Proposal

UCSD 97-1147

PROPOSAL SUMMARY

Principal Investigator David T. Sandwell

Scripps Institution of Oceanography, La Jolla, CA 92093-0225

Ph. 619 534-7109 sandwell@geosat.ucsd.edu

Co-Investigator Walter H. F. Smith

NOAA Geosciences Laboratory, N/OES12 Silver Spring MD 20901

Global Bathymetric Prediction for Ocean Modelling and Marine Geophysics

Proposed Costs: Yr 1 $63,915 Yr2 $74,282 Yr3 $63972

(d) ABSTRACT

We propose to construct a complete bathymetric map of the oceans at a 3-10 km resolution by combining all of the available depth soundings collected over the past 30 years with high resolution marine gravity information provided by the Geosat, ERS-1/2, and Topex/Poseidon altimeters. Detailed bathymetry is essential for understanding physical oceanography and marine geophysics. Currents and tides are controlled by the overall shapes of the ocean basins as well as the smaller sharp ocean ridges and seamounts. Because erosion rates are low in the deep oceans, detailed bathymetry reveals the mantle convection patterns, the plate boundaries, the cooling/subsidence of the oceanic lithosphere, the oceanic plateaus, and the distribution of off-ridge volcanoes. Current global digital bathymetry maps (e.g. ETOPO-5) lack many important details such as a 400 km-long ridge that rises to within 135 m of sea level (Figure 1). Moreover, they are contaminated by long-wavelength errors (~2000 km) which prevent accurate identification of seafloor swells associated with mantle plumes [Smith, 1993].

We propose to:

* Accumulate all available depth soundings collected over the past 30 years. (funded by NSF)

* Use the short wavelength (< 160 km) satellite gravity information to interpolate between sparse ship soundings.

* Improve the resolution of the marine gravity field using enhanced estimates along repeat altimeter profiles together with the dense altimeter measurements.

* Refine/Improve bathymetric predictions using the improved resolution gravity field and also by investigating computer-intensive methods for bathymetric prediction such as inverse theory.

* Produce a Globe of the Earth similar to the globe of Venus prepared by the NASA Magellan investigation. This will also include the best available digital land data.

Smith, W. H. F. and D. T. Sandwell, Bathymetric prediction from dense satellite altimetry and sparse shipboard bathymetry, J. Geophys. Res., 99, 21803-21824, 1994.

Sandwell, D. T. and W. H. F. Smith, Marine Gravity from Geosat and ERS-1 Altimetry, J. Geophys. Res., in press (anonymous ftp baltica.ucsd.edu).

(e) PROJECT DESCRIPTION

Prior Support

We were funded under the NASA Geodynamics Program for the geophysical analysis of Geosat, ERS-1, and Topex Altimetry. Total funding under NASA NAGW-3035 was $218,000 for a 4.5-year period 1/1/92 until 6/30/96 ($48k/yr). Due to the unexpected declassification of all of the Geosat/GM data on 7/95, this funding was insufficient to complete the research and publicize the results. Support for our recent satellite altimetry research has been "bootlegged" from NSF seagoing grants and other sources such as unrestricted donations from petroleum exploration companies and poster sales.

Scientific Publications

Sandwell, D. T., Antarctic Marine Gravity Field from High Density Satellite Altimetry, Geophys. J. Int., 109 , 437-448, 1992.

Small, C. and D. T. Sandwell, A Comparison of Satellite and Shipboard Gravity Measurements in the Gulf of Mexico, Geophysics, 57, 885-893, 1992.

Sandwell, D. D., L. A. Lawver, and I.W.D. Dalziel, W.H.F. Smith and M. Wiederspahn, ANTARCTICA Gravity Anomaly and Infrared Satellite Image, Scripps Institution of Oceanography, Geological Data Center, 1992. USGS MAP 1-2284

Neumann, G. A., D. W. Forsyth and D. Sandwell, Comparison of marine gravity from shipboard and high-density satellite altimetry along the mid-Atlantic Ridge, 30.5[ring]S-35.5[ring]S, Geophys. Res. Letts., 20, 1639-1642, 1993.

Small, C. and D. T. Sandwell, Imaging mid-ocean ridge transitions with satellite gravity, Geology, 22, 123-126, 1994.

Levitt, D. A. and D. T. Sandwell, Lithospheric bending at subduction zones based on depth soundings and satellite gravity, J. Geophys. Res., 100, 379-400, 1995.

Smith, W. H. F. and D. T. Sandwell, Bathymetric prediction from dense satellite altimetry and sparse shipboard bathymetry, J. Geophys. Res., 99, 21803-21824, 1994.

Smith, W. H. F. and D. T. Sandwell, Seafloor Topography Predicted from Satellite Altimetry and Ship Depth Measurements (Map), World Data Center-A for Marine Geology and Geophysics, Report MGG-09, National Geophysical Data Center, Boulder, Colorado, 80303.

Yale, M. M., D. T. Sandwell and W. H. F. Smith, Comparison of along-track resolution of stacked Geosat, ERS-1 and Topex satellite altimeters, J. Geophys. Res., 100, 15117-15127, 1995.

Sandwell, D. T., Exploration of the remote ocean basins with satellite altimeters, 1996 McGraw-Hill Yearbook of Science and Technology, p. 178-182, McGraw-Hill, Inc., New York, 1995

Sandwell, D. T. and W. H. F. Smith, Marine Gravity Anomaly from Satellite Altimetry, map Geological Data Center, Scripps Institution of Oceanography, December, 1995. (digital file, anonymous ftp baltica.ucsd.edu)

Sandwell, D. T. and W. H. F. Smith, Marine Gravity from Geosat and ERS-1 Altimetry, J. Geophys. Res., in press, July, 1996. (anonymous ftp baltica.ucsd.edu)

Small, C. and D. Sandwell, Sights Unseen, Natural History, March, 1996, p. 28-32.

Sandwell, D. T. and W. H. F Smith, Gravity Anomaly from Geosat and ERS-1 Altimetry, Versions 1-7, 1992-1995. (map available from SIO Geological Data Center, digital file available from anonymous ftp baltica.ucsd.edu)

Public Education

Recently Smith and Sandwell have worked with the media to publicize results from satellite altimeter missions. This activity began on October 23, 1995 when NOAA held a press conference to highlight the marine gravity field derived from ERS-1 geodetic phase altimetry and declassified Geosat Geodetic Mission altimetry data. Gravity anomalies are a rather technical subject so developing a story that is both accessible to the average reader and reasonably accurate involved a lot of interaction with the media. A partial list of the popular science publications follows.

In addition to the news and magazine articles, we have distributed both digital and print copies of these new gravity data. Since November 29, 1995, when Version 7.2 of the gravity grid was completed, there have been 9605 connections to our anonymous ftp site at baltica.ucsd.edu. Of these, 2319 visits had transfer times ranging from 10 minutes to 21 hours suggesting the 64 Mbyte (compressed) gravity grid was copied. There are mirror sites in Europe and Australia to distribute these data. Every day we fill requests for tape copies and high resolution graphics. The Scripps Geological Data Center has distributed 2300 copies of a large poster based on these data.

