Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Post-glacial rebound wikipedia , lookup
Seismic communication wikipedia , lookup
Shear wave splitting wikipedia , lookup
Mantle plume wikipedia , lookup
Magnetotellurics wikipedia , lookup
Plate tectonics wikipedia , lookup
Earthquake engineering wikipedia , lookup
Surface wave inversion wikipedia , lookup
Reflection seismology wikipedia , lookup
Seismic inversion wikipedia , lookup
In this chapter 11.1 Introduction 311 11.2 Wide-angle acquisition 311 Seismic sources 312 Receivers 314 11.3 Modelling wide-angle data 316 Travel-time analysis 317 Amplitude modelling 318 11.4 Examples of wide-angle seismic interpretations Oceanic crust 319 Lithosphere stretching in the North Sea Volcanic continental margins 321 Sub-basalt imaging 324 References 328 320 318 11 Wide-angle refraction and reflection R.S. White Bullard Laboratories, Department of Earth Sciences, University of Cambridge, Cambridge, United Kingdom 11.1 Introduction Although near-offset seismic reflection profiling has dominated sub-surface exploration and mapping for half a century, the wide-angle refractions (often called ‘diving waves’) and reflections carry considerable additional information, particularly on the seismic velocity of the sub-surface. This enhanced velocity information may be crucial in understanding and imaging deep crustal structure. Broadly speaking, conventional near-offset reflection data provide a good image of the shapes of sub-surface interfaces, while the wide-angle data provide good control on the velocities and hence lithologies of the sub-surface. Therefore, normal incidence and wide-angle data provide complementary information. Wide-angle data may also be of particular use for specialist problems such as imaging sediments beneath high-velocity layers caused by basalt or tabular salt, for undershooting complex structures such as those created by salt domes, and for generating velocity models for pre-stack depth migration. In the following sections, I discuss first the methods of acquiring wide-angle data, then the main processing and modelling techniques used to aid interpretation of the data, and finally show a variety of case examples that illustrate the power of the technique in regional studies. 11.2 Wide-angle acquisition The term ‘wide-angle’ is often used for arrivals at offsets greater than those recorded by conventional seismic reflection surveys, capable of returning diving waves (which are usually muted before conventional seismic reflection processing as they exhibit non-hyperbolic moveout), or wide-angle reflections approaching or beyond the critical angle. The offsets required to record wideangle arrivals depend on the depth of the target: as a very crude rule-of-thumb, Principles of Geologic Analysis DOI: 10.1016/B978-0-444-53042-4.00011-X Copyright © 2012 by Elsevier B.V. All rights of reproduction in any form reserved. 311 Principles of Geologic Analysis offsets of four to five times the target depth are necessary in order to record diving waves from the target depth. So, for example, in a whole-crustal study targeted at determining the Moho depth beneath a sedimentary basin, offsets of over 100 km are required. If the target is relatively shallow, such as potentially hydrocarbon-bearing sediments beneath basalt flows, offsets of 10 20 km may suffice. But one common factor in all wide-angle profiles is that powerful seismic sources are usually required, because seismic energy suffers a large decrease in energy with offset due both to geometric spreading and absorption along the travel path. Furthermore, as every cycle of a seismic wave suffers absorption, the energy of high-frequency waves is reduced much more rapidly than that of low-frequency waves in travelling the same distance. As a consequence, arrivals from deep in the crust or those that have travelled a long way as diving waves tend to contain mainly low-frequency energy, as the high frequencies have been absorbed. For deep penetration, it is therefore necessary to focus the seismic source to produce a waveform rich in low frequencies (typically centred on about 10 Hz for deep crustal studies). A corollary of the low-frequency nature of the seismic returns is that the achievable resolution is of necessity reduced for deep or distant targets. Seismic sources The very earliest determinations of crustal structure used naturally occurring earthquake sources with limited numbers of seismometers, usually in permanent installations in observatories. Nevertheless, the major features of the crust and upper mantle were identified using earthquake arrivals by the early twentieth century. They included the definition of the Moho (strictly Mohorovočić discontinuity), now usually identified as the base of the crust, and the observation that the continental crust is usually about 35 km thick, and considerably thicker under mountain belts such as the Himalayas. Much of the nomenclature for seismic phases developed during these early studies remains in use today. Most notably, PmP is used universally to denote a wide-angle reflection off the Moho, Pg to denote a refraction through the crust (which was originally considered to be of granitic composition, hence the subscript ‘g’) and Pn for the refraction through the upper mantle beneath the Moho. Earthquake sources are large, and the arrivals can be recognised over distances of hundreds of kilometres, so they are well suited to whole-crustal regional studies. However, they are not so useful for high-resolution studies of the upper crust or sedimentary basins, nor are they useful for directed studies of particular regions. Therefore, for more detailed studies, artificially generated seismic sources (often called ‘controlled sources’) and portable seismometers were developed so that any desired region could be targeted. Interestingly, in recent years, the use of earthquakes as seismic sources has again become popular, owing partly to the availability of relatively cheap but nonetheless