I. Vasconcelos

Full-wavefield, Towed-marine Seismic Acquisition and Applications

A four-component (4C) streamer recording pressure as well as the three-component particle velocity vector, addresses long-standing geophysical problems such as receiver-side sampling and deghosting. In this paper, we introduce multicomponent marine seismic sources generating monopole and dipole responses in the water. We describe a few different alternatives for generating such a source using existing technology. Three different application areas are described in some detail: source-side deghosting, source-side wavefield reconstruction and, finally, a vector-acoustic reverse-time imaging approach that requires monopole and dipole data on both the source and receiver side of the acquisition.

Marchenko Imaging of Volve Field, North Sea

Marchenko redatuming estimates the full response (including internal multiples) from a virtual source inside of an medium, using only reflection measurements at the Earth’s surface and a smooth estimate of the velocity model. As such, it forms a new way to obtain full field propagators to form images of target zones in the subsurface by means of Marchenko imaging, without necessarily have to create detailed models of overburden structure. One of the main obstacles to the application of such novel techniques to field datasets is the set of requirements of the reflection response: it should be wideband, acquired with wide aperture, densely sampled arrays of co-located sources and receivers, and should have undergone removal of direct waves, source and receiver ghosts and free-surface multiples. We use a wave-equation approach to jointly redatum, demultiple, and source designature to transform data recorded using ocean-bottom acquisition systems into a suitable proxy of the reflection response required by the Marchenko scheme. We briefly review the Marchenko redatuming scheme, and present the first encouraging field results of 2D target-oriented imaging of an ocean-bottom cable dataset, acquired over the Volve field. We further discuss the ‘challenge of convergence’ of the Marchenko redatuming scheme for real data.

An interferometric interpretation of Marchenko redatuming

Recently, an iterative scheme was introduced to retrieve up- and downgoing Green’s functions at an arbitrary location F in the subsurface. The scheme uses the reflection data as acquired at the surface as input, together with an estimate of the direct arrival from the surface to location F, which is referred to as the initial focusing function. We interpret the overall action of the scheme as the successive actions of various linear filters, acting on the initial focusing function. These filters involve multidimensional crosscorrelations with the reflection response, time reversals and truncations in time. Inspired by literature on seismic interferometry, we interpret multidimensional crosscorrelation in terms of the subtraction of traveltimes along stationary raypaths. The scheme has been designed for layered media with smooth interfaces. Our interferometric interpretation reveals some of the scheme’s limitations when it is applied to more complex configurations. It can be concluded that (downgoing or upgoing) internal multiples that arrive at F with a particular angle can be retrieved only if the initial focusing function (i.e., the direct wave) has visited F with this angle. Consequently, shadow zones that cannot be imaged with primary reflections can theoretically also not be imaged with internal multiples, when the current iterative scheme is used for their retrieval. Finally, we observe that the current scheme does not yet optimally perform in media with point scatterers, since an underlying assumption (generally referred to as the ansatz) is not perfectly obeyed in this case. It is envisioned that this can be improved if truncations in time that are implemented after each iteration are replaced by more advanced filtering methods.

Reverse-time Imaging of Dual-source 4C Marine Seismic Data Using Primaries, Ghosts, and Multiples

Novel technologies in marine seismic data acquisition allow for the recording of full vectoracoustic (VA) data: four-component recordings (pressure and three-component acceleration) of fields excited both by pressure-only as well as dipole/gradient sources. Here we present a wave-equation, nonlinear, reverse-time imaging method that takes full advantage of acoustic dual-source (monopole and dipole sources) multi-component data. The method’s formulation relies on source-receiver scattering reciprocity relations that make proper use of VA fields both in the wavefield extrapolation and imaging condition steps in a self-consistent manner. The new VA imaging method is capable of simultaneously focusing energy from upgoing and both receiver- and sourceside ghost arrivals, while jointly using information fromprimaries and multiples contained in each of these fields. Additionally, VA imaging handles image amplitudes better than conventional reversetime migration because it does not require far-field radiation assumptions, properly handling finite-aperture directivity information directly from dual-source four-component data. While the method does not require any deghosting as a preprocessing step, it can use previously separated up and downgoing fields to generate independent subsurface images. This allows for new approaches in the design of marine seismic acquisition. We demonstrate the method using synthetic examples, which include complex stratigraphic and structural subsurface features.

