Technical Library

A robust FWI gradient for highresolution velocity model building
Author: Jaime RamosMartinez, Sean Crawley, Zuihong Zou, Alejandro Valenciano, Elena Klochikhina, Nizar Chemingui
SEG  1 October 2016We describe a robust method to produce longwavelength updates in gradientbased Full Waveform Inversion (FWI). The gradient is computed by applying dynamic weights in the velocity sensitivity kernel derived from impedance and velocity parameterization of the classical objective function. The new kernel implementation effectively eliminates the migration isochrones produced by the specular reflections and emphasizes the lowwavenumber components in the gradient in heterogeneous media. The new gradient is able to provide velocity updates beyond penetration depth of diving waves. We use a synthetic example to illustrate how this dynamically weighted FWI gradient successfully recovers the background velocity from precritical reflections. We apply the new approach to 2D and 3D dual sensor data from deepwater Gulf of Mexico. Results show how the dynamically weighted FWI gradient can combine both transmitted and reflected energy in a global FWI scheme and provide highresolution velocity models without migration imprint in the updates.

Quantification of the Resolution in the Inversion results from Towed Streamer EM Data
Author: Joel Skogman, Johan Mattsson,
SEG  1 October 2016The resolution of resistivity models from 3D inversion of Towed Streamer EM data is analyzed using the resolution matrix. In particular, a new resolution length derived from the resolution matrix is introduced. This measure is obtained by dividing the volume of an inversion cell with the corresponding diagonal element in the resolution matrix. Hence, the measure is independent on the actual cell discretization of the inversion domain. Resolution lengths are calculated both for a hypothetical case with noise free data and for resistivity sections obtained with regularized 3D inversion. The resolution of the ideal case with no noise constitutes the upper limit of the best possible resolving power of the Towed Streamer EM data. This limit is then compared against the results where smoothing regularization has been applied in the 3D inversion. A comparison is illustrated for inversion results of Towed Steamer EM data from a survey line in the Barents Sea.

Sparsity Promoting Morphological Decomposition for Coherent Noise Suppression: Application to Streamer Vibration Related Noise
Author: Pierre Turquais, Endrias Asgedom, Walter Söllner
SEG  1 October 2016We address the signal and noise separation problem where the noise is coherent. We use a dictionary learning method to learn a dictionary of unit vectors called atoms; each one representing an elementary waveform redundant in the noisy data. In such a learned dictionary, some atoms represent signal waveforms while others represent noise waveforms. Using a multivariate Gaussian classifier trained on a noise recording, the atoms representing noise waveforms are discriminated and separated from the atoms representing seismic waveforms and two subdictionaries are created; one describing the morphology of the signal, the other describing the morphology of the noise. Using these subdictionaries, a morphological component analysis problem is set to separate the seismic signal and the coherent noise. In contrast to fixing transforms for representing the noise and the signal, our method is entirely adapting to the morphology of the signal and the noise. We present an application for removing streamer vibration related noise and show successful denoising results on synthetic and field data examples.

Postmigration image optimization: a Gulf of Mexico case study
Author: Sergey Frolov, Samuel Brown, Sriram Arasanipalai, Nizar Chemingui
SEG  1 October 2016There are many factors which affect the quality of a subsurface image: acquisition geometry, data quality, accuracy of the subsurface model, imaging algorithm, image postprocessing steps, etc. Despite improvements in seismic acquisition, such as full azimuth (FAZ) surveys, and advanced imaging algorithms such as reverse time migration (RTM), focusing energy beneath a complex salt overburden remains a challenging problem. We propose a postimaging workflow to mitigate migration artefacts, limited illumination problems, and velocity model inaccuracies that cause image distortion. RTM images derived with an inverse scattering imaging condition are decomposed into angle/azimuth domain gathers. The angles and azimuths which constructively interfere at each image point to produce the best image are chosen so as to remove both coherent and incoherent noise and produce an optimal stack. Finally, structureconformable filtering is applied. We apply this workflow to an RTM image from Garden Banks in the Gulf of Mexico .

Adaptive multiple subtraction by statistical curvelet matching
Author: Truong Nguyen, Richard Dyer
SEG  1 October 2016In this paper, we present a novel algorithm that performs adaptive subtraction of a multiple model in the curvelet domain. The algorithm is based on the observation that the predicted model is an imperfect estimate of the actual multiples, containing two types of error: 1. a systematic error that manifests as an approximately constant phase and amplitude error within each subband and 2. a localized error that potentially varies from coefficient to coefficient. Adaptation of the model is automatically controlled by parameters provided by a statistical modelling step. Results show that the algorithm works well with different types of multiples and levels of noise.