high-fidelity three-component, broad-band seismometers, and partly to the 312 Principles of Geologic Analysis increasing environmental regulation that makes it more difficult and expensive to use large explosive charges, which are the natural controlled source replacement for earthquakes. Furthermore, earthquake sources are rich in shear waves, whereas most controlled sources are poor at generating shear waves. The combination of information on compressional wave velocities and shear wave velocities provides much better constraints on lithology than do compressional wave velocities alone, and shear waves are particularly sensitive to the presence of fluids, so are useful, for example, for studying hydrocarbon reservoirs. Two main techniques that use earthquake sources are in common use today. The first is to use microearthquakes to make 3D tomographic maps of small volumes. This is only useful where the target area of interest is in the same region as the earthquakes, which of course is rare: two settings where it does apply are mapping regions around a sub-surface reservoir where there is active microseismicity as a result of hydrocarbon extraction, and around active volcanic regions. The more useful method, which makes use of large global earthquakes at a regional scale, is to map the main features of the crust and upper mantle using arrays of broad-band seismometers by a technique known as mapping receiver functions. Large earthquakes, typically with magnitudes greater than 6, are required, and seismometers are usually left in place for 12 18 months or more in order to record sufficient well-distributed global earthquakes with good signal to noise ratios. The advantage of this method is that any chosen area can be investigated as the earthquakes sources are global, and it is mainly just a matter of waiting until a good suite of earthquakes has been recorded. The main limitation of this technique is that only low-frequency energy (1 Hz or lower) propagates globally, so the resolution is correspondingly limited. Other particular circumstances that may limit the usefulness of receiver functions include the presence of sedimentary sections that create multiples at the same time as the primary arrivals of interest (often the Moho), and the absence of sharp interfaces capable of generating the required mode conversions. The main use of receiver functions is in mapping the depth of the Moho and of horizons within the upper mantle. For more detailed wide-angle acquisition on land, explosives remain the main source capable of producing long-range seismic arrivals, and are still used for regional studies. Swept-frequency Vibroseis causes somewhat less environmental impact, but generally does not produce sufficient low-frequency energy to propagate more than a few tens of kilometers through the crust. So Vibroseis is most useful for wide-angle studies of the uppermost crust, rather than the deeper or whole-crustal section. At sea, the use of airguns offers powerful, non-invasive seismic sources that can produce waveforms tuned to specific targets by adjusting the chamber volume, tow depth and firing sequence of individual guns in an array. For example, lowfrequency (c. 10 Hz) sources can be made by towing deep (typically 15 20 m), 313 Principles of Geologic Analysis by using large chambers (up to 1000 cu. in. or even larger), or by tuning to enhance the bubble pulse rather than tuning on the peak pulse as is conventional (Lunnon et al., 2003). By using arrays of up to 50 guns totaling c. 10,000 cu. in. volume, arrivals with good signal to noise ratio have been recorded to offsets of more than 300 km (Staples et al., 1999). Further advantages of large airgun arrays over explosive sources are the highly repeatable waveform available from airguns and the ability to fire them at short intervals, thereby providing much denser shot spacing than explosives. Receivers Because wide-angle arrivals usually require large offsets between the source and the receiver, much wide-angle recording has used stand-alone, and relatively sparse, receivers. At sea, following initial experiments with two ships, one of which lowered a hydrophone and the other of which fired a shot, the technique was developed of using free-floating sonobuoys with hydrophones suspended beneath them (Fig. 11.1A). Typically 5 10 sonobuoys were deployed with explosive or airgun shots fired out to any desired range. Of course, the sonobuoys usually drifted and data were sparse, so most interpretations using these techniques assumed a 1D crustal model with no lateral variation, at least not locally. The main improvement to this technique is to move the receivers to the seabed, where they remain fixed (Fig. 11.1B). An added bonus of seabed receivers is that three component seismometers can be recorded in addition to a hydrophone, opening the possibility of recording shear (S) waves on horizontal components in addition to compressional (P) waves and of removing water multiples by PZ summation. By the early twenty-first century, large deployments of up to 100 ocean bottom seismometers (OBS) were common (e.g. White et al., 2002), in order to provide better control on lateral inhomogeneity. For regional studies at sea, OBS remain the receiver of choice. On land, the equivalent is free-standing 3-component seismometers using a GPS signal to provide a reference time base. The disadvantage of using free-standing seismometers is that of necessity the recording array has to be relatively sparse and that it has a large spacing between seismometers. This is adequate for determination of the velocity field, but is not generally sufficiently dense for making a seismic image of the subsurface, such as is produced by conventional normal incidence or near-offset reflection profiles. One partial solution to this problem is to use a seabed cable (Fig. 11.1C). Such cables were initially only a few kilometres long, but with current technology it is possible to deploy up to 40 km in one length, with a surface ship attached to record the multichannel data in real time. However, although this allows true multichannel data to be recorded at wide angles, it is still restricted to one location and it is difficult and expensive to move the seabed cable. 