Broadband Marine Research Articles

New technologies for seismic resolution enhancement and bandwidth expansion: Applications in SE Asian Basin

Over the last few years, the petroleum giant and its partner have effectively implemented new marine seismic technologies in this part of the world. During the acquisition phase, the industry has observed the introduction of novel broadband acquisition techniques such as dual-sensor cable, multiple towed streamers (over/under), slanted and variable depth streamers, and isometric areal recording. The application of these techniques has resulted in a noteworthy enhancement of the recorded data’s bandwidth. The focus of this paper is on the improvement of seismic bandwidth expansion resulting from advancements in marine broadband acquisition, innovative processing techniques, and inversion methodology. The outcome leads to a noteworthy enhancement in the vertical resolution, which proves to be valuable in the identification of thin stacked pay beds and the creation of a reservoir model that is constrained by seismic data. This paper focuses on the imaging aspect, particularly the complex structures. The utilization of wide and full azimuth (Circular) recording and the resolution of Gas wipeout issues through seabed acquisition with Ocean Bottom Cables (OBC) have been advantageous. By utilizing de-ghosted or far-field signature modeled techniques on the processing side, it is possible to increase the bandwidth for both legacy and modern high-quality data, such as Q-marine. A novel de-convolution technique has been developed, which can enhance signal frequency to a considerable extent while minimizing or eliminating any impact on noise. The article presents various wavelet transform techniques that can effectively enhance the signal-to-noise ratio (S/N). The methods are noteworthy due to their ability to be swiftly executed post-stack while incurring minimal expenses. Further, the inversion techniques, with a focus on stochastic elastic inversion, generates broadband data, enabling the identification of pay sand that falls below the quarter of wavelength criteria. The developed techniques are novel and have been validated using authentic Offshore Malaysia data.

On electro-acoustic characteristics of a marine broadband sparker for seismic exploration

On electro-acoustic characteristics of a marine broadband sparker for seismic exploration

Three-dimensional anisotropic inversion and electrostratigraphic imaging of marine magnetotelluric data to understand the control of crustal deformations by pre-existing lithospheric structures in the Mexican Ridges Fold belt, Southwestern Gulf of Mexico

SUMMARY3-D imaging of the lithosphere in the Mexican Ridges fold belt is important for understanding how the crustal deformations in this basin relate to deep tectonic processes and structures inherited from extinct Jurassic seafloor spreading. Here, we use broad-band (0.0001–0.4642 Hz) marine magnetotelluric data from the basin to reconstruct the 3-D anisotropic resistivities of the lithosphere and their spatial gradients. The resistivity gradients maxima enabled independent definition of important geological boundaries (seen on collocated seismic reflection data) and estimation of crustal thickness. We found anomalous layered zones of low resistivity and high electrical anisotropy at 5–8 km depth (coinciding with the regional detachment zone in Eocene shales in 3-D seismic data) and in the upper mantle which we interpret as indicating intense deformation and/or recent magmatic influence. We also found a banded crystalline basement structure across the fossil spreading centre comprising WSW–ENE trending, 6–10 km wide, electrically resistive subvertical sheets with conductive and anisotropic borders, which merge into a basal resistive stock-like body at 15–20 km depth. These are cut or bounded by later NNW trending major faults. These WSW and NNW structural trends correlate with the previously interpreted transform and normal faults that formed during the Late Jurassic opening of the Gulf of Mexico only if rotated clockwise by 25–30°. Surprisingly, the rugged thrust-related seabed is offset at the projected positions of the steep resistive-conductive basement sheets (which also have spatially coincident high magnetic intensity and seismicity) enabling us to infer they represent magmatic intrusions facilitated by pre-existing faults. Their conductive borders spatially coincide with possibly fluid-filled vertical fracture-sets in the overlying sediments seen in seismic data which we interpret as hydrothermal fluid pathways. We infer that a magmatic body recently intruded our study area, its ascent controlled by pre-existing basement structures, and influenced the deformation of the Neogene sequences and the seafloor topography.

