Signal Apparition Research Articles

Multi-purpose high-resolution seismic acquisition: the deep-sea mining case

Summary The oil and gas industry has developed highly sophisticated technology for offshore hydrocarbon exploration. The traditional focus has been on hydrocarbon exploration and production targets. These targets are commonly buried under a few kilometres of sedimentary layers and 3D seismic technology has been the main type of data acquired for characterizing these targets. A secondary focus has been on the shallow section, and it has mostly been driven by shallow hazard investigations to aid the drilling of those targets. This characterization is commonly done with 2D high-resolution seismic referred to as site surveys. In recent years, shallower targets have been sought for carbon capture and storage (CCS). It is best to store carbon dioxide in its critical state which is achieved at burial depths of about 800 m. Thus, the goal is to locate porous rocks with a natural seal at depths of 800 m-1500 m below the seabed. Deeper reservoirs can be used for CCS, but shallower ones are more economical. In addition, offshore mineral exploration is at the point of becoming a commercial activity. To characterize these mineral reservoirs or deposits, the selected type of data needs to resolve the very near surface (first few decametres) at a very high resolution in an efficient way that enables the location of targets with an area extension of 100 to 300 m. Thus, in 2021 3D seismic is aimed at best resolving the very shallow and the very deep. These facts motivated the set of experiments acquired in the AM20-lab in the Norwegian Atlantic Margin in 2020. In this paper, we focus on AM20-lab test 2. While the focus of test 2 is to achieve ultra-high resolution near-surface 3D seismic for mineral exploration, the data provides multipurpose value for medium and deep targets as well. The survey was designed and acquired with a novel signal apparition decasource encoding and was benchmarked against pentasource data from a production multiclient survey which was designed for hydrocarbon exploration

Perfect partial reconstructions for multiple simultaneous sources

ABSTRACTA major focus of research in the seismic industry of the past two decades has been the acquisition and subsequent separation of seismic data using multiple sources fired simultaneously. The recently introduced method of signal apparition provides a new take on the problem by replacing the random time‐shifts usually employed to encode the different sources by fully deterministic periodic time‐shifts. In this paper, we give a mathematical proof showing that the signal apparition method results in optimally large regions in the frequency–wavenumber space where exact separation of sources is achieved. These regions are diamond shaped and we prove that using any other method of source encoding results in strictly smaller regions of exact separation. The results are valid for arbitrary number of sources. Numerical examples for different number of sources (three, respectively, four sources) demonstrate the exact recovery of these diamond‐shaped regions. The implementation of the theoretical proofs in the field is illustrated by the results of a conducted field test.

Perturbations of Time Shifts in Signal Apparition

In the field of exploration geophysics, the method known as offers a different perspective on how to separate signals acquired from simultaneously fired seismic sources. The method uses a periodic sequence of small time shift variations to encode the different sources, and this choice leads to exact separation of signals in diamond-shaped regions of the frequencywavenumber domain which are twice as large compared to those recovered by other methods. In this paper we investigate the signal apparition method's sensitivity to perturbations of the periodic time shift sequence. We model (measured or unknown) perturbations in a stochastic fashion and prove that the resulting inverse problem of separating the data is still well-posed, and we demonstrate the stability by numerical simulations on a finite-difference synthetic data set generated over a complex salt model. (Less)

Multisource encoding and decoding using the signal apparition technique

Signal apparition is a method for encoding sources in simultaneous multisource seismic acquisition and decoding the multisource response of the earth into its single-source responses. For [Formula: see text] sources, encoding is performed by applying periodic sequences of period [Formula: see text] to each of the sources along source lines. Decoding is achieved in the wavenumber domain for each frequency by solving an [Formula: see text] linear system of equations. The system’s matrix is the product of a Fourier matrix and an encoding matrix, the latter containing the information of the codes. The solution of the system is unique when the encoding matrix is invertible. When the encoding sequences consist of time delays applied to sources’ firing times, the determinant of the encoding matrix becomes a polynomial. A unique solution to decoding then exists if the roots of the polynomial avoid the unit circle. Periodic time-shift sequences for two, three, four, and six sources are discussed. A model example of simultaneous four-source data acquisition illustrates the performance of the encoding/decoding technique for the spatially nonaliased case.

