The low frequency output of marine air‐gun arrays

Hegna, Stian; Parkes, Gregg

doi:10.1190/1.3628192

Cited by 21 publications

(15 citation statements)

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“…This means that ultralow frequency amplitude and phase control would be impossible in seismic processing and/or inversion. Hegna and Parkes (2011) also demonstrate that the bubble oscillation period gets shorter with increasing hydrostatic pressure and, hence, increasing tow depth. For a particular gun volume and pressure, therefore, the power of the signals in the ultra-low frequency end emitted by an air-gun decrease with increasing tow depth.…”

Section: Fighting the Inescapable Laws Of Physicsmentioning

confidence: 89%

“…Removal of either/both the source-side and receiver-side ghost effects will evidently allow the recovery of ultra-low frequency amplitudes penalised by ghosting. Parkes and Hegna (2011) and Hegna and Parkes (2012) describe a solution to complete ghost removal that is AVA/AVO-compliant (amplitude vs angle or offset), and phasecompliant for all scenarios of interest to seismic inversion and quantitative interpretation (discussed later). Figure 5 illustrates the ultra-low frequency benefits to seismic images after complete ghost removal.…”

Section: Unwanted Reflections From the Free-surface Of The Oceanmentioning

confidence: 99%

“…Hegna and Parkes (2011) refer to the Rayleigh-Willis equation, which describes a proportional relationship between the bubble oscillation period from an air gun and the cube root of the air gun volume. A longer bubble period equates to lower characteristic frequency, with the frequency corresponding to the most significant low-end amplitude (Fig.…”

Section: Fighting the Inescapable Laws Of Physicsmentioning

confidence: 99%

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Low-frequency seismic: the next revolution in resolution

Long

Reiser

2014

The APPEA Journal

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Ultra-low seismic frequencies less than about 7 Hz cannot be produced by conventional air gun arrays, for any configuration and for any towing depth. There is a profound difference between improving low-frequency recovery by removing source and receiver ghosts (achievable) and improving lowfrequency injection on the source side (an unrealised dream).If 1-7 Hz amplitudes could be usefully injected into the earth, it would be possible to facilitate much sharper seismic representation of geological contacts and internal features, and seismic inversion would yield robust and precise predictions of reservoir properties-without well control. The net result is fewer exploration and appraisal wells, greatly reduced exploration and development risks, and optimised recoverable reserves.Furthermore, an emerging seismic pursuit known as full waveform inversion (FWI) makes the bold promise that raw seismic field gathers can be directly used to invert for the highest achievable velocity models, almost without any human intervention. These models will bypass the traditional lack of low-frequency information in band-limited seismic data, and facilitate the aforementioned ambition of seismic inversion without well control. FWI, however, is confronted by the paradox that ultra-low-frequency seismic gathers are the necessary input for stable results. This paper describes new technologies that may enable the injection of strong 2-7 Hz amplitudes into the earth, and explains in simple terms how FWI can already be pursued as a robust complement to the prediction of accurate reservoir properties.The low-frequency revolution is already here. KEYWORDSLow-frequency, ghosts, deghosting, dual-sensor, seismic inversion, FWI, reservoir characterisation, seismic. INTRODUCTIONConventional air gun source arrays used for towed streamer (marine) seismic surveys cannot produce significant amplitudes below about 7 Hz. This 0-7 Hz range will be referred to as 'ultralow frequencies' . Although it may be counterintuitive, ultra-low frequency amplitudes can greatly improve vertical (temporal) seismic resolution, as demonstrated in Figure 1. Assuming that equal amplitudes are recovered in processing for all frequencies of relevance, additional low-frequency amplitudes will significantly attenuate sidelobe amplitudes on the zero phase wavelet representing elastic impedance boundaries in the earth. Note also in Figure 1 how additional low-frequency amplitudes do not affect the width of the central event. Overall, the interference between neighbouring reflection events in vertical columns of thin stratified layers will be minimised, thus optimising vertical resolution.Ultra-low frequency amplitudes are also critical for quantitatively precise estimation of elastic reservoir properties, which will be discussed later. Figure 2 is a comparison of two-dimensional towed streamer results over the Møre Margin area of the Norwegian North Sea. Note how the additional low-frequency amplitudes recovered in the ghost-free data have contributed to reduced sidel...

