Download European Research Council funds 2D ultra

news feature
first break volume 33, June 2015
European Research Council funds 2D ultra-deep
seismic reflection data acquisition across the
Atlantic Ocean
Professor Satish Singh* of the Institut de Physique du Globe de Paris (IPGP) reports on a
European Research Council (ERC) funded project to study the oceanic lithosphere down to
its base at up to 100 km depth.
The IPGP has recently completed the
landmark Trans-Atlantic Imaging of
Lithosphere-Asthenosphere Boundary
(LAB) project to study oceanic lithosphere up to a depth of 100 km. The
data were acquired using WesternGeco
IsoMetrix marine isometric seismic
technology aboard the vessel Western
Trident during March-April 2015 and
is now being interpreted.
The term Lithosphere, derived from
the Greek words, ‘lithos’ for ‘rock’,
and ‘sphaira’ for ‘sphere’, defines the
solid and rigid plate that moves over
highly viscous, mechanically weak and
ductile material within the upper part
of the Earth’s mantle or Asthenosphere
(Greek ‘astheno’ for ‘weak’). The lithosphere is the main building block of
the plate tectonics and LAB is the most
prevalent boundary on the Earth.
The most direct evidence for the
presence of the base of the lithosphere
has come from surface wave studies
where the lithosphere is associated with
the presence of a high shear-wave velocity above a low shear wave velocity and
high attenuation asthenosphere below
with a thick (30-40 km) transition
between the two. Recently, a receiver
function study, a method similar to the
reflection method, based on P to S conversion, has led to the suggestion that
the LAB could be a sharp boundary,
approximately 10 km thick. However,
the receiver function studies are limited to a few locations due to a sparse
spatial combination of earthquakes
sources and receivers on the Earth.
Seismic reflection techniques are
an efficient method to provide images
of the subsurface over a large area,
typically with a resolution of tens to
hundreds of metres. Can this technique
be used to image the LAB down to
100 km depth? Is there a sharp boundary that can be imaged by seismic
reflection techniques? Four important
problems arise when imaging deep
structures in an oceanic environment:
(1) deployment of a low frequency
source for deeper penetration of the
seismic waves, (2) removal of source
and receiver ghosts, (3) scattering from
the seafloor, and (4) water bottom multiples. The TransAtlanticILab project
proposes to address these problems and
image the LAB down to 100 km.
The experiment utilized a 12 kmlong IsoMetrix streamer deployed at
30 m water depth. The energy source
was a 10,170 cubic inch air-gun array
comprised of six sub-arrays with eight
guns each, deployed at 15 m depth,
targeting a very low frequency output.
The multi-measurement towed-streamer
system recorded both total pressure and
particle acceleration vectors using densely sampled micro-electrical mechanical
system (MEMS) accelerometers. The
shot interval varied from 50 m to 75 m,
and consequently the record length from
20 s to 30 s, depending upon the target
depth along the profile.
The equatorial Atlantic Ocean
was chosen for this study because the
oceanic fracture zones in this region
maintain their azimuth over 2000 km
and thus enable a straight 2D profile to
be shot along one segment of the oceanic
lithosphere. However, the recent United
Nation Law of Sea has allowed the
countries to claim sea up to 350 miles
from their continental shelf (instead of
the earlier 200 miles Exclusive Economy
Zone), and hence permits from surrounding Sub-Saharan African countries
Figure 1 M/V Western Trident during the Trans­
AtlanticILab experiment.
*
Corresponding author, E-mail: [email protected]
© 2015 EAGE www.firstbreak.org
47
news feature
first break volume 33, June 2015
Figure 2 Bathymetric map showing the TransAtlanticIlab profile (white) with age of the lithosphere (light
blue lines). The numbers indicate the age of lithosphere in a million years. The red dashed line marks the
position of the Mid-Atlantic Ridge, where the age of lithosphere is zero. The yellow lines mark the position of the transform faults (solid) and fracture zones (dashed). Black dashed lines mark the Greenwich
meridian and the Equator.
were required prior to data acquisition,
which was not possible due to limited
time. This restriction had two effects: (1)
the maximum age of the lithosphere that
can be imaged decreased from 100 Ma
to 75 Ma and (2) two extra-turns were
acquired to remain within the international waters.
A total of 2775 km ultra-deep seismic reflection data was acquired starting
from Greenwich Meridian at 1º S, at
about 75 Ma of oceanic lithosphere
(Figure 2) in a nearly E-W direction,
crossing the Mid-Atlantic Ridge at 1.3º
S, corresponding to zero age of the
lithosphere. This part of the profile spans
0-75 Ma of the oceanic lithosphere on
the African Plate. The profile extends
approximately 500 km west of the MidAtlantic Ridge and transects 0-25 Ma
of the oceanic lithosphere on the South
American Plate.
This E-W profile is connected with
an N-S profile that traverses the two
most prominent fracture zones and a
transform fault on the Earth, Chain
Fracture Zone, Romanche Transform
Fault and St Paul Fracture Zone. These
fracture zones are responsible for the
shape of Equatorial Africa and Brazil
and the 2000 km E-W coastline along
the Equatorial Africa.
The age contrast across the Chain
Facture Zone is about 15 Ma (i.e. an
offset of 300 km). Across the Romanche
48
Transform Fault the age contrast is
45 Ma, with an offset of 900 km, and
across St Paul it is 35 Ma with an offset
of 700 km. The Romanche transform
fault is the largest transform fault on
Earth and has hosted a series of large
earthquakes, including the 1994 Mw=7.1
earthquake. It is approximately 40 km
wide, and consists of a deep valley reaching to a water depth of 6200 m along our
profile, bounded by ridges that rise up
to 1700 m below the sea surface. These
fracture zones have also been responsible
for the unmixing of the water between
the southern and northern oceans, and
hindering the migration of the ecosystem
from south to north across the Equator.
IsoMetrix data require special processing, and WesternGeco provided an
infield geophysics team to perform initial preconditioning of the raw measurements into total pressure (P), plus vertical (Az) and horizontal (Ay) pressure
gradients, at 3125 m receiver intervals
in SEGD format. A key objective of the
project was to extract usable frequency
content down to 1.5 Hz, and hence care
was taken during the noise removal
process to preserve low frequencies. The
pressure and vertical gradient data were
combined to remove the effect of the
receiver-side ghost. Since the seafloor
is very rugose across the survey area,
wavefield scattering is a key issue, as
well as the related multiples. Because
of the very strong velocity contrast
on the seafloor, 4th and sometimes 5th
order multiples are observed. Thus, the
removal of scattered noise and multiples
is essential for extraction of the weak
signals from the mantle and LAB at
50-70 km depth. These are challenging
problems, to say the least.
Apart from imaging the base of the
lithosphere, the possible presence of melt
lenses in the mantle beneath the ridge
axis down to 20-40 km depth should
also be imaged. The 12-km streamer
data provides offsets that will allow
us to characterize the nature of oceanic crust using full waveform inversion
based techniques. The crustal structure
beneath the transform faults and fracture zones are poorly known, and these
data should also provide new insight
into these features. As the lithosphere
gets older, it cools and contracts, leading
to faulting in the mantle, which should
also be imaged using these data.
The reflection experiment will be
complemented by refraction data acquisition along one part of the profile, providing velocity information that would
be useful to migrate these data, and
also characterize the velocity structure
down to 20-30 km depth. This part
of the project will be jointly carried
out by GeoMar in Germany and IPGP
in France. Furthermore, 30 broadband
seismometers will be deployed along the
same profiles in order to characterize
the large-scale velocity structure of the
upper mantle, and possibly image the
LAB using receiver function technique.
The British Natural Environmental
Research Council and an ERC grant to
Dr Kate Rychert will fund this part of
the project. Magnetotelluric instruments
along the same profile are also likely to
be deployed to provide information on
the resistivity structure. All these data
will be jointly interpreted to develop a
model of the formation and evolution
of the oceanic lithosphere. The reflection data from this experiment will be
released to the scientific community after
initial publication of results. However,
scientists interested in testing their new
algorithms on these data can request a
part of the data beforehand.
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