Marine Geosciences

 

OTC 11948

Marine Seismic Investigations: A Perspective from France and Europe

J.-C Sibuet and H.D. Needham, Ifremer Centre de Brest, B.P. 70, 29280 Plouzané, France

Copyright 2000, Offshore Technology Conference

This paper was prepared for presentation at the 2000 Offshore Technology Conference held in Houston, Texas, 1­4 May 2000.

This paper was selected for presentation by the OTC Program Committee following review of information contained in an abstract submitted by the author(s). Contents of the paper, as presented, have not been reviewed by the Offshore Technology Conference and are subject to correction by the author(s). The material, as presented, does not necessarily reflect any position of the Offshore Technology Conference or its officers. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of the Offshore Technology Conference is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of where and by whom the paper was presented.

---

Abstract
Research institutions in European countries conduct seismic experiments to investigate the geology of all the provinces of the ocean floor. Some objectives of deep reflection-refraction studies of continental margins are of mutual interest to academia and the oil industry. An inventory of existing facilities in Europe for carrying out these studies suggests that, chartering of commercial ships aside, the research potential could be enhanced by combining forces (MCS equipment, ocean floor seismographs, ships) and, in due course, by the operation of a dedicated or semi-dedicated seismic vessel for large experiments.

Introduction
In the early days of marine seismic exploration "oceanographers tried to get support from the oil companies in adapting geophysical methods for use at sea, but it was said that no one would ever want to look for oil underwater"1. Today, hydrocarbons are being produced in water depths down to 1.5 km, petroleum reconnaissance extends into water depths greater than 3 km and 3D multichannel seismic (MCS) reflection investigations using multiple streamers have become the norm. The new "triangular" exploration vessels, starting with the Ramform Explorer in 1996, about 40-m wide at the stern, tow two arrays of guns, shot alternatively, and up to twelve 6-km long streamers spaced every 25 m, with 16 streamers already envisaged. These ships currently conduct 3D surveys of areas of up to 10,000 km2.
Oceanographic research institutions cannot compete with service companies in 3D MCS data acquisition but, for them, seismic exploration of the ocean floor covers a far broader range of geological objectives than those of the petroleum industry. Here, we refer to different kinds of seismic experiments of interest to both academia and industry in terms of depth of targets and resolution. We focus in particular on low frequency, deep penetration, investigations and consider the potential of European research institutions to conduct them.

Background
The first seismic experiments at sea were made on the New Jersey continental shelf in the late 1930's 2 and in the English Channel 3. Renewed interest in the ocean floor after World War II, coupled with the availability of surplus explosives and more sensitive hydrophones, gave an impetus to the extension of seismic exploration into deep water. Vertical incidence reflections were recorded during the 1947-1948 Swedish deep-sea expedition 4. Numerous reflection profiles were shot in the North Atlantic and deep-water refraction data collected there 5-8 and in the Pacific. By about 1960, the principal features of the seismic layering of the oceanic crust had been determined by refraction measurements 9,10 and the first order thickness and distribution of continental margin, mid-oceanic ridge and abyssal floor sediments had been established.
Although dynamite provides high explosive energy, logistics were difficult and low firing repetition rates were a hindrance to the correlation of sub-sea floor echoes. A number of other sound sources for shallow penetration were built in the 1950's, including the electrical sparker, the boomer, gas exploders and low-frequency echo-sounders. For deep penetration, dynamite and other explosives continued to be used well into the 1960's. In the early 1970's, again to overcome disadvantages of the dynamite charge (e.g. radiation of near-field destructive energy in all directions), a new generation of devices was developed, principally to meet the requirements of the petroleum industry (e.g. Lugg 11). A few, among many, examples include mechanically operated explosive sources such as Maxipulse and Flexotir, implosive sound sources such as Flexichoc and Vaporchoc, and the Simplon Water Gun. The single most important advance in controlled source design was the concept of the (Bolt) airgun, invented in the Lamont Geological Observatory in the middle 1960's. Today, dynamite is employed only for very specific purposes (e.g. some studies of the oceanic crust). Standard sources for high and medium energy requirements are air guns (Bolt (PAR), sleeve air guns and/or the recent Generator-Injector (GI) guns 12).
The first recording arrays consisted of a towing cable with a few hydrophones suspended from it. One technique for improving the signal/noise ratio, the so-called slacking method, was to pay out the cable at the speed of the ship each time the dynamite charge was fired, and haul it in after recording the seismic signal. Hydrophone arrays were subsequently enclosed in a plastic oil filled sleeve and the principle of the modern streamer thus developed. Starting in the early 60's, when digital recording laboratories became available, the first multichannel streamers were built to obtain multi-fold coverage. During the same period, ocean bottom seismographs (OBSs) began gradually to replace sonobuoys for most purposes. In more recent years, MCS recording technology has been improved by the introduction of digital streamers, digital recording has increased the capacity of OBSs and the quality of the records and the development of high-speed computers has enabled the introduction of 3D seismic investigations.