Attenuation of long period multiple using F-k filter and surface-related multiple elimination methods on 2D broadband seismic data from Morowali Waters, Sulawesi

Wide range frequency bandwidth on seismic data is a necessity due to its close relation to resolution and depth of target. High-frequency seismic waves provide high-resolution imaging that defines thin bed layers in shallow sediment, while low-frequency seismic waves can penetrate into deeper target depth. As a result of broadband seismic technology, its wide range of frequency bandwidth is a suitable geophysical exploration method in the oil and gas industry. A major obstacle that is frequently found in marine seismic data acquisition is the existence of multiples. Short period multiple and reverberation are commonly attenuated by the predictive deconvolution method on prestack data. Advanced methods are needed to suppress long period multiple in marine seismic data. The 2D broadband marine seismic data from deep Morowali Waters, Sulawesi, contains both short and long period multiples. The predictive deconvolution, which is applied to the processing sequences, successfully eliminates short period multiple on prestack data. The combination of F-k filter and Surface Related Multiple Elimination (SRME) methods are successful in attenuating long period multiple of the 2D broadband marine seismic data. The Prestack Time Migration section shows fine resolution of seismic images.

Broadband Marine Seismic Acquisition Technologies: Challenges and Opportunities

The paper addresses marine broadband seismic data acquisition technologies, including conventional seismic oil exploration (frequencies no higher than a few tens of hertz) to map the subsurface down to several kilometers and engineering seismic surveying (frequencies from a few hundred to a few thousand hertz) to characterize sediments within the upper several hundred meters below the seabed. Various methods and approaches used in the two kinds of surveying have a lot in common, although they belong to different specializations. For the first time, this problem is discussed from a single point of view. We used published materials by foreign geophysical companies and experimental data obtained with acquisition techniques that we modified for two-tiered observations. It is shown that the modified techniques are efficient in the Arctic, especially for studying permafrost in upper subseabed sediments. This paper also discusses the application of towed recording systems and ocean floor multicomponent streamer cables and stations, including those using fiber optic technologies. Fiber optic receiver systems are most efficient for seismic time-lapse monitoring of oil and gas reservoirs during production (4D seismic). Analysis of the common problems with conventional and high-frequency marine seismic surveys allows the conclusion that it is time to revise the long-standing concept of marine seismic data acquisition. One possible way is to develop an integrated system for the acquisition, processing, and interpretation of seismic data in all frequency ranges. Our findings can aid in developing a methodology for geophysical surveying in hard-to-reach water areas.

Hunting Altitude of Eleonora's Falcon (Falco eleonorae) Over a Breeding Colony

We investigated the flight altitude of hunting Eleonora's Falcons (Falco eleonorae) around their colony near the island of Crete (Greece). We used a broadband marine surveillance radar positioned on the coast of Crete and detected the falcons’ movements during the breeding period. The birds were monitored using a Java application that enabled us to manually track radar targets directly on the radar screen and transfer the data into a GIS environment. Thirty hours of radar monitoring between 14 September and 22 September 2017 produced 4774 records of the species (1218 over inland and 3556 over the sea). Hunting falcons flew on average at 1292 ± 11 masl (range = 17–3475, median flight altitude 1156 m), though 70% of them were tracked flying between 17 and 1750 m. The range of flight altitude recorded was greater than that previously published, and was consistent with the prey selection during the nestling-rearing period, i.e., migratory passerines. Eleonora's Falcons hunted more frequently over the sea than over the mainland and at a higher altitudes during the morning than at midday or in the afternoon. The daily flight pattern probably reflected the higher intensity of passerine migration in the early morning and late afternoon, as well as an opportunistic prey shift from birds to insects during midday when passerine migration is weak. Compared to other techniques previously used for tracking this species, namely visual observations (1970s), optical range finder (1990s), and satellite telemetry (2000s), broadband marine radar is not limited by small sample size, geolocation error, weather conditions, or poor visibility at high altitudes.