314 Principles of Geologic Analysis Figure 11.1 Methods of acquiring wideangle seismic data: (A) free-floating sonobuoys with suspended hydrophones (which may record data internally, or radio-telemeter it to the firing ship); (B) ocean bottom seismometers, usually with a hydrophone plus three-component geophone, with internal recording and clock; (C) ocean bottom cable, with closely spaced receivers (typically 25 m apart), both hydrophone and three-component geophones, and recording at a surface seismic vessel; (D) expanding spread configuration using two ships to produce a multifold common mid-point gather; (E) large aperture acquisition using multiple passes of two seismic vessels sailing in line astern at fixed separations to synthesise large offsets. A Sonobuoys (hydrophone only) B Ocean bottom seismometers (3 component + hydrophone) C Ocean bottom cable (3 component + hydrophone) D Two-ship expanding spread (common mid-point) 6000 PASS 1 PASS 2 6000 6000 6000 18000 6000 E Two-ship synthetic aperture profile (offsets 0–30,000 m) The power of conventional streamer acquisition is the ease of obtaining multichannel coverage of the sub-surface. The development of digital, small-diameter streamers has made it possible to tow ever-longer hydrophone streamers. The increase from a maximum length of 2400 m in the 1970s to 12,000 m by the end of the 1990s enables wide-angle data to be recorded using a single ship with a source and towed streamer. With a 12,000 m streamer, critical angle reflections may be recorded from the uppermost few kilometres below seabed, which is adequate for many targets. 315 Principles of Geologic Analysis However, two innovative strategies using two-ship acquisition have been developed that enable offsets to be recorded to any chosen range while still retaining the advantages of a densely sampled multichannel streamer. The first of these is called an ‘expanding spread configuration’ (Fig. 11.1D; Buhl et al., 1982). By sailing two seismic vessels away from a common mid-point on reciprocal courses, the shots from one ship can be recorded across the streamer towed by the other ship. After appropriate sorting and stacking, this produces essentially a giant common mid-point gather out to any desired offset (limited only by the signal to noise ratio). Such a profile is good for wide-angle interpretation, seamlessly linking the normal incidence to the wide-angle profile. The main drawback of an expanding spread profile (ESP) is that it provides poor control on lateral variations in structure. Other than the shallow section, the assumption has to be made of 1D (i.e., laterally uniform) velocity structure. Nevertheless, provided care is taken to position the profile along a track with minimum lateral variability, such as shooting along strike, this can be a useful way of rapidly acquiring multiple wide-angle profiles out to long offsets, each with high-density, stacked data. A real example of such a survey comprising multiple ESPs is the determination of the crustal velocity structure across the continental margin west of Rockall in the North Atlantic discussed in a later section. The second technique is to steam two seismic vessels in line astern, two streamer lengths apart, with the ships shooting alternately in a flip-flop fashion (Fig. 11.1E). This is sometimes termed a ‘synthetic aperture’ profile as it is possible to enhance the aperture of recorded data beyond that of a single streamer length. Assuming reciprocity of travel times from source to receiver, the recorded multichannel data can be binned to produce a ‘supergather’ that mimics a single shot into a streamer that is three times the length of either of the single streamers. The apparent maximum offset of the supergather can be increased simply by making a second and further passes of the two ships along the same profile at successively greater separations (Fig. 11.1E). This technique retains all the advantages of multichannel processing, although at the expense of recording only half the fold of cover compared to a single ship acquisition, as a result of the flip-flop shooting. An example of the use of this technique was the recording of three passes of a pair of seismic vessels, giving offsets at all ranges up to 38,000 m on profiles over the Faroes shelf (White et al., 1999). These results were targeted successfully at sub-basalt imaging, as discussed further in a later section. 11.3 Modelling wide-angle data The workhorse of standard multichannel seismic reflection processing is stacking along trajectories with hyperbolic moveout. However, as angles of incidence increase toward the critical angle, the simple hyperbolic assumption for reflections breaks down, even in the 1D case. The occurrence of anisotropy, which 316 Principles of Geologic Analysis can reach 20% in some sediments, of velocity gradients and of laterally inhomogeneous structures, all cause the variation of travel time with offset to be much more complicated than the simple hyperbolic case. The deviation from hyperbolic moveout becomes greater, the larger the offset and the farther the seismic energy has travelled. The variations in travel times of diving waves are close to linear with offset, although they too are equally subject to perturbations by the effects of anisotropy, of velocity gradients and of lateral variations in structure. Therefore, different modelling strategies need to be applied to wide-angle data than are generally used for conventional seismic reflection data. Travel-time analysis Most seismic modelling starts with matching the observed variation of travel time against offset with the travel times predicted by a model. In the simplest case, and in the early days of analysis before computers were sufficiently