Sandwell and Smith have spent about 10-30% of their time on the phone to users and writers explaining what the data are and how they should be used. The people interested in these data range from "the real estate agent who goes sailing in the western Pacific" to "the amateur scientist who thinks the earth is expanding" to the "freelance writer" to the "marine geophysicist planning a cruise" to the "Navy scientist in a classified lab wanting to compare methods" to the "geophysicist at EXXON who wants to know the exact characteristics of the low-pass filter applied to the data". We see this as a great opportunity to inform the public about oceanography and enhance their awareness of the ocean basins and of earth science in general. The basic message is: one can learn a lot about the earth from the unmanned space program, but one also needs ships to provide detailed measurements and verification. People are happy to hear this story and they see it as a productive use of their tax dollars. Over the past 8 months we have spent about $55k on printing costs, postage, mailing tubes, SyQuest Disks, 8mm tapes, phone charges, etc. We attempted to pass some of this responsibility on to the National Data Centers but things happened much too quickly.

Finally, Smith and Sandwell work with graduate and undergraduate students on Earth Science research. (Smith is a research associate at SIO and a lecturer at the Johns Hopkins University.) This student interaction involves formal classroom teaching, informal seminars and daily interaction on research topics. In the budget we have included funding for a graduate student.

Magazine Articles

Carrol-Strait, G. (1996, May, 1996). Beneath the Feet of Neptune. The World & I p. 158-165.

Kunzig, R. (1996, March, 1996). The Seafloor from Space. Discover p. 58-64.

Lawler, A. (1995, November 1995). Sea-Floor Data Flow from Postwar Era. Science, p. 727.

McNutt, M. (25 January, 1996). The 5-billion-dollar bumps, Nature.

Monastersky, R. (1995, 16 December, 1995). A New View of the Earth. Science News p. 410-411.

Reichhardt, T. (1996, 28 May, 1996). Water World. Popular Science p. 70-71.

Small, C., & Sandwell, D. (1996, March, 1996). Sights Unseen. Natural History p. 28-33.

Yulsman, T. (1996, June, 1996). The Seafloor Laid Bare. Earth p. 42-51.

Newspaper Articles

Anonymous (25 October, 1995). Military Data Helps Detail Ocean Map. Sudbury Times

Anonymous (November, 1995). New Seafloor Map Released by NOAA, Scripps. Sea Technology

Anonymous (24 October, 1995). Scientists make detailed ocean map. Daily Republic

Bokstede, H. (26 January, 1996). Under ytan lurar berg och dalar. Svenska Dagbladet

Boyd, R. (24 October, 1995). A detailed map of the entire ocean floor. Philadelphia Inquirer

Boyd, R. (24 October, 1995). Mapping secrets of the deep. Press-Telegram

Boyd, R. (24 October, 1995). Once-secret photos help map the oceans. Sun News

Boyd, R. (24 October, 1995). A road map to the ocean floor. Miami Herald

Boyd, R. (24 October, 1995). Satellite data helps scientists create map of ocean floor. San Jose Mercury News

Boyd, R. (24 October, 1995). Satellite map of ocean floor offers detailed 'data feast'. News Tribune

Boyd, R. (24 October, 1995). Scientist's full-color satellite maps shine light on ocean floor. Los Angeles Times

Boyd, R. (24 October, 1995). Scientists create detailed map of ocean floor. Detroit Free Press

Boyd, R. (24 October, 1995). Spectacular Views of Earth's Oceans. Duluth News-Tribune

Broad, W. (24 October, 1995). Map Makes Ocean Floors as Knowable as Venus. The New York Times

Broad, W. (25 October, 1995). Map of Ocean Floor Opens a Window on the Mysteries of the Sea. International Herald Tribune

Broad, W. (24 October, 1995). New map of ocean floors provides glimpse of sunless depths. Ottawa Citizen

Carlowicz, M. (31 October, 1995). New Map of Seafloor Mirrors Surface. EOS, Transactions, AGU, p. 441-442.

Hill, R. ( 22 November 1995). Deep Details. Oregonian, p. A16.

Ladbury, R. (September 1995). Satellite Mapping of Terra Incognita Provides Welcome Relief. Physics Today

Lane, E. (24 October, 1995). Bottom of Sea Viewed in Detail. Newsday

Lane, E. (24 October, 1995). Detailed new map made by satellite shows ocean floor. Seattle Times

Miller, K. (24 October, 1995). Spy data brings ocean floor to light. USA Today

Miller, K. (24 October, 1995). Spy satellite data provide clear view of ocean floor. Courier-Post

Miller, K. (24 October, 1995). Spy Satellite Info Helps Scientists Map Ocean Floor. Chicago Sun-Times

Morgan, N. (October 12, 1995). Scripps team roams oceans with space maps. The San Diego Union Tribune

Associated Press (25 October, 1995). Map provides new details about ocean floor. Daily Herald

Associated Press (24 October, 1995). Scientists make first detailed map of ocean floor. Washington Times

Associated Press (29 October, 1995). Using satellite 'data feast,' detailed pictures of ocean floors emerge. Chicago Tribune

Schmid, R. (8 November, 1995). Declassified Data Reveals New Views of Ocean Bottom. Washington Post

Schmid, R. (21 November, 1995). Satellites help scientists map ocean floor. Capper's

Schmid, R. (24 October, 1995). Scientists Map Ocean Floor with Declassified Spy Sat. Data. Washington Post,

P-I. News Service (24 October, 1995). Map opens new avenues on the ocean floor. Seattle Post-Intelligencer

Snavely, R. (November, 1995) Declassified Navy Data Helps SIO Map Ocean. The Guardian

Spotts, P. (25 October, 1995). Seabed Exposed in Unparalled Detail. Christain Science Monitor

Introduction

A detailed knowledge of topography is fundamental to the understanding of most earth processes. On the land, weather and climate are controlled by topography on scales ranging from large continental land masses to small mountain valleys. The land is shaped by tectonics, erosion, and sedimentation and thus detailed topography is an essential component of any geological investigation. Hydrogeologic and biological processes are also largely controlled by local relief. The planned Shuttle mission to map land topography at 30 m horizontal resolution will provide much of the detailed information for land studies.

In the Oceans, detailed bathymetry is also essential for understanding physical oceanography and marine geophysics. Currents and tides are controlled by the overall shapes of the ocean basins as well as the smaller sharp ocean ridges and seamounts (Figure 1). Because erosion and sedimentation rates are low in the deep oceans, detailed bathymetry reveals the mantle convection patterns, the plate boundaries, the cooling/subsidence of the oceanic lithosphere, the oceanic plateaus, and the distribution of off-ridge volcanoes. Finally biological processes are largely controlled by ocean depth and terrain. In contrast to the Shuttle mission which can recover the land topography in 10 days, it would take approximatelt 125 ship-years to map the ocean basins at a 100 m horizontal resolution. Here we propose to map the topography of the ocean basins at a 3-10 km resolution using data collected by satellite altimeters and shipborn echo sounders. This is a diverse effort that is currently partly funded by the Marine Geology and Geophysics Division Program at NSF and was partly funded under the NASA Global Geodynamics Program.

Figure 1. (next page -top) Seafloor depth based on ETOPO-5 lacks the topographic expression of a 400 km long ridge as well as the rugged topography of the Eltanin and Udintsev Fracture Zone Systems. The ridge (53[ring]S, 140[ring]W, minumum depth 135 m) was first surveyed by a French expedition in December of 1996. These topographic features effect the flow of the the Antarctic Circumploar Current.

(middle) Predicted seafloor depth based on ship soundings and declassified Geosat/GM data. The Sub-Antarctic Front (SAF-red) [Gille, 1994] passes directly over the NW-trending ridge. The Polar Front (PF) is centered on the 6000m deep valley of the Udintsev transform fault.

(bottom) Ship soundings used in the bathymetric prediction. Predicted depths are constrained to agree with measured depths along these tracklines.

