Marine Geosciences

 

MARINE SEISMIC INVESTIGATIONS:

SOME THOUGHTS ON FUTURE DEVELOPMENTS
FOR EUROPEAN RESEARCH INSTITUTIONS

Sibuet J.-C. and Needham H.D., 1998, Proceedings of the 1st "Assemblea Luso-Espanhola de Geodesia e Geofisica", in press

 

Abstract

Today, sixty years after the first seismic experiments at sea, multichannel (MCS) reflection investigations by the petroleum industry have reached an unprecedented level of sophistication. The situation for oceanographic research institutions, concerned with a wider range of objectives and competing priorities, is different. In Europe, very high resolution and high resolution studies are within the ordinary compass of individual institutions, including the development of deep towed packages. So is the operation of ocean bottom seismographs (OBSs). On the other hand, at least for some countries, investing in a major deep penetration MCS facility on an institutional or national basis raises questions.

Five EU countries (France, Germany, Italy, Spain, the United Kingdom) and Norway conduct MCS investigations at the present time but at a restricted level of investigating power. Most of the gun array capacities of the seven systems (there are 2 in Germany) are limited. Only two vessels have big built-in compressors. Streamers are of medium length and all except one are analogue. The net result is that European oceanographic research institutions, and hence Europe, cannot work at the front line of deep exploration of the lithosphere. Yet some fundamental scientific problems can be addressed only there, a better understanding of deep structure than exists today will be required to target future deep drilling objectives and there are new possibilities for collaboration with the petroleum industry in this area of fundamental research.

Only a few ships in the research fleets of European countries could be more or less readily adapted to accomodate a large, modern seismic facility. Were one or more suitably equipped vessels to exist, this could, in principle, open the way to their use by scientists of other countries, for example through bilateral agreements or perhaps through an EU support programme for the use of sea-going facilities. Other possibilities include chartering industrial vessels or acquiring a shared research facility for deep penetration investigations as a complement to the more conventional or specific facilities which research institutions need to maintain and improve for their own purposes in different countries.

 

MCS SYSTEMS OF EUROPEAN COUNTRIES

 

	FRANCE (Ifremer)	GERMANY* (BGR)	UK (NERC)	SPAIN (NRA)	NORWAY (Bergen Univ.)	ITALY** (OGS Explora)
SOURCE	12 GI Sodera 2520 c.i. or 8 bolts (max 8000 c.i.) 2000 psi	20 air guns in 2 arrays 3124 c.i. 1920 psi	14 air guns max. 6400 c.i. max. 2000 psi	7 air guns in 2 arrays 1930 c.i. 2000 psi	7 air guns in 1 array 1956 c.i. 2000 psi	32 air guns 4 subarrays 5000 c.i. 2000 psi
STREAMER	analogue AMG (Sercel) 96 channels 2400 m	digital Syntron 120 channels 3000 m	analogue Teledyne 96 channels 2400 m	analogue Teledyne 40508 96 channels 2400 m	analogue Fjord Instrum. 120 channels 3000 m	analogue   120 up to 180 ch 3000 to 4500 m
ACQUISITION SYSTEM	SERCEL 358 DMX SEGD	SYNTRON Syntrax 480MSRS SEGD	SERCEL SN 358 SEGB	TEXAS Inst. DFS V SEGD (exabyte)	TEXAS Inst. DFS V SEGY	SERCEL 358 DMX SEGD
AGE	1988	1995	1983	1991/1994	1996	1988
MEAN UTILISATION	3 months/yr	3 months/yr	0.5 month/yr	1.5 month/yr	0.7 month/yr	3 months/yr?
FUTURE	increase streamer length and GI source	increase streamer length and source	new DAT lab 1-2 months/year increase shiptime availability	upgrade source	no plans	ship availability?

 

* Information incomplete (Alfred Wegener Institute system not included)

** Information incomplete (University of Bologna system not included)

Technical characteristics of MCS systems presently used in European countries. The future evolutions of the systems as indicated are either planned or are under discussion. The light University of Bologna system, not included in the table (2 GI guns of 7 l total volume, 600-m long analogue streamer), is used about 3 months/year. AWI, Alfred Wegener Institute; BGR, Bundesanstalt für Geowissenschaften und Rohstoffe; NERC, Natural Environment Research Council; NRA, National Research Agency; OGS, Osservatorio Geofisico Sperimentale; Darwin, RSS Darwin; Disc., RSS Discovery; Hesper., R/V Hesperides; Polarst., R/V Polarstern; Ross, RSS James Clark Ross; Sonne, R/V Sonne.