powerful, this was done using analytic solutions of the travel times, which almost invariably assumed a 1D structure. As computers increased in power, and as the recorded spatial data density increased, it became feasible to incorporate into the models first 2D and more recently 3D variations in velocity structure. Travel times through a trial model distribution of velocity are calculated using a ray-tracer which uses Snell’s Law to calculate ray-paths through the model. The model is then adjusted iteratively until there is a satisfactory fit between the modelled and observed travel times. The model may either be adjusted manually by an experienced operator, or an inverse method may be adopted, which automatically modifies and iterates the model until the difference between the observed and modelled travel times falls beneath a defined threshold. Normally, the iterations are continued until the travel times are fitted to within their estimated picking uncertainty. The bulk of ray-trace modelling is performed using only the compressional, or P-waves. Exactly the same technique may be applied to shear waves, but is far less commonly done, partly because explosive sources generate few shear waves (and none if they are in water), and so it is necessary to rely on the vagaries of mode conversion between P- and S-waves. Another reason that shear waves are rarely used is that S-waves are slower than P-waves and so the arrivals are always buried in the coda of earlier arriving energy and so are harder to define and to pick. Nevertheless, shear waves are valuable in understanding the sub-surface because the ratio of P- to S-wave velocities of a rock is far more diagnostic of the rock type and properties, and especially of the presence of fluids such as hydrocarbons, than are the P-wave velocities alone. Wide-angle data are far richer in converted S-waves than are near-offset data, because mode conversion occurs most efficiently at oblique angles of incidence, so S-waves are likely to become more important in the future with the increasing availability of good three-component receivers (e.g., Eccles et al., 2009). 317 Principles of Geologic Analysis Travel times are reduced at 7 km/s (i.e., a linear moveout of 7 km/s has been applied to the data). Note the strong wide-angle Moho reflection and the marked increase in amplitude of this reflection at about 40 km offset, as the critical distance is approached. 0 10 20 30 40 50 60 70 80 90 100 110 120 130 1 Time - Offset / 7 (s) White et al., 2002). Offset (km) 0 2 3 4 ve wa ter Wa Figure 11.2 Example of an ocean bottom wide-angle seismic profile acquired over the Norwegian Sea and Faroes continent-ocean boundary (from Crustal diving wave flection Mantle re 5 6 Water multiple 7 8 It is important to remember that the velocity model derived by travel-time modelling is non-unique. In principle, there are numerous different models that may fit the travel times equally well. Therefore, an important part of interpreting wide-angle data is to investigate the extent of the model space that satisfactorily fits the observations, and to explore the resolution of different areas of the model by making extensive resolution tests. One way of doing this is to perturb the travel times within the limits of their uncertainty, and then to make multiple tomographic inversions. The average and variance of all these inversions give a good measure of which features of the model are well constrained and which less so: we show an example later (Korenaga et al., 2000; White and Smith, 2009). Amplitude modelling The wide-angle seismic energy carries far more information than the travel times alone. Perhaps the most important characteristic is the amplitude of the arrivals, and their variation with offset. As the critical angle is approached, the amplitude increases greatly, as shown by the wide-angle Moho reflection illustrated in Fig. 11.2. The exact form of the variation of amplitude with angle of incidence is controlled primarily by the nature of the P- and S-wave velocity contrasts across the interface causing the reflection. As with travel-time modelling, use of this amplitude information started with 1D cases, progressing to more complex 2D and 3D models, and has developed from forward modelling controlled by operator intervention to more rigorous inversion schemes. 11.4 Examples of wide-angle seismic interpretations In the following four examples, I show results from four typical and influential wide-angle surveys that have had a high impact in geological understanding of the structure of oceanic crust, of the development of rifted sedimentary basins, of continental margins, and of imaging through thick volcanic layers overlying sediments. Each used different techniques which amplify the points discussed above. 318 Principles of Geologic Analysis Oceanic crust One of the major triumphs of wide-angle seismic profiling as early as the 1950s was to show that oceanic crust is much thinner than continental crust by a factor of 5 or more, that it varies relatively little in thickness across the globe and that the characteristic velocities of the crust are quite unlike those found in continental crust. All these observations were explained soon thereafter by the theory of seafloor spreading. As oceanic crust covers 70% of the surface of the earth, and wide-angle seismics were (and still are) the only way of measuring the crustal thickness, this was an early and crucial contribution of wideangle seismics to understanding the large-scale geology of the earth. The classic work on oceanic structure was published first by Maurice Hill, based at Cambridge in the United Kingdom. He used 41 wide-angle seismic profiles from the Atlantic and Pacific oceans to define the mean crustal thickness (excluding the variable sediment cover) as 6.4 1.9 km. By 1963, Russ Raitt, operating from Scripps Institution of Oceanography in the United States, had increased the number of profiles to over 125, and the geographic coverage to include the