 

Depth of Targets and Resolution
In addition to geological context, depth of targets and water depth, a number of interdependent variables and constraints enter into the design of an experiment at sea (characteristics and geometry of seismic sources and receivers, firing rate, survey speed, time and cost). We follow the convention of grouping experiments into three categories, corresponding to the inverse relationship of frequency and depth of penetration: Very High Resolution (VHR), High Resolution (HR) and Low Frequency (LF). We arbitrarily associate VHR experiments with a source frequency range of 500-2000 Hz, HR investigations with a frequency range of 100-500 Hz and LF, deep target shooting with a frequency range of 5-50 Hz.
Most MCS instrumentation has been developed to meet the specifications of petroleum companies for images of potentially oil-bearing strata in the sedimentary basins of the continental shelf and slope. Except for reconnaissance profiling, oil company geophysicists have always searched for the best available resolution at the depth of their objectives (reservoirs) within the sedimentary sequence overlying basement.
VHR systems are widely used by service companies to acquire information for anchoring drilling platforms and for detecting shallow hazards (e.g. gas pockets) on the continental shelf and upper slope. HR systems are used for investigating deeper targets, down to about 2 km, and in water depths of less than about 1500 m. For continental shelf objectives, petroleum geophysicists combine high frequency sources with MCS streamers with short (6.25-12.5m) hydrophone group (channel) intervals. Long cables are now used for amplitude versus offset (AVO)/seismic attribute/facies analysis and great insight is given into geotechnical and shallow hazards issues via coherence cubes. For surveys requiring deep penetration and for deep water work, which imply decay in the amplitude of the signal and in resolution, petroleum geophysicists employ LF sources and MCS streamers with relatively long group intervals. Long streamers are equally needed for multiple suppression and to increase the resolution of stacking velocities.
Objectives in fundamental research concern all the provinces of the ocean floor and range from the stratification of superficial and deep sedimentary sequences, to the depth and morphology of basement features and to the structure of the lower crust and upper mantle. Consequently, seismic investigations at all frequencies have been conducted by research institutions since the early days of modern oceanography.

VHR Investigations
Short, single-channel VHR seismic data have long been gathered on the continental shelf as analogue and, more recently, digital data (e.g. Lericolais et al. 13). VHR data are used to build and refine models of sediment transport, erosional and depositional mechanisms and patterns (e.g. Rabineau et al. 14) and to explore relationships between sediment stratigraphy and sea-level variations (e.g. Posamentier et al. 15). Favored sources are electrical sparkers. The typical, effective maximum depth of penetration, constrained by the depth of the first multiple, is about 100 m. This is approximately the same as the maximum reach of low frequency (2.5-5 kHz) echo sounders (ultra-high frequency sources in the seismic jargon), which yield good records only for very fine grained sediments. The vertical resolution aimed at for VHR investigations is about 1 m to a few tens of centimeters. New developments concern the use of multichannel streamers (Fig. 1) and 3D acquisition systems coupled with conventional or specific processing tools (Fig. 2).

Because of the decrease of source energy by geometrical spreading within the water column and of resolution with depth, VHR data in the upper range of frequencies cannot be acquired from the sea surface in areas deeper than about 300 m. They can be collected in deep water only with a combination of a deep-towed source and streamer, such as was developed by the Naval Ocean Research and Development Activity (NORDA) in the late 70's and early 80's 16. Suitable new sources include chirps. A source of this type is being considered for the deep tow EGR (Ensemble Géophysique Remorqué) system, the successor, now under study, of the Ifremer 6000m Sar/Pasisar package. The possible frequency range is 250-2000 Hz. For the lowest frequencies in this range, the potential depth of investigation of fine-grained sedimentary sequences may be as much as 1500 m or more provided that the source energy turns out to be powerful enough. Deep-towed systems of this sort offer interesting possibilities for the fine-scale study of deep sea Quaternary sequences.

 

High Resolution Investigations

Sources today include mini GI and sleeve air guns and provide penetration of up to about 2000 m of sediment (about 1.5 second two-way travel time), depending on the source frequency and energy content. Depth of penetration was initially limited by the presence of the first multiple. Developments include the use of mutichannel streamers (with short (e.g. 6.25 m) group intervals) which enables multiples to be eliminated by data processing. With improvements of this kind, studies of seismic sequence stratigraphy in shallow water can be extended to most of the late Tertiary.
The width of the first Fresnel zone (width of the area over which the seismic information is stacked) increases significantly as a function of water depth and there is thus no way to acquire HR data (lateral resolution <15 m) in deep water (>1500 m) with conventional devices (source and recording system at the sea-surface). The only way is to use either a hybrid arrangement (source at the sea-surface and recording system near the seafloor) or a fully deep towed device (source and recording system near the seafloor). For a 300 Hz frequency source, for example, the 120 m width of the Fresnel zone for a conventional device decreases to 25 m for a hybrid system and to 15 m for a fully deep towed device. For an intermediate frequency (60 Hz) source, the 600 m width of the Fresnel zone for a conventional arrangement decreases to 130 m for a hybrid system and to 80 m for a fully deep towed device (Fig. 3).

