Making the transition from discrete shot records to continuous seismic records and source wavefields, and its potential impact on survey efficiency and environmental footprint
Author: Stian Hegna, Tilman Kluver, Jostein Lima, Jens Wisløff
Geophysical Prospecting - 1 July 2019
A marine seismic method based on continuous source and receiver wavefields has
been developed. The method requires continuous recording of the seismic data. The
source that may consist of multiple source elements can emit signals continuously
while moving. The ideal source wavefield to be used with this method should be as
white as possible both in a temporal and a spatial sense to avoid deep notches in the
spectrum enabling a stable multi-dimensional deconvolution. White noise has such
properties. However, equipment that can generate white noise does not exist. In order
to generate a continuous source wavefield that is approaching the properties of white
noise using existing equipment onboard marine seismic vessels, individual air-guns
can be triggered with short randomized time intervals in a near-continuous fashion.
The main potential benefits with the method are to reduce the environmental impact
of marine seismic surveys and to improve acquisition efficiency. The peak sound
pressure levels are significantly reduced by triggering one air-gun at a time compared
to conventional marine seismic sources. Sound exposure levels are also reduced in
most directions. Since the method is based on continuous recording of seismic data
and the air-guns are triggered based on time and not based on position, there are less
vessel speed limitations compared to conventional marine seismic data acquisition.
Also, because the source wavefield is spread out in time, the wavefields emitted from
source elements in different cross-line positions can be designed such that the emitted
wavefield is spatially white in this direction. This means that source elements in
multiple cross-line positions can be operated simultaneously, potentially improving
the cross-line sampling and/or the acquisition efficiency.
Real-Time Marine Seismic Acquisition and Processing
Author: Andrew Long
Industry Insights - 25 June 2019
The SEG will host a post-convention workshop at the annual meeting in San Antonio, Texas (September 2019) entitled ‘Real-Time Processing for Large-Scale Streaming Seismic Data’. Two relevant questions on the processing of seismic data streamed in real time from field acquisition are, ‘How can real-time processing make a difference in operations and decisions and what are the current bottlenecks?’ and ‘How do data acquisition systems shape real-time processing workflows?’ I address these issues in the context of towed streamer marine seismic operations where streaming is dependent upon available geostationary satellite bandwidth.
Diving deeper to reveal hydrocarbon potential in the Barents Sea
Author: Sören Naumann, Rune Sakariassen
GeoExpro - 4 June 2019
The south-western Barents Sea is characterised by a complex geological regime with a heterogeneous overburden. A key challenge in producing an accurate image of the subsurface lies in creating a velocity model which describes the recorded data well. Refraction-based Full Waveform Inversion (FWI) has become the standard tool for high resolution velocity model building in the Barents Sea. Nevertheless, due to the lack of recorded long offsets, model depths have been limited to the shallow overburden in the past.
In 2018 PGS and TGS utilised a novel acquisition setup for acquiring an ultra high density 3D seismic dataset in the Barents Sea, covering parts of the Hammerfest Basin and Finnmark Platform. In addition to 16 densely spaced streamers, three streamers were extended from 7 km to 10 km length, allowing the recording of deeper diving waves (refractions) and thereby enabling FWI to produce velocity updates to greater depths.
Robust FWI Updates in the Presence of Cycle Skipping
Author: Jaime Ramos-Martinez, Alejandro Valenciano, Xiaoyang Jiang, Nizar Chemingui
EAGE - 3 June 2019
Full Waveform Inversion (FWI) success depends on producing seamless short- and long-wavelength model updates while avoiding cycle skipping. In its traditional implementation, FWI risks converging to an inaccurate result if the data lacks sufficient low frequencies or the starting model is far from the true one. Additionally, the model
updates may display a reflectivity imprint before the long-wavelength features are fully recovered. A solution to these fundamental challenges combines the quadratic form of the Wasserstein distance (W2-norm) for measuring the data misfit with a robust implementation of a velocity gradient. The W2-norm reduces the risk of cycle skipping
whereas the velocity gradient effectively eliminates the reflectivity imprint and emphasizes the long-wavelength model updates. We illustrate the performance of the new solution on a field survey acquired offshore Brazil. There, we demonstrate how FWI successfully updates the earth model and resolves a high-velocity carbonate layer that was missing from the starting model.
Source Deghosting and Demultiple for Calm and Rough Weather Conditions
Author: Elsa Cecconello, Endrias Asgedom, Walter Söllner
EAGE - 3 June 2019
Source deghosting and demultiple algorithms have been extensively studied but mostly in the case of a flat seasurface. In this work, we consider time-varying sea-surfaces in different types of weather conditions and derive an inversion approach for removal of sea-surface effects. Starting from Rayleigh’s reciprocity theorem, we model seismic data including time-dependent wavefields scattered at the sea-surface, and highlight the temporal variation of these wavefields through simple synthetic examples, comparing two different weather conditions (calm and rough). We also reveal a limitation of source deghosting in the context of time-dependent wavefields: source deghosting affects the sea-surface multiples and can compromise the success of demultiple processing, which is applied in a later step. Results shows that this limitation is also important under calm weather conditions. To overcome this limitation, we propose simultaneously source deghosting and demultiple, therefore, removing all seasurface effects in one-step. Synthetic data examples are shown using the Sigsbee2B geological model.