Work Statement

Now is the time to undertake this bathymetric mapping because radar altimeters aboard the ERS-1/2, Geosat, and Topex/Poseidon spacecraft have surveyed the marine gravity field over nearly all of the world's oceans to a high accuracy and spatial resolution. On March 15, 1995 ERS-1 completed its dense mapping (~8 km track spacing at equator) of the marine gravity between latitudes of +81.5[ring]. On July 28, 1995 all of the Geosat altimeter data were declassified (~4 km track spacing at the equator; latitudes between +72[ring]). Moreover, the Topex/Poseidon altimeter has accumulated many years of data having exceptional quality. With NASA and other funding we have assimilated most of these data into a global marine gravity anomaly grid (Figure 2) [Sandwell and Smith, 1996].

Figure 2 (top) Tracks of stacked Geosat/ERM (17-day repeat cycle) (22.5[ring]-25[ring] N), Geosat/GM (20[ring]-22.5[ring] N), ERS-1 Geodetic Phase (168-day repeat cycle) (17.5[ring]-20[ring] N) and stacked ERS-1 (35-day repeat) (15[ring]-17.5N). (bottom) Vertical gravity gradient (i.e., curvature of ocean surface) around Hawaii derived from all 4 data sets. Contours at 50 and 100 Eotovos units are shown to highlight seamount/island signatures (http://topex.ucsd.edu/mar_grav.html).

In the wavelength band 15 to 200 km, variations in gravity anomaly are highly correlated with seafloor topography. Since many southern ocean areas and some northern ocean areas are sparsely surveyed, these new satellite altimeter data reveal many previously unsurveyed features such as ridge axes, seamounts and fracture zones.

The conceptual approach is to use the sparse depth soundings to constrain the long-wavelength depth while the shorter-wavelength topography is predicted from the downward-continued satellite gravity measurements [Smith and Sandwell, 1994]. Over the short wavelength band, the topography/gravity ratio is regionally calibrated using available soundings. We have found that major errors in the ETOPO-5 bathymetric model make it unsuitable for either constraining the long-wavelength depths or for calibrating the topography/gravity ratio. Thus for an accurate prediction, it is essential to go back to the raw ship soundings.

The elements of our "NSF-funded" research are to:

* Accumulate all available depth soundings collected over the past 30 years and place them in a GMT data base. The main work here is the editing and cleaning of the GEODAS-3 data recently distributed by NOAA on CD-ROM. Much of this work is done.

* Use the short wavelength (< 160 km) satellite gravity information to interpolate between ship soundings to produce a global grid of predicted bathymetry; refine Nettleton's method (described below) and investigate linear inverse theory methods for bathymetric prediction. A third draft of the global Nettleton approach was just completed.

* Identify well navigated cruises in the GMT data base and force the bathymetric prediction to match those soundings.

* Extract regional variations in the topography/gravity transfer function and relate them to tectonic provinces (i.e. seamount chains, ridge axes, fracture zones, sedimented abyssal plains).

* Examine variations in topography/gravity transfer function at spreading ridges with regard to spreading rate and distance to ridge/transform intersection. Also examine possible anisotropic topography/gravity transfer functions at spreading ridges.

We propose the following elements for a "NASA-funded" part of the program:

* Improve the accuracy and resolution of the gravity field along repeat tracks of ERS-1/2, Geosat/ERM and Topex/Poseidon and test these results with precise shipboard gravity. This will involve incorporating the latest orbits and tide corrections into the raw satellite altimeter measurements as well as averaging profiles along repeat tracklines to improve accuracy and resolution. Cross-track geoid gradients from global vertical deflection grids will be used to correct the repeat profiles to a common trackline. Ocean waves limit the accuracy/resolution of gravity field recovery so optimal weighting schemes will be investigated.

* Construct new gravity grid using enhanced estimates along repeat profiles together with the less accurate but more dense geodetic mission measurements.

* Refine/Improve bathymetric predictions - The major limitations to bathymetric prediction are the quality of the ship soundings, the resolution of the gravity field, and the stability of the downward continuation. We propose to enhance this effort through refinement of our data bases and investigation of computer-intensive downward continuation methods such as Inverse Theory.

* Outreach Program/Globe of the Earth - When a high-quality version of the bathymetric prediction is available, will produce of a globe of the Earth similar to the globe of Venus prepared by the NASA Magellan investigation. This will also include the best available digital land data. In addition, we propose to continue to publicize results from satellite altimeter/ocean research.

1. Improved Resolution of ERS-1/2, Geosat/ERM, and Topex/Poseidon Profiles

Much of our proposal is a refinement of work in progress. The dense altimeter coverage from Geosat/GM and ERS-1/GM has only been available for a little more than 1 year. As noted in a preprint by Rapp and Yi [1996], there is room for improvement in gravity resolution, especially along repeat profiles. The improvements will come from more accurate orbits/corrections as well as an improved understanding and perhaps removal of the oceanographic signals. We feel that the greatest benefits will come from averaging many cycles of ERS-1/2 data along its 35-day repeat path because this longer repeat cycle offers the best spatial coverage of any repeat orbit data.

Upward continuation of the gravity field from the ocean floor to the ocean surface causes a dramatic attenuation of the short wavelength gravity anomaly. Consider a gravity anomaly on the ocean floor having a wavelength of 4 times the ocean depth (eg., 20 km). The anomaly at the sea surface will be attenuated by a factor of e-[pi]/2= 0.2. This attenuation makes it very difficult to resolve wavelengths shorter than 20 km in the deep oceans and causes the downward continuation associated with bathymetric prediction to be unstable. Features such as sedimentary basins and salt domes are typically narrower than 10 km and are buried by 1-10 km of sediment. Better resolution of these shorter wavelengths is a primary concern of the exploration industry since all of the major exploration companies now use satellite altimetry for reconnaissance information in remote marine environments [Biegert and Herring, 1996] such as the Caspian Sea and Southeast Asia. We propose to produce high resolution gravity anomaly profiles along repeat altimeter tracklines so these data can be merged with more accurate shipboard gravity observation. The good agreement shown in Figure 3 suggests that such a merging will be possible and fruitful. The method used to convert along-track sea surface slope profiles into along-track gravity anomaly profiles [Small and Sandwell, 1992] relies on having a good knowledge of the cross-track sea surface slope which is available from Version 7.2 of our gravity grid. A primary component of our proposed research will be to construct these high resolution gravity profiles and make them available to the geophysical community. Topex, with more than 130 repeat cycles and a higher accuracy altimeter, will offer the highest resolution gravity profiles.

This improvement in satellite-derived gravity profiles will require an upgrade of the Geosat and ERS-1/2 altimeter measurements with the latest precise orbits [Scharroo, 1996; Ries et al., 1996], the new high accuracy tide models [Bettapur and Eanes, 1994, CSR V3.0] as well as other ionospheric/atmospheric delay corrections. John Lillibridge and Walter Smith at NOAA are currently performing these upgrades in collaboration with Chester Koblinsky at NASA. The improved tide models are particularly important for short-wavelength marine gravity recovery on the shallow continental margins. After upgrading the raw data, all of the available repeat slope profiles from all three satellites will be averaged (stacked) using the methods described in Yale et al. [1995] where cross-track sea surface slopes will be used to correct the repeat profiles for deviations from the model ground track.