Indian Ocean, and came up with a similar average thickness of 6.6 1.6 km (Table 11.1). The acquisition methodology for these profiles was to use single sonobuoy receivers, sometimes with explosive sources, but more commonly with airguns, while the interpretation assumed a small number of uniform velocity layers with plane interfaces between them. It soon became apparent that the oceanic crust could be modelled by 3 layers, with layer 1 the variable thickness sediments overlying layer 2, interpreted as mainly extrusive basaltic volcanic rocks above a basal layer 3, later found to be gabbroic igneous rocks. This prosaic division of the basement crustal section into Table 11.1 Normal oceanic crustal structure from wide-angle seismic data Raitt (1963) (using straight-line fits to travel time vs. distance) Thickness (km) Velocity (km/s) Layer 2 1.71 0.75 5.07 0.63 Layer 3 4.86 1.42 6.69 0.26 8.13 0.24 Mantle Igneous Crustal Thickness: 6.57 1.61 km White et al. (1992) (using velocity structures constrained by amplitude modelling) Layer 2 2.11 0.55 2.5–6.6 Layer 3 4.97 0.90 6.6–7.6 Mantle >7.6 Igneous Crustal Thickness: 7.08 0.78 km 319 Principles of Geologic Analysis layers 2 and 3 became standard nomenclature which has persisted for over 40 years, in all subsequent studies of oceanic crust. The assumption of uniform velocity layers was a necessary consequence of the simple analytical interpretation technique, which was to fit straight-line segments to the first arrival diving waves on a distance versus travel-time plot. A new compilation of normal oceanic crustal structure by White et al. (1992), using results only from well-controlled interpretations of profiles that had been modelled using the amplitudes as well as the travel times of the wide-angle arrivals, yielded a surprisingly similar average thickness of 7.1 0.8 km. The increase of about 10% over the thicknesses interpreted using simple straight-line fits to the travel-time data is due to the amplitude-controlled interpretation constraining the velocity gradients properly rather than assuming uniform velocity layers. Nevertheless, it is striking that the first look at oceanic structure from wide-angle seismics in the midtwentieth century gave a surprisingly good picture of the structure of oceanic crust, which underpinned the plate tectonic revolution and our understanding of the role of mid-ocean spreading ridges in crustal formation. Work over the past half-century has of course refined the understanding of oceanic crustal structure and enabled us to map and to understand the causes of deviations from the norm. The chief of these are the generation of thicker oceanic crust than normal in the vicinity of thermal anomalies in the mantle caused by mantle plumes, of locally thin crust in oceanic transform faults, sometimes associated with serpentinisation, and of a reduction in crustal thickness as the full spreading rate drops below a critical value of about 15 mm/a because of conductive cooling of the upwelling mantle causing a reduction in the amount of melt generated. Almost all this understanding has come from the use of wide-angle seismic profiles: after 40 years of deep sea drilling, the deepest borehole into oceanic crust still only reaches a little over 2 km below the seafloor, so our knowledge of the deeper in situ section is still reliant primarily on seismic data. Lithosphere stretching in the North Sea In 1978, Dan McKenzie of Cambridge University, United Kingdom, published a model for the development of sedimentary basins by lithospheric stretching and thinning, which now underpins much basin research. This model was primarily based on the observation that the thermal time constant of the subsidence which provided accommodation space for the sediments in many basins was similar to the thermal time constant for the subsidence of oceanic lithosphere, which is formed by a well-understood process of rifting at mid-ocean ridges. Beneath sedimentary basins like the North Sea, there are large normal fault systems, showing that extensional rifting occurred immediately prior to sediment infill. Therefore, the available evidence pointed to an extensional cause for basin formation. However, initially the model of lithospheric stretching was not universally accepted, primarily because the normal faults imaged on seismic reflection profiles do not appear to supply sufficient extension for the observed subsidence 320 Principles of Geologic Analysis (the solution to this apparent contradiction is that much of the extension is taken up at a scale below that resolved by the seismic profiles, and that there is also evidence of depth-dependent stretching, with the upper crust locally exhibiting less extension than the whole lithosphere). The key to widespread acceptance of the lithosphere stretching model was the proof from a wide-angle seismic profile across the North Sea Basin that the crust was indeed thinned by extension under the sedimentary basin, and that the amount and inferred timing of the thinning was entirely consistent with the age, thickness and subsidence pattern of the overlying sediments (Barton and Wood, 1984). As in the case of oceanic crustal structure described above, this is an example of wide-angle seismics playing a crucial role in the understanding and acceptance of a major step forward in the way the earth is understood. The wide-angle profile was recorded in 1981 1982 along a 530 km transect crossing the Central Graben of the North Sea, using 12 seafloor seismometers and 16 explosive shots ranging in size from 75 to 1000 kg (Fig. 11.3A). By modern standards, this is a small amount of data, but it demonstrated the key result of crustal thinning under the centre of the basin (Fig. 11.3B). The travel times of the arrivals were modelled using a ray-tracing technique through a 2D model, with further constraints added from the major amplitude variations. The thinning is equivalent to 70 km of stretching, which equates to a stretching factor (b) of 1.6. By the