FLIEDNER, MORITZ M., ROBERT S.WHITE, and JUERGEN FRUEHN, University of Cambridge
Abstract: Determining the Structure of Basalt Flows with Wide-Angle Seismic Data
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 chances 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 form 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 ought
therefore to be 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.
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.
The simplest, but not very
reliable way to estimate the thickness of the basalt layer is to use the
(gradient-dependent) penetration depth of the seismic ray at the termination
of the first basalt arrival (the range at which the step-back to the basement
arrival occurs). A more.direct way is the tracing of the base-basalt wide-angle
reflection if it can be identified. An independent method is provided by
modeling the amplitude-versus-offset behavior of the first arrival. For
simple gradient-layer models the first-arrival amplitude reaches a maximum,
the location of which depends on the thickness and gradient of the basalt
layer (Figure 1). The velocity
gradient also determines the width and height
of the peak and (for a given intrinsic attenuation) the slope of the AVO
curve. Modeling real data typically requires more than a single gradient
layer (Figure 2).
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 3a) 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
a discrimination by a criterion like dip that is usually available in real
data. The field example (Figure 3b) 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 3) is hence more important, and highlights
arrivals not seen on conventional migrations.
Figure 1. Basalt first-arrival
AVO curves from synthetic shot gathers (reflectivity method). A high-velocity
layer of varying thickness and
velocity
gradient overlies a constant low-
velocity
layer.
Figure 2. 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).
Figure 3( a) First two panels
are details of a synthetic gather. Traveltime is reduced by 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. (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. SF sea floor, TB top
basalt, BB base basalt, B basement, 1M first basalt multiple, 2M second
basalt multiple.
AAPG Search and Discovery Article #90923@1999 International Conference and Exhibition, Birmingham, England