Digital Streamer Research Articles

Acquisition and preliminary analysis of multi-channel seismic reflection data, acquired during the oceanographic cruises of the TOMO-ETNA experiment

<p>The TOMO-ETNA experiment was performed in the framework of the FP7 “MED-SUV” (MEDiterranean SUpersite Volcanoes) in order to gain a detailed geological and structural model of the continental and oceanic crust <span>concerning </span>Etna and Aeolian Islands volcanoes (Sicily, Italy), by means of active and passive seismic exploration methodologies. Among all data collected, some 1410 km of marine multi-channel seismic (MCS) reflection profiles were acquired in the Ionian and Tyrrhenian Seas during two of the three oceanographic cruises of the TOMO-ETNA experiment, in July and November 2014, with the aim of shading light to deep, intermediate and shallow stratigraphy and crustal structure of the two above mentioned areas. The MCS sections, targeted to deep exploration, were acquired during the oceanographic cruise on board the R/V “Sarmiento de Gamboa”, using an active seismic source of 16 air-guns, for a total volume of 4340 cu. in., and a 3000 m long, 240-channels digital streamer as receiving system. High-resolution seismic profiles were instead collected through the R/V “Aegaeo”, using two smaller air-guns (overall 270 cu. in. volume) and a 96 channels, 300 m long digital streamer. This paper provides a detailed description of the acquisition parameters and main processing steps adopted for the MCS data. Some processed lines are shown and preliminarily interpreted, to highlight the overall good quality and the high potential of the MCS sections collected during the TOMO-ETNA experiment.</p>

Seismic evidence of Caledonian deformed crust and uppermost mantle structures in the northern part of the Trans-European Suture Zone, SW Baltic Sea

Collisional structures from the closure of the Tornquist Ocean and subsequent amalgamation of Avalonia and Baltica during the Caledonian Orogeny in the northern part of the Trans-European Suture Zone (TESZ) in the SW Baltic Sea are investigated. A grid of marine reflection seismic lines was gathered in 1996 during the DEKORP-BASIN '96 campaign, shooting with an airgun array of 52 l total volume and recording with a digital streamer of up to 2.1 km length. The detailed reflection seismic analysis is mainly based on post-stack migrated sections of this survey, but one profile has also been processed by a pre-stack depth migration algorithm. The data provides well-constrained images of upper crustal reflectivity and lower crustal/uppermost mantle reflections. In the area of the Caledonian suture, a reflection pattern is observed with opposing dips in the upper crust and the uppermost mantle. Detailed analysis of dipping reflections in the upper crust provides evidence for two different sets of reflections, which are separated by the O-horizon, the main decollement of the Caledonian deformation complex. S-dipping reflections beneath the sub-Permian discontinuity and above the O-horizon are interpreted as Caledonian thrust structures. Beneath the O-horizon, SW-dipping reflections in the upper crust are interpreted as ductile shear zones and crustal deformation features that evolved during the Sveconorwegian Orogeny. The Caledonian deformation complex is subdivided into (1) S-dipping foreland thrusts in the north, (2) the S-dipping suture itself that shows increased reflectivity, and (3) apparently NE-dipping downfaulted sedimentary horizons south of the Avalonia–Baltica suture, which may have been reactivated during Mesozoic normal faulting. The reflection Moho at 28–35 km depth appears to truncate a N-dipping mantle structure, which may represent remnant structures from Tornquist Ocean closure or late-collisional compressional shear planes in the upper mantle. A contour map of these mantle reflections indicates a consistent northward dip, which is steepest where there is strong bending of the Caledonian deformation front. The thin-skinned character of the Caledonian deformation complex and the fact that N-dipping mantle reflections do not truncate the Moho indicate that the Baltica crust was not mechanically involved in the Caledonian collision and, therefore, escaped deformation in this area.

OPTICAL RECORDING: A NEW TECHNOLOGY FOR SEISMIC DATA ACQUISITION

The demand for improved resolution in marine seismic surveys has led in the last few years to the development of recording systems with significantly improved spatial and temporal sampling rates. The ability to maximize data quality by means of array forming (i.e., combining particular combinations of hydrophone elements) is becoming increasingly important and new digital streamer systems now provide the capability to record as many as 1,000 channels of data.