time this profile was recorded, there was a good set of industry wells available, and subsidence analysis of the drilled sedimentary sections confirmed the stretching pattern mapped by the wide-angle seismic profile (Barton and Wood, 1984). Volcanic continental margins The third example of the impact of wide-angle seismics on advances in geological understanding of the planet comes from the rifted continental margins which are produced between the oceanic and continental domains discussed in the first two examples above. Rifted continental margins occur where the stretching that generates a sedimentary basin does not stop, but continues until the continental lithosphere is broken and a new oceanic basin forms between the two portions of continental lithosphere. The examples shown here are from a series of two-ship ESPs shot in 1985 across the Hatton continental margin west of Britain on the northwest European continental margin, and a dip-line profile shot using 85 OBS in 2002. The latter demonstrates the increase in velocity resolution achieved using a denser OBS array. The 1985 profiles are important because they were one of the first to show clearly the presence of voluminous lower crustal intruded igneous rocks in addition to large-scale extrusive volcanic flows on what are now known as ‘volcanic’ continental margins. The lower crustal igneous rocks are too deep to be drilled, and never have been, but they were imaged clearly on the wide-angle seismic profiles by their characteristic high (for continental lower crust) P-wave velocities of 7.3 7.5 km/s. Such seismic velocities can be explained by the presence of 321 Principles of Geologic Analysis 59 N4 S7 S8 57 S6 F7 F4 (figures from Barton and Wood, 1984). N2 N5 C Gr entr ab al en S1 F2 Shots < 400 kg Shots > 400 kg PUSS array 55 A 2 0 4 2 6 crust CE N TA 2.0 sediments DIS mantle 8.4 km / s CI TY VE LO DEPTH E W F2 5.8 6.2 20 40 F7 F4 0 km Figure 11.3 Wideangle experiment shot across the North Sea Basin in 1981 1982 which confirmed the lithosphere stretching model for rifted basin formation. (A) Map showing position of seabed pull-up shallow-water seismometers (PUSSes), and explosive shots (stars), with main normal faults marked. (B) Final velocity model along profile with velocity contour interval 0.2 km/s, sediments marked by hatched lines, no vertical exaggeration N1 N3 MOHO S1 5.8 6.6 7.0 8.0 8.1 8.3 8.3 0 B S6 S7 N1 S8 N2 8.1 8.3 N3 6.2 N4 N5 MOHO 6.6 7.0 50 DISTANCE km high-magnesium picritic melts generated by decompression melting of the mantle by the interaction between rifting lithosphere and a thermal anomaly in the mantle, often known as a ‘mantle plume’ or ‘hot-spot’ (White and Smith, 2009). The crucial factor that enables their presence to be identified in the deep crust is their high seismic velocities, and these can only be determined with wide-angle profiles. The data acquisition in summer 1985 recorded seven ESPs orientated parallel to the margin in order to reduce the effect of lateral variations in structure (Fig. 11.4A). The receiver array was a 2400 m streamer with 25 m groups, and 322 Principles of Geologic Analysis (figure adapted from Fowler et al., 1989); (C) velocity structure from Monte Carlo average modelled using tomographic inversion of travel times recorded by OBS at positions marked by ellipses on the seafloor (from White and Smith, 2009). ICELAND BASIN 60N H G E F C D A B N O TT A H 59N K N A B HATTON BASIN 58N km 0 22W A H 20W 50 F E OCEANIC C D B A 5.0 5.5 2.0 4.0 2.0 Depth (km) 16W Location G 6.5 10 100 18W 0 5.0 6.0 7.0 6.0 7.3 6.5 CONTINENTAL COT 8.0 7.0 MANTLE 20 7.3 B Fowler et al. (1989) 8.0 0 5.5 6.0 5.0 4.5 5.0 Depth (km) Figure 11.4 Crustal structure of the Hatton-Rockall volcanic continental margin calculated from a series of two-ship expanding spread profiles (ESPs) shot along strike over the Hatton Bank margin and adjacent oceanic crust, and from a dense OBS profile: (A) map showing location of ESPs (broken lines) with dip profile down their mid-points, and location of OBS profile (OBS at circles); (B) structure across the continental margin inferred from the series of 1D constraints on the velocity-depth variation provided by the ESPs. Note the lower crustal igneous intrusions that exhibit characteristic seismic velocities above 7.3 km/s 10 COT 7.0 6.5 OCEANIC MANTLE 7.5 7.4 CONTINENTAL 20 30 C White and Smith (2009) Monte Carlo average NW SE 100km the profiles were shot twice using both explosives and airguns. The use of airguns had the advantage of producing a conventional seismic reflection profile at the same time as the ESPs were recorded, while the explosive shots ensured penetration to the base of the crust. With modern larger and much more powerful airguns capable of producing arrivals that can be recorded to ranges of over 120 km (e.g., Fig. 11.2), the use of explosives is no longer necessary for such targets. The second, OBS profile (Fig. 11.4C) used a 6340 cubic inch (104 l) array of airguns. A cross-section of the structure across the rifted margin (Fig. 11.4B) shows the main results from modelling two-ship ESPs (Fowler et al., 1989). The nearby parallel OBS profile (Fig. 11.4C) shows the average from 100 automated inversions, with small perturbations within the estimated picking uncertainty applied to the travel-time data prior to each inversion (White and Smith, 2009). The final velocity model is therefore a robust measure of the structure and shows better resolution of the high-velocity region in the continent-ocean transition. The 323 Principles of Geologic Analysis crust thins from the continental to the oceanic end, and extrusive volcanics in the upper section just beneath the seafloor are imaged by the seismic reflection profiles and by their P-wave velocities of 4.0 5.5 km/s, while the multiple sill intruded lower crust is demarcated by the region of high seismic velocities above 7.3 km/s. Similar results have been found subsequently on both the Greenland margin that is conjugate to the Hatton margin, and