Marine Streamer Acquisition Research Articles

Seismic acquisition and imaging strategies for unlocking subsurface complexities in Malaysian basins

Seismic data acquisition and imaging technologies have made important contributions to hydrocarbon discoveries and enhancing recovery from existing reservoirs in Malaysian basins. PETRONAS has been leveraging these technologies to address the exploration and development challenges encountered in Malaysian basins and other parts of the world and to support its play-based exploration. Some of the key technology applications include imaging below shallow gas and carbonates, imaging of complex geology for exploring deep water and deep plays, and high-resolution imaging of shallow clastic plays. Dual and multimeasurement streamers, multiazimuth and full-azimuth seismic, triple- and penta-source blended acquisition, and two- and four-component seabed seismic technologies have been integrated with high-end processing and imaging technologies such as advanced demultiple techniques, deblending, full-waveform inversion, reverse time migration, and PS-converted wave imaging to address complex subsurface challenges. In this article, we present an overview of the evolution and application of innovative seismic acquisition and imaging technologies in Malaysian basins. Selected case histories demonstrate how these technologies have enabled explorers to unlock subsurface complexities, adding value to exploration and development activities. We share the advancements in 3D seismic survey design, marine streamer acquisition, seabed seismic acquisition, and seismic imaging technologies, and how an integrated approach helped PETRONAS address geologic challenges to enhance its exploration success.

Full-wavefield marine survey design using all multiples

Despite a tremendous leap in efficiency and wavefield sampling over the past two decades, it is sometimes still difficult to achieve adequate coverage and resolution with marine streamer acquisition. It is therefore necessary to carefully study the acquisition geometry, especially with respect to resolution and image quality in the crossline direction. We have extended the existing focal beam method to analyze marine streamer geometries with and without using all multiples. The focal beam analysis method provides a direct link between the acquisition parameters at the surface (number of sources, source locations, number of detectors, detector locations, and which sources are sensed by which detectors) and the image quality at a target location in the subsurface. We have further derived the concept of weighted focal beams to assess the angular aperture available for a specific acquisition configuration at the considered depth point. So far, this method has only addressed illumination by primaries. Multiples are traditionally suppressed but can instead be used in the imaging process, in which they often contribute positively by opening up the available aperture. Therefore, we have extended the focal beam analysis concept to account for multiple-reflected waves. Focal beams are more densely sampled in the ray-parameter domain when using the full wavefield (primaries and multiples). We especially addressed coverage deficiencies that can occur when the survey geometry deviates from the ideal, for example, due to feathering. As a result, extra infill lines must be acquired. We have determined how infill analysis can be performed in the depth domain via the focal beam theory. Our analysis provides the geophysical-based infill specifications to assess the impact of coverage holes on data quality. This leads to more accurate infill decisions compared with traditional common midpoint-based criteria.

An algorithm adapting encoded simultaneous-source full-waveform inversion to marine-streamer acquisition data

In recent years, the encoded simultaneous-source full-waveform inversion (ESSFWI) algorithm, which can reduce the computational cost of the individual-source full-waveform inversion (FWI), has been studied by many researchers. Although the ESSFWI technique can be successfully applied to fixed-spread acquisition data, the application of this technique to marine-streamer data is difficult because of the acquisition geometry. The difference in the acquisition geometry between the observed and modeled data produces missing trace parts in a stacked seismogram and unwanted events in the residual seismogram. To avoid these unwanted residuals, the correlation-based ESSFWI technique has been suggested by several researchers. However, the correlation-based approach unpredictably modifies the residual seismograms, which may result in distortion of the inverted velocity model. We developed an alternative approach to properly cope with the acquisition geometry problem. Our algorithm modified the offset-limited observed data to full-offset data by combining the original observed data with the modeled data. This modification led to an undistorted residual seismogram for the simultaneous multiple shot data. However, if we applied this modification process at every iteration of the inversion, the computational cost increased. To accelerate our ESSFWI algorithm, we proposed a strategy that modified the observed data at the iterations corresponding to the square numbers. Through the numerical examples, we discovered that our ESSFWI strategy better described the velocity structure of Marmousi-2 model than the correlation-based approach. Based on the comparison of computational speed-up for several strategies, we also confirmed that our ESSFWI algorithm using square numbers was a reasonable strategy. An example using real data found that our algorithm is slightly sensitive to noise, but we could minimize this sensitivity to noise by using a reasonable strategy to reduce the problem of missing traces. As a result, we could perform an effective ESSFWI even for marine-streamer acquisitions.

Application of multi‐source waveform inversion to marine streamer data using the global correlation norm

ABSTRACTConventional multi‐source waveform inversion using an objective function based on the least‐square misfit cannot be applied to marine streamer acquisition data because of inconsistent acquisition geometries between observed and modelled data. To apply the multi‐source waveform inversion to marine streamer data, we use the global correlation between observed and modelled data as an alternative objective function. The new residual seismogram derived from the global correlation norm attenuates modelled data not supported by the configuration of observed data and thus, can be applied to multi‐source waveform inversion of marine streamer data. We also show that the global correlation norm is theoretically the same as the least‐square norm of the normalized wavefield. To efficiently calculate the gradient, our method employs a back‐propagation algorithm similar to reverse‐time migration based on the adjoint‐state of the wave equation. In numerical examples, the multi‐source waveform inversion using the global correlation norm results in better inversion results for marine streamer acquisition data than the conventional approach.

Reflection moveout inversion for horizontal transverse isotropy: Accuracy, limitation, and acquisition

Horizontal transverse isotropy (HTI) is the simplest azimuthally anisotropic model used to describe vertical fracturing in an isotropic matrix. Assuming that the subsurface is laterally homogeneous, and using the elliptical variation of P-wave NMO velocity with azimuth measured in at least three different source‐to‐receiver orientations, we can estimate three key parameters of HTI media: the vertical velocity, anisotropy, and the azimuth of the symmetry axis. Such parameter estimation is sensitive to the angular separation between the survey lines in 2-D acquisition or, equivalently, to source‐to‐receiver azimuths in 3-D acquisition and the set of azimuths used in the inversion procedure. The accuracy in estimating the azimuth, in particular, is also sensitive to the strength of anisotropy, while the accuracy in resolving vertical velocity and anisotropy is about the same for any strength of anisotropy. To maximize the accuracy and stability in parameter estimation, it is best to have the azimuths for the source‐to‐receiver directions 60° apart when only three directions are used. This requirement is feasible in land seismic data acquisition where wide azimuthal coverage can be designed. In marine streamer acquisition, however, the azimuthal data coverage is limited. Multiple survey directions are necessary to achieve such wide azimuthal coverage in streamer surveys. To perform the inversion using three azimuth directions, 60° apart, an HTI layer overlain by an azimuthally isotropic overburden should have a time thickness, relative to the total time, of at least the ratio of the error in the NMO (stacking) velocity to the interval anisotropy strength of the HTI layer. Having more than three source‐to‐receiver azimuths (e.g., full azimuthal coverage), however, provides a useful data redundancy that enhances the quality of the estimates, thus allowing acceptable parameter estimation at smaller relative thicknesses.