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Development of a three-dimensional velocity model for the crust and upper mantle in the Barents Sea, Novaya Zemlya and Kola-Karelia regions
26th Seismic Research Review --- Trends in nuclear explosion monitoring, стр. 50--60. Orlando, Florida, (сентября 2004)
Аннотация
The principal objective of the present study is to compile a three-dimensional
(3D) seismic velocity model of the crust and upper mantle for the
larger Barents Sea region, at a spatial resolution of nominally 50×50
km. The main accomplishments so far have been concerned with compilation,
collation and review of primary existing geophysical data, including
first of all deep seismic wide-angle profiles (OBS and ESP – two-ship
expanded spread profiles), deep multichannel seismic reflection (MCS)
profiles, and shallower 1D velocity profiles. The main source of
data has been a data base compiled over many years at the University
of Oslo (UiO), supplemented by data compiled at the United States
Geological Survey (USGS) and from collaboration partners in Norway
and in Russia. Subsequently, detailed comparisons of data and models
between the UiO and USGS have been performed, and unified criteria
for quality assessment have been developed. The result is a full
integration of the underlying data, and a common unified model. The
50×50 km grid tiles in the target region have been defined in an
optimum way such that the tiles form a fully equidistant grid. The
filling of the grid tiles so far shows a very good coverage in the
western Barents Sea, a reasonable coverage also in the Novaya Zemlya
region, but is less-constrained in the northern and northeastern
parts of the target region. The results show that the depths to Moho
vary from about 10 km in the oceanic crustal domain to more than
40 km in coastal regions of Norway and Russia and in the Kara Sea,
while sediment thicknesses are 15-20 km in the south-western and
eastern parts of the Barents Sea. Following the USGS methodology
each grid tile is represented with layers for ice, water, soft sediments,
hard sediments, and crystalline upper, middle and lower crust. Finally
there is a layer describing the seismic velocity and density of the
uppermost mantle, which is controlling Pn and Sn travel times. We
have also acquired an upper mantle model (Shapiro and Ritzwoller,
2002) that eventually may be integrated with our new crustal model
and tested for a Ground Truth (GT) data base of about 50 events that
also has been established as a part of this study. Since some large
regional distances also will be used when comparing observed GT travel
times with computed travel times through the established model, a
mantle velocity model down to about 400 km is needed. To facilitate
this travel time testing a number of 2D profiles have been established
through the regions that are well covered with initially sampled
1D velocity functions, followed by different smoothing and interpolating
techniques, dependent on which modelling method that will be applied.
So far 2D ray tracing and finite difference methods have been used
here, with preliminary testing also of 3D methods. Since the grid
tiles filled with primary data are unevenly distributed it has been
necessary also to develop and to test methodologies for interpolation
and extrapolation, in order to have all tiles filled. In regions
where the data coverage is not dense, geological provinces are defined
which are supposed to hold similar tectonic histories. In this case
a method is being tested in which the velocity profiles have been
used to calculate the crystalline crustal thickness as a function
of sediment thickness. Since the crustal rock velocity distribution
for a particular geological province is known, a regional depth-to-basement
map can used to calculate representative 1D velocity-depth functions
for the entire crust. This technique looks promising as a means for
providing an equally sampled crustal model, as is required for seismological
purposes.