 

Seismic sources

 

FRANCE (Ifremer)	12 GI Sodera 2520 c.i. or 8 air guns max 3000 c.i.
GERMANY (BGR)	20 air guns 3125 c.i.
UK (NERC)	14 air guns max. 6400 c.i. max.
SPAIN (NRA)	7 air guns 1930 c.i.
NORWAY (Bergen Univ.)	7 air guns 1956 c.i.
ITALY (OGS Explora)	32 air guns 5000 c.i.

 

In general, seismic sources are too weak for deep seismic penetration.

Streamers

 

 

FRANCE (Ifremer)	96 channels 2400 m
GERMANY (BGR)	120 channels 3000 m
UK (NERC)	96 channels 2400 m
SPAIN (NRA)	96 channels 2400 m
NORWAY (Bergen Univ.)	120 channels 3000 m
ITALY (OGS Explora)	120 up to 180 channels 3000 to 4500 m

 

- All streamers are at least 96 channels

- No very long streamers

- All analogue except for BGR digital streamer.

 

Annual MCS utilisation

 

FRANCE (Ifremer)	3 months
GERMANY (BGR)	4 months
UK (NERC)	1 month
SPAIN (NRA)	1.5 month
NORWAY (Bergen Univ.)	0.7 month
ITALY (OGS Explora)	3 months

 

 

 

Very low or low rate of utilisation for all countries.

Summary

 

1) 7 MCS systems in Europe but most with weaknesses (e.g. seismic sources, streamer length, compressors ...) and question of shiptime availability.

2) Rate of utilisation very low for each individual system.

3) Total utilisation of MCS equipment in Europe:

- about 13 months/yr

- Mob/demob and transits: about 13 months/yr

(equivalent to more than one full time operated seismic system for all European countries).

4) Large investment in Europe: 20 MEcus?

or more?

5) Taking the European countries as a group:

- waste of money and inefficiency,

- difficulty to tackle most deep crustal problems during the next 10 years.

 

Observations

1) Large European facilities:

- GEOMAR seismic reflexion processing Center

- Pool of OBSs under discussion.

2) Several "Margins" initiatives exist, will exist or will be supported in Europe (UK, France...).

3) The scientific community is hurting. It needs good modern seismic data. There is a clear need for high quality crustal investigations (both MCS and seismic refraction) requiring:

- long digital streamer, minimum 6 km up to 12 km long

- large seismic source volume (more than 10,000 c.i.)

- 480 channels recording lab

- 30 to 50 OBSs are needed to constitute a significant pool.

4) Operating this system would require a chartered seismic commercial ship or a dedicated seismic vessel (cf. US and Japan).

Questions

1) Use the "EuroMargins" initiative (ESF) to test interest of European countries in supporting a project for a European seismic research vessel?

2) If enough support, proposition to Brussels (via ESF?, via countries?)?

3) Approach to oil companies?

4) Ridge investigations?

5) Co-ordinated access of ship to all participating European oceanographic countries, large or small.

6) The ship a shared tool, not a programme?

7) Essential to keep (and if possible to improve) existing seismic systems.

EGR deep-tow

(Ensemble Géophysique Remorqué)

1. Swath bathymetry and sidescan sonar (50-60 kHz) for a 2-km swath

2. Mud penetrator and seismic devices

- Source: Chirp low frequency on the weight of the deeptow (250-3000 Hz) or conventional surface source

- Streamer: digital (10-5000 kHz), 50-m long, expected penetration 2 km with the chirp source

- Mud penetrator: streamer to increase longitudinal directivity, a few transversal antennas (every 5°) to increase lateral directivity (pseudo 3D)

3. Magnetometer and gravimeter

- proton or 3-component magnetometer

- gravimeter later

4. Sea-water physical and chemical properties

- CTD, nephelometer, ADCP

- in situ chemical analyses (Mn, H2S)