A disadvantage of the hybrid system is that the geometry of acquisition (source-hydrophone array-seafloor) induces large incidence angles of the seismic signal and necessitates appropriate specific processing 17. In addition, absorption (at least for very high frequencies) and wave front expansion of the signal decrease its amplitude as a function of the length of the propagation path through the water column. In spite of these difficulties, hybrid systems with the recording array near the seafloor can provide, in at least some geological contexts, a close approach to reasonably good quality HR data for the upper two kilometers or so of the sediment column in water depths down to 6000 m. The point is illustrated by comparing a portion of a record (Fig. 5b) acquired with the deep towed Pasisar streamer perpendicular to the deep Galicia margin with a conventional MCS section shot along the same line (Fig. 5a). The hybrid, Pasisar data show, for example, that the tilted block in the profile is not bounded by a single listric fault, but by several 20° dipping normal faults with vertical offsets of several hundred meters.

Deep Objectives

Significant work has been done by European research institutions on the seismic structure of the Mid-oceanic Ridge, back-arc basin spreading centers and old oceanic crust, plateaus and large igneous provinces.
Reflection studies have been used to image the Moho in old crust and have been successful, for example, along the fast spreading East Pacific Rise in identifying Moho and mapping the roofs of magma chambers there 18,19 (Fig. 6), first imaged in 1981 with a single channel streamer and one Simplon Water Gun 20. More recent investigations have involved the coupling of 2D reflection profiling with the deployment of closely spaced seafloor instruments for refraction studies and, in large operations, networks of many instruments together with the incorporation of data from earthquake seismology. The unprecedented MELT experiment, conducted on the East Pacific Rise under the auspices of the US Ridge program, combined seismic and electromagnetic observations to determine the pattern of upwelling beneath the ridge and the geometry of the zone of partial melting 21. Seismicity and refraction data recorded by OBSs have begun to shed new light on difficult problems such as along and across-strike, intra segment variations of the crustal architecture and thickness of slow spreading ridges.

In continental margin research, seismic objectives have, with time, become increasingly deep in order to approach questions such as the structure and rheology of the lower crust and upper mantle, melt generation and migration, or the nature of the seismogenic zone beneath active margins and the origin of fluid circulation in accretionary prisms. All these studies have been carried out either in areas of no interest to the petroleum industry (ridges and deep oceanic basins) or, in the case of continental margins, beyond its requirements in terms of depth of penetration.
To obtain deep reflections, research institutions have increased the length of their streamers and the capacity and efficiency of their seismic sources. A major breakthrough has been the single bubble "Monobulle" principle of increasing the power content of air guns in the low (10-25 Hz) frequency range and reducing energy in the higher frequencies (>25 Hz), which introduces noise that can pollute the signal in the useful, low frequency range 22,23. The result is an increase in useful energy by a factor of about 2.5-3. For example, the energy provided in the 10-25 frequency band by a 4000 c.i. gun array in the "Monobulle" mode is equivalent to that of a 10 000-12 000 c.i. conventional array.

Some critical information cannot be obtained without refraction data (to give velocity structure), acquired by using two ships or with standard OBSs, or with ocean bottom vertical arrays (OBVSAs), which represent a leap forward in terms of signal/noise ratio and velocity resolution (Avedik, 1986 #138).
The value of combining reflection and refraction methods is illustrated here by a profile (IAM11) of the Iberian margin (Fig. 4), shot by the commercial ship Geco Sigma using a 120 l airgun source and a 4.8-km streamer with 192 channels24. In this profile, the rotated fault blocks, bounded on the western side by normal faults and well expressed in the upper part of the margin, overlie the strong, smooth S reflector, which disappears east of the ODP site 639 (Fig. 7a). Mantle reflections are displayed only in the upper part of the margin (km 30 to 60). At the base of the continental slope is a prominent north-south feature, the Peridotite Ridge, composed of serpentinized peridotites 25,26. The velocity model derived from iterative pre-stack depth migration (Fig. 7b) does not provide any velocity information for the lower crust and upper mantle. However, the model (Fig. 8a) resulting from a seismic refraction experiment using OBSs does do so. It is consistent with the reflection velocity model, established from MCS data for the sediments and for the upper portion of the tilted fault blocks (Fig. 8b), and it shows that the continental crust thins westward to 2 km east of the Peridotite Ridge, which is located near the ocean-continent transition. West of the ridge, the oceanic crust is about 3 km-thick but it thickens oceanward to 7 km. The ridge is thus shown to cap a ~ 60-km-wide lens-shaped serpentinized peridotite body underlying both thinned continental and thin oceanic crust 27.
The structural interpretation of line IAM11 shown in Fig. 7b includes constraints derived from both reflection and refraction data. It demonstrates, for example, that the Moho is displayed continuously in the reflection data in the eastern part of the profile but can be deduced, at the base of the serpentinized peridotite body, only from the refraction data because the velocity contrast beneath the body is too weak to be detected as a reflector.
In the next decade, important advances in understanding the formation of continental margins and of the oceanic crust will depend on increasingly better information of the type illustrated by the data acquired along the IAM11 line.

