M1-P6 2019

Name: M1-P6 2019
Start: 2019-03-25
End: 2019-03-28
Location: Ligurian Sea

Type	Training - University
Ship	Téthys II
Ship owner	CNRS until 2019 - IFREMER since 2020
Dates	25/03/2019 - 28/03/2019
Chief scientist(s)	MIGEON Sébastien
	LABORATOIRE GEOAZUR - UMR 7329 / UR082 250 rue Albert Einstein CS 10269 Campus Azur 06905 Sophia-Antipolis +33 (0)4 92 94 26 02 commesi@geoazur.unice.fr https://www.oca.eu/fr/acc-geoazur
Objective	This marine geophysics training is organized as part of the Master STePE (Earth and Planetary Sciences, Environment), GEO-P course (Geosciences-Planetology), of Sorbonne University. It concerns more particularly the level of Master1 and the module 4UG11 (second semester of the academic year). It takes place at the Institut de la Mer de Villefranche (IMEV). The goal is to give students a first applied training on geophysical acquisition methods through the implementation of a multitrace seismic-reflection tool aboard the RV Tethys II. The training consists of marine acquisitions (two days per student) and classroom work (courses / practicals on the principles of acquisition, processing and interpretation of seismic profiles). Because of its proximity to the Villefranche observatory where the training is held, the geological object studied is the North Ligurian continental margin, offshore Nice / Monaco. This margin is a good example of a passive margin that has recorded various tectonic and sedimentary processes. During the training, the days at sea make it possible to understand the acquisition parameters of seismic profiles and their influence on the seismic image. These images allow students to apply the principles of seismic stratigraphy by identifying seismic facies, reflector geometries, reflector termination patterns. Once interpreted, these images make it possible to map in 2D and then 3D seismic units on the first kilometers of the basement and give them a geological significance. The whole sequence of "acquisition - processing - interpretation of data" is therefore practiced very concretely during the training.

Scientific context

Main results

Data acquired and analyses carried out at sea and on shore

The acquisitions were made using the seismic tool of the Géoazur laboratory. This seismic tool includes a multichannel streamer (24 channels, 12.5 m / channel), a Sercel mini-GI air gun (24 ci / 24 ci), a Luchard compressor and a Delph digital acquisition system.

For this "training" campaign, the 18 students were divided into 2 groups who were to go out at sea in turn. Group 1 acquired one seismic profile during day 1. Group 2 was unable to go out at sea during day 2 due to poor weather conditions. Group 1 performed tests, such as a change in the depth of immersion of the streamer and the air gun (by changing the length of the ends connecting the air gun/the head of the streamer to the buoy ensuring its flottability), a change in the frequency of the shots, during the day 3 along the profile acquired during the day 1 in order to demonstrate and understand the impact of the acquisition parameters on the final seismic image. Group 2 was able to acquire one seismic profile during the day 4 which was reduced to the morning only to allow the disembarkation of the seismic material during the afternoon.

The 2 acquired seismic profiles were analyzed and processed in the classroom by the students, using a standard processing chain, perfectly suited to the acquisition device. This chain was developed for this data. It consists of a suite of scripts using executables of the open source CWP / Seismic Unix processing software. This modularization in the form of scripts makes it possible to divide the processing into different stages, for a good understanding of the contribution of each by the students who can see the contribution on the data. The chain includes:

1- Import of raw SEGY data, converted to SU format and resampled (with anti-aliasing filter);

2- Examination of data under their initial CSG (Common Shot Gathers) sequence and examination of trace headers;

3- Extraction of the sets in common offsets for analysis of the raw data by classes of offsets (Quality Control): identification of the artefacts (multiples, multiples folded from previous shots, diffractions) and analysis of the direct wave at the different offsets to reconstruct the characteristics of the acquisition geometry (measurement of effective offsets); spectral analysis for measuring the cutoff frequencies (notches) from which the depths of each element of the streamer are deduced - reconstruction of the immersion profile of the streamer;

4- Editing of headers to define a CMP sequence;

5- Editing traces to suppress the signal of any noisy channels;

6- Sorting in CMP and examining data in this form;

7- Correction NMO and stack, either at the speed of the water, or with a gradient under the bottom of the water (previously pointed then on the section with minimum offset) - examination on CMP after NMO to check the good flattening of the thinking hyperbolas

8- Possible test of different stack speeds to show the low dependence of the result, due to the short range of offsets covered by the device;

9- Comparison of the stacked section with the common offset sections previously extracted to verify the improvement of the signal / noise ratio by summation (in particular on deep reflectors);

10- Stolt post-stack migration at water speed;

11- Post-processing with Fourier domain filtering, either uniform or time-varying filter with a narrower bandwidth under the Messinian horizons to take advantage of the lower frequency content under these horizons; equalization of ranges by AGC;

12- Meticulous mute above the bottom of the water and SEGY re-export for large format final plot (for interpretation).