Figure 3. Comparison between a shipboard gravity profile in the western South Atlantic (solid) and a gravity profile along a track corresponding to a 62-fold stack of Geosat altimeter profiles (dashed). The difference was not computed at -21.5[ring]S latitude where the ship track deviates from the satellite track to avoid an island. This shipboard gravity profile was collected along a Geosat repeat profile in a quiet oceanographic environment in order to assess the accuracy of the Geosat altimeter data [Jung and Vogt, 1992]. Accurate tie points and partial GPS navigation were used along with the best shipboard gravimeter to achieve ~1 mGal accuracy; most older marine gravity measurements have an rms accuracy of only 10 mGal because of inaccurate Eotovos correction related to inaccurate navigation [Wessel and Watts, 1988].

2. Construction of High Resolution Gravity Field

An example of the gravity field derived from Geosat and ERS-1 altimetry data is shown in Figure 2 [Sandwell and Smith, 1996]. The grid was constructed by combining high spatial density profiles and repeat track profiles from both Geosat and ERS-1. Repeat profiles from T/P data were not used in this initial grid because the fast gridding method relies on dense coverage for rapid convergence. Assessments of these grids using shipboard gravity measurements show accuracies of 3-7 mGal and resolutions 23-30 km wavelength (0.5 coherence) [Marks 1996; Sandwell and Smith, 1996; Rapp and Yi, 1996]. The highest resolution and accuracy is achieved along repeat profiles of Geosat/ERM or the ERS-1 35-day cycle [Yale et al., 1994].

At the very shortest wavelengths (10-80 km), prediction of seafloor topography corresponds to downward continuation of the gravity field from the ocean surface to the mean ocean depth. Downward continuation amplifies both the signal and noise so an increase in the signal-to-noise ratio is needed for improvements in the bathymetric prediction, especially for wavelengths less than 25 km. Thus a second major aspect of our work will be to merge the higher-quality gravity profiles from the repeat tracks with the lower quality but higher spatial density profiles from the geodetic phases of Geosat and ERS-1 in order to further refine the global gravity model for bathymetric prediction. The advantage here is that the routines blockmedian and surface [Smith and Wessel, 1990] from the GMT [Wessel and Smith, 1995] software package can be used in a very controlled way to achieve optimal results on a massive data set. A previous version of our gravity grid, made prior to the declassification of the Geosat data (Version 5), was constructed by using this approach and the results were excellent.

3. Refined Bathymetric Predictions

Data quality is the most important aspect of bathymetric prediction. High resolution satellite gravity data is needed not only to interpolate among the sparse soundings but also to identify which soundings are bad. Many soundings from remote areas of the oceans were collected before shipboard computers and satellite navigation were available. Moreover, all of the ship sounding data were collected over a 30 year period on a variety of ships from many countries and with many different chief scientists aboard. Thus the quality of the data is highly variable and many entire cruises or sections of cruises are bad [Smith, 1993]; only the most recent (since ~1987) GPS-navigated multibeam data are reliable. Typical errors include: navigation errors, digitizing errors, typographical errors due to hand entry of older sounding, reporting the data in fathoms instead of meters, incorrect sound velocity measurements and even computer errors in reading punch cards One bad section of a cruise in an isolated region will introduce a seafloor topographic feature that does not exist. Some named examples are the Islas Orcadas Seamounts in the Weddell Sea and the Novara Knoll in the Southern Indian Ocean [Canadian Hydrographic Service, 1984]. The high resolution gravity fields provides the information needed to assess the accuracy of the ship sounding data. Our approach is to identify the bad cruises through a comparison with an initial prediction based on the gravity and either eliminate them or attempt to fix problem areas (data rescue); rescue is especially important for soundings that fill a large data gap. We maintain 4 data bases of standard underway geophysical data (navigation, depth, gravity, and magnetics) in a GMTPLUS-format that is easily assessable through GMT routines. There is a lot of overlap among the data bases although each contains some unique cruises. Some statistics on the number of good and bad cruises in each data base follows

Good Bad

WS 2185 564

SIO 1415 182

NGDC 1253 813

BB 125 848

WS - Wessel Smith data base which is a derivative of the original Lamont Data base. SIO - Scripps data base, Geological Data Center. NGDC - National Geophysical Data Center data base. BB - Brownbook derivative of Lamont data base. (WS and BB have some identical data so WS is searched first.)

The automation, maintenance, and rescue of the ship data is largely funded by the NSF Division of Ocean Sciences. In addition to these data we are preparing for the possible declassification of a the US Navy Ocean Survey data [Medea Report, 1995].

Data preparation and assembly is an ongoing process; the current data are sufficiently good to construct a global bathymetric grid (http://topex.ucsd.edu/marine_topo/mar_topo.html). Here is one recipe (Nettleton's Method) that we are developing.

1) Grid available bathymetric soundings on a 2 minute Mercator grid that matches our gravity anomaly grid. To avoid seams, all work is done on a global grid between latitudes of +72[ring]. Coastline points from GMT provide the zero-depth estimates. A finite-difference, minimum-curvature routine is used to interpolate the global grid [Smith and Wessel, 1990]. This gridding program requires at least 256 Mbytes of computer memory.

2) Separate the grid into low-pass and high-pass components using a Gaussian filter (0.5 gain at 160 km). Filtering and downward continuation are performed with a multiple strip, 2-D FFT that spans 0-360[ring] longitude to avoid Greenwich edge effects.

3) Form high-pass filtered gravity using the same Gaussian filter.

4) Downward continue the high-pass filtered gravity to the low-pass filtered bathymetry assuming Laplace's equation is appropriate. A depth-dependent Wiener filter is used to stabilize the downward continuation.

5) Accumulate high-pass filtered soundings and corresponding high-pass filtered/downward-continued gravity into small (160 km) overlapping areas and perform a robust regression analysis. In sediment-free areas, the topography/gravity transfer function should be flat and equal to 1/2[pi]G[Delta][rho] so in the space domain, a linear regression is appropriate. This works well on young seafloor but not on old seafloor where sediment cover destroys the correlation between topography and gravity. In these cases we assume the seafloor is flat and set the topography/gravity ratio to zero. Finally there are intermediate cases where topographic depressions will be sediment filled while the highs protrude above the sediments so the topography/gravity relationship is non-linear. It is these partially sedimented areas that make the bathymetric problem difficult and inherently non-linear. Continental margins and shelves pose similar problems.

6) Regional topography/gravity ratio estimates are gridded and multiplied by the high-pass filtered/downward-continued gravity to form high-pass filtered predicted bathymetry.

7) The total predicted bathymetry is equal to the sum of the high-pass filtered predicted bathymetry and the low-pass filtered bathymetry.

8) Finally, the pixels constrained by ship soundings or coastline data are reset to the measured values and the finite-difference, minimum curvature routine is used to perturb the predicted values toward the measured values. Measured depths are flagged so they can be extracted separately. This final step dramatically increases the accuracy and resolution of the bathymetric grid in well surveyed areas so it agrees with the best hand-contoured bathymetric charts (Figure 4).

Figure 4 (next pages) Contours of seafloor depth (1000 m interval) south of Japan. (top) digitized contours taken from GEBCO sheet 06. (bottom) same depth contours derived from version 3.0 of our bathymetric prediction. (next page) same depth contours from ETOPO-5. This area was selected for comparison because it is well surveyed and the digital GEBCO contours were carefully prepared by the Joint Oceanographic Data Center, Tokyo, Japan. The agreement between the GEBCO and the Prediction is excellent suggesting that in well surveyed areas our procedures are correct. In sparsely surveyed areas the agreement is only good where there is control from ship soundings. We believe that, in the data gaps, the Prediction is more accurate than the GEBCO charts. The ETOPO-5 grid is not suitable for these types of quantitative test.