Seismic Equipment of European Research Institutions
MCS Systems. Most of the main oceanographic countries in Europe have operated deep water MCS systems for ten years or more and continue to do so: France, Germany, the UK, Spain and Norway (Table 1) and, until very recently, Italy.

Table 1 - Technical characteristics of MCS systems presently used in European countries. The future evolution of the systems as indicated are either planned or are under discussion. Italy is not included in the table: the light University of Bologna system (2 GI guns of 7 l total volume, 600-m long analogue streamer), is used about 3 months/year; the R/V Explora (Osservatorio Geofisico Sperimentale), equipped with a 3000 to 4500 m long analogue streamer and a 80 l total volume air gun array, was decommissioned in 1998. AWI, Alfred Wegener Institute; BGR, Bundesanstalt für Geowissenschaften und Rohstoffe; NERC, Natural Environment Research Council; NRA, National Research Agency; Darwin, RSS Darwin; Disc., RSS Discovery; Hesper., R/V Hesperides; Polarst., R/V Polarstern; Ross, RSS James Clark Ross; Sonne, R/V Sonne.
	FRANCE	GERMANY		UK	SPAIN	NORWAY
	(Ifremer)	(BGR)	(AWI)	(NERC)	(NRA)	(Bergen Univ.)
SOURCE	14 GI Sodera 3 G Sodera 8 Bolt array of 48 l max. (3000 c.i.) at 20 s 140 bars	20 air guns in 2 arrays 51,2 l (3125 c.i.)   134 bars	8 air guns 24 l (1500 c.i.)     120 bars	14 air guns max. 102 l max. (6400 c.i.)   140 bars	7 air guns in 2 arrays 31 l (1930 c.i.)   140 bars	7 air guns in 1 array 32 l (1956 c.i.)   140 bars
COMPRESSORS	2160 m3/h     portable	Sonne 3470 m3/h   portable 1300m3/h	Polarst. 1800 m3/h   portable 480m3/h	Darwin 630 m3/h Disc. 3790 m3/h Ross 2220 m3/h portable 630 m3/h	Hesper. 630 m3/h   portable 630 m3/h	600 m3/h     portable
STREAMER	digital Sercel 360 channels 4500 m	digital Syntron 120 channels 3000 m	analogue 96 channels 2400 m	analogue Teledyne 96 channels 2400 m	analogue Teledyne 40508 96 channels 2400 m	analogue Fjord Instrum. 120 channels 3000 m
ACQUISITION LABORATORY	Sercel SEAL (SN 408) SEGD	Syntron Syntrak 480MSRS SEGD	Geometrics ES2420 SEGD	Geometrics Strata View RX96 SEGD(DAT)	Texas Inst. DFS V SEGD (exabyte)	Texas Inst. DFS V   SEGY
AGE	1999	1995	1994	1984/1998	1991/1994	1996
MEAN UTILISATION	3 months/yr	4 months/yr	3 months/yr	1 month/yr	1.5 month/yr	0.7 month/yr
NEAR FUTURE	increase compressor air-capacity	increase streamer length and source	new streamer in 2000 upgrade source with GI guns	1-2 months/yr increase in ship- time availability upgrade source	upgrade source	digital streamer?

The seismic sources of all systems consist of air-gun arrays, commonly used at a pressure of 140 bars. The arrays vary from 30 to 100 l in air-capacity. Some are filled only by portable compressors able to deliver 600-2000 m3/h. The RSS Discovery and R/V Sonne have large powerful built-in units (3790 and 3470 m3/h respectively) and the RSS James Clark Ross (2220 m3/h), the R/V Polarstern (1800 m3/h), the R/V Hesperides (630 m3/h) and the R/V Darwin (630 m3/h) have complementary, built-in compressor capabilities. Only the German BGR system, and since 1999, the Ifremer system in France, are fully digital. Streamer lengths vary between 2400 and 3000 m (96 to 120 channels with a 25-m group interval) except for the new 4500 m Ifremer streamer (360 channels with a 12.5 m group interval).
The total investment in MCS equipment in Europe is about 30 million US dollars (about 4 to 5 M dollars for each of the 7 systems). The average rate of utilization per system is about 2 (0.7 to 4) months a year, which reflects the interaction of demand and logistical constraints (e.g. shiptime, technical teams, budgets). Compared with the annual utilization rate of MCS equipment in the US or Japan, the total for the European systems of about 15 months a year (with, in addition, about 12 months for mob/demob time and for transits) represents a high overall effort.
The age of most of the MCS equipment in Europe is less than 10 years but the technology is at least 15 years old, except for that of the German (BGR) and French (Ifremer) digital streamers. No institution operates a seismic ship such as, until last year, the Australian R/V Rig Seismic, or a semi-dedicated vessel specially adapted for seismic work such as the 75 m R/V Maurice Ewing in the United States or the 105 m R/V Karei in Japan, commissioned in 1997 for use as a platform for seismic operations and the 11,000 m ROV "Kaiko". In France, the R/V Nadir is now fitted out mainly for seismic cruises (MCS, Pasisar, etc.) but is near the end of its natural life.
The new Ifremer system places it only a couple of steps behind that of the R/V Ewing, the current, but not ultimate reference for a research institutional sea going, single streamer seismic facility. The R/V Ewing (a former industrial seismic ship), operated by the Lamont-Doherty Earth Observatory and used primarily for expeditions supported by the National Science Foundation, has an excellent gun-array towing system and was re-equipped in 1997 with a 6-km long digital streamer (240 channels with 25-m group interval, with varying group intervals down to 6.25 m), a 480 channel recording laboratory and a tuned array of 20 air guns with a maximum total volume of 180 l.
In general, MCS data processing in Europe is carried out by individual laboratories for their own purposes. Geomar (Kiel, Germany) runs a high performance processing center partly funded by the European Union (EU) and open to EU scientists.

