See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/224634874 Bathymetry from satellite altimetry: Present and future Conference Paper · February 2005 DOI: 10.1109/OCEANS.2005.1640160 · Source: IEEE Xplore CITATION READS 1 210 3 authors, including: Walter HF Smith David T. Sandwell National Oceanic and Atmospheric Administrat… University of California, San Diego 116 PUBLICATIONS 28,038 CITATIONS 334 PUBLICATIONS 17,258 CITATIONS SEE PROFILE SEE PROFILE Some of the authors of this publication are also working on these related projects: Marine Gravity View project GMTSAR View project All content following this page was uploaded by Walter HF Smith on 29 May 2015. The user has requested enhancement of the downloaded file. Bathymetry From Satellite Altimetry: Present And Future Walter H. F. Smith U.S. National Oceanic and Atmospheric Administration Silver Spring Maryland David T. Sandwell Scripps Institution of Oceanography La Jolla, California R. Keith Raney Johns Hopkins University Applied Physics Laboratory Laurel Maryland Abstract - Bathymetric survey lines cover the remote ocean basins about as sparsely as the Interstate Highway System covers the United States. Therefore the most complete global bathymetric models employ reconnaissance deep sea bottom topography (“bathymetry from space”) combining conventional acoustic soundings with detailed marine gravity field information derived from densely spaced satellite altimeter profiles of sea surface slope. Gravity and bathymetry may be correlated over spatial scales (half-wavelengths) of roughly 5 to 80 km. The vertical precision and horizontal resolution of derived bathymetry depends on the signal-to-noise characteristics of the satellite altimetry over the correlated band. Comparison between gravity anomalies derived from existing satellite altimeter data and gravity anomalies measured with shipboard gravimeters shows root-mean-square differences around 5 milliGals (mGal) and spectral coherency (signal exceeding noise) for half-wavelengths longer than about 13 km. Reconnaissance bathymetry estimates derived from these data have a similar scale of resolution (roughly 15 km half wavelength) and vertical errors of approximately 125 to 250 meters, depending on conditions such as regional water depth and the spectrum of the local topographic signal. The most challenging error situation is predicting the summit depth of a narrow and steep seamount rising from deep water. In the area of the USS San Francisco collision, for example, the altimetric bathymetry map shows a ridge with a local summit at 278 meters depth near the crash site, rising from a regional background depth of more than 3000 m of water. Landsat imagery near the crash site suggests that the seamount the San Francisco hit is probably shallower than 40 m at its summit. State-of-the-art shipboard gravimetry has an error level around 1 mGal, and cross-spectral comparisons of shipboard measurements of gravity and bathymetry show coherency down to half wavelengths as short as 5 km. Thus if a new mission could reduce the error in gravity maps derived from satellite altimetry by as much as a factor of five, one could expect significant improvements in the horizontal resolution and vertical error of estimated reconnaissance bathymetry. While the error propagation from gravity to estimated bathymetry depends on the local conditions as above, we can expect a five-fold reduction in the bathymetry error from a five-fold reduction in gravity error. Recent advances in altimetry – the delay-Doppler technology – make this five-fold gain readily achievable with a low-cost mission. The limiting error in altimetry of sea surface slopes is random noise in the altimeter’s range measurement, and the delay-Doppler altimeter is superior to conventional altimeters in this respect by a factor of at least two. The mission should have an orbit with a ground track pattern that does not repeat for at least 1.2 to 1.5 years or so, in order to obtain dense spatial sampling to support short-wavelength horizontal resolution. Thus a 5 to 6 year mission would yield four-fold data redundancy, reducing the error another factor of two. An additional noise reduction factor of roughly 1-1/4 or so can be gained by choosing an optimal orbital inclination. I. THE NEED FOR GLOBALLY UNIFORM BATHYMETRY Bathymetry is foundational data, providing basic infrastructure for scientific, economic, educational, managerial, and political work. Surface and submarine navigation, communications cable and pipeline route planning, resource exploration, habitat management, and territorial claims under the Law of the Sea all require reliable bathymetric maps to be available on demand. A variety of applications also require ocean-basin-wide or global-scale bathymetric information with a geographically uniform level of detail. Examples include • Understanding the geologic processes responsible for ocean floor features unexplained by simple plate tectonics, such as abyssal hills, seamounts, microplates, and propagating rifts. • Improving tsunami hazard forecast accuracy by mapping the deep ocean topography that steers tsunami wave energy. • Determining