Using wide-angle seismic data for basalt and sub-basalt imaging

M. M. Fliedner and R. S. White, University of Cambridge.

Summary

The imaging of basalt flows and underlying structures can be improved by using data recorded at long offsets. Travel-times and amplitudes of wide-angle refractions and reflections allow the construction of better velocity models. The high near-critical amplitudes of wide-angle seismic data and the absence of low-velocity multiples can be exploited in imaging basalt and sub-basalt arrivals by pre-stack migration of selected portions of the wide-angle wavefield.

Introduction

High-velocity basalt flows create a major obstacle to imaging lower-velocity underlying sedimentary structures. The highly reflective top of the basalts scatters much of the incident seismic energy; short-period ringing, simple and peg-leg multiples obscure weak sub-basalt reflections with similar move-out; the high-velocity basalt layer absorbs preferentially the higher frequencies in the incident wavelet, degrading the achievable resolution of a sub-basalt image; and strong ray-bending may distort the sub-basalt image.

Recording the seismic wave-field at longer offsets (with super-long arrays or by two-ship marine acquisition) may improve the possibilities for sub-basalt imaging in several ways: it allows the recognition of sub-basalt low-velocity layers by their shadow-zone effect on the wide-angle wave-field (step-back from the first arrivals in the basalt flows to the sub-sedimentary basement arrivals); higher reflection amplitudes may be recorded near the critical distance; multiples created in sedimentary layers above the basalt will be absent at wide-angles; travel-time data from wide-angle arrivals allow an improved migration-velocity model to be constructed; there is the possibility of identifying arrivals in the pre-stack gathers for selective imaging.

Influence of basalt velocity structure on wide-angle wavefield

In order to assess what information can be extracted from the wide-angle wave-field produced by a layer of basalt flows and the underlying structure, we calculate synthetic reflectivity seismograms with basalt velocity structures based on well-log and geologic mapping data. Due to the low resolution of the low-frequency seismic energy that penetrates through the basalt and the convergence in time of seismic arrivals with increasing offset, even greatly simplified versions of the realistic velocity model produce similar wide-angle wave-fields. It ist therefore possible to derive a seismic velocity model of the basalt from wide-angle travel-times and amplitudes that is sufficiently accurate for the migration of sub-basalt events. Whereas travel-time modeling and inversion by ray-tracing of refracted and reflected arrivals to derive velocity models is a well-established technique, modeling the amplitudes of these arrivals is rarely used for that purpose (Spence et al., 1989; Fliedner et al., 1998).

In wide-angle data, the first arrival from the basalt flows is easy to identify and its amplitude can be modeled as a function of the elastic velocity and density structure of the basalt layer. The velocities and densities of the overlying sediments are usually well known, as is the average compressional-wave velocity of the basalt (from seismic travel-times). Limited shear-wave data from bore holes indicate that the V_p/V_s-ratio of basalts varies mostly in a narrow band between 1.8 and 2.0 and varies little throughout the layer. Densities can be estimated from established velocity-density relations. Intrinsic attenuation in basalts has been found to be low (Q < 150).

Since most of the effective attenuation in basalt flows is kinematic, it is thus correctly modeled in a reflectivity synthetic seismogram. Amplitude modeling can yield a more detailed basalt velocity model than travel-time ray-tracing alone (Figure 1). It is most sensitive to significant low-velocity layers within the basalt (e.g. weathered zones or sedimentary layers between individual flows or flow units), as these cut off refracted arrivals from deeper flows.

Figure 1: Real data example of modeled basalt amplitude. The starting model was the result of ray-tracing (dashed velocity-depth curve where it deviates from the final model).

Imaging of sub-basalt events by pre-stack depth migration
When a good velocity model is available, it becomes possible to migrate sub-basalt data into an image where otherwise faint events are boosted because of the higher amplitudes of the wide-angle data and the reduced contamination with multiples. It is necessary to select carefully the parts of the wide-angle wave-field that contribute to the stack. This selection will be guided by the first-order structural interpretation from the wide-angle (ray-tracing) velocity analysis. The synthetic data example (Figure 2a) demonstrates the information that can be recovered under ideal circumstances (perfect velocity model). In this case it is possible to distinguish unequivocally between primary and other events in the stack, even though this 1-D example does not allow discrimination by a criterion like dip that is usually available in real data. The field example (Figure 2b) is a single CMP from a fairly one-dimensional area with a velocity structure similar to the one used in the synthetic. It shows that under more realistic conditions, the near-vertical migration alone contains less useful sub-basalt information than the noise-free and perfectly migrated synthetic; the contribution from selected wide-angle data (inset on third panel of Figure 2) is hence more important, and highlights arrivals not seen on conventional migrations.

Figure 2: (a) First two panels are details of a synthetic gather. Traveltime is reduced at 5000 m/s. Third panel shows 1-D pre-stack depth migrated stack of the synthetic data overlaid by the velocity model; for the inset only selected wide-angle data were migrated, showing enhancement of sub-basalt reflections. (b) First two panels are details of wide-angle gather acquired on the North-Atlantic volcanic margin. Inset in depth-migrated third panel contains selected wide-angle data only, showing enhancement of sub-basalt reflectors. SF sea floor, TB top basalt, BB base basalt, B basement, 1M first basalt multiple, 2M second basalt multiple.

Example of selective wide-angle migration

The idea of selectively migrating long-offset gathers has been implemented for a short marine line in (Figure 3. Offsets between 5 and 18 km were used to image the base of a North-Atlantic basalt flow and a prominent sub-basalt reflector.

Figure 3: Kirchhoff pre-stack depth-migration of selected events from marine wide-angle data. The compressional wave image is a composite of the P-wave base-basalt and a sub-basalt reflection. The converted-wave image is the base basalt reflector imaged by the energy that travels as shear wave through the basalt layer.

This technique works in principle equally well with compressional and shear wave data. The interface between sediments and basalt (top basalt) can be an efficient converter of P-wave to S-wave energy if the P-wave velocity of the sediments matches the S-wave velocity of the basalts (White and Stephen, 1980). A shear-wave image may in this case contain better information than a P-wave image. In the example of Figure 3, though, the sedimentary P-wave velocities are significantly lower than the basalt S-wave velocities. As a result, little conversion takes place and the converted-wave image of the base-basalt reflector is weak.

Conclusions

Wide-angle data contain useful information that helps to overcome the limitations of conventional near-vertical imaging in areas that are covered by high-velocity basalt flows. Standard techniques of wide-angle data interpretation like travel-time ray-tracing can be improved by modeling the amplitudes of the basalt arrivals. The resulting velocity models allow direct imaging of basalt and sub-basalt reflectors by migrating selected parts of the wide-angle shot-gathers.

References

Fliedner, M.M., White, R.S., and Smallwood, J.R., Seismic velocity structure of basalt flows, Society of Exploration Geophysicists International Exposition and 68th Annual Meeting, New Orleans 1998.

Spence, G.D., White, R.S., Westbrook, G.K., and Fowler, S.R., 1989, The Hatton Bank continental margin - I. Shallow structure from two-ship expanding spread seismic profiles, Geophys. J. Roy. astr. Soc., 96, 273-294.

White, R.S., and Stephen, R.A., 1980, Compressional to shear wave conversion in oceanic crust, Geophys. J. Roy. astr. Soc., 63, 547-565.