OBS Pools. There is a long tradition in some research institutions in the use and development of sea floor recording instruments. Today, except for a pool of 30 OBHs, located at Geomar and presently co-funded by the EU for cooperative programs, OBS equipment in Europe is operated by approximately 10 individual geophysical groups in different countries who use OBSs for their own research programs. The total number of instruments has increased in recent years and now numbers over 100 of different make. Recent acquisitions have been mainly Cambridge or Geomar built instruments. Aside from the drawbacks of combining diverse types of instruments, the need for large numbers of OBSs for some experiments could be already at least partially met by collaboration among geophysical teams in Europe.

Combinations of Equipment and Ships. With a combination of the seismic equipment of two nations (e.g. the Ifremer digital streamer and laboratory, a gun array combining the sources of two countries and a British or German ship with inboard compressors), the result would be a performant 2D MCS system except for streamer length. However, if another country were to acquire a Sercel compatible digital streamer, this could then be combined with the Ifremer streamer. In this regard, it is worth noting that CGG (Compagnie Générale de Géophysique), the mother company of Sercel has recently acquired the Syntron company.
Extended multiple coverage and velocity structure could be obtained by using two ships to shoot an identical profile with different constant offsets 28. In this approach, which requires very precise navigation, one ship would tow the Ifremer (4.5 km) streamer and the other, say, the BGR (3 km) streamer, with each ship operating an identical gun array drawn from existing equipment in, say, France, Germany or the UK. With three passes over the same profile, the resulting MCS profile would be equivalent to a profile obtained using one ship and a single, 27-km long streamer. Such experiments are ideal for addressing the problem of imaging structures below volcanic and salt deposits and to characterize the velocity field down to the upper mantle. Another way to get even longer range information is to use as many as a hundred (or more) OBSs.

Research Programs
In addition to studies of their Atlantic margins and in the Mediterranean Sea, European countries work in all oceans. The UK, France and Germany, in particular, have conducted, and conduct seismic cruises worldwide, including in polar regions. The Alfred Wegener Institute system is specially adapted, and used only, for work in ice infested areas 29. Spain and Italy have concentrated their distant activity mostly around Antarctica and in the south Pacific and Atlantic oceans. The overall European effort has concerned oceanic crustal studies as well as those of continental margins. Margin studies are the principal vector in Europe today for getting support to update and improve seismic facilities. Although objectives in studies of deep oceanic basins and ridges are different to those in continental margin research, the necessary seismic facilities are essentially the same.
Ridge research in all the oceans has been stimulated during the past 10 years by the existence of the US RIDGE program, the international InterRidge program and the many national ridge programs in Europe and around the world which liase with InterRidge. Results of the recent, multi disciplinary work on ridges has emphasized the need for seismic data and enable more educated selections of targets than was previously possible. Future priorities are likely to include a big experiment of the MELT type 21 on a slow-spreading ridge and comparative studies of the 3D architecture of crustal units at the segment scale.
The US "Margins" program, initiated in 1997, with an emphasis on active processes, has been followed by national margins programs in France and the UK. An international initiative, InterMargins, a homologue of InterRidge, has been started, with Germany (Geomar) as the first host country. A European Science Foundation network "Ocean Margins" has been created. Furthermore, the recently accelerated interest of petroleum companies in, for them, ultra-deep (>1500 m) water renews opportunities for European research institutions to work interactively with industry, as many did before the early 80's when the oil companies curtailed their deep water reconnaissance programs.
The margins initiative in France "Groupement de Recherche Marges" (GDR Marges) brings together the principal research institutions concerned (CNRS-INSU, Ifremer, BRGM and IRD (previously ORSTOM)), together with IFP and partners from the petroleum industry (Elf and Total-Fina). The GDR Marges supports projects of common interest agreed on by the participating members. In addition, sea-going collaborative actions between research institutions and industry have recently been renewed. The most important of these is the co-financed Ifremer-Elf, 3-4 year ZAIANGO project concerning the west African continental margin. The project involves a total of several months at sea and draws on a broad base of Ifremer scientific expertise, facilities and instrumentation (multibeam bathymetry and imagery, SAR (deep-tow) imagery, magnetics, gravimetry, high and low frequency seismics with simultaneous use of OBSs, the deep towed seismic recording package Pasisar, sediment coring, heat flow measurements, investigations with the ROV Victor and biological sampling, including traps moored on the sea floor).
In the UK, the new 5-year, multidisciplinary LINK program was initiated with the purpose of improving knowledge of the geology of ocean margins. The program is designed to encourage the UK research community, in partnership with industry, to address challenges in exploring for, and developing, deep-water oil fields and aims to encourage industry and academic technological collaboration. The Natural Environment Research Council (NERC), which will contribute up to £4.5 M, and government departments will provide 50% of the funding, with the other 50% to be provided by industry in cash or kind.
In Norway, scientific collaboration has traditionally existed between Norwegian universities, industry and the National Petroleum Directorate (NPD), which supervises hydrocarbon exploitation. This collaboration provides an efficient framework for the exploitation of data acquired by the petroleum industry, or provided to the Norwegian authorities by foreign companies working in the Norwegian sector. A national program is now under consideration, reflecting official encouragement to have an organized participation in InterMargins.
The ESF (European Science Foundation) sponsored network, "Ocean Margins", created in 1999 and to run for 18-24 months, will review the current situation of margins research in Europe with a view to proposing a program of common interest to participating European institutions. A principal focus is likely to be the deep structure of selected conjugate margins of the Atlantic. Possible sources of funding are the European Union's Framework Program 5 (EU-FP5), national funding agencies and the oil industry.