the effects of bathymetry and seafloor roughness on ocean circulation, mixing, climate, and biological communities, habitats, and mobility. II. THE ROLE OF SATELLITE ALTIMETRY Despite the fundamental nature of bathymetry, we have much better maps of Earth’s Moon, Mars, Venus and some asteroid surfaces than we have of Earth’s ocean floors. Seafloor maps are inadequate for many of the purposes above because there has been no systematic ocean mapping effort. Existing surveys cover only a small fraction of the ocean floor and in an irregular pattern. The most detailed mapping possible would employ ships or robotic vehicles equipped with acoustic swath mapping systems, but a complete survey would take hundreds of years of vessel time at a cost of billions of dollars [1]. In shallow areas the bottom may be visible to airborne or spaceborne optical or hyperspectral sensors, but these systems are useful only in water depths less than several tens of meters, at best. The only technique for globally uniform reconnaissance of deep-sea bottom topography is spaceborne radar altimetry of ocean surface height anomalies. These anomalies combine a time-invariant signal reflecting the equipotential of the Earth’s gravity field with other, mostly time-varying, signals associated with tides, currents and eddies, climatic fluctuations, and other physical oceanographic signals. The bathymetric signal is expressed in short-spatial scale (less than 80 km half-wavelength) and time-invariant sea surface slopes, and this signal is readily distinguished from other oceanographic signals such as El Niño and the dynamics of basin circulation. III. FACTORS LIMITING RESOLUTION Decades of marine geophysical research have confirmed that in the deep-sea, where sediments are thin, bottom topography is usually well-correlated with sea surface gravity anomalies over a band of horizontal wavelengths. At the largest scales, “isostatic compensation” attenuates the gravitational attraction of the seafloor topography, cancelling it by adding an opposite attraction caused by compensating masses at greater depth. This attenuation is spread over a band of wavelengths generally longer than 160 km but with details depending on the mechanical strength of the lithosphere [2, 3]. As a consequence, satellite altimeter data can most usefully be applied to mapping of smaller sized features, such as seamounts, abyssal hills, and the seafloor topography built by the seafloor spreading process [4, 5]. The shortest resolvable scale depends ultimately on the signal-to-noise ratio in the satellite altimeter data at spatial scales shorter than 80 km. The sea floor topography spectrum tends to be red, due to its quasi-fractal nature and finite variance. The gravitational attraction at the sea surface produced by the seafloor topography is further reddened by “upward continuation”, which attenuates anomalies by a factor exp[-2πd/L], where d is the mean depth between sea surface and sea floor, and L is the full-wavelength of the topographic signal. Thus the signal of interest diminishes rapidly as the horizontal scale of a feature shrinks to approach the typical depth in the deep ocean, on the order of 5 km. Inversion of gravity data to estimate bathymetry requires the inverse operation of downward continuation, producing exponential growth of amplitude with decreasing wavelength. To prevent the estimate from being swamped by noise one must limit the estimation to a band of wavelengths. The shortest scale in the band is somewhat variable, as the best possible resolution is achieved by designing the roll-off of the band-pass filter to consider the spectra of the signals to be resolved and the error processes in satellite altimeter data at the scales of interest. Reference [2] discusses the optimization of the filter roll-off and the factors limiting resolution in detail. IV. THE CURRENT SITUATION To be bathymetrically useful, an altimeter mission must collect data over a network of ground tracks sufficiently dense to resolve signals at half-wavelengths shorter than 80 km. Two such missions have been flown, the U.S. Navy’s GeoSat Geodetic Mission of 1985-86, and the European Space Agency’s ERS-1 Mission Phases E and F of 1994-95. Combining these data one obtains a sea surface gravity field which shows high correlation with shipborne gravimetry at half-wavelengths longer than 12 km. Root-mean-square differences between satellite altimeter gravity models and shipborne gravimetry suggest that the typical sea surface slope error in the models is around 3 micro-radians. These data have been combined with publicly available conventional bathymetric survey data to estimate global reconnaissance bathymetry [4]. The results show good correlation with truth data at half-wavelengths longer than 13 km. Differences between estimated and true point values are less than 100 m about 50% of the time, but errors can exceed 250 m in areas of very rugged topography and/or very sparse control sounding data. An example of a rugged area with poor control is the Caroline Islands region of Micronesia in the Western Pacific, where the USS San Francisco nuclear submarine ran into a seamount