While this recipe provides accurate results, it may not be optimal so we propose two additional types of investigations. First, in areas where ship soundings are very sparse, we will experiment with lengthening the wavelength of the low-pass filter from 160 km to perhaps 700 km. Including these longer wavelengths will introduce an additional parameter (flexural parameter Sichoix and Boneville, 1996) into the topography/gravity transfer function estimation. A second, completely different approach would be to develop a spatially varying Green's function to map gravity into topography over a given wavelength band. Here the concepts of linear inverse theory [Parker, 1994] will be used to stabilize the problem and provide error bounds on the predicted depths as a function of distance from ship soundings.

4. Geology, Geophysics, and Outreach

This new grid will enable one to revisit several important unresolved problems in marine geology and geophysics. One important issue is the debate on the cooling history of old oceanic lithosphere as seen in the differences between the Parsons and Sclater [1977] depth/age model PSM and the Stein and Stein [1992] GDH-1 model. There is a dramatic difference in implied thermal structure of the old oceanic lithosphere because GDH1 is about 500 m shallower than PSM. This difference has implications for the buoyancy and driving force of subducted lithosphere, the rheological characteristics of the ductile part of the lithosphere, and the amount of earth cooling that is not related to large-scale mantle convection. We believe that the GDH1 model is 500 m too shallow because it is based on ETOPO-5 and also that it ignores the non-Gaussian statistics of seafloor depth. Seamounts and plateaus make the seafloor shallower but there are no comparable features that produce similar localized deepening and so the depth distribution is skewed toward shallow depths. In areas where seamount population is high, the mean depth is biased by about 500 m. A second issue is the size and extent of the South Pacific Super Swell. In the Southern Hemisphere, ETOPO-5 is based on the 500 m GEBCO depth contours and there is a tendency for the ETOPO-5 depths to occur at these multiples of 500 m. When the slope of the seafloor is very low, such as around the eastern part of the Super Swell, this effect can produce long-wavelength artifacts [Smith, 1993; Levitt and Sandwell, 1996].

There are many other important problems to be addressed using a high-resolution bathymetric grid such as: distributions and volumes of seamount chains, the relationships of volcanic chains to hot-spot swells, the volumes of the oceanic plateaus and their relationships to ancient plumes or major plate reorganizations and the distribution of buoyant, depleted mantle beneath the lithosphere.

Finally, a significant part of our proposed effort will be to make the digital files and printed maps available to both the scientific community and the general public. Because of our unexpected experience with declassified Geosat data, we have a better idea of how to make this process efficient. Some of the costs of this effort are included in the budget, especially those related to the scientific distribution. However, we have also found that the public and the petroleum exploration companies are willing to pay for the cost of this distribution. In particular we propose to produce a 35" x 54" poster of global topography to match a marine gravity anomaly poster distributed by the SIO Geological Data Center. The topography grid will include the best land grids that are available. We propose to distribute the map products through NGDC and the SIO Geological Data Center. In addition we propose to produce a Globe of the Earth similar to the Globe of Venus produced by JPL, Sky Publishing Co. and Replogle Globes Inc..

References

Bettadpur, S. V., and R. J. Eanes, Geographical representation of radial orbit perturbations due to ocean tides: Implications for satellite altimetry, J. Geophys. Res., 99, 24883-24898, 1994.

Biegert, E. K. and A. T. Herring, Exploration applications of satellite gravity, EOS Trans. AGU., v. 77, no. 17, p. S40, Spring AGU Meeting, 1996.

Gille, S.T., Mean surface height of the Antarctic Circumpolar Current from Geosat data: Method and application, J. Geophys. Res., 99, 18255-18273, 1994.

Jung, W.Y. and P.R. Vogt, Predicting bathymetry from Geosat-ERM and shipborne profiles in the South Atlantic Ocean, Tectonophysics, 210, 235-253, 1992.

Levitt, D. A. and D. T. Sandwell, Modal depth anomalies from multibeam bathymetry: Is there a South Pacific superswell?, Earth. Planet. Sci. Lett., 139, 1-16, 1999.

Marks, K. M., Resolution of the Scripps/NOAA marine gravity field from satellite altimetry. Geophys. Res. Lett., 23(16), 2069-2072, 1996.

Medea Report, Scientific Utility of Naval Environmental Data, Medea Office, Mitre Co., McLean, VA, July, 1995.

Parsons, B., and J. G. Sclater, An analysis of the variation of the ocean floor bathymetry and heat flow with age, J. Geophys. Res., 82, 803-827, 1977.

Rapp, R., and Y. Yi, Role of ocean variability and sea surface topography in the recovery of the mean sea surface and gravity anomalies from satellite altimeter data, J. Geodesy, submitted August, 1996, 1996.

Ries, J.C., J.J. Bordi, C.K. Shum, and B.D. Tapley, Assessment of precise orbit accuracy for ERS-1 and ERS-2, EOS Trans. AGU., v. 77, no. 17, p. S76, Spring AGU Meeting, 1996.

Sichoix, L., and A. Bonneville, Prediction of bathymetry in French Polynesia constrained by shipboard data, Geophys. Res. Lett., 23, 2469-2472, 1996.

Sandwell, D.T. and W.H.F. Smith, Marine Gravity from Satellite Altimetry, Version 7.2 (map and digital file), Geological Data Center, Scripps Inst. of Oceano., 9500 Gilman Dr., La Jolla, CA, 92093-0223, anonymous ftp to baltica.ucsd.edu, 1995.

Sandwell, D. T. and W. H. F. Smith, Marine Gravity from Geosat and ERS-1 Altimetry, J. Geophys. Res., in press, August, 1996. (anonymous ftp baltica.ucsd.edu)

Small, C. and D. T. Sandwell, A Comparison of Satellite and Shipboard Gravity Measurements in the Gulf of Mexico, Geophysics, 57, 885-893, 1992.

Smith, W. H. F., On the accuracy of digital bathymetry data, J. Geophys. Res., 98, 9591-9603, 1993.

Smith, W. H. F., and P. Wessel, Gridding with continuous curvature splines in tension, Geophysics, 55, 293-305, 1990.

Scharroo, R., Topex-class orbits for the ERS Satellites, EOS Trans, AGU, 1996 Spring Meeting, v.77, no. 17, p. S76, 1996.

Smith, W. H. F. and D. T. Sandwell, Bathymetric prediction from dense satellite altimetry and sparse shipboard bathymetry, J. Geophys. Res., 99, 21803-21824, 1994.

Stein, C. A., and S. Stein, A model for the global variation in oceanic depth and heat flow with lithospheric age, Nature, 359, 123-129, 1992.

Wessel, P. and A.B. Watts, On the accuracy of marine gravity measurements, J. Geophys. Res., 93, 393-413, 1988.

Yale, M.M., D.T. Sandwell, and W.H.F. Smith, Comparison of along-track resolution of stacked Geosat, ERS-1 and TOPEX satellite altimeters, J. Geophys. Res., 100, 1995.

(f) MANAGEMENT APPROACH

The proposed research involves geodetic, geophysical, and oceanographic components using both satellite altimeter and shipboard measurements. In addition, two institutions are involved, Sandwell at SIO and Smith at NOAA. Most of the work will be performed at SIO. The work is divided into 3 sections:

1. Sandwell, Smith, and Yale (graduate student) are primarily responsible for the pre-processing of the T/P, ERS-1/2 and Geosat altimetry, the improvements in the marine gravity field from repeat-orbit satellite altimeter profiles (year 1).

2. Sandwell and Smith will combine the high-resolution repeat altimeter profiles with the dense altimeter measurements from the geodetic phases of Geosat and ERS-1 to construct a gravity field model and vertical deflection models. These grids along with the profile will be made available for public access through ftp (years 1 and 2).