Data enhancement from a 500-channel streamer

Seismic reflection data obtained with a 500-channel digital streamer cable can be processed by array-forming techniques such as optimal weighting of individual channels and beam-steering. Such processing can improve the resolution of horizons at early and intermediate times as well as enhance the continuity and clarity of later reflections. The data may also be processed to simulate results obtainable with a wide variety of conventional streamers. To demonstrate data enhancement obtained by these processing techniques, a selected line was surveyed in the Gulf of Mexico, first with the 500-channel digital system and then with a conventional 48-channel streamer. Comparison of stacked sections shows that: 1) long steered arrays can enhance deeper events while retaining the high-frequency content of shallow data, and 2) short arrays at small group intervals allow finer vertical resolution of shallow events, as well as finer lateral resolution at all depths. When the digital streamer data were processed to duplicate the 48-channel conventional data, the digital system yielded somewhat better results; this improvement may be attributed in part to inherent advantages of digital telemetry over analog telemetry.

Special issue on oceanic seismic exploration - Introduction to the issue

CEANIC SEISMIC EXPLORATION constitutes an important area of bottom-interacting ocean acoustics. It is concerned with the probing of the ocean bottom and subbottom by means of acousticsignals artificiallygeneratedin the upper layers of the ocean. The signals involved are of low frequencies, ranging typically from nearly zero to a few kilohertz. Due partly to the increased interest and activity in offshore petroleum exploration, oceanic seismic exploration has received considerable impetus in recent years, and is likely to draw increasing attention in the years to come as developing technology enables humankind to begin untapping the immense organic, fluid, and mineral resources lying below our oceans. It is therefore fitting that in the month in which the Institute of Electrical and Electronic Engineers enters the second century of its existence, this journal should devote attention to this important area. In this special issue there are seven papers of particular interest covering various topics in this field. The first paper by D. A. Fisher and G. H. F. Gardner is concerned with the problem of fonvard modeling of the seafloor and basement from seismic reflection data. The common and relative merits of numerical and physical modeling are discussed in the context of two examples. The next two papers, respectively by J. A. DeSanto and D. J. Thomson, are devoted to the issue of inversion of acoustic field data. The paper by DeSanto presents a modal (full-wave) method, based on the Born approximation, for retrieving the ocean sound-speed profile from the values of the transmitted acoustic field measured by an array. On the other hand, the paper by Thomson separately reconstructs the density and sound-speed profiles of the ocean subbottom, by a method for which the theoretical foundations were provided by Candel et al. [ 13 ~ and presents some numerical simulations of this approach for a geoacoustical model of the Hatteras Abyssal Plain deep sea sediments. A closely related topic is discussed in the paper by N. R. Chapman, I. Barrodale, and C. A. Zala, who present a novel approach for obtaining sound speed and sound-speed gradient in surficial seafloor sediments from bottom-reflected seismic signals by an I1-nomz (least absolute value) deconvoltion method. The next two papers are concerned respectively with the analysis and measurement of ocean-acoustic data from towed line arrays. The paper by L. R. LeBlanc presents a method for separating the data measured by a linear acoustic towed array into statistically uncorrelated components and then interpreting the results in terms of angular distribution of energy of the components. The paper by W. Dragoset and K. Lamer demonstrates the data enhancement capabilities of a spstem which uses a recently developed 500-channel digital streamer cable in seismic reflection data collection. Finally, the paper by J. M. Berkson and J. E. Matthews presents an interesting characterization and discussion of the statistical properties of seafloor roughness gleaned from narrow bandwidth echo sounding and seismic reflection profiling. In closing, we would like to thank the authors for having so diligently prepared their contributions for this issue in a very short period of time and to Stan Ehrlich, Editor of this JOURNAL, for his help and advice in all matters concerning this issue.

Data Enhancement from 500-Channel Streamer: ABSTRACT

Seismic reflection data obtained with a 500-channel, digital streamer cable can be processed by array-forming techniques that include optimal weighting and time-shifting of individual channels. Such processing can improve the vertical resolution of horizons at early and intermediate times as well as enhance the continuity and clarity of later reflections. In addition to the recording of individual channels, a separate recording can be made aboard ship with the individual channels array formed to simulate data obtainable with any of a wide variety of conventional End_Page 921------------------------------ streamers. To demonstrate data enhancement obtained by these techniques, a line was surveyed in the Gulf of Mexico, first with the 500-channel system and then with a conventional 48-channel streamer. When the 500-channel data were processed to duplicate the conventional streamer data, the larger system yielded better results for late as well as early times; this improvement may be attributed partly to reduced cable noise. Comparisons made with record sections and stacked sections show that: (1) long, beam-steered arrays can enhance deeper events while retaining the high-frequency content of shallow data, and (2) short arrays at small group intervals allow fine resolution of shallow events. The ultimate approach to preserving broadband information is full 500-channel processing of individually recorded channels. With such processing, individual hydrophone groups can be considered essentially as points. When the Gulf of Mexico data are processed in this way, signal continuity persists to frequencies above 125 Hz, and lateral changes are observed to the limit of resolution provided by the 3-m spacing of the output stacked traces. End_of_Article - Last_Page 922------------