Source deghosting by depth apparition

Marine seismic data are distorted by ghosts as waves propagating upward reflect downward from the sea surface. Ghosts appear on the source side and the receiver side. However, whereas the receiver-side ghost problem has been studied in detail, and many different solutions have been proposed and implemented commercially, the source-side ghost problem has remained largely unsolved with few satisfactory solutions available. We have developed a new and simple method to remove sea-surface ghosts that is related to the recently introduced concept of signal apparition. As opposed to the temporal/spatial source signature modulation functions used in the original signal apparition theory, our source deghosting method relies on using sources at different depths but not at the same lateral positions. The new method promises to be particularly suitable for 3D applications on sparse or incomplete acquisition geometries.

Full-fold, multi-source acquisition through shot isolation technology and its implications on productivity gains

Shot isolation enabled full-fold, multi-source acquisition is achieved by applying the novel technology of signal apparition to the encoding and decoding of fully superimposed shot records from non-distance separated sources. The introduction of periodically varying shot-to-shot modulation functions for encoding injects energy at predetermined positions within distinct wavenumber ranges, which then enables a deterministic decoding of individual shots in data processing. We demonstrate the method for single vessel operations, first on a dual source configuration for seabed acquisition, extracted from the SEG Advanced Modeling Program (SEAM) Phase I dataset, and then extend the methodology to triple-sources through the use of an emulated marine real data example from the North Sea. With shot isolation enabled multi-source acquisition being successfully applied in a marine and seabed seismic context, scope for significant productivity gains is created. Compared to established acquisition techniques, gains arise from the ability to acquire full-fold data in a shorter time frame without compromising the subsurface bin size. For marine seismic acquisition, this also implies that the number of towed streamers can be reduced, giving leeway to additional strategic considerations on subjects such as capital expenditures and the optimization of vessel deployment plans.

Flawless diamond separation in simultaneous source acquisition by seismic apparition

Abstract In this paper we prove that the recently introduced method of signal apparition optimally separates signals from interfering sources recorded during simultaneous source seismic data acquisition. By utilizing a periodic sequence of source signatures along one source line, that wavefield becomes separately partially visible in the spectral domain where it can be isolated from interfering signals, processed, and subtracted from the original recordings, thereby separating the wavefields from each other. Whereas other methods for simultaneous source separation can recover data in a triangle-shaped region in the spectral domain, signal apparition allows for the exact separation of data in a diamond-shaped region that is twice as large thereby enabling superior reconstruction of separated wavefields throughout the entire data bandwidth.

Signal apparition applied to towed marine simultaneous sources – a case study on synthesized real data from the Viking Graben

Encoding sources using random dithers for simultaneous source acquisition offers significant scope for enhanced towed marine operations. However, so far it has proven to be challenging to reduce the decoding residual noise from these techniques to a level that is widely accepted by the industry. The emerging technology of signal apparition offers a new approach to overcome this challenge. In contrast to established thinking, the signal apparition concept capitalizes on periodic rather than random shot-to-shot modulation functions to facilitate shot separation. The approach allows us to efficiently populate the available data space in, for instance, the frequency wavenumber domain with energy from different simultaneous sources. The introduction of periodic modulation functions in seismic acquisition produces an effect where parts of the energy of one or more sources are shifted to different empty parts within the frequency wavenumber (f-k) domain. This so-called apparated energy can then be used to perfectly predict at low frequencies the remaining part of the signal in the regions of the f-k domain where the wavefields from the different sources overlap and to deterministically separate the sources. On a synthesized real data set from the Viking Graben in the North Sea, the suitability of this concept to marine seismic simultaneous sources is demonstrated. An operational scenario for a single vessel is designed where the source arrays are excited simultaneously. Having two closely spaced simultaneous sources is a challenging case for shot separation but, if successful, such a cost-effective single-vessel configuration offers highly significant productivity gains. The wavefield decoding results analysed pre-stack and post-stack are very encouraging and indicate that the signal apparition concept indeed has the potential to deliver the high quality source decoding required for many seismic applications.

Signal apparition for simultaneous source wavefield separation

Abstract A new method for discrete sampling of signals is presented with specific applications to the reconstruction of recorded interfering wavefields from two or more sources excited simultaneously at discrete positions along lines. By utilizing a periodic sequence of source signatures along one of the source lines, the corresponding wavefield becomes separately visible in a part of the spectral domain where it can be isolated, processed and subtracted from the interfering wavefields. As a result, interfering wavefields from multiple sources recorded at a single location can be fully separated from each other. The concept is referred to as signal apparition which we suggest refers to ‘the act of becoming visible’. It may find applications in a wide range of disciplines relying on wave experimentation, such as acoustic, seismic and electromagnetic imaging of the Earth's interior for instance to significantly enhance resolution of subsurface images.