3. Sandwell and Smith will refine bathymetric predictions using Nettleton's method in year 1 and explore new methods in years 2 and 3.

A graduate student will participate in all aspects of the research and much of this research will form a Ph. D. thesis.

(g) PERSONNEL

NAME: David T. Sandwell BORN: April 7, 1953

ADDRESS: Scripps Institution of Oceano. PHONE: (619) 534-7109 (off.)

La Jolla, CA 92093-0225 INTERNET: sandwell@radar.ucsd.edu

FAX: (619) 534-9873

PRESENT POSITION:

Professor of Marine Geophysics, Scripps Institution of Oceanography

EDUCATION:

Ph.D., 1981, University of California at Los Angeles, Geophysics and Space Physics.

M.S., 1978, University of California at Los Angeles, Geophysics.

B.S., 1975, University of Connecticut, Major Physics, Minor Mathematics

PROFESSIONAL EXPERIENCE:

1989-93 Scripps Institution of Oceanography. Associate Professor.

1985-89 The University of Texas at Austin. Research Scientist, joint appointment, Institute for Geophysics and Center for Space Research.

1982-85 National Geodetic Survey, Rockville, MD. Research Geophysicist.

1976-81 University of California, Los Angeles, Research Assistant.

OTHER EXPERIENCE:

10/95-present AGU Editor of Earth Interactions, Electronic Journal

9/95-present Member of NRC Space Studies Board, Committee on Earth Studies

5/95 - present Member of NRC, US Committee on Geodynamics

1/95 - 10/95 Associate Editor, Journal of Geophysical Research

9/94 - present SIO Representative to UCSD Academic Senate

9/93 - 8/94 Chair of UCSD Academic Senate Committee on Computing

2/92 - 12/95 Office of Technology Assessment panel on Earth Observing Systems

10/90 - 1/93 Member of NASA's topographic mapping mission working group

6/90 - 1/95 Member of National Research Council, Committee on Geodesy

1/88 - 1/90 Panel member of NSF RIDGE Initiative, Theoretical/Experimental/Analytical

12/86 - 1/91 Science steering group member for NASA's satellite gravity program

1/87 - 12/90 Associate Editor, Reviews of Geophysics and Space Physics

2/85-1/89 Associate Editor, Journal of Geophysical Research

RESEARCH FUNDING:

9/95-present NSF Isostasy of the Southern Oceans

7/92-12/95 NSF Stretching of the Central Pacific Lithosphere

1/92- 3/96 NASA Global Geodynamic Applications of Satellite Altimetry

5/90 - 1/95 NASA Magellan Guest Investigator,

CRUISE PARTICIPATION:

5/83 Participating scientist on Bermuda Swell heat flow experiment

3/87 Assistant chief scientist on R/V Washington to explore Seasat gravity lineations

2/89 Assistant scientist on R/V Surveyor to map the Shackleton Fracture Zone

1/93 Chief scientist on R/V Melville to map En-Echelon Ridges, Gloria Leg 4

1/94 Co-chief scientist on R/V Melville to map Eltanin and Udintsev Fracture Zones

MEMBERSHIPS:

6/77-present American Geophysical Union

6/80-present International Association of Geodesy

RELEVANT PUBLICATIONS: (from more than 70)

Sandwell, D. T., and G. Schubert, Geoid Height versus Age for Symmetric Spreading Ridges, J. Geophys. Res., 85, 7235-7241, 1980.

Sandwell, D. T., Thermal Isostasy: Response of a Moving Lithosphere to a Distributed Heat Source, J. Geophys. Res., 87, 1001-1014, 1982.

Sandwell, D. T., and G. Schubert, Geoid Height-Age Relation from Seasat Altimeter Profiles across the Mendocino Fracture Zone, J. Geophys. Res., 87, 3949-3958, 1982.

Sandwell, D. T., A Detailed View of the South Pacific Geoid from Satellite Altimetry, J. Geophys. Res., 89, 1089-1104, 1984.

Sandwell, D. T., Thermomechanical Evolution of Oceanic Fracture Zones, J. Geophys. Res., 89, 11401-11413, 1984.

Schubert, G. and D. T. Sandwell, Crustal Volumes of the Continents and of Onceanic and Continental Submarine Plateaus, Earth Planet. Sci. Lett., 92, 234-246, 1989.

Sandwell, D. T. and K. R. MacKenzie, Geoid Height Versus Topography for Oceanic Plateaus and Swells, J. Geophys. Res., 94, 7403-7418,1989.

Small, C. and D. T. Sandwell, An Abrupt Change in Ridge-Axis Gravity with Spreading Rate, J. Geophys. Res., 94, 17383-17392, 1989.

Sandwell, D. T. and B. Zhang, Global Mesoscale Variability from Geosat Exact Repeat Mission: Correlation with Ocean Depth, J. Geophys. Res., 94, 17971-17984, 1989.

Sandwell, D. T., Antarctic Marine Gravity Field from High Density Satellite Altimetry, Geophys. J. Int., 109 , 437-448, 1992.

Small, C. and D. T. Sandwell, A Comparison of Satellite and Shipboard Gravity Measurements in the Gulf of Mexico, Geophysics, 57, 885-893, 1992.

Small, C. and D. T. Sandwell, Imaging mid-ocean ridge transitions with satellite gravity, Geology, 22, 123-126, 1994.

Levitt, D. A. and D. T. Sandwell, Lithospheric bending at subduction zones based on depth soundings and satellite gravity, J. Geophys. Res., 100, 379-400, 1995.

Smith, W. H. F. and D. T. Sandwell, Bathymetric prediction from dense satellite altimetry and sparse shipboard bathymetry, J. Geophys. Res., 99, 21803-21824, 1994.

Smith, W. H. F. and D. T. Sandwell, Seafloor Topography Predicted from Satellite Altimetry and Ship Depth Measurements (Map), World Data Center-A for Marine Geology and Geophysics, Report MGG-09, National Geophysical Data Center, Boulder, Colorado, 80303.

Sandwell, D. T., E.L. Winterer, J. Mammerickx, R. A. Duncan, M. A. Lynch, D. A. Levitt, and C. L. Johnson, Evidence for diffuse extension of the Pacific plate from Pukapuka ridges and crossgrain gravity lineations, J. Geophys. Res., 100, 15087-15099, 1995.

Yale, M. M., D. T. Sandwell and W. H. F. Smith, Comparison of along-track resolution of stacked Geosat, ERS-1 and Topex satellite altimeters, J. Geophys. Res., 100, 15117-15127, 1995.

Sandwell, D. T., Exploration of the remote ocean basins with satellite altimeters, 1996 McGraw-Hill Yearbook of Science and Technology, p. 178-182, McGraw-Hill, INc., New York, 1995

Levitt, D. A. and D. T. Sandwell, Modal depth anomalies from multibeam bathymetry: Is there a South Pacific Superswell?, Earth Planet. Sci. Lett., 139, 1-16, 1996.