on many other volcanic margins worldwide, including the northern North Atlantic, the South Atlantic and the Indian continental margins. In all these cases, wide-angle seismic profiles continue to provide the best constraints on the volume of igneous rock added to the crust, and provide a crucial constraint in modelling the development and subsidence of the continental margins and their hinterland. The improved velocity field from wide-angle seismic profiles allows better pre-stack depth migrations to be made of conventional seismic reflection profiles. I show an application of this in Fig. 11.5, which is across the North Atlantic volcanic continental margin near the Faroe Islands (White et al., 2008). The velocity structure across the continent-ocean transition (Fig. 11.5A) is remarkably similar to that found on the Hatton Bank margin (Fig. 11.4C), with abnormally high velocities in the lower crust. However, it also shows basalt flows (in green, Fig. 11.5A) extruded above sediments (in blue, Fig. 11.5A) beneath the Fugloy Ridge. On the pre-stack depth migrated reflection image (Fig. 11.5B), the extruded lavas produce seaward-dipping reflectors in the upper crustal section, and the intrusions produce sub-horizontal layering visible in the mid-crust. Sub-basalt imaging The previous case examples were from, respectively, oceanic crust, continental crust and the boundary between the two found at continental margins. In addition to illustrating the importance of wide-angle data in understanding the geology of three of the earth’s major settings, they also demonstrate the use of three of the main techniques for seismic acquisition. The oceanic crust studies used primarily sonobuoys (Fig. 11.1A), and the North Sea continental basin study used seabed receivers (Fig. 11.1B) while the volcanic continental margin profiles used two-ship expanding spread and dense OBS profiles (Fig. 11.1B and D). The final example of the use of wide-angle data discussed in this section used the two-ship method of recording synthetic aperture profiles (Fig. 11.1E), in this case with three passes giving offsets from 0 to 38,000 m. The target for this was primarily the potentially hydrocarbon-bearing sub-basalt sediments. This example illustrates well the integration between near-normal incidence profiling and wide-angle profiling, as data from the near and far offsets are combined seamlessly to improve both the velocity resolution (which carries information primarily on rock types), and the reflection images (which carry information primarily on the geometric structure of the sub-surface). Combination of the near-offset and wide-angle data gives much better understanding of the subsurface than using either alone, and each benefits from the other. 324 Norwegian Basin 0 Fugloy Ridge Faroe-Shetland Basin Depth (km) 5 10 15 20 mantle mantle 25 NW A SE 30 0 50 100 150 200 250 0 Two-Way Time (s) seaward-dipping reflectors lower-crustal intrusions 5 10 NW SE 0 50 100 150 Distance from Chron 22 (km) B 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 200 6.0 6.5 250 7.0 7.5 Velocity (km/s) Figure 11.5 (A) Seismic velocity structure of crust across the continent-ocean transition near the Faroe Islands. Velocities of post-rift Cenozoic sediments (purple) are constrained by semblance moveout analysis of reflections recorded on the 12-km-long hydrophone streamer and sub-sediment crustal velocities are constrained by tomography using wide-angle reflections and diving waves from ocean bottom seismometer data (OBS locations shown by circles at seafloor). Colour bands are at 0.1 km/s intervals, with contours above 7.0 km/s spaced every 0.1 km/s to highlight the lower-crustal velocity. (B) Section of multichannel seismic reflection profile with superimposed P-wave velocity field from (A). Extrusive lava flows are imaged as seaward-dipping reflectors between 50–90 km along profile and as subhorizontal layered basalts at distances >90 km. Lower-crustal layering coincident with high (>7.0 km/s) velocities, caused by igneous intrusions, lies beneath the seaward-dipping basalts on the continent-ocean transition, with a termination at c. 90 km against continental crust with lower velocities. Processing of the reflection profile included source designature, multiple suppression and post-stack time migration (diagram modified from White et al., 2008). 325 7W 6W 5W 4W 3W 2W 62N 1 7 61N 500 A 60N 6W 0 Depth (km) 0 10 5W 4W 3W 40 Distance (km) 80 4.80 basalt 5.20 5.50 5.65 6.00 2W 120 2.20 3.80 4.20 5.80 160 sed. basement 6.78 20 7.00 6.80 7.80 mantle 30 7.20 7.80 FLARE-1 40 B Faroe Is. Depth (km) 0 Faroe Shelf Faroe-Shetland Basin basalt sed. 10 basement 20 30 C 40 intruded? mantle FLARE-1 Figure 11.6 (A) Map showing location of long-offset two-ship profiles 1 and 7 of the FLARE project. Purple line marks southeastern limit of Tertiary basalt flows. Bathymetric contours every 100 m, with heavier contour at 1000 m. Inset shows location of survey area on the Northwest European continental margin; black lines mark location of all 12 FLARE profiles. (B) Representative wide-angle ray-paths through the crustal velocity model along the FLARE-1 profile, including both offshore supergathers and onshore land station recording on Suduroy. Red rays are those turned in the basalt layers, or reflected off their bottom. Blue rays are reflected off the top of the basement which marks the base of the sub-basalt sediments, or returned as diving rays through the top of the basement. Green rays are reflections off the Moho. Only every tenth ray is shown for clarity. Selected velocities shown in km/s. (C) Interpreted cross-section. Interfaces constrained well by seismic data are shown as solid lines. Note the crustal thinning eastward toward the centre of the Faroe-Shetland Basin, and the basalt flows (red) extending eastward from their source in the vicinity of the Faroe Islands and over-riding a pre-existing sedimentary section (yellow) which lies above the basement (brown). Green section beneath Faroe Shelf and Islands represents basement with velocities modified by weathering, igneous intrusion or emplacement of tuffs. 