after regularization, the amplitude versus offset (AVO) phenomena, and the VVAz inversion results are relatively consistent (or matched) with the model. A similar result is found for the case of real 3D seismic data. A significant positive correlation between the fracture intensity of FMI and the real seismic data of about 0.9 is obtained. Therefore, CRS can be used as a regularization and interpolation method before the VVAz inversion of the relatively narrow azimuth 3D seismic data.

Robust Maritime Target Detector in Short Dwell Time

Detection of small-sized maritime targets is an important task for a marine surveillance radar. Recently, with the emergence of a marine surveillance radar system that has a narrow azimuth beamwidth and rapidly rotating antennas, the available dwell time for detecting a maritime target is usually very short. This short dwell time considerably degrades the performance of conventional detectors, especially those focusing on small-sized targets. In this paper, we propose an efficient detector for small-sized maritime targets to provide a reliable detection performance, even in short dwell times. The proposed scheme is based on a new joint metric, which results from the product of the magnitude and difference features in the Doppler spectra. We discriminate the target bins from sea clutter bins using a statistical discriminator based on the joint metric, whose probability density function follows the product distribution of standard gamma distributions. Compared to conventional detectors, the proposed scheme can provide a robust performance in terms of the average signal-to-clutter ratio as well as the detection rate, especially in shorter dwell times.

Steered sample algorithm for acoustic source localization.

High-precision source localization depends on many factors, including a suitable location method. Beamforming-based methods, such as the steered response power (SRP), are a common type of acoustic localization methods. However, these methods have low spatial resolution. The SRP method with phase transform (SRP-PHAT) improves the spatial resolution of SRP and is one of the most effective and robust methods for source localization. However, the introduction of a phase transform to SRP might amplify the power of the noise and result in many local extrema in the SRP space, which has a negative impact on source localization. In this paper, a steered sample algorithm (SSA) based on the reciprocity of wave propagation for acoustic source localization is proposed. The SSA localization process is similar to the hyperbolic Radon transform, which is theoretically analyzed and is the most essential difference form the SRP/SRP-PHAT. Compared with the SRP-PHAT, the experimental results demonstrate that the SSA perform better when it comes to array signal positioning with limited array elements and narrow azimuth signal, where SSA can achieve high precision positioning with lower SNR.

Three-dimensional Marchenko internal multiple attenuation on narrow azimuth streamer data of the Santos Basin, Brazil.

ABSTRACTIn recent years, a variety of Marchenko methods for the attenuation of internal multiples has been developed. These methods have been extensively tested on two‐dimensional synthetic data and applied to two‐dimensional field data, but only little is known about their behaviour on three‐dimensional synthetic data and three‐dimensional field data. Particularly, it is not known whether Marchenko methods are sufficiently robust for sparse acquisition geometries that are found in practice. Therefore, we start by performing a series of synthetic tests to identify the key acquisition parameters and limitations that affect the result of three‐dimensional Marchenko internal multiple prediction and subtraction using an adaptive double‐focusing method. Based on these tests, we define an interpolation strategy and use it for the field data application. Starting from a wide azimuth dense grid of sources and receivers, a series of decimation tests are performed until a narrow azimuth streamer geometry remains. We evaluate the effect of the removal of sail lines, near offsets, far offsets and outer cables on the result of the adaptive double‐focusing method. These tests show that our method is most sensitive to the limited aperture in the crossline direction and the sail line spacing when applying it to synthetic narrow azimuth streamer data. The sail line spacing can be interpolated, but the aperture in the crossline direction is a limitation of the acquisition. Next, we apply the adaptive Marchenko double‐focusing method to the narrow azimuth streamer field data from the Santos Basin, Brazil. Internal multiples are predicted and adaptively subtracted, thereby improving the geological interpretation of the target area. These results imply that our adaptive double‐focusing method is sufficiently robust for the application to three‐dimensional field data, although the key acquisition parameters and limitations will naturally differ in other geological settings and for other types of acquisition.