Sandwell, D. T. and W. H. F. Smith, Marine Gravity Anomaly from Satellite Altimetry, map Geological Data Center, Scripps Institution of Oceanography, December, 1995. (digital file, anonymous ftp baltica.ucsd.edu)

Sandwell, D. T. and W. H. F. Smith, Marine Gravity from Geosat and ERS-1 Altimetry, accepted J. Geophys. Res., June 1, 1996. (anonymous ftp baltica.ucsd.edu)

CURRICULUM VITAE

Walter H. F. Smith

Date of Birth: September 14, 1961 Social Security No: 557-27-7426

Place of Birth: Boston, Massachusetts Citizenship: United States

EDUCATION

Degrees Earned:

October 1990 Ph. D., Marine Geophysics, Columbia University, New York Dissertation: Marine Geophysical Studies of Seamounts in the Pacific Ocean Basin

May 1986 M. A., Geological Sciences, Columbia University, New York

May 1984 B. S., Geological Sciences, University of Southern California, Los Angeles

Other relevant studies:

July 1984 Ecole d'Eté: La Geophysique Interne et l'Espace, Centre National d'Etudes Spatiales, Toulouse, France

1981-82 Undergraduate studies at Cuesta College, San Luis Obispo, CA (transferred to USC in 1982).

PROFESSIONAL EXPERIENCE

Positions Held:

Spring 1996 Lecturer, the Johns Hopkins University, Baltimore, MD.

1992 - Geophysicist, Geosciences Laboratory, Nat'l Ocean Service, Nat'l Oceanic & Atmospheric Admin., U.S. Department of Commerce, Silver Spring, MD.

1990 - 92 Postgraduate Researcher, Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, University of California, San Diego, CA

1984 - 90 Graduate Research Assistant, Lamont-Doherty Geological Observatory of Columbia University, Palisades, NY

Spring 1986 Teaching Assistant in Structural Geology, Columbia University, New York, NY

Field and Sea Experience:

November 1991 R/V Thomas Washington cruise Tunes-05: Co-Chief Scientist, Western Pacific Guyots, Kwajalein - Guam

July 1990 R/V Moana Wave cruise MW-9009: Co-Chief Scientist, Marshall Islands Guyots ODP Site Survey Augmentation, Pohnpei - Honolulu

May 1988 R/V Thomas Washington cruise RNDB-II: responsible for acquisition and reduction of gravity data for Hawaiian Flexural Moat Geophysical and Coring Survey, Honolulu - Honolulu

September 1985 R/V Robert Conrad cruise RC-2610: responsible for acquisition of data for thesis research, including gravity, Seabeam, and dredging operations, AT&T Cable Survey / Western Pacific Seamounts, Honolulu - Guam

Summer 1983 Summer Field Camp, White-Inyo Mountains, CA, Univ. of Southern California

Spring 1983 Field Mapping, Mojave Desert, CA, Univ. of Southern California

PROFESSIONAL SOCIETY MEMBERSHIPS

1990 Sigma Xi

1989 Society of Exploration Geophysicists

1984 American Geophysical Union

HONORS AND AWARDS

1995 Gold Medal, U.S. Department of Commerce.

1994 Alumnus of the Year, Cuesta College.

1990 - 92 Cecil and Ida Green Foundation Scholar, Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography

1984 - 90 Faculty Fellowship, Columbia University

1985 - 86 Amoco Foundation Doctoral Fellowship, Columbia University

1983 Summer Internship, Lamont-Doherty Geological Observatory

1982 - 84 Keck Foundation Merit Award, University of Southern California

SERVICE TO THE SCIENTIFIC AND EDUCATIONAL COMMUNITY

Since 1988, co-author and distributor of the GMT-System software. This is a collection of C language tools, data, and on-line help for creating PostScript maps and scientific illustrations in the UNIX computing environment. It is distributed free of charge over the Internet (http://www.soest .hawaii.edu/soest/gmt.html) and is used by more than 5000 scientists on every continent (including Antarctica) and on board ships and aircraft. Although it was designed with the needs of Earth and physical scientists in mind, it is also used in medical tomography and political science applications.

Since 1990, supplied a "white paper" and several other documents and maps to the Oceanographer of the Navy, and aggressively argued the scientific rationale for declassification of Geosat altimeter data. These data were released in July, 1995.

Committee on Education and Human Resources, American Geophysical Union (Member, 1994-96; Chair, 1996-98).

Committee on Earth Gravity from Space, National Research Council, 1996.

Scientific Advisor to the International Hydrographic Organization and the Inter-governmental Oceanographic Commission of UNESCO for GEBCO, the General Bathymetric Chart of the Oceans. (Since May, 1993.) Chair of a working group investigating machine algorithms for gridding of digitized contour data.

Member, Working Group 107 - Improved Global Bathymetry, Scientific Committee on Oceanic Research, International Council of Scientific Unions (since January 1996).

Pro bono consultant to the National Geographic Society for the revision of its World Physical Map. Directed artist Tibor Toth to paint sea floor relief in accordance with new results from satellite altimetry. Map appeared in the February 1994 issue of National Geographic magazine.

Peer reviewer of research proposals submitted to the National Science Foundation and the National Aeronautics and Space Administration, and of research articles submitted to Earth and Planetary Science Letters, Geophysical Journal International, Geophysical Research Letters, Journal of Geophysical Research, Marine Geodesy, Nature and Tectonics.

PUBLICATIONS

Books and Review Articles in preparation:

W. H. F. Smith, The mapping and interpretation of Earth's gravity anomalies, a book in the series "Critical Topics in Modern Earth Sciences", to be published by Columbia University Press in Spring, 1997.

Peer-Reviewed Research Articles:

Wessel, P., and W. H. F. Smith, A global self-consistent hierarchical high-resolution shoreline database, J. Geophys. Res., 101, 8741-8743, 1996.

Wessel, P., and W. H. F. Smith, New version of the Generic Mapping Tools released, EOS Trans. Amer. Geophys. Un., 76, 329, 1995, with electronic supplement on the World Wide Web at http://www.agu.org/eos_elec/95154e.html. (First-ever electronic publication by the AGU, August 15, 1995.)

Yale, M. M., D. T. Sandwell, and W. H. F. Smith, Comparison of along-track resolution of stacked Geosat, ERS-1, and TOPEX satellite altimeters, J. Geophys. Res., 100, 15,117-15,127, 1995.

Phipps Morgan, J., W. J. Morgan, Y.-S. Zhang, and W. H. F. Smith, Observational hints for a plume-fed, sub-oceanic asthenosphere and its role in mantle convection, J. Geophys. Res., 100, 12,753-12,767, 1995.

Smith, W. H. F., and D. T. Sandwell, Bathymetric prediction from dense satellite altimetry and sparse shipboard bathymetry, J. Geophys. Res., 99, 21,803-21,824, 1994.

Abbott, D., R. Drury, and W. H. F. Smith, Flat to steep transition in subduction style, Geology, 22, 937-940, 1994.

Abbott, D., L. Burgess, J. Longhi, and W. H. F. Smith, An empirical thermal history of the Earth's upper mantle, J. Geophys. Res., 99, 13,835-13,850, 1994.

Smith, W. H. F., On the accuracy of digital bathymetric data, J. Geophys. Res., 98, 9591-9603, 1993.

Müller, R. D., and W. H. F. Smith, Deformation of the oceanic crust between the North American and South American plates, J. Geophys. Res., 98, 8275-8291, 1993.

Marks, K. M., D. C. McAdoo, and W. H. F. Smith, Mapping the Southwest Indian Ridge with Geosat, EOS Trans. Am. Geophys. U., 74, 81, 86, 1993.

Morgan, J. P., and W. H. F. Smith, Flattening of the sea-floor depth-age curve as a response to asthenospheric flow, Nature, 359, 524-527, 1992.

Wessel, P., and W. H. F. Smith, Free software helps map, display data, EOS Trans. Am. Geophys. U., 72, 441, 446, 1991.