Broadband Marine Towed Streamer Acquisition; South China Sea Case Study

The following paper details the analysis of a 2D broadband marine seismic project acquired offshore China. The deployment configuration included overunder sources and three streamers towed at different depths(5, 17 and 23m). This specific configuration allows for analysis of dual tow depth as a method for increasing bandwidth but also allows for evaluation of two distinct dual tow depth combination solutions over/under and sparse/under. This paper will review the theory behind the two combination techniques, compare the seismic datasets, and finalize with some conclusions on the relative benefits both in terms of seismic imaging and acquisition efficiency.

DISCover - Broadband Towed Streamer Acquisition

In this paper we present a new method for broadband marine acquisition and processing capable of imaging deep targets while maintaining full high frequency bandwidth data. The method uses a deep interpolated streamer coverage approach (DISCover), which comprises a conventional 3D shallow towed-streamer spread, and a deep streamer spread which is more sparsely sampled in the crossline direction. The measurement from the shallow streamer plane is complemented with a low frequency limited measurement from the deep streamer plane; In essence boosting the low frequencies of the shallow streamer measurement, which are heavily attenuated by the shallow tow ghost response. Because the deep streamers are only going to provide the low-frequency part of the bandwidth, we can more sparsely sample these data thus enabling efficient acquisition scenarios as fewer streamers are required. The data are combined in processing, optimizing the signal-to-noise ratio over the entire bandwidth. The resulting data exhibit both high resolution and deep penetration, characteristics suitable for deep imaging below attenuative overburdens. Velocity model building, imaging and impedance inversion also benefit from the broadband result. In addition, data acquired in this way are more robust to poor weather conditions than conventionally acquired data. Data from a 3D case study using this new acquisition method were acquired off the NW Shelf of Australia. The streamer spread consisted of six shallow streamers towed at a depth of 6 m and two deeper streamers (below shallow streamers 2 and 5) towed at a depth of 20 m. The correct relative position of the streamers was controlled by means of streamer steering. A novel over/under source design was also used to bias source output toward low frequencies and further enhance the low-frequency signal-to-noise ratio of the acquired data.

Design of Marine Broadband System for Beach front Angling and Its Applications

Design of Marine Broadband System for Beach front Angling and Its Applications

Integration of broadband seismic data into reservoir characterization workflows: A case study from the Campos Basin, Brazil

Towed-streamer marine broadband data have been key contributors to recent petroleum exploration history, in new frontiers and in mature basins around the world. They have improved the characterization of reservoirs by reducing the uncertainty in structural and stratigraphic interpretation and by providing more quantitative estimates of reservoir properties. Dedicated acquisition, processing, and quality control (QC) methods have been developed to capitalize on the broad bandwidth of the data and allow their rapid integration into reservoir models. Using a variable-depth steamer data set acquired in the Campos Basin, Brazil, we determine that particular care that should be taken when processing and inverting broadband data to realize their full potential for reservoir interpretation and uncertainty management in the reservoir model. In particular, we determine the QC implemented and interpretative processing approach used to monitor data improvements during processing and preconditioning for elastic inversion. In addition, we evaluate the importance of properly modeling the low frequencies during wavelet estimation. We find the benefits of carefully processed broadband data for structural interpretation and describe the application of acoustic and elastic inversions cascaded with Bayesian lithofacies classification, to provide clear interpretative products with which we were able to demonstrate a reduction in the uncertainty of the prediction and characterization of Santonian oil sandstones in the Campos Basin.