in January of 2005. The altimetry-based estimate ([4], version 8.2, November, 2000) shows a ridge with summits shallower than 300 m. Later inspection of a Landsat7 image in the area suggests that depth may be less than 50 m near the collision site. It seems then that altimetrically-estimated bathymetry performs worst where it is needed most: in areas of rugged topography with poor ground-truth sounding control. However, this is a consequence of the noise level in the currently available data, which necessitates filtering the estimated depths to wavelengths longer than about 25 km. In rugged areas there is significant topographic variation at shorter scales, and so the filtered estimate under-predicts seamount summits. A new mission with a lower noise level would need less filtering, resulting in a product much more faithful to the actual topography. V. A FUTURE MISSION? We have been researching for some years the possibility of a future mission. At present it seems that the delay-Doppler altimeter [6] offers the best way forward. This device employs signal-processing innovations from synthetic aperture radar in a nadir-looking ocean height instrument, achieving several advantages over the conventional instruments used to date. First, the instrument has lower random noise level, by a factor of 2 in calm seas and more than 2 in rough seas. Operating this instrument for 5 to 6 years, as opposed to the 11 months of the ERS-1 or 18 months of the Geosat mapping campaigns, would cut the noise level in the resolved gravity field by a factor of at least 4 (16 in variance); optimizing the orbital inclination would achieve further gains. This reduced noise level would immediately improve the resolution and reduce the error in estimated bathymetry by comparable amounts. Second, the delay-Doppler instrument yields finer spatial sampling in the direction of flight. This can support finer spatial resolution of sea surface slope for gravity and bathymetry. Importantly, it can also support smarter surface tracking, so that the instrument can acquire data much closer to shore than a conventional instrument. Third, the delay-Doppler instrument achieves these performance gains by making smarter use of the returned radar energy, and this also results in much greater efficiency. These gains permit instrument power levels to be reduced, leading to small and lightweight electronics with very long expected life at modest cost. The website [7] describes a mission scenario with links to some additional information, including a mission design study [8] and some animations demonstrating the recovery of mesoscale oceanography from a geodetic orbit and the sensitivity of tsunami propagation to deep-water bathymetry. A mission to achieve these goals could be realized now at a cost under $100M. Because ocean bathymetry is a fundamental measurement of our planet, there is a broad spectrum of interest from government, the research community, industry, and the general public. It is important to stress here that a low-cost and focused mission design with a simple instrument package would be multi-purpose, and simultaneously so. The bathymetric measurement goal is essentially a gravity measurement goal, and so the mission will furnish knowledge of the marine gravity field. This knowledge is directly useful in geophysical studies, in resource exploration, and in inertial navigation. The gravity and bathymetry application requires only a low-noise measurement on a densely spaced network of ground tracks, such as from an orbit that does not repeat for at least a year. The temporal sequence with which those ground tracks are overflown is irrelevant to the bathymetry mission, and so the orbit can be designed for any desired sampling of tides or mesoscale ocean signals. In particular, though “nonrepeat” it can be rich in “near repeats” and thereby furnish observations of the mesoscale ocean eddies. Because the necessary measurement is band-limited, absolute accuracy of the total sea-surface height, or long-term stability in that absolute accuracy, are not required. This makes the instrument package much cheaper and simpler than those of missions like TOPEX or Jason, which are designed to monitor climate variability signals such as El Niño and global sea level rise. It should be noted that radar altimetry of the ocean surface, by measuring the sea surface shape anomalies caused by gravity anomalies, measures gravity at sea level. Gravity at sea level suffers only about 4 km of upward continuation from the sea floor to the sea surface, and so resolves bathymetric signals. Space gravity missions such as the Gravity Recovery And Climate Experiment (GRACE) measure gravity at orbital altitude, 400 km above the sea floor, where upward continuation wipes out the bathymetric signal. Thus advances in global ocean mapping will require a new altimeter mission; they will not come from missions such as GRACE. ACKNOWLEDGMENTS This mission concept has matured through its development as a NASA ESSP-3 proposal in 2001 (“ABYSS: Altimetric Bathymetry from Surface Slopes”), in a NASA/NOAA/NSF funded workshop in 2002 (http://www.igpp.ucsd.edu/bathymetry_workshop), