Article

Marine vibrators: the new phase of seismic exploration

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Abstract

Marine seismic vibrators are generally considered to be less intrusive than airguns from an environmental perspective. This is because they emit their energy spread out in time, rather than in a single, high intensity pulse. There are also significant geophysical benefits associated with marine vibrators, and they stem from the ability to specify in detail the output acoustic waveform. The phase can be specified independently at each frequency. Such detailed control cannot be achieved with conventional airgun sources, where the phase can only be modified by the use of simple overall time delays. The vibrator phase can be employed in several different ways: it can be applied to the overall source phase in a sequence so that it varies from one source point to the next; it can be applied to the individual vibrators within the source array so the source directivity is changed; it can be applied to the overall source phase of each source in a simultaneous source acquisition. Carefully designed phase‐sequences can attenuate the residual source noise, and this in turn allows extra source points to be interleaved between the conventional ones. For these extra source points, the relative phase of the vibrators within the array can be chosen to create a transverse gradient source, which illuminates the earth predominantly in directions out of the plane of the sail line without left/right ambiguity. If seismic vibrator data are acquired using interleaved conventional and transverse gradient sweeps, more information is collected per kilometre of vessel travel than is the case in conventional acquisition. This richer data acquisition leads to the possibility of acquiring all the necessary seismic data in a shorter time. Three‐dimensional reconstruction techniques are used to recover the same image quality that would have been obtained using the conventional, more time‐consuming acquisition. For a marine vibrator to be suitable for these techniques it must, in general terms, have ‘high fidelity’. The precise device specifications are defined through realistic end‐to‐end simulations of the physical systems and of the processing. The specifications are somewhat more onerous than for a conventional vibrator, but they are achievable. A prototype vibrator that satisfies these requirements has been built. In a simulated case study of a three‐dimensional deep‐water ocean bottom node survey, the seismic data could have been acquired using marine vibrators in one third of the time that it would have taken using airguns. This article is protected by copyright. All rights reserved