A bigger picture view of separated wavefields and marine broadband seismic

The advent of dual-sensor recording for towed seismic streamers enabled the true separation of the up-going and down-going pressure wavefields (Carlson et al., 2007), and heralded the ‘broadband’ seismic revolution seen in marine seismic exploration over the past decade (Widmaier et al., 2015). In reality, ‘broadband’ typically means deghosting accompanied by some additional forms of spectral conditioning. In most cases, the post-stack interpretation of deghosted data is enhanced by improved geological texture, improved event coherence and deeper signal penetration. Recent attention has focused on the benefits of low frequencies in broadband seismic data (ten Kroode et al., 2013), and indeed where ultra-low frequency phase integrity is preserved as a complement to accurate amplitude-versus-angle fidelity, deghosted data also enables more accurate pre-stack quantitative interpretation (Reiser et al., 2015a, 2015b). However, it has also become clear that some long-standing imperfections in the seismic method remain. The rapid decay in air gun output below about 7 Hz (Parkes and Hegna, 2011) is not satisfactorily addressed by deghosting, compensation for high-frequency attenuation remains a fundamental challenge, and traditional challenges to wavelet processing, denoise, multiple removal, velocity estimation and imaging may in fact be more complicated for some broadband methods, or unaffected for others. The key benefit of dual-sensor methods are accurate access to the various separated wavefields, in addition to enhanced recoverable frequency bandwidth. Reservoir monitoring is enhanced by the elimination of the down-going pressure wavefield from 4D differencing, reservoir illumination can be enhanced by the incorporation of the down-going pressure wavefield into wave theoretic imaging, reflectivity inversion can be enhanced by isolating the up-going pressure wavefield in imaging, and overall we expect to see ‘complete wavefield’ reservoir imaging and characterization solutions mature rapidly. Furthermore, increased focus on spatial frequency content courtesy of ‘wave equation inversion’ imaging solutions is also taking broadband seismic past the historical focus upon only temporal frequency content.

Variable source depth acquisition for (an overall) improved signal-to-noise ratio in marine broadband seismic data: A modeling study

Marine seismic data acquisition has conventionally been carried out with source arrays positioned at a single depth. The resulting source-ghost notch is often taken as the limit for the bandwidth of the data. In broadband seismic data acquisition, multilevel sources (MLSs) are now being used to fill in these notches. Here we present a modeling study in which variable source-depth acquisition (VSDA) is compared with conventional single-level source and MLS acquisition. Our results indicate that an overall (not all over) improvement of signal-to-noise ratio is obtained using VSDA compared with the other source strategies. Due to the nature of VSDA, having different far-field signatures for different source depths, the amplitude variation with offset behavior of the processed VSDA data is not ideal. In stacked images, these variations are mostly seen in very shallow reflectors ([Formula: see text]).

Back to basics on broadband seismic amplitudes, phase and resolution

Marine broadband seismic results typically have observed amplitude spectra that are inconsistent with the earth reflectivity spectra measured from spatially coincident well data. The most notable inconsistency is a visual bias towards stronger ultra-low frequency amplitudes in the 0–10 Hz range on seismic data. Whilst useful in principle for more accurate elastic seismic inversion with less dependence upon low frequency model (LFM) building and for more stable full waveform inversion (FWI), the low frequency bias will degrade seismic resolution if not balanced properly during processing. Furthermore, the observed seismic amplitude spectra on pre-stack data vary with increasing offset and the phase can strongly vary in a frequency-dependent manner. These variations are attributed to three first-order processes: 1. Progressive attenuation of higher frequencies with longer travel paths, 2. NMO stretch, and 3. Offset-dependent tuning, and may easily be compounded by inappropriate parameterization of various processing and imaging steps. A key component of elastic seismic inversion is the extraction and scaling of several angle-dependent and depth-dependent wavelets. These wavelets are ideally independent of the aforementioned processes but inevitably they are not, and the angle ranges used in the inversion impact the significance of NMO stretch and offset-dependent tuning. Even if these various considerations can all be accommodated during processing, the resolution of seismic images is limited by the resolution of the velocity model, the scattering assumptions of the imaging algorithms used, and the a priori information used to constrain imaging and inversion. After reviewing the principles of seismic wavefield propagation in the contemporary context of broadband seismic methods, largely pursued with the ambition of removing sea surface-related ghost effects, I discuss the additional uncertainties introduced into seismic signal processing by broader bandwidth data — notably at the low frequency end. I consider two rather different ‘broadband seismic’ perspectives: 1. The industry must progress towards higher fidelity and resource-consuming measurements of every source event and every receiver measurement in order to effectively deconvolve the ‘system response’ from the data, or, 2. High-end imaging solutions can automatically eliminate the ‘source term’ without requiring detailed source information — assuming that wavefield separation has robustly accounted for dynamic variations in receiver geometry. A longer term consideration of imaging suggests that the classical sequential processing paradigm is in fact dead, that the definition of ‘noise’ is changing, and that advances in hardware are enabling solutions to long-standing challenges with cross-talk artifacts and irregular illumination. The addition of appropriate a priori information into joint migration and inversion allows historical assumptions about the unknown velocity model having a smooth background to be dismissed, and step changes in velocity model resolution may be achievable. I conclude by discussing how higher resolution velocity models will translate to less non-physical efforts in processing that can corrupt pre-stack amplitude, pre-stack phase and (elastic) image resolution.