A Practical Approach to Vector-acoustic Imaging of Primaries and Free-surface Multiples

Free-surface multiples travel different paths and illuminate different volumes of the subsurface than primaries. When used jointly with primaries to image the subsurface by means of forward and backward extrapolation of separated down- and up-going wave components respectively, free-surface multiples have been shown to improve the continuity of shallow parts of the subsurface image by suppressing acquisition related footprints. We show that by carefully combining the full pressure and particle velocity data by means of newly developed, vector-acoustic boundary conditions, wavefronts can be forward and backward propagated without ambiguity in their propagation direction. Wavefield decomposition is thus naturally incorporated within the extrapolation procedure. Moreover, ocean-bottom acquisition geometries generally present source coverage that is wider than the receiver array. A strategy is proposed to incorporate in our imaging scheme energy of primary events whose direct source illumination lies outside of the receiver aperture. This is achieved by combining a directly modelled source illumination with the recorded (down-going) data.

Marchenko imaging below an overburden with random scatterers

Marchenko imaging is a new way to deal with internal multiple scattering in migration. It has been designed for layered media with smooth interfaces. Here we analyze the performance of the Marchenko scheme for a medium with many point scatterers. Although the conditions for Marchenko imaging are violated, we observe from a numerical experiment that the signal-to-noise ratio of the obtained image is significantly higher than with standard imaging.

Retrieving Reservoir-only Reflection and Transmission Responses from Target-enclosing Extended Images

The Marchenko redatuming approach reconstructs wavefields at depth that contain not only primary reflections, but also multiply-scattered waves. While such fields in principle contain additional subsurface information, conventional imaging approaches cannot tap into the information encoded in internal multiples in a trivial manner. We discuss a new approach that uses the full information contained in Marchenko-redatumed fields, whose output are local reflection and transmission responses that fully enclose a target volume at depth, without contributions from over- or under-burden structures. To obtain the Target-Enclosing Extended Images (TEEIs) we solve a multi-dimensional deconvolution (MDD) problem that can be severely ill-posed, so we offer stable estimates to the MDD problem that rely on the physics of the Marchenko scheme. We validate our method on ocean-bottom field data from the North Sea. In our field data example, we show that the TEEIs can be used for reservoir-targeted imaging using reflection and, for the first time, local transmission responses, shown to be the direct by-product of using internal multiples in the redatuming scheme. Finally, we present local, TEEI-derived reflection and transmission images of the target volume at depth that are structurally consistent with a benchmark image from conventional migration of surface data.

Full-wavefield Redatuming of Perturbed Fields with the Marchenko Method

Wavefield extrapolation, or redatuming, is a critical step for imaging. It is particularly challenging in areas such as subsalt or under complex overburdens. The framework of Marchenko redatuming allows for the retrieval of up- and downgoing fields at chosen locations in the subsurface that contain primary arrivals and internal multiples, while requiring relatively little knowledge of the subsurface model. In this paper, we present a new form of the Marchenko system for perturbed fields. Based on this system, we present a new iterative scheme that explicitly reconstructs only the unknown perturbations to the Marchenko focusing functions, and by consequence only the perturbed/scattered up- and downgoing Green’s functions. This new scheme departs from previous versions of the method in that it requires additional inputs, which include an extra initial focusing operator and perturbations to the surface reflection data. We validate our method with numerical tests, showing that it is particularly well-suited to properly handle complex models with large/sharp constrasts such as salt boundaries. We foresee this new approach to be of use not only in general imaging applications, but also for time-lapse studies as it can directly redatum time-lapse changes.

An Interferometry-Based, Subsurface-Domain Objective Function for Targeted Waveform Inversion

Estimating reservoir parameters from surface data is a challenging problem, particularly when the reservoir is seated beneath a complex overburden whose properties are also unknown. Here, we present a new metric for detecting and quantifying errors in subsurface models that is well-suited for target-oriented inversion. We refer to this metric as an ``interferometric misfit because it relies on wavefield extrapolation from both convolution- and correlation-type reciprocity used in seismic interferometry. When the source point is outside the volume, both forward- or reverse-time extrapolation produce the same field. Although their results are the same, the physical interactions between the components of the data and of the extrapolators are different in forward or reverse time. Because of this, the forward- and reverse-time extrapolated fields are only equal when the model used is consistent with the real subsurface model. We thus use the difference between the forward- and reverse-time extrapolated fields to define a subsurface-domain metric that quantifies model errors. This misfit relies on full-waveform information from data and is nonlinear on the medium parameters. Because it is designed to sense medium parameters only inside a target volume of interest, our approach comprises a metric for target-oriented inversion in the subsurface domain.