Sub-basalt P- and S-wave imaging – west of Hebrides
J. Fruehn, GX Technology EAME Ltd., London
Summary
Multiples or residual multiples very often mask this
arrival on the near-vertical offsets.
P- and S-wave (converted phase) sub-basalt arrivals
were identified at different offset ranges of a long-offset
seismic line: P-waves at near-vertical offsets (0-6km),
and P- and S-waves at wide-angle offsets (between 8 and
12.5 km). Prestack depth migration and detailed depth
focusing analysis applied on these arrivals yielded three
different depth sections, which are discussed and
compared in this paper. The converted phase image
shows strong continuous sub-basalt reflections. The
P-wave images are less continuous, but they constrain
the S-wave reflections locally. The sub-basalt migration
velocities are generally low, indicating (1) inter-fingering
with sediment and low-density volcanics (as found in a
borehole to the SE of the profile) and (2) highly variable
basalt composition (fresh, altered and weathered basalt as
found in a borehole to the NW of the line). Acoustic and
elastic synthetic seismic modeling was used to
investigate the nature of the sub-basalt reflections (intrabasalt, base basalt, sub-basalt sediment, and multiple).
Introduction
Conventional seismic reflection profiling (surface
recorded near-vertical data) commonly fails to image
coherent sub-basalt reflections, the main problem being
high-amplitude multiples that obscure relatively weak
sub-basalt reflections. Therefore, a variety of large
aperture acquisition geometries were designed to record
wide-angle arrivals from beneath the basalt. At far
offsets, beyond the water-wave cone and the first waterbottom multiple, the sub-basalt arrivals are less affected
by multiples and more easily identifiable by their
amplitude behavior and traveltime pattern.
Standard seismic processing and controlled stacking of
mode converted energy has proven successful in
sub-basalt imaging along a long-offset line that was
acquired in 1996 by Phillips Petroleum over Tertiary
flood basalt in the Rockall Trough, NW of the Hebrides
(Emsley et al., 1998). In this paper, we present new
results from migration of this dataset.
Method
The new element in our approach is migration of P- and
S-wave arrivals from the same sub-basalt reflectors. We
thereby achieve three independent depth sections that
show overlapping and complementing structural
elements. We identify the arrivals relevant for migration
in different offset ranges and at different traveltimes. The
P-wave events arrive earlier and are found in the nearvertical and the wide-angle offset ranges, whereas
S-wave events arrive up to 1 second later and are more
easily identified on the far offsets (Figures 1 and 2).
Figure 1: Example common depth point (CDP) gather showing
the arrivals used for migration. White triangle at far offsets
delimits the region that was used for P-wave wide-angle
migration.
Our procedure involves careful traveltime analysis,
synthetic modeling and prestack depth migration. The
basalt traveltimes (top and base basalt reflection, basalt
refraction) form a characteristic pattern at intermediate to
far offsets, on which their identification is based (Figure
2). Increasing top basalt amplitudes at the critical
distance (4000 m offset in Figures 1 and 2) mark the
onset of turning rays (refraction) that continue toward far
offsets as a linear event; intra-basalt and base basalt
reflections approach the basalt refraction asymptotically
and eventually merge with it at far offsets (Figure 2).
There are considerable variations along the line in this
traveltime pattern, which are mainly induced by
bathymetric changes and the rough and anticlinal top of
the basalt (Figure 3).
Synthetic modeling suggests that conversion from p to s
(and from s to p for upward rays) occurs at the
acoustically hard top of the basalt. In Figure 1 this arrival
(inset B) is only visible at far offsets between 6.5 s and
7.5 s two-way traveltime (TWT) mainly because of the
Sub-basalt imaging – west of Hebrides
strong multiples at near offsets. However, Figure 2
shows that even without multiples the converted wave,
base (S), is difficult to identify because the amplitudes
are very low in the near-vertical range. They start
increasing at intermediate offsets.
The wide-angle P-wave image (Figure 3c) was
calculated from the arrivals beyond the water-wave cone
only (white triangle in Figure 1). The section exhibits
strong sub-basalt reflections between CDP 300 and CDP
1000 (coincident with the S-wave reflection) and, at
greater depth, bright reflections (SB4) from the far
offsets of the gathers.