Full Waveform Inversion of Simultaneous Long-Offset Data
Author: Nizar Chemingui, Alejandro Valenciano
EAGE - 3 June 2019
The standard workflow for velocity model building (VMB) in complex regimes is an interpretive process that requires time-consuming manual intervention, and remains an error-prone process that can produce suboptimal results.
Here we discuss an application of Full Waveform Inversion (FWI) to automate the refinement of legacy velocity models generated by conventional workflows. We demonstrate our solution on a full-azimuth (FAZ) survey acquired in the Gulf of Mexico using dual-sensor streamers and blended sources in the form of simultaneous long-offsets (SLO). The dual-sensor acquisition provides low-frequency data while the SLO configuration enables the recording of long offsets in excess of 16 km. The long offsets and low frequencies were key to using both refractions and reflections to update the deeper parts of the velocity model. Our solution directly inverts the simultaneous data as
acquired in the field. We also employed an FWI velocity gradient that eliminates the migration isochrones, removing the reflectivity imprint from the model updates. The FWI application to the field survey successfully refined the geometry of the salt bodies including the base salt and the intra-salt enclosures. It also improved the RTM image
particularly the salt flanks and the subsalt reflectors.
A Robust Iterative Deblending Method for Simultaneous Source Acquisition
Author: Lian Duan, Maiza Bekara, Edwin Hodges, Paolo Terenghi
EAGE - 3 June 2019
This paper presents a new method for simultaneous source deblending developed in the context of spatial coherency preservation. The deblending is formulated as an inverse problem which is solved in local overlapping windows extracted from the blended data. To constrain the solution, the unknown sources are assumed to be quasipredictive in the spatial domains. During the signal estimation, this will promote tolerance on impulsive blending noise and provide flexibility and robustness in coherency preservation depending upon the signal to noise ratio at a given frequency or within a given window. The proposed method is generic and can be applied to all configurations of simultaneous source acquisition and can deal with an arbitrary number of sources. Tests on numerically blended real towed streamer marine data show good deblending results with an excellent compromise between signal preservation and cross-talk contamination across a wide frequency range, particularly towards the low end. The proposed method is robust and flexible and can be applied in the early stages of a typical marine seismic processing
Methodology Utilizing Continuous Source and Receiver Wavefields - Signal to Noise Ratio Considerations
Author: Stian Hegna, Tilman Klüver, Jostein Lima
EAGE - 3 June 2019
The peak sound pressure levels (SPL) and the sound exposure levels (SEL) are significantly reduced with the seismic methodology based on the emission and recording of continuous source and receiver wavefields. The peak sound pressure levels are reduced because the emitted energy is spread out in time approaching the properties of white noise, in contrast to conventional marine sources where the emitted wavefield is approaching the properties of a spike. The energy levels emitted by marine seismic sources and its impact on the signal to noise ratio has been discussed in a number of papers. The ability to achieve comparable penetration in depth with the continuous wavefield method despite of significantly reduced peak SPL and SEL levels compared to the conventional marine seismic method is related to several aspects such as attenuation of shot generated noise, improved spatial sampling of source positions, and improved ability to attenuate noise with long continuous records. In this paper we will discuss the phase spectrum of the wavefield emitted by a source, and its influence on the signal to noise ratio after deconvolving the emitted wavefield from the received wavefield.
Continuous Wavefields Method – a New Marine Seismic Acquisition and Processing Approach
Author: Stian Hegna, Tilman Klüver, J. Lima
EAGE - 3 June 2019
There are several challenges with the conventional 3D marine seismic method that are addressed with a novel seismic methodology based on the emission and recording of continuous source and receiver wavefields. The challenges that are discussed in this paper are coarse spatial sampling of source positions, environmental impact of marine seismic sources, and efficiency of the acquisition. The coarse spatial sampling of source positions is addressed by emitting continuous source wavefields. The environmental impact of marine seismic sources is reduced by spreading the emitted energy out in time. The efficiency of marine seismic acquisition can be improved with the continuous wavefields method because there are no vessel speed limitations caused by the method as such, and because source elements can be distributed crossline in multiple positions improving the coverage in each sail line and hence reducing the number of sail lines needed to cover a certain area.
An innovative approach automation for velocity model building
Author: Tony Martin, Marcus Bell
First Break - 3 June 2019
Tony Martin and Marcus Bell describe the use of a Monte Carlo simulation, enabling multiple realizations of the solution in order to derive estimates of the uncertainty of an individual model, as well as drive velocity model building in an automated fashion.