A Global Digital map of

Sediment Thickness


Gabi Laske and Guy Masters


LAST UPDATE (September 06, 10): The anonymous ftp site for the model has moved. This did not affect the tar file containing all files. The links to individual files have been reopened.
IMPORTANT NOTICE (June 23,99): If you have used the whole model (thickness+elastic parameters) using the tar file that was created before June 99, please replace it with the current model. The old model had a bug that caused the shear velocity in the first layer to be too small (sometimes negative) in very thin sediments (<100m) in some oceanic areas. This affected about 30 of the 64800 1x1 deg cells. We greatly appologize for any inconvenience.

Sediment Thickness
Why is this map important?
Elastic Properties
A comparison with CRUST5.1
Related sites
Publications
References
Acknowledgments
download section


Sediment Thickness

This global sediment map is digitized on a 1x1 scale. In the oceans, published digital high-resolution maps were averaged (e.g. Pacific, Indian and South Atlantic oceans). In areas for which such files are not available (e.g. Arctic and North Atlantic ocean), the sediment thickness was hand-digitized using atlases and maps. The sediment thickness in most of the continental areas (incl. shelves) were obtained by digitizing the Tectonic Map of the World provided by the EXXON production research group (1985).

This map (incl. the postscript file) is available as part of a package including the whole 3-layer sediment model. Go to download section for details.

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Why is this map important?

Many of the global seismic long-period data used in mantle tomography are sensitive to variations in crustal structure but are not able to resolve it. For example, the contribution of the crust to global phase velocity maps of 150s Rayleigh waves is about half of the total signal (and is of opposite sign of that of the underlying mantle). Most authors handle this by applying a "crustal correction" to the data before an inversion for mantle structure. Since the inversion techniques can erroneously map crustal structure down to great depth, accurate crustal corrections to the data sets are extremely important. A significant contribution comes from thick sedimentary basins which have rather low values of seismic velocities and density. Hence, accurate knowledge of the structure of the "sedimentary layer" in a crustal model is crucial.

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Elastic Properties

Sediments have rather low seismic velocities and densities. In order to evaluate the seismic and gravity signal of large sedimentary basins, in situ information of these parameters become important. Seismic velocities (Vp and Vs) and density are specified on the same 1x1 grid and are also available with the map.
Since the seismic velocity changes rather rapidly with depth, it is necessary to subdivide the sedimentary cover. We chose to specify the elastic parameters in a 3-layer model.
While the top layer covers the first 2km of sediments, the second one (both shown here) covers the 5km underneath. For areas with sediments thicknesses larger than 7km, a third layer covers the rest (not shown here).
For much of the oceanic areas, detailed information of P-velocities are available in form of regional velocity functions (v=v0 +Kt, where t is one-way vertical travel time to layer mid-points). The plot shown here summarizes areas where velocity functions are available (areas of the same color do NOT have the same velocity function).
The velocity functions will be summarized in a future publication and are available upon request
( gmasters@ucsd.edu).
On continents, the velocities are less well known, as most refraction seismic profiles do not yield detailed information on the sedimentary cover. We therefore adopt the values given in CRUST 5.1 (2.1-2.5km/s in the first 2km, then 3.2-5.3km/s). The plot shown here summarizes Vp in the first two layers. Details on CRUST 5.1 can be found in the next section.
Since in situ information on shear velocity and density are not yet available on global scale, we use the curves of Ludwig, Nafe and Drake to obtain these values.

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A comparison with CRUST 5.1

CRUST 5.1 ( Mooney et al., 1998) is a global crustal model, specified on a 5x5 grid. This model specifies compressional and shear velocity as well as density in seven layers (ice; water; soft and hard sediments; upper, middle and lower crust).
This model has been completed before the new 1x1 sediment model was available.

We average the new sediment map to 5x5 cells and compare the resulting map to the sedimentary cover of CRUST5.1 (layers 3 and 4, soft and hard sediments).
There is general agreement between the two maps and CRUST5.1 represents the actual thickness in most of the oceanic areas. Significant differences (up to 2.9km) can be found in some shelf regions and sedimentary basins (e.g. Arctic, north of Greenland). Sediments on continents appear slightly overpredicted by CRUST5.1, especially in Eurasia and the Western U.S..
The Arctic and Antarctic Regions.

Due to low values of seismic velocities and density, even small errors in the representation of the sedimentary cover can lead to significant changes in the seismic signal (e.g. for surface wave dispersion).
To illustrate this, we calculate phase velocity maps for Rayleigh wave at 35 seconds (28mHz) using our new sediment map and our global crustal model CRUST 5.1. In the top map, the sediments of CRUST 5.1 have been replaced with the new model, while the lower panel shows the predictions of CRUST 5.1 (with the CRUST 5.1 sediments).
In the difference plot, many blue areas correlate with red areas in the sediment difference map (e.g. Artic Ocean north of Greenland). A lack of sediments in CRUST 5.1 yields high phase velocities that are too high. Overestimation of the sediment thickness in large parts on the continents in CRUST 5.1 (e.g. Eurasia) leads to velocities which are too low.
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Related Sites

Note: see also NGDC web site. The thickness map shown here basically uses the same maps as the NGDC map in the Indian, South Atlantic and Pacific oceans (with a few exceptions).
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Publications

A manuscript describing the model in detail is in preparation. Please refer to the following publication when you use our model:

G. Laske and G. Masters, A Global Digital Map of Sediment Thickness, EOS Trans. AGU, 78, F483, 1997.
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References

Ludwig, W.F., J.E. Nafe and C.L. Drake, Seismic Refraction, in "The Sea, Vol. 4, Ideas and Observations on Progress in the Study of the Seas", A.E. Maxwell (ed.), Wiley-Interscience, New York, 1970.

Mooney, W.D., G. Laske and G. Masters, CRUST5.1: A global crustal model at 5x5. J. Geophys. Res., 103, 727-747, 1998.

Sediment Maps used in this study

ReferenceRegion Covered
Tucholke, 1986North Atlantic (DNAG)
Tucholke and Uchupi, 1989aNorth Atlantic (atlas)
Tucholke and Uchupi, 1989bMid Atlantic (atlas)
Divins and Rabinowitz, 1989South Atlantic
Salvador, 1986Gulf of Mexico (DNAG)
Jackson and Oakey, 1986Arctic Ocean (DNAG)
Hayes, 1991Antarctica, Southern Oceans
Ludwig and Houtz, 1979North and South Pacific Ocean
Renkin and Sclater, 1988North Pacific Ocean
Rabinowitz et al., 1988Indian Ocean
Exxon, 1985global sediment map (here, Continents)
Beloussov and Pavlenkova, 1984Eurasia
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Acknowledgments

We would like to thank Jean-Yves Royer, Lisa Gahagen and John Sclater for providing us with digital versions of maps of the Indian and Southern Oceans, as well as the Pacific and South Atlantic Oceans.
We are also grateful to Robert Houtz who provided velocity functions and pointers to the literature.

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Download section

Click here to get a README and download the sediment map and files of the whole 3-layer sediment model. Please check the README file for instructions.
To download a compressed tar file with everything included (use gzip to uncompress) Click here.
As we are interested in feedback, please let us know for what you use the model and/or which area could be improved.

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Gabi Laske ( glaske@ucsd.edu)

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