Shared Interests of Academia and Industry
In general, subjects of mutual interest include (i) sedimentary architecture and dynamics (ii) fluid flow and gas hydrates, (iii) the development of predictive models of continental margin formation, which need to take in account relationships between factors such as basement tectonics, the amount of crustal thinning, the strength of the extended continental lithosphere, the thermal effects of magmatism (underplating and intrusives) and the flux of sediments.
Although the motivations of the petroleum industry in continental margin exploration (deep water hydrocarbon production) and those of research scientists (defining the structure and evolution of the crust and deep basins) differ, many of the required data are the same. For example, because temperature controls degree of maturation and the viscosity of hydrocarbons, a good estimation of the past temperature regime in time and space is needed in order better to define source rock-maturation-migration-reservoir systems. The development of realistic thermal models to constrain temperature regimes depends on a better understanding of the mechanisms of continental margin formation (and hence of deep structure), which are first order questions in fundamental research. Another example of convergence of interest in LF seismic work concerns continental margins with volcanic sills or salt deposits within the sedimentary column. These produce topographic complexities and inversions of seismic velocities. Therefore, they act as seismic screens over deep structures of interest to academic scientists and over hydrocarbon source rocks and reservoirs of interest to the oil industry. Penetration through the volcanic and salt layers poses problems for the design of seismic experiments which research institutions are well placed to address.
Continental margin segmentation is another subject of shared interest. The origin of the segmentation, its relationship to pre-existing zones of weakness and mantle dynamics and the implications of segmentation in terms of the structure of the adjacent oceanic lithosphere is of great interest to academic scientists. The significance of the transition between volcanic and non-volcanic margin segments is one example of unanswered questions. Transfer zones between segments are of interest to industry because some major oil fields are associated with them. Progress in this area requires dense nets of MCS lines and ocean floor instruments; though being discussed, no major experiments of this type have yet been undertaken by research institutions.

Ships and Equipment: National and European Approaches
The total investment represented by the sum of the MCS equipment in Europe, the duplication of much of this equipment and its aggregate use of approximately 1 year per year (Table 1) suggests an opening for a vessel optimally equipped to address front line problems concerning the deep structure of continental margins and the oceanic crust.
Large MCS and refraction experiments pose the problem of support vessels because of the size and weight of the material and other operational constraints. A few ships in European research institutions could well accommodate the necessary equipment, having big stern desks and built-in compressors; none has an ideal gun-array towing system. The presence of a fully equipped vessel in any one country would, in principle, provide a basis for limited time sharing under bilateral or other exchange agreements, or for access to other European institutions through EU support for the use of large facilities.
Another possibility is that of a European ship. This idea, evoked periodically by European marine geophysicists and which we raised again recently in at an exploratory ESF ocean margins workshop in 1998, does not appear to find favor or be thought viable at the present time.
Apart from questions concerning the technical and administrative organization of such a project, a European ship, whose on-site working time would be about 7-8 months a year, would imply new money and/or impact on the sea going time for seismic research in the participating countries. At least most European institutions and nations currently operating MCS systems would likely want to maintain and update them to meet requirements not covered by limited access to a European vessel. These requirements vary from country to country. They include medium penetration exploration profiling and surveys in areas of particular interest, specific projects involving surface ship and deep-towed equipment for high resolution surveys, work in ice infested seas with adapted MCS equipment and collaborative projects with industry or non European countries. Furthermore, institutional and national seismic systems are useful for testing and using new methodologies, techniques and instruments (gun configurations and arrangements, hybrid or fully deep towed systems and so forth) and, in some countries, for training and for coastal management surveys, including in co-operation with developing countries.
An alternative way to have access to a fully equipped seismic ship is to go the chartering route. A single streamer commercial vessel could be hired at about the same cost as that of operating a standard multipurpose oceanographic vessel. For a seismic ship with a 12 streamer 3D system, the daily rate is at least an order of magnitude higher (a few hundred thousand dollars). The issue of cost aside, there are also disadvantages. Objectives of research institutions fall within only a small proportion of the geographical operating sphere of industrial vessels and projects would have to be mounted case by case, both logistically and financially, unless the lease was an extended one. Furthermore, sub-contracting does not particularly favor the development of an intellectual infrastructure for seismic research, or readily allow for modification of data acquisition modes or for the incorporation of innovative technical developments. Nor would it readily enable Europe to participate in big international experiments far afield.