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.

Estimation of source signatures from air guns fired at various depths: A field test of the source scaling law

Recent advances in marine broadband seismic data acquisition have led to a range of new air-gun source configurations. The air-gun arrays have conventionally been kept at a constant depth, but to attenuate the source-side ghost reflection, new source strategies involving multiple source depths have been proposed. The bubble-time period for an air-gun bubble is dependent on, among many parameters, the firing depth. We use quasi near-field measurements of air-gun signatures to validate a version of the well-known source scaling law in which the characteristic bubble-time period is used as the scale. We find that the source scaling law can be used to estimate a source signature from one depth knowing the source signature at a different depth from the same gun. Furthermore, we derive a correction term to the Rayleigh-Willis bubble-time equation to correct for the fact that interaction between the bubble and free surface reduces the bubble-time period. This correction term improves our results significantly for air guns positioned close to the air-water interface. The error between the estimated and measured source signatures is dependent on the difference in source depth. For a depth difference of [Formula: see text], we estimate signatures that have NRMS differences ranging between 5% and 6% from the measured signature at the given depth and between 8% and 12% when the difference is [Formula: see text].

Full waveform inversion comparison of conventional and broadband marine seismic streamer data, NW Shelf Australia

In this paper, we illustrate the application of frequency- domain FWI to two 2D data sets acquired simultaneously offshore North West Australia (NWA); one data set was recorded with a conventional streamer, and the other with the use of variable depth streamer configuration. First we discuss the two data sets, and our frequency-domain FWI algorithm. We then present each FWI result, and discuss the superiority of the broadband FWI results obtained with the variable depth streamer data. SUMMARY The lack of low-frequency information in conventional marine seismic streamer data inhibits the success of frequency-domain full waveform inversion (FWI). Low frequencies are typically absent in marine seismic data due to the low-cut spectral responses of airgun sources and hydrophone receivers, and the fact that the air-water interface produces source and receiver ghost reflections which create notch frequencies in the data amplitude spectrum. Advances in broadband streamer acquisition, such as the variable depth towed streamer, allow us to extend the low and high bounds of the useful frequency bandwidth in the seismic data spectrum. We illustrate the application of frequency-domain FWI to two 2D seismic data sets acquired simultaneously offshore North West Australia. Both data sets were acquired together, one with a conventional streamer, and the other with a variable depth streamer configuration. Our examples demonstrate that the FWI results are clearly superior when using the broadband variable depth streamer data, compared to using the conventional streamer data.

Reservoir property estimation using only dual-sensor seismic data – a case study from the West of Shetlands, UKCS