This processing is done during 2 half-days of work in the computer room with the students who practice the whole chain individually (3h of courses, 5h of practicals).

The training is also supplemented by a 4-hours course / practicals on wide-angle seismic, emphasizing the complementarity of this geophysical prospecting method with the seismic reflection practiced during the training.

Finally, interpretation of seismic-reflection images allows an application of the principles of seismic stratigraphy seen theoretically during courses, with an initiation to the identification of sequences of reflectors and seismic units based on the seismic facies, geometries of boundary reflectors, etc. The whole is restored in the evolving tectono-sedimentary framework of the margin and the Ligurian basin.

Bibliography

Publications

Migeon Sebastien, Mulder Thierry, Savoye Bruno, Sage Francoise (2012). Hydrodynamic processes, velocity structure and stratification in natural turbidity currents: Results inferred from field data in the Var Turbidite System. Sedimentary Geology, 245, 48-62. https://doi.org/10.1016/j.sedgeo.2011.12.007

Sage F., Beslier M. -O., Thinon I., Larroque C., Dessa J-X, Migeon S., Angelier J., Guennoc P., Schreiber D., Michaud F., Stephan J. -F., Sonnette L. (2011). Structure and evolution of a passive margin in a compressive environment: Example of the south-western Alps-Ligurian basin junction during the Cenozoic. Marine And Petroleum Geology, 28(7), 1263-1282. https://doi.org/10.1016/j.marpetgeo.2011.03.012

Migeon Sébastien, Mulder Thierry, Savoye Bruno, Sage Françoise (2006). The Var turbidite system (Ligurian Sea, northwestern Mediterranean) - morphology, sediment supply, construction of turbidite levee and sediment waves: implications for hydrocarbon reservoirs. Geo-Marine Letters, 26(6), 361-371. Publisher's official version : https://doi.org/10.1007/s00367-006-0047-x , Open Access version : https://archimer.ifremer.fr/doc/00000/2258/

References of International Seminar Communications

Sage, F., Beslier, M.-O., Dessa, J.X., Schenini, L., Watremez, L., Mercier de Lépinay, B., Gaullier, V., Larroque, C., Béthoux, N. ,Corradi, N., Bigot, A., Migeon, S., Ruiz Constán, A. (2014). Inversionof back-arc basins : example of the Ligurian Basin, Western Mediterranean. Geophysical Research Abstracts, n°EGU2014-15668, European Geosciences Union (EGU) General Assembly 2014, 27 April - 2 May, 2014 Vienna (Austria).

Sage F., M.O. Beslier, B. Mercier de Lepinay, N. Béthoux, V. Gaullier, C. Larroque, N. Corradi, L. Schenini, J.X. Dessa, A. Bigot, S. Migeon, A. Ruiz Constán (2013). Localized deformation along an inverted rifted margin: Example of the Northern Ligurian margin, Western Mediterranean. AGU Fall meeting, Poster T21B?2550.

Sage F., Beslier, M.O., Gaullier, V., Larroque, C., Dessa, J.X., Mercier de Lepinay, B., Corradi, N., Migeon, S., Katz, H., Ruiz Constan, A. (2013). Partitioning of deformation along a reactivated rifted margin: example of the northern Ligurian margin. Geophysical Research Abstracts Vol. 15, EGU2013?12902?3.

Hassoun V., Migeon S., Larroque C., Cattaneo A., Mercier de Lépinay B. (2011). Will the French Riviera disappear? Ancient and present day submarine landslides along the north-western Ligurian Margin (NW Mediterranean) under the microscope, American Geophysical Union (AGU), n°OS13B-1521, AGU Fall Meeting, 5-9 december, San Francisco (USA).

References of National Seminar Communications

Beslier, M.-O., F. Sage, J.-X. Dessa, V. Gaullier, L. Schenini, B. Mercier de Lépinay, C. Larroque, L. Watremez, N. Bethoux, N. Corradi, A. Bigot, S. Migeon, A. Ruiz-Constán, A. Leprêtre (2014). Inversion of back-arc basins : example of the Ligurian Basin, Western Mediterranean. 24ème réunion des Sciences de la Terre, Pau, 27-31 octobre 2014.

Thesis using campaign data

K. SAMALENS (2018). Mouvements de terrain sous-marins sur la Marge Ligure : enregistrement sédimentaire de l'activité sismique ?. Thèse de l'Université de Nice Sophia Antipolis.

V. HASSOUN (2016). Interaction aléa gravitaire et déformation sur la marge nord-Ligure. Thèse de l'Université de Nice-Sophia Antipolis.