Figure 2: Synthetic CDP gather showing basalt arrivals;
wb=water bottom, base(S) is the converted phase reflected off
the base basalt.
The main contribution to our study however, comes from
the SIRIUS™ imaging loop, which consists of a
Kirchhoff-type prestack depth migration scheme
(PSDM) and depth-focusing analysis (DFA). The
interactiveness of DFA allows us to assess the effect of
velocity changes on the common reflection point (CRP)
gathers in real time. In a top-to-bottom approach, the
optimal migration velocities are updated iteratively and
are closely linked to the imaged structures (velocity
boundaries and imaged horizons are coincident). Finite
difference migration was also tested and compared with
the Kirchhoff results.
Additional constraints on the nature of the sub-basalt
reflections were taken from synthetic modeling that was
mainly used to exclude a number of the most common
multiples that may have been imaged.
Examples
The images shown in Figure 3 result from several
iterations of combined PSDM and DFA. The S-wave
image shows a strong top of the basalt (Figure 3a TB)
and three, largely continuous, sub-basalt reflections
(SB1-SB3). The basalt S-wave migration velocities vary
between 1600 m/s and 2300 m/s along the line.
The near-vertical P-wave image (Figure 3b) shows only
few sub-basalt reflections, most continuously between
CDP 1250 and CDP 1750. Here, P- and S-wave images
are almost coincident (white dashed line is SB3 from
S-wave image). Velocities range between 3900 m/s and
4200 m/s.
Figure 3: PSDM of P- and S-wave arrivals; (a) migration with
S-wave velocities, (b) and (c) migration with P-wave velocities.
White dashed line is SB3 from S-wave image.
The migration velocities are generally low compared
with basalt velocities found elsewhere along the North
Atlantic margin e.g., Faeroe-Shetland basalt (Fruehn et
Sub-basalt imaging – west of Hebrides
al., 1999). Regional boreholes, however, show that the
basalt from the West Hebrides area consist of various
volcanic facies (tuffs, fresh, altered, weathered basalt,
sills), with seismic velocities ranging between 4000 m/s
and 5500 m/s. Additionally, the volcanic units closer to
the continent show strong inter-fingering with sediment
(tuffaceous siltstone), which lowers the average seismic
velocities even more (3500-4500 m/s). Similarly low
velocities were reported from the Møre basin where
flood basalt inter-fingers with sediment (Planke et al.,
1999).
Conclusions
The most continuous sub-basalt image was obtained
from migration with S-wave velocities. The P-wave
migrations, however, can be used to constrain this image
by providing independent evidence of the same
reflectors.
Superimposing the sub-basalt horizon SB3, as imaged in
the S-wave migration, on the P-wave sections (dashed
line in Figures 3b and 3c), we observe locally continuous
P-wave reflections at similar depths. The near-vertical
P-wave section (Figure 3b) constrains the S-wave image
in the SE, and the wide-angle P-wave image (Figure 3c)
in the NW. Additionally, the wide-angle migration
images a very deep arrival (SB4), probably reflected off
the underlying Lewisian basement that was drilled in a
borehole to the NE of the profile. The relatively low
migration velocities are in agreement with regional
borehole information showing inhomogeneous basalt
composition. Data analysis suggests SB3 to be the base
of the basalt. Synthetic modeling has helped to exclude a
number of multiples, which potentially could have been
migrated.
References
Emsley D., Boswell, P. and P. Davis, Sub-basalt imaging
using long offset reflection seismic data, Extended
abstracts to the 60th EAGE Conference and Technical
Exhibition, 8-12 June 1998, Leipzig, Germany 1998.
Fruehn J., White, R. S., Fliedner, M., Richardson, K. R.,
Cullen, E., Latkiewicz, C., Wayne, K. and J.
Smallwood, Large-aperture seismic: Imaging beneath
high-velocity strata, World Oil, vol. 220, no. 1, 109113, 1999.
Planke, S., Alvestad, E., and O. Eldholm, Seismic
characteristics of basaltic extrusive and intrusive
rocks, The Leading Edge, vol. 18, no. 3, 342-348,
1999.