Virtual Sources of Body Waves from Noise Correlations in a Mineral Exploration Context

Abstract The extraction of body waves from passive seismic recordings has great potential for monitoring and imaging applications. The low environmental impact, low cost, and high accessibility of passive techniques makes them especially attractive as replacement or complementary techniques to active-source exploration. There still, however, remain many challenges with body-wave extraction, mainly the strong dependence on local seismic sources necessary to create high-frequency body-wave energy. Here, we present the Marathon dataset collected in September 2018, which consists of 30 days of continuous recordings from a dense surface array of 1020 single vertical-component geophones deployed over a mineral exploration block. First, we use a cross-correlation beamforming technique to evaluate the wavefield each minute and discover that the local highway and railroad traffic are the primary sources of high-frequency body-wave energy. Next, we demonstrate how selective stacking of cross-correlation functions during periods where vehicles and trains are passing near the array reveals strong body-wave arrivals. Based on source station geometry and the estimated geologic structure, we interpret these arrivals as virtual refractions due to their high velocity and linear moveout. Finally, we demonstrate how the apparent velocity of these arrivals along the array contains information about the local geologic structure, mainly the major dipping layer. Although vehicle sources illuminating array in a narrow azimuth may not seem ideal for passive reflection imaging, we expect this case will be commonly encountered and should serve as a good dataset for the development of new techniques in this domain.

A Wideband Beamforming Antenna Array for 802.11ac and 4.9 GHz in Modern Transportation Market

In this work, a novel antenna structure has been proposed, which consists of multiple sub-array features i.e., a field selectable beam (90°, 180°, 270°, and 360°) and the choice of gain (11.16, 14.59 and 17.25 dBi) that can be easily adapted to cater for the dynamic scenarios in the transportation environment. The sub-arrays were designed using the microstrip patch antenna (MPA) concept with capacitive feed and dual substrate stacked up configuration for superior operating bandwidth covering the entire 802.11ac (5.17 to 5.85 GHz Industrial Scientific and Medical (ISM) band), in addition to the extended coverage for 4.92 to 4.98 GHz licensed band with narrow azimuth beamwidth of 24°. The sub-array was designed, simulated and experimentally evaluated and the beamforming results revealed that the antenna structure can be integrated with beamforming concepts to provide an enhanced wireless link between the ground base station and the mobile terminals that allows beam steering to focus on the targeted direction and null the interference directions with small beam width. It is expected that the proposed configurable gain/beam beamforming antenna array will further reduce the deployment cost and enhance the anti-interference performance by two-fold, and shall bring the user experience in the transportation market to the next level.

OPTIMALISASI DAN ANALISIS DESAIN PARAMETER SEISMIK 3D DARAT BERDASARKAN MODEL GEOLOGI LAPANGAN “RL”

The design of 3D land seismic parameters is a preliminary step before the acquisition of 3D land seismic data to obtain seismic data quality and efficient survey budget. The objective of this research is to obtain high quality of 3D land seismic data based on fold coverage spread on each target layer. The method used is based on the principle of wave propagation or ray tracing method on each target layer, wherein each wave reflection point (bin) will have a different value of fold so that the required configuration of the stretch that will result in the optimal spread of the fold. The applied stretch configurations are orthogonal, brick, zig-zag and slanted where each stretch configuration uses two types of templates that is narrow and wide azimuth. From the simulation analysis results of several stretch configurations in the survey area, then the optimal stretch configuration will be obtained in the survey area. The stretch configuration will be applied is the configuration of orthogonal stretch with narrow azimuth type template. Consideration of the selection of this stretch configuration because it has a more evenly folding effect on the target layer than with other stretch configurations. Furthermore, to obtain the configuration of the appropriate stretch of field conditions, the source point is moved outside the obstacle zone so that the quality of seismic data is maintained.

Processing the first full azimuth OBN survey in Australia: a step change in imaging quality

SummaryOcean Bottom Node (OBN) acquisition has been widely used in oil and gas industry to provide improved reservoir imaging when compared to conventional narrow azimuth (NAZ) towed streamer data....