Imaging the total wavefields by reflectivity inversion using amplitudenormalized wavefield decomposition: field data example
Author: Alba Ordoñez, University of Oslo, Walter Söllner, Tilman Klüver
SEG  1 October 2016Based on an example acquired with dualsensor towedstreamers and time and depth distributed sources, we image the total upand downgoing wavefields using primaries and multiples. The imaging framework is based on computing the subsurface impulse response (i.e., reflectivity). At every depth level, the latter can be obtained by inverting the matrix form of an integral equation defined in terms of the amplitudenormalized upgoing pressure and downgoing vertical velocity wavefields. This procedure gives the reflectivity matrix. The total upgoing wavefield used in the imaging scheme is composed of the scattered energy from primaries and multiples. The primary reflected wavefield is generated by a direct downgoing source wavefield, which is mostly passing the acquisition surface at offsets smaller than the nearest data channel. Hence, the most relevant part of the direct wavefield is not measured in the studied example. From the nearfield pressure measurements, we predict the missing direct arrivals needed to image the primary reflected wavefield; and we then synthesize the total downgoing wavefield by adding the downgoing scattered energy. By downward extrapolating the total up and downgoing wavefields, the information of the subsurface is extracted from the reflectivity matrix in the spatial and angular domains.

A practical crosstalk attenuation method for separated wavefield imaging
Author: Shaoping Lu, Dan Whitmore, Alejandro Valenciano, Nizar Chemingui, Grunde Rønholt
SEG  1 October 2016Images of primaries and multiples can be contaminated by erroneous phantom images caused by the crosstalk of multiply scattered events. Using a causality property of the events in separated up and downgoing wavefields, we present a practical crosstalk prediction and attenuation method when imaging using surface reflected wavefields. Our method does not explicitly separate multiples of any specified order. Through both synthetic and field data examples, we show how the proposed method reliably predicts the crosstalk in separated wavefield imaging. Consequently, it provides higher quality images with balanced amplitudes and improved illumination.

Full waveform inversion with steerable variation
Author: Lingyun Qiu, Kathy Zou, Alejandro Valenciano, Nizar Chemingui
SEG  1 October 2016We propose a new regularization scheme for Full Waveform Inversion (FWI). The new method makes use of a priori information on the spatial variability of the earth model to overcome the limitations of the inversion in the presence of high velocity contrast geobodies and cycle skipping. It comprises two additional regularization terms to the FWI objective function. The first term evaluates the L1 norm of total variation (TV) of the model, while the second term steers the solution based on local prior information of the model spatial variability. Both regularization terms can be made spatially variant to accommodate different geological features in the model, i.e. sediments (smooth changes), salt bodies (piecewise constant). Our procedure makes use of the split Bregman iterations, an effective algorithm for solving the L1 optimization problems. The result is a computationally efficient and accurate implementation. We show the potential of the method by using the BP 2004 velocity benchmark model. There, our regularization scheme allows the inversion to start from a simple velocity model and delivers a highquality reconstruction of salt bodies.

Advances in broadband quantitative interpretation
Author: Cyrille Reiser, Tim Bird
SEG  1 October 2016For almost 10 years now marine broadband seismic has provided the industry access to seismic data with a significant increase of seismic frequency bandwidth on both the low side of the frequency spectra and also on the high side. Seismic analysis and case studies in recent years using broadband seismic have revealed several benefits of broader seismic bandwidth for reservoir geoscientists  both for structuralstratigraphic interpretation and for quantitative seismic reservoir characterization and properties estimation. Prestack seismic inversion has been an excellent means to derive the full benefits and the value of acquired broadband prestack seismic for seismic reservoir characterisation and reservoir imaging.
This paper presents case studies of recent analysis at various stages of the explorationproduction asset life as well as some potential pitfalls to be aware of when using prestack broadband seismic data for improved target delineation, estimation of reservoir properties and ultimately derisking of a prospect or well positioning. A case study for a shallow reservoir in a frontier exploration setting will also demonstrate some recent developments in the integration of Full Waveform Inversion and imaging using multiples for an improved low frequency model and more reliable prestack seismic inversion.

Extending the 3D primary image with multiples and mirrors
Author: Dan Whitmore
SEG  1 October 2016This presentation reviews imaging technologies applied to both primaries and multiples, with particular emphasis on the value of multiples in wide azimuth scenarios. Included is a discussion of imaging principles, extraction of offset and angle dependent measurements, and mitigation of crosstalk. Whether multiples are considered as a replacement for unrecorded primaries, or complementary (or redundant) measurements to primaries, a more complete image of the subsurface can be produced by including both primaries and multiples in the imaging process. In some cases, the multiples image is superior to the primaries. Examples show the additional value multiples contribute in 3D wide azimuth scenarios: multistreamer surface acquisition, ocean bottom seismic and 3D VSP’s. Included is a prognosis for future  including advances in imaging methods and the tradeoffs of acquisition sampling (cost) and image quality.