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Section: Fighting the Inescapable Laws Of Physicsmentioning

confidence: 89%

Section: Unwanted Reflections From the Free-surface Of The Oceanmentioning

confidence: 99%

Section: Fighting the Inescapable Laws Of Physicsmentioning

confidence: 99%

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Low-frequency seismic: the next revolution in resolution

Long

Reiser

2014

The APPEA Journal

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“…The severity of these notches and depressions depends on the free-surface reflection coefficient for the ghost notches and the primary to bubble ratio for the bubble related notches, respectively. The low-frequency output from an air-gun array is almost unaffected by the source depth (Hegna and Parkes, 2011;Hopperstad et al, 2012;Landrø and Amundsen, 2014). There are two major effects related to the depth of the source that influences the low-frequency output from an air gun.…”

Section: Theory Notches In the Air-gun Spectrummentioning

confidence: 93%

“…The low-frequency response from an air-gun array is only weakly dependent on the source depth. If we position the source deeper, it will result in a source ghost response more favorable for the low frequencies, but the corresponding reduction in the bubble time period yields less low-frequency output (Hegna and Parkes, 2011;Hopperstad et al, 2012;Landrø and Amundsen, 2014). However, the main effect and goal of this acquisition strategy is to obtain diversity of the notches related to the ghost and notches or depressions caused by the bubble.…”

Section: Discussionmentioning

confidence: 99%

Variable source depth acquisition for improved marine broadband seismic data

Haavik¹,

Landrø²

2015

GEOPHYSICS

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In marine seismic data acquisition, varying the source depth along a sail line gives diversity in sequential shot gather frequency spectra. Undesired alterations of the frequency spectra are created by the source ghost and by air-gun bubble oscillations. By deliberately varying the source depth along a sail line, it is possible to obtain a seismic data set that will have energy more evenly distributed within the main frequency band of the source output. This is obtained when data acquired with different source depths are stacked in imaging. We formulated a simple inverse problem that seeks to find the optimal distribution of source depths over a sequential series of shots that shape the amplitude spectrum of the final image into a desired shape. We assumed that the data are receiver-side deghosted, that designature could be applied to each shot gather, and that the shot gathers could be redatumed to a common datum prior to imaging.

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Reprocessing 2-D airgun seismic reflection data SALTFLU (salt deformation and sub-salt fluid circulation in the Algero-Balearic abyssal plain) in the Balearic promontory and the Algerian basin

Blondel

Ford

Lockwood³

et al. 2023

Mar Geophys Res

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In an ever more challenging context for the acquisition of seismic data in the Mediterranean Sea, reprocessing to improve the quality of legacy data has become increasingly important. This work presents the newly reprocessed, open access dataset SALTFLU acquired in the Algerian basin by the National Institute of Oceanography and Applied Geophysics (OGS) in 2012. We apply a ‘broadband’ reprocessing strategy adapted for offset-limited (3 km streamer for a target 4 km below the sea level) airgun reflection seismic data acquired in deep water settings. We then assess if the reprocessed images provide new geological insights on the Mediterranean sub-surface. The workflow relies on an integrated approach combining geophysics and geological interpretation to iteratively build the velocity model. In this way we aim to tackle some of the challenges linked to imaging deep complex geological structures containing high velocity contrasts with 2-D, offset-limited seismic data. We first broaden the bandwidth of the data through multi-domain de-noising, deghosting and a source designature using an operator derived from the seabed reflection. We then perform iterative migration velocity analysis, pre-stack time migration and multiple attenuation in the Radon domain to obtain time-migrated images. The initial velocity model is derived from the resulting time migration velocities, and geologically driven model updates are generated using a combination of travel-time tomography, seismic interpretation of the major salt horizons and velocity gradient flooding. The gradient flooding aims to reproduce the large scale first-order velocity variations, while the travel-time tomography aims to resolve the smaller second-order velocity variations. The results improve our deep geological knowledge of the under-explored Algerian basin down to the base salt and the pre-salt. Fluid indicators are imaged within the Plio-Quaternary of the Algerian basin, which we interpret as thermogenic or biogenic gas sourced from either the Messinian Upper Unit or from the pre-salt, migrating through a hydro-fractured salt. The reprocessed data image lateral and vertical seismic facies variation within the Messinian units that could shed new light on the tectono-stratigraphic processes acting during the Messinian Salinity Crisis. It also reveals numerous previously unresolved volcanic structures within the Formentera basin.

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The low frequency output of marine air‐gun arrays

Cited by 21 publications

References 3 publications

Low-frequency seismic: the next revolution in resolution

Low-frequency seismic: the next revolution in resolution

Variable source depth acquisition for improved marine broadband seismic data

Reprocessing 2-D airgun seismic reflection data SALTFLU (salt deformation and sub-salt fluid circulation in the Algero-Balearic abyssal plain) in the Balearic promontory and the Algerian basin

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