326 Principles of Geologic Analysis The Faroes Large Aperture Research Experiment (FLARE) profiles were recorded in 1996 by an industry consortium (White et al., 1999), across a transect of the Faroes Shelf with large sub-surface basalt flows extending southeastward from the Faroes Islands (Fig. 11.6A). Travel-time tomography using a ray-tracing method showed unambiguously, for the first time in this area, that a thick sediment layer lay beneath the basalts, and that the basalt layer thinned from several kilometres thick near the Faroe Islands to a feather edge some 150 km away (Fig. 11.6B). The 38 km maximum offsets provided by the synthetic aperture profiles were sufficient to provide good wide-angle arrivals from the basalts, and from the sub-basalt basement, but did not reach sufficiently great offsets to give strong wide-angle arrivals from the Moho at the base of the crust. However, on the profile illustrated here, fixed three-component land seismometers were deployed at one end of the profile, the Faroese Island of Suduroy, which provided strong arrivals to much greater offsets (Richardson et al., 1999). These constrained the Moho depth over a short segment of the profile, which then was extrapolated along the full length of the profile by using measurements of the gravity anomaly (Fig. 11.6C). Measurement of the velocity structure provides powerful insights into the geology of this region, in this case showing the presence of sub-basalt sediments. However, the good velocity control provided by the wide-angle data also 0 SE NW FL A RE-1 1 Velocity (km/s) Depth (km) 2 Basalt 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 3 4 Sediment 5 6 7 Basement 8 9 0 0 WSW 0 20 40 80 100 20 Distance (km) 40 60 80 60 100 ENE 120 FL A RE-7 1 2 Depth (km) Figure 11.7 Composite profiles (from White et al., 2003) along FLARE-1 (down dip) and FLARE-7 (along strike; see Fig. 11.6A for location), generated by merging the conventional depthconverted nearoffset seismic section above the base of basalt with the pre-stack depth migrated base-basalt and sub-basalt arrivals, and the pre-stack depthmigrated basement arrivals. Colours corresponding to velocities derived from ray-tracing modelling and used for pre-stack depth migration are superimposed on the seismic profiles. 3 4 5 Basalt Sediment 6 7 8 Basement 9 327 Principles of Geologic Analysis provides two other major benefits. First, it enables a well-controlled pre-stack depth migration of the conventional seismic data to be made, markedly improving the image. Second, the wide-angle arrivals have large amplitudes because they approach the critical angle, so they themselves can be migrated back to normal incidence to provide good images, such as those shown in Fig. 11.7, of reflection horizons (in this case the base of the basalt layer and the underlying basement) that are too weak to image well with conventional near-offset seismic data (White et al., 2003). References Barton, P.J., Wood, R.J., 1984. Tectonic evolution of the North Sea basin: crustal stretching and subsidence. Geophys. J. R. Astron. Soc. 79, 987 1022. Buhl, P., Diebold, J.B., Stoffa, P.L., 1982. Array length magnification through the use of multiple sources and receiving arrays. Geophysics 47, 311–315. Eccles, J.D., White, R.S., Christie, P.A.F., 2009. Identification and inversion of converted shear waves: case studies from the European North Atlantic continental margins. Geophys. J. Int. 179, 381–400. doi:10.1111/j.1365-246X.2009.04290.x. Fowler, S.R., White, R.S., Spence, G.D., Westbrook, G.K., 1989. The Hatton Bank continental margin-II. Deep structure from two-ship expanding spread seismic profiles. Geophys. J. 96, 295–309. Korenaga, J., Holbrook, W.S., Kent, G.M., Kelemen, P.B., Detrick, R.S., Hopper, J.R., Dahl-Jensen, T., 2000. Crustal Structure of the Southeast Greenland margin from joint refraction and reflection seismic tomography. J. Geophys. Res. 105, 21591–21614. Lunnon, Z.C., Christie, P.A.F., White, R.S., 2003. An evaluation of peak and bubble tuning in sub-basalt seismology: modelling and results. First Break 21, 51–56. Raitt, R.W., 1963. The crustal rocks. In: Hill, M.N. (Eds.), The Sea, vol. 3. Wiley Interscience, New York, pp. 85 102. Richardson, K.R., White, R.S., England, R.W., Fruehn, J., 1999. Crustal structure east of the Faroe Islands. Pet. Geosci. 5, 161–172. Staples, R.K., Hobbs, R.W., White, R.S., 1999. A comparison between airguns and explosives as wide-angle seismic sources. Geophys. Prospect. 47, 313–339. White, R.S., McKenzie, D., O’Nions, R.K., 1992. Oceanic crustal thickness from seismic measurements and rare earth element inversions. J. Geophys. Res. 97, 19683–19715. White, R.S., Fruehn, J., Richardson, K.R., Cullen, E., Kirk, W., Smallwood, J.R., Latkiewicz, C., 1999. Faroes Large Aperture Research Experiment (FLARE): imaging through basalts. In: Fleet, A.J., Boldy, S.A.R. (Eds.), Petroleum Geology of Northwest Europe: Proceedings of the 5th Conference. Geological Society of London, London, pp. 1243–1252. White, R.S., Christie, P.A.F., Kusznir, N.J., Roberts, A., Davies, A., Hurst, N., Lunnon, Z., Parkin, C.J., Roberts, A.W., Smith, L.K., Spitzer, R., Surendra, A., Tymms, V., 2002. iSIMM pushes frontiers of marine seismic acquisition. First Break 20, 782–786. White, R.S., Smallwood, J.R., Fliedner, M.M., Boslaugh, B., Maresh, J., Fruehn, J., 2003. Imaging and regional distribution of basalt flows in the Faroe-Shetland Basin. Geophys. Prospect. 51, 215–231. White, R.S., Smith, L.K., Roberts, A.W., Christie, P.A.F., Kusznir, N.J., iSIMM Team, 2008. Lowercrustal intrusion on the North Atlantic continental margin. Nature 452, 460–464. doi:10.1038/nature06687. White, R.S., Smith, L.K., 2009. Crustal structure of the Hatton and the conjugate east Greenland rifted volcanic continental margins, NE Atlantic. J. Geophys. Res. 114, B02305, doi:10.1029/2008JB005856. 328