Multi-Azimuth 3-D Surface-Related Multiple Elimination ? Application to Offshore Nile Delta

In recent years, a full 3-D extension of Surface-Related Multiple Elimination (SRME) has become available to address complex multiples. True-azimuth 3-D SRME (TA 3-D SRME) aims to predict the full 3-D nature of all surface-related multiples, while making no assumptions about the subsurface geology. The application of TA 3-D SRME on data acquired using multi-azimuth towed streamer configurations has led to significant improvements in multiple removal compared to the application on data acquired in conventional narrow-azimuth towed streamer mode. Multi-azimuth data from the Nile Delta is used to compare results between narrow-azimuth 3-D SRME and true-azimuth 3-D SRME. Multiples are among the toughest challenges in processing seismic data in the Nile Delta, which is characterized by a structurally complex dipping water bottom, shallow gas charged channel systems in a compaction driven overburden, overlying a complex anhydrite rich layer of Messinian age. This overburden generates complex diffracted multiples, both in the shallow section (water bottom multiples) and in the deeper section (Messinian peg-legs). The results indicate that the multi-azimuth 3-D SRME leads to better results than conventional narrow azimuth 3-D SRME. The azimuthal diversity inherent in the multi-azimuth data allows for better multiple prediction and data recons truction of missing data when TA 3-D SRME is applied, compared to conventional narrow-azimuth 3-D SRME.

Introduction to this special section: Impact of compressive sensing on seismic data acquisition and processing

The last 30 years have seen ever-increasing amounts of seismic data being acquired. It is now common to see 12 or more streamers deployed with a single seismic vessel, ocean-bottom sensors deployed by the thousands, or land surveys with channel counts into the many tens of thousands. The volume of data has been defined not only by the sensor density but also by the number of components recorded at each location. Radial data coverage has grown from narrow azimuth to multiazimuth to wide azimuth to full azimuth. A higher sensor count has only been a part of the data explosion story. Source effort has evolved as well, with continuous recording and a variety of simultaneous-source schemes being utilized. Data density and field effort have grown, and continue to grow, with the imaging and interpretational demands of the industry.

Sparse well pattern and high-efficient development of metamorphic buried hills reservoirs in Bohai Sea area, China

In order to achieve the development objective of fewer wells with higher production of metamorphic buried hill reservoirs in the Bohai Sea area, the JZ251S oilfield at Bohai Bay Basin was taken as an example to carry out elaboration of reservoir fracture, quantitative characterization of water displacing oil mechanism at dual-porosity reservoir, optimization of new well pattern mode, formulation of rational development technology policy, maintaining productivity and controlling water rising based on development experience of similar oil reservoir, thus forming the key high efficiency development technique of sparse well pattern of offshore metamorphic rock reservoir. Based on the characteristics of the JZ251S buried hill reservoir, forward simulation of wave equation for fracture anisotropy was carried out, verifying the effectiveness of narrow azimuth seismic data to fractures detection in work area. Multi-parameters prestack inversion and geostress field simulation were applied to forecast location of fractures and direction of fractures respectively. Based on the large-scale 3D physical model and numerical simulation, a new top-bottom interlaced 3D injection-production well deployment model concerning the horizontal well was presented. Considering the production demand, the reasonable oil production rate of JZ251S oil reservoir shall be controlled at 3%-4%, depletion development until that the formation pressure level maintains at 70% of initial formation pressure can be implemented for the oil field at the initial stage, and then the development mode of water flooding to keep pressure can be carried out. Considering the pilot production data at the work zone, different water breakthrough models of the horizontal well were simulated to form four diagnosis charts, which help to stabilize oil and control water effectively. The field practice shows that these techniques greatly increase the crude oil output and improve the water-injection development effect.

Highly repeatable 3D compressive full-azimuth towed-streamer time-lapse acquisition — A numerical feasibility study at scale

Most conventional 3D time-lapse (or 4D) acquisitions are ocean-bottom cable (OBC) or ocean-bottom node (OBN) surveys, as these surveys are relatively easy to replicate compared to towed-streamer surveys. To attain high degrees of repeatability and survey replicability, dense periodic sampling has become the norm for 4D surveys and renders this technology expensive. Conventional towed-streamer acquisitions suffer from limited illumination of the subsurface due to narrow azimuth. Although, acquisition techniques such as multiazimuth, wide-azimuth, rich-azimuth acquisition, etc., have been developed to illuminate the subsurface from all possible angles, these techniques can be prohibitively expensive for densely sampled surveys. This leads to uneven sampling — i.e., dense receiver and coarse source sampling or vice versa — to make these acquisitions more affordable. Motivated by the design principles of compressive sensing (CS), we perform a numerical study in which we acquire economic, randomly subsampled (or compressive), and simultaneous towed-streamer time-lapse data without the need of replicating surveys. We recover densely sampled time-lapse data on one and the same periodic grid by using a joint-recovery model that exploits shared information among different time-lapse recordings, coupled with a computationally inexpensive and scalable rank-minimization technique. The acquisition is low cost since we have subsampled measurements (about 70% subsampled) simulated with a simultaneous long-offset acquisition configuration of two source vessels traveling across a survey area at random azimuths. We analyze the performance of our proposed compressive acquisition and subsequent recovery strategy by conducting a synthetic at-scale seismic experiment on a 3D time-lapse model containing geologic features such as channel systems, dipping and faulted beds, unconformities, and a gas cloud. Our findings indicate that the insistence on replicability between surveys and the need for OBC/OBN 4D surveys can, perhaps, be relaxed. Moreover, this is a natural next step beyond the successful CS acquisition examples discussed in this special section.

Broadband marine controlled-source electromagnetic for subsalt and around salt exploration

Salt basins, mainly Tertiary basins with mobilized salt, are notoriously difficult places to explore because of the traditionally poor seismic images typically obtained around and below salt bodies. In areas where the salt structures are extremely complex, the seismic signal-to-noise ratio may still be limited and, therefore, complicate the estimation of the velocity field variations that could be used to migrate the seismic data correctly and recover a good image suitable for prospect generation. We have evaluated the results of an integrated seismic-electromagnetic (EM) two-step interpretation workflow that we applied to a broadband marine controlled-source EM (mCSEM) research survey acquired over a selected ultra-deepwater area of Espirito Santo Basin, Brazil. The presence of shallow allochthonous salt structures makes around salt and subsalt seismic depth imaging remarkably challenging. To illustrate the proposed workflow, we have concentrated on a subdomain of the mCSEM data set, in which a shallow allochthonous salt body has been interpreted before. In the first step, we applied a 3D pixel-based inversion to the mCSEM data intending to recover the first guess of the geometry and resistivity of the salt body, but also the background resistivity. As a starting model, we used a resistivity mesh given by seismic interpretation and resistivity information provided by available nearby wells. Then, we applied a structure-based inversion to the mCSEM data, in which the retrieved model in step one was used as an input. The goal of that second inversion was to recover the base of the salt interface. The top of the salt and the background resistivities remained fixed throughout the process. As a result, we were able to define better the base of the allochthonous salt body. That was reinterpreted approximately 300–700 m shallower than interpreted from narrow azimuth seismic.

Drilling closer to salt

Drilling up-dip wells, in close proximity to salt bodies is common practice in exploration for oil and gas. Imaging of sand layers close to salt however is difficult in many cases owing to the complex geometrical shape of the salt body as well as the rapid change in material properties between the sedimentary section and the salt body. In many cases this results in either drilling into the salt body by mistake, or too low in the target formations. This difficulty is very well known in the Gulf of Mexico shelf which has been a prolific oil and gas producing region for more than 70 years. Over the years, seismic data used for interpretation and prospect generation in this area has been sub-optimal in many cases. Many of the producing fields in the Gulf of Mexico shelf consist of steeply dipping hydrocarbon-bearing sands truncated against salt domes. Unfortunately, in many cases the salt bodies defining the reservoir edges are not well imaged on associated seismic data, making the accurate mapping of the producing reservoir very difficult (Foley et. al., 1991). One solution to the seismic imaging problem can be achieved by design and acquisition of new seismic data. In the past few years, more effort has taken place to acquire new data on the Gulf of Mexico shelf, but dense spacing of surface platforms makes acquiring new surface streamer data difficult. The newer data is mainly acquired using ocean bottom node technology which results in wide azimuth seismic data. The new data has the potential to have much better seismic resolution than the older narrow azimuth streamer data used by the industry for many years. In addition, nodes can be placed much closer to surface installations creating better illumination in these areas. However, because of the complexity of the geology in close proximity to salt bodies, the clear imaging of sedimentary layers near salt remains challenging even when the newest seismic data is used.