 

Conclusions
Coupled with drilling, experimental deep penetration marine seismic reflection and refraction investigations (in an active phase at the present time) will, for the foreseeable future, remain absolutely central to advancing knowledge about the submarine lithosphere and overlying sediments. There are present opportunities for European geophysicists to reset technical and methodological objectives for achieving scientific goals.
Significant work can already be done with existing equipment in some countries. The potential can be enhanced through joint projects in which facilities (and in some cases ships) are combined. Mounting and dismounting equipment from different institutions cruise by cruise is not a formula for routine efficiency.
Taking into account the inseparability of policies concerning ships, large equipment and logistical issues, operating specialized or semi-specialized ships is worth discussing within the European theater. A fully equipped ship with a first rate seismic system for big experiments, as a complement to more routine national facilities, would not necessarily have to be a single purpose vessel. It could be, for example, a seismic and "station" (e.g. ROV) ship for use in alternative modes. It could be operated by a national agency, at least in the first instance, even if it is co-funded by the EU.

 

Acknowledgments
Information concerning the technical characteristics of MCS systems presently used in the European countries and comments on a first version of the paper were kindly provided by Paolo Berger (OGS Trieste, Italy), Angelo Camerlinghi (OGS Trieste, Italy), José Diaz (CSIC, Barcelona, Spain), Jacques Hervéou and Patrick Farcy (Ifremer, Brest, France), Karl Hinz (BGR, Hannover, Germany), Wilfried Jokat (AWI, Bremerhaven, Germany), Rob Larter (BAS, Cambridge, UK), Rolf Mjelde (Norway), Ken Robertson (RVS, Southampton, UK), Robert Whitmarsh (SOC, Southampton, UK) and Nevio Zitellini (University, Bologna, Italy). The IAM 11 MCS line was acquired through the Joule programme (IAM project). The data were processed by Véronique Louvel and Hervé Nouzé. Yann-Hervé De Roeck, Bruno Marsset and Jacques Meunier provided figures relative to VHR seismic developments. We thank Mike Coffin, chairman of the American Geophysical Union session at the OTC 2000, for encouraging us to submit this paper, an earlier, unpublished version of which was prepared following the 1st "Assemblea Luso-Espanhola de Geodesia e Geofisica", held in Almeria 9-13 February 1998. We are grateful to Vincent Renard, David Roberts, Robert Whitmarsh and particularly to Félix Avedik, for their constructive and critical suggestions.

 