Staudigel, H., K.-H. Park, M. Pringle, J. L. Rubenstone, W. H. F. Smith, and A. Zindler, The longevity of the south Pacific isotopic and thermal anomaly, Earth Planet. Sci. Lett., 102, 24-44, 1991.

Smith, W. H. F., and P. Wessel, Gridding with continuous curvature splines in tension, Geophysics, 55, 293-305, 1990.

Smith, W. H. F., H. Staudigel, A. B. Watts, and M. S. Pringle, The Magellan Seamounts: Early Cretaceous record of the South Pacific Isotopic and Thermal Anomaly, J. Geophys. Res., 94, 10,501-10,523, 1989.

Technical and Cruise Reports, Maps, Data and Software Products, Electronic Publications, Conference Proceedings, Forum Statements, etc:

Sandwell, D. T., and W. H. F. Smith, Global marine gravity field "world_grav.img.7.2", digital data set available by ftp over Internet, and printed map available from Geological Data Center, Scripps Institution of Oceanography, La Jolla, Calif., November, 1995. (Reprocessing with new filters; an improvement over world_grav.img.6.2.)

Dessler, A. E., W. Smith, and R. Lopez, The future employment of geophysicists, EOS Trans. Amer. Geophys. Un., 76, 372, 1995. (Statement of the AGU Committee on Education and Human Resources in response to the employment situation for recent PhDs.)

Sandwell, D. T., and W. H. F. Smith, Global marine gravity field "world_grav.img.6.2", digital data set available by ftp over Internet, and printed map available from Geological Data Center, Scripps Institution of Oceanography, La Jolla, Calif., September, 1995. (Extended version of world_grav.img.6 using a 2-minute grid.)

Sandwell, D. T., and W. H. F. Smith, Global marine gravity field "world_grav.img.6", digital data set available by ftp over Internet, and printed map available from Geological Data Center, Scripps Institution of Oceanography, La Jolla, Calif., August, 1995. (Contains Geosat GM data declassified in July 1995 and ERS-1 GM OPRs through December 1994.)

Wessel, P., and W. H. F. Smith, GMT-The Generic Mapping Tools version 3.0, free software and data distributed over the World Wide Web at http://www.soest.hawaii.edu/soest/ gmt.html, August, 1995.

Sandwell, D. T., M. M. Yale, and W. H. F. Smith, Global marine gravity field "world_grav.img.5", digital data set available over Internet, and printed map available from Geological Data Center, Scripps Institution of Oceanography, La Jolla, Calif., May, 1995. (Contains ERS-1 GM IGDRs for both cycles, as well as Geosat and Topex.)

Sandwell, D. T., M. M. Yale, and W. H. F. Smith, Global marine gravity field "world_grav.img.4", digital data set available over Internet, and printed map available from Geological Data Center, Scripps Institution of Oceanography, La Jolla, Calif., December, 1994. (Contains 1st cycle of ERS-1 GM IGDRs, as well as Geosat and Topex.)

Smith, W. H. F., and D. T. Sandwell, Sea floor topography predicted from satellite altimetry and ship depth measurements, World data center for marine geology and geophysics report MGG-09, National Geophysical Data Center, U.S. Dept. Commerce, Boulder, Colo., 1994. (Digital data file available from NGDC on CD-ROM, and map printed by U.S. Gov't Printing Office).

Sandwell, D. T., and W. H. F. Smith, Global marine gravity field "world_grav.img.3", digital data set available over Internet, and printed map available from Geological Data Center, Scripps Institution of Oceanography, La Jolla, Calif., May, 1994. (Produced using stacked ERS-1, Geosat and Topex profiles.)

Smith, W. H. F., and D. T. Sandwell, Scientific rationale for declassification of Geosat altimeter data, unpublished "white paper" prepared for the Oceanographer of the Navy at the request of the American Geophysical Union, February, 1994.

McAdoo, D. C., K. M. Marks, W. H. F. Smith, and J. L. Lillibridge III, Gravity field of the Ross Sea region from satellite altimetry: geodynamic implications for West Antarctica, Proceedings Second ERS-1 Symposium - Space at the Service of our Environment, Hamburg, Germany, 11-14 October 1993, pp. 1227-1231, European Space Agency (ESA) Special Publication ESA SP-361, January, 1994.

Wessel, P., and W. H. F. Smith, The GMT-System Version 2.1 Release 4 Technical Reference and Cookbook, SOEST Univ. Hawaii and NOAA Geosciences Laboratory internal report, August, 1993. (Electronic publication).

Marks, K. M., D. C. McAdoo, and W. H. F. Smith, Marine gravity field from Geosat GM data south of 30[ring]S, digital data set available on CD-ROM, and printed map, Natl. Geophys. Data Center, U.S. Dept. Commerce, Boulder, Colo., 1993.

Wessel, P., and W. H. F. Smith, GMT version 2.1.3, CD-ROM distribution, in Prime Time Freeware, 2, number 1, Sunnyvale, CA, 1993.

Sandwell, D. T., and W. H. F. Smith, Global marine gravity field "world_grav.img.2", digital data set available over Internet, and printed map available from Geological Data Center, Scripps Institution of Oceanography, La Jolla, Calif., May, 1993. (Contains ERS-1 35 day, Geosat ERM and declassified GM, and Seasat where needed; filters and POCS method retuned in response to ship comparisons.)

Sandwell, D. T., and W. H. F. Smith, Global marine gravity field from ERS-1, Geosat and Seasat data "world_grav.img", digital data set available over Internet, and printed map available from Geological Data Center, Scripps Institution of Oceanography, La Jolla, Calif., December, 1992.

Sandwell, D. T., L. A. Lawver, I. W. D. Dalziel, W. H. F. Smith, and M. Wiederspohn, Antarctica gravity anomaly and infrared satellite image, map, Geological Data Center, Scripps Institution of Oceanography, La Jolla, Calif., 1992.

Wessel, P., and W. H. F. Smith, The GMT-System Version 2.0 Technical Reference and Cookbook, SOEST Univ. Hawaii and IGPP Univ. Calif. San Diego internal report, June, 1991. (Available as electronic publication).

Bergersen, D. D., and W. H. F. Smith, Moana Wave cruise MW 9009, Marshall Islands Guyots, Pohnpei to Honolulu, Cruise Report, Hawaii Institute of Geophysics, 1990.

Wessel, P., and W. H. F. Smith, The GMT-SYSTEM Version 1.0 Technical Reference and Cookbook, Internal Report, Lamont-Doherty Geological Observatory, 1988.

(h) FACILITIES AND EQUIPMENT

SIO has unique facilities for performing this type of oceanographic research. Sandwell and Smith maintain a data base of all of the satellite altimeter data collected by Geos-3, Seasat, Geosat, ERS-1 and Topex/Poseidon and all data are accessible from a 2-terabyte Metrum disk/tape storage system. Smith and Sandwell also maintain all of the available digital marine geophysical data that has been collected over the past 30 years using the GMTPLUS system developed by Smith and Wessel (Hawaii) while they were graduate students at Lamont. This data base system was redesigned in 1992 to accommodate more complete records. Currently we maintain all of the data from 4 sources, NGDC-GEODAS, Lamont-"Brownbook", Lamont-"Wessel/Smith" and SIO-underway data. In addition, the SIO Geological data center maintains all of the multibeam echo-sounder data collected by R/V Washington and R/V Melville along with multibeam data from other institutions. We are preparing for the possible declassification of all of the Navy Ocean Survey Data that has been collected over the past 30 years. Instant access to these data bases using mature and tested software will facilitate the research described above. Smith is also an author of the GMT graphics system so we are able to modify and upgrade this system as necessary.