Reflection seismology is nowadays the preferred technique in the oil and gas industry to estimate the properties of the Earth's subsurface. The method typically includes a series of procedures that fit in three broad categories: • seismic data acquisition; • data processing and imaging; • interpretation and reservoir characterization. This thesis mainly focuses on the first category and aims at improving both the operational productivity of seismic surveys in terms of costs, and the quality of the data in terms of signal-to-noise ratio and frequency content. Hereafter, we present a novel approach to seismic data collection named Dispersed Source Array (DSA) acquisition. It is proposed to replace traditional broadband sources with a set of devices dedicated to different and complementary frequency bands. Modern multiple driver loudspeaker systems are based on the same key concept and their improved performance is demonstrated. During field operations, it is often impossible to accurately implement nominal survey geometries in practice. Frequently, acquisition geophysicists are required to cope with unforeseen circumstances such as obstacles in the field and inaccessible or restricted areas. These complications may compromise the quality of the data or lead to delays, and thus extra expenses, during acquisition. In this thesis, we propose two automated approaches to survey design focused on avoiding spatial discontinuities in the recorded data and on guaranteeing adequate data quality. The two methods are based on the reorganization of regular (centralized) and irregular (decentralized) source acquisition grids, respectively, and provide a practical acquisition plan for seismic crews. In this thesis, based on theoretical considerations and numerical data inversion and imaging examples, the feasibility of Dispersed Source Array acquisitions is demonstrated. Additionally, we show that it is possible to reliably recover subsurface information based on irregularly sampled datasets. We show how, despite the significant mismatch between baseline and monitor survey geometries, decentralized DSA surveys are also suitable for time-lapse studies. @en

Although seismic sources typically consist of identical broadband units alone, no physical constraint dictates the use of only one kind of device. We propose an acquisition method that involves the simultaneous exploitation of multiple types of sources during seismic surveys. It is suggested to replace (or support) traditional broadband sources with several devices individually transmitting diverse and reduced frequency bands and covering together the entire temporal and spatial bandwidth of interest. Together, these devices represent a so-called dispersed source array. As a consequence, the use of simpler sources becomes a practical proposition for seismic acquisition. In fact, the devices dedicated to the generation of the higher frequencies may be smaller and less powerful than the conventional sources, providing the acquisition system with increased operational flexibility and decreasing its environmental impact. Offshore, we can think of more manageable boats carrying air guns of different volumes or marine vibrators generating sweeps with different frequency ranges. On land, vibrator trucks of different sizes, specifically designed for the emission of particular frequency bands, are preferred. From a manufacturing point of view, such source units guarantee a more efficient acoustic energy transmission than today's complex broadband alternatives, relaxing the low- versus high-frequency compromise. Furthermore, specific attention can be addressed to choose shot densities that are optimum for different devices according to their emitted bandwidth. In fact, since the sampling requirements depend on the maximum transmitted frequencies, the appropriate number of sources dedicated to the lower frequencies is relatively small, provided the signal-to-noise ratio requirements are met. Additionally, the method allows to rethink the way to address the ghost problem in marine seismic acquisition, permitting to tow different sources at different depths based on the devices' individual central frequencies. As a consequence, the destructive interference of the ghost notches, including the one at 0 Hz, is largely mitigated. Furthermore, blended acquisition (also known as simultaneous source acquisition) is part of the dispersed source array concept, improving the operational flexibility, cost efficiency, and signal-to-noise ratio. Based on theoretical considerations and numerical data examples, the advantages of this approach and its feasibility are demonstrated.

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In Dispersed Source Arrays (DSA) acquisitions, traditional broadband seismic sources are replaced (or supported) with dedicated narrower band devices with different central frequencies, blended together to cover the entire temporal and spatial bandwidth of interest. The recent advances in unmanned systems technology and the improved operational flexibility enabled by the limited dimensions of most DSA devices may be beneficial to the data acquisition efficiency. In fact, with DSAs the use of relatively simple autonomous devices becomes a practical proposition for seismic surveys. In a marine environment we might consider employing several autonomous source boats at the same time, while on land a combination of autonomous source trucks of varied dimensions and designs is suggested. This abstract presents a real-time decentralized and automated approach to acquisition design in order to handle the larger number of sources simultaneously operational in the field. Additionally, using the Simultaneous Joint Migration Inversion (SJMI) technology it is possible to reliably recover time-lapse information despite the significant mismatch between baseline and monitor survey geometries introduced by the decentralized acquisition method.@en

A major practical advantage of the Dispersed Source Array (DSA) concept is that most of its source units are smaller and less powerful than conventional sources providing the acquisition system with increased operational flexibility. To handle the larger number of sources simultaneously operational in the field, we propose to organize the acquisition system in an automated and decentralized manner. We demonstrate that, by applying this strategy it is possible to produce valid image results even without rigorously strict survey design. Finally, we prove that using the Simultaneous Joint Migration Inversion (SJMI) technology it is possible to reliably recover time-lapse information even when a significant mismatch between baseline and monitor survey geometries is introduced by the system decentralization.@en

In the last few years, the interest towards the utilization of Dispersed Source Arrays (DSAs) in seismic acquisition has considerably grown. The proposed approach offers a wide range of practical advantages, while no physical constraint restrains us from utilizing diverse sources with different spectral properties during seismic surveys. As a consequence, the use of simple autonomous source boats with airgun arrays of different sizes or marine vibrators producing sweeps with different frequency ranges (in marine) and simple autonomous source trucks (on land) becomes a practical proposition in DSA acquisitions. This concept could give to the system additional operational flexibility and facilitate the automation of seismic data collection. Therefore, with this study we intend to investigate the advantages that DSAs and system decentralization would bring to seismic data acquisition. Although the main focus of this research is on the marine environment, a generalization to the applications of the method on land is possible. Preliminary examples show that it is possible to produce valid migration outputs from 3D decentralized DSA data. @en

A seismic acquisition method is proposed that involves the exploitation of inhomogeneous sources. The constraint of employing only identical units can be abandoned. We suggest to replace (or reinforce) traditional broadband sources with narrow(er)band devices, together representing a dispersed source array (DSA). The whole inhomogeneous ensemble of sources incorporated to the arrays is required to cover the entire temporal and spatial bandwidth of interest. The DSA concept can be an important step towards the robotization of the seismic acquisition process and an improved operational flexibility. In addition, narrow band source design can be better optimized, no compromise between the emission efficiency around the limits of the bandwidth of interest is requested. Furthermore specific attention can be addressed to choose source depths and spatial sampling intervals that are optimum for particular devices. Valid and encouraging numerical migration results are shown. @en