References
1. Gaskell, T. F.: Under the deep oceans; Twentieth century voyages of Discovery, Eyre and Spottiswoode, London (1960).
2. Ewing, M., Crary, A. P. and Rutherford, H. M.: "Geophysical investigations in the emerged and submerged Atlantic and coastal plain, Part 1: methods and results," Bulletin of the Geological Society of America (1937) 48, 753.
3. Bullard, E. C. and Gaskell, T. F.: "Submarine seismic investigations," Proceedings of the Royal Society (1941) A222, 408.
4. Weibull, W.: "Sound explosions: Reports of the Swedish Deep-Sea expedition,"1947-1948 (1954) 4, 1.
5. Hersey, J. B. and Ewing, M.: "Seismic reflections from beneath the ocean floor," Transactions American Geophysical Union (1949) 30, 5.
6. Ewing, M., Worzel, L., Hersey, J. B., Press, F. and Hamilton, G. R.: "Seismic refraction measurements in the Atlantic Ocean basin, Part 1," Bulletin of the seismological Society of America (1950) 40, 233.
7. Officer, C. B., Ewing, M. and Wuenschel, P. C.: "Seismic refraction measurements in the Atlantic Ocean, Part IV: Bermuda, Bermuda Rise and Nares Basin," Bulletin of the Geological Society of America (1952) 63, 777.
8. Hill, M. N.: "Seismic refraction shooting in an area of the eastern Atlantic," Philosophical Transactions of the Royal Society of London (1952) 244, 561.
9. Hill, M. N.: "Recent Geophysical exploration of the ocean floor," Progr. Phys. Chim. Earth (1957) 2, 129.
10. Raitt, R. W.: The Sea, Hill, M. N. (ed.), Interscience, New York, (1963) 85.
11. Lugg, R. Developments in geophysical exploration methods ­ 1, Fitch, A. A. (ed.), Applied Science Publishers, Barking, England (1979) 143.
12. Pascouet, A. P.: "Something new under the water, the bubbleless air gun," The leading edge (1991) 10, 79.
13. Lericolais, G., Allenou, J.-P., Berné, S. and Morvan, B.: "A new system for acquisition and processing of very high-resolution seismic reflection data," Geophysics (1990) 55, 1036.
14. Rabineau, M. et al.: "3D architecture of lowstand and transgressive Quaternary sand bodies on the outer shelf of the Gulf of Lion, France," Marine and Petroleum Geology (1998) 15, 439.
15. Posamentier, H. W., Jervey, M. T. and Vail, P. R. Sea-level changes - an integrated approach, Wilgus, C. K. et al. (eds.), SEPM Spec. Pub., Tulsa (1988) 42, 102.
16. Faot, M. G. Acoustics and the seabed, Pace, M. (ed.), Bath Univ. Press, New-York, (1983) 369.
17. Nouzé, H., Marsset, B., Savoye, B., Sibuet, J.-C. and Thomas, Y.: "Pasisar: performances of a high and very high resolution deep-towed seismic device," Marine Geophysical Researches (1998) 19, 379.
18. Detrick, R. S. Ophiolite genesis and evolution of the oceanic lithosphere, Peters, T. (ed.), (1991) 7.
19. Vera, E. E. et al.: "The structure of 0- to 0.2-m.y.-old oceanic crust at 9°N on the East Pacific Rise from expanded spread profiles," Journal of Geophysical Research (1990) 95, 15,529.
20. Avedik, F. and Géli, L.: "Single channel seismic reflection data from the East Pacific Rise axis between latitude 11°50' and 12°54' N," Geology (1987) 15, 857.
21. The MELT Seismic Team.: "Imaging the deep seismic structure beneath a Mid-ocean Ridge: the MELT experiment," Science (1998) 280, 1215.
22. Avedik, F., Renard, V., Allenou, J.-P. and Morvan, B.: ""Single bubble" air-gun array for deep exploration," Geophysics 58, (1993) 366.
23. Avedik, F. et al.: ""Single bubble" marine source offers new perspectives for lithospheric exploration," Tectonophysics (1996) 276, 57.
24. Banda, E., Torné, M. and IAM Group: "Iberian Atlantic Margins group investigates deep structure of ocean margins," EOS, Transactions, American Geophysical Union (1995) 76, 25.
25. Sibuet, J.-C., Mazé, J.-P., Amortila, P. and Le Pichon, X. Initial Reports of Ocean Drilling Project, Boillot, G., Winterer, E. L. and Meyer, A. W. (eds), U.S. Government Printing Office, Washington, (1987) 77.
26. Boillot, G., Winterer, E. L., Meyer, A. W. and Shipboard Scientific Party. Initial Reports of Ocean Drilling Project, Boillot, G., Winterer, E. L. and Meyer, A. W. (eds), U.S. Government Printing Office, Washington, (1987) 3.
27. Whitmarsh, R. B. et al.: "The ocean-continent boundary off the western continental margin of Iberia - III. Crustal structure west of Galicia Bank," Journal of Geophysical Research (1996) 101, 28,291.
28. Fruehn, J. et al.: "Large-aperture seismic: imaging beneath high-velocity strata," World Oil, (1999) 1999, 109.
29. Jokat, W., Buratsev, V. Y. and Miller, H.: "Marine seismic profiling in ice covered regions," Polarforschung (1994) 64, 9.
30. De Roeck, Y.-H., Marsset, B., Meunier, J., Noble, M. and Girault, R.: "Multichannel processing of very high resolution seismic," European Association of Exploration Geophysicists, Vienna, Austria, 6-10 June 1994, abstract (1994).
31. Meunier, J., Marsset, B., Missiaen, T., Pennec, S. and Woerther, P.: "3D Very High Resolution Marine Seismic Data Acquisition," Environmental and Engineering Geophysics (EEGS European Section) Nantes, France, 2-4 Septembre 1996, abstract (1996).
32. Henriet, J.-P., Verschuren, M. and Versteeg, W.: "Very high resolution 3D seismic reflection imaging of small-scale structural deformation," First Break (1992) 10, 81.
33. Marsset, B. et al.: "Very high resolution 3D marine seismic data processing for geotechnical applications," Geophysical Prospecting (1998) 46, 105.
34. Thomas, Y., Sibuet, J.-C., Nouzé, H. and Marsset, B.: "Etude détaillée de la structure d'un bloc basculé de la marge continentale de la Galice (Ouest-Ibérie) à l'aide de données sismiques acquises près du fond (système Pasisar) : conséquences sur la structuration de cette marge," Bulletin de la Société Géologique de France (1996) 167, 559.
35. Detrick, R. S. et al.: "Multichannel seismic imaging of a crustal magma chamber along the East Pacific Rise," Nature (1987) 326, 35.