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... Pramik et al., 2013;Ozasa, 2019). As a matter of fact, marine vibrator has the advantage in the controllability of the signal phase in the seismic emission (Laws et al, 2019). Marine vibrators could have the advances not only in are not only in the environmental but in the geophysical aspects. ...
... This problem has been studied by many authors (e.g. Dragoset, 1988;Schultz et al, 1989;Hampson and Jakubowicz, 1990;Noss et al, 1999;Laws et al., 2019), and the processing in frequency-space domain (Schultz et al., 1989;Noss et al., 1999) or in a way to use source and receiver locations discretized in timespace domain (Laws et al., 2019). The problem of moving sources may no longer be of the disadvantages. ...
... This problem has been studied by many authors (e.g. Dragoset, 1988;Schultz et al, 1989;Hampson and Jakubowicz, 1990;Noss et al, 1999;Laws et al., 2019), and the processing in frequency-space domain (Schultz et al., 1989;Noss et al., 1999) or in a way to use source and receiver locations discretized in timespace domain (Laws et al., 2019). The problem of moving sources may no longer be of the disadvantages. ...
Conference Paper
We propose a new processing approach not only to correct but to exploit the effects of the motion of marine vibratory source deployed an offshore seismic survey. We decompose the survey data into plural sections as a function of time, space and signal frequency. The instantaneous spatial locations of shot/receivers as a function of time and signal frequency are used in the data processing. Our processing method is applied to numerical simulation data for a moving source generating a waveform of a finite time duration encoded with a frequency sweep, and to a marine seismic reflection survey data acquired by a mechanical-driven marine seismic vibrator that was designed to verify its feasibility. We found it is possible to remove the source motion effect by applying our processing approach with the conventional migration processing. Furthermore, we perceived the enhancement of the lateral resolution after taking the motion of the source into account in marine vibrator survey. The utilization of marine vibratory sources has the potential to realize high accuracy survey when exploiting the continuity in time and space for seismic emission.
... Marine Vibrators. Marine vibrators (vibroseis) are experimental, non-impulsive seismic sources that have been investigated as a potential replacement for airguns, particularly for surveys in shallow water or in environmentally sensitive areas [10,82,83]. Using hydraulics or electromagnetics, vibroseis sources generate a broadband seismic signal that mimics the low-frequency characteristics of airguns and can be designed to include frequencies (~100 kHz) for higher resolution imaging, while suppressing energy at the highest frequencies (greater than ~150 kHz [10]). ...
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Marine acoustic sources are widely used for geophysical imaging, oceanographic sensing, and communicating with and tracking objects or robotic vehicles in the water column. Under the U.S. Marine Mammal Protection Act and similar regulations in several other countries, the impact of controlled acoustic sources is assessed based on whether the sound levels received by marine mammals meet the criteria for harassment that causes certain behavioral responses. This study describes quantitative factors beyond received sound levels that could be used to assess how marine species are affected by many commonly deployed marine acoustic sources, including airguns, high-resolution geophysical sources (e.g., multibeam echosounders, sidescan sonars, subbottom profilers, boomers, and sparkers), oceanographic instrumentation (e.g., acoustic doppler current profilers, split-beam fisheries sonars), and communication/tracking sources (e.g., acoustic releases and locators, navigational transponders). Using physical criteria about the sources, such as source level, transmission frequency, directionality, beamwidth, and pulse repetition rate, we divide marine acoustic sources into four tiers that could inform regulatory evaluation. Tier 1 refers to high-energy airgun surveys with a total volume larger than 1500 in3 (24.5 L) or arrays with more than 12 airguns, while Tier 2 covers the remaining low/intermediate energy airgun surveys. Tier 4 includes most high-resolution geophysical, oceanographic, and communication/tracking sources, which are considered unlikely to result in incidental take of marine mammals and therefore termed de minimis. Tier 3 covers most non-airgun seismic sources, which either have characteristics that do not meet the de minimis category (e.g., some sparkers) or could not be fully evaluated here (e.g., bubble guns, some boomers). We also consider the simultaneous use of multiple acoustic sources, discuss marine mammal field observations that are consistent with the de minimis designation for some acoustic sources, and suggest how to evaluate acoustic sources that are not explicitly considered here.
... However, marine vibrators are not only attractive for their low environmental impact, but also advantageous from an exploration point of view because they provide data with lower frequencies compared to those acquired using airguns (Guitton et al., 2021). Further, by blending phase-encoded sources, marine vibrators enable simultaneous acquisition (Laws et al., 2019), reducing acquisition time, which is more challenging to achieve using conventional air-guns (e.g., Wason and Herrmann (2013)). However, marine vibrators are in continuous motion and have a long sweep duration, introducing Doppler effects and time-dependent source-receiver offsets in ocean-bottom acquisition. ...
... Marine vibrators are not just advantageous for environmental reasons, but also have geophysics-related merits. From a seismic exploration perspective, marine vibrators provide more low-frequency content than air-guns , and enable simultaneous acquisition by blending phaseencoded sources (Laws et al., 2019). However, processing and imaging challenges emerge because of the motion and long duration of the marine vibrator signal, including Doppler effects and time-dependent source-receiver offsets in oceanbottom acquisition. ...
... The optimal depth must be able to provide both minimum and maximum intended frequencies within the expected sweep time. With continuous sweeps, phase encoding can eb used to remove residual shot noise (Laws et al., 2018b). ...
... We can maximize the acoustic energy by matching the depth of each transducer to the frequency band of its sweeps, but the resulting multi-depth geometry results in mechanical challenges. Similarly, producing a directional signal by combining two transducers into a gradient source (Laws et al., 2018b) places stringent demands on the precision of the towing geometry. We develop finite-element models of potential hardware designs and apply them in simulations under realistic current and sea-state conditions. ...
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