Marine broadband towed streamer seismic analysis and case studies in recent years have revealed several benefits of extended seismic bandwidth for the reservoir geoscientist – both for structural and quantitative seismic interpretation. Since 2007, dual-sensor towed streamer seismic technology (Tenghamn et al., 2007) has provided industry-wide access to seismic data with a significantly broader range of low and high frequencies. Benefits on the interpretation side include improved vertical resolution, enhanced geological texture, cleaner event continuity and character, sharper fault plane truncations and improved seismic-to-well correlations. The key benefit for quantitative seismic interpretation (QI) is a significant reduction of the low frequency model requirement and this allows seismic inversion to be derived from the seismic data rather than from a model extrapolation of a priori information. In addition, the quality and prediction of the reservoir properties derived from seismic data, without well log information, has significantly improved – resulting in the potential de-risking of prospects and discoveries. This paper describes the data analysis of dual-sensor 3D seismic from the West of Shetlands area and specifically focuses on wavelet extraction and low frequency phase stability. The study explores the relevance of these aspects to the inversion and investigates the estimation of reservoir properties, including porosity prediction, without the direct use of well information. The match of results from seismic only inversion to ‘blind wells’ demonstrates the potential of dual-sensor pre-stack data to reliably estimate elastic and reservoir properties in an exploration setting. Estimating reliable absolute reservoir properties away from wells has been a continuous challenge for reservoir geoscientists. Where it can be achieved, the value of the information can be significant in de-risking an opportunity and/or better characterising a prospect. In addition, in appraisal, development and reservoir optimisation, any reliable elastic information extracted from seismic data can potentially assist in optimising a well location and its trajectory for maximum recovery.

Single-sensor density estimation of highly broadband marine mammal calls

Odontocete echolocation clicks have been used as a preferred cue for density estimation studies from single-sensor data sets. Such sounds are broadband in nature, with 10-dB bandwidths of 20 to 40 kHz or more. Estimating their detection probability is one of the main requirements of density estimation studies. For single-sensor data, detection probability is estimated using the sonar equation to simulate received signal-to-noise ratio of thousands of click realizations. A major problem with such an approach is that the passive sonar equation is a continuous-wave (CW) analysis tool (single-frequencies). Using CW analysis with a click’s center frequency while disregarding its bandwidth has been shown to introduce bias to detection probabilities and hence to population estimates. In this study, the methodology used to estimate detection probabilities is re-evaluated, and the bias in sonar equation density estimates is quantified by using a synthetic data set. A new approach based on the calculation of arrivals and subsequent convolution with a click source function is also presented. Application of the new approach to the synthetic data set showed accurate results. Further complexities of density estimation studies are illustrated with a data set containing highly broadband false killer whale (Pseudorca crassidens) clicks. [Work supported by ONR.]

Marine broadband technology: History and remaining challenges from an end-user perspective

Broadband marine seismic data is evolving as the new standard in petroleum geoscience. Much of the dialogue to date has been focused on the seismic acquisition systems and deghosting aspects of this technology. The first requirement of any broadband project should be to “do not harm.” This is especially true in the acquisition arena, since the penalty for deficient data is significant. A reasonable objective would be to demonstrate equal, or better, data quality than conventional “high-resolution” shallow-tow surveys, increased operational efficiency, and overall improved signal/noise compared to processing-only-based broadband solutions. It is probably not a reasonable expectation that a single acquisition system always will provide more value/cost benefit than alternative options. A simple metric for the effectiveness of any broadband technique is demonstration that better results can be achieved over more conventional processing flows. An understanding of data fidelity and signal attenuation (Q) is essential in generating the most usable end products. Key criteria for successful final deliverables include a well-balanced spectrum not suffering from resolution loss or excessive noise and minimal artifacts from the deghosting process. It is important to continue the rigorous debate over which specific implementation results in the best solution for a specific situation. A clear understanding of end-user expectations and workflows is essential in achieving this goal.

Variable source depth acquisition for improved marine broadband seismic data

In marine seismic data acquisition, varying the source depth along a sail line gives diversity in sequential shot gather frequency spectra. Undesired alterations of the frequency spectra are created by the source ghost and by air-gun bubble oscillations. By deliberately varying the source depth along a sail line, it is possible to obtain a seismic data set that will have energy more evenly distributed within the main frequency band of the source output. This is obtained when data acquired with different source depths are stacked in imaging. We formulated a simple inverse problem that seeks to find the optimal distribution of source depths over a sequential series of shots that shape the amplitude spectrum of the final image into a desired shape. We assumed that the data are receiver-side deghosted, that designature could be applied to each shot gather, and that the shot gathers could be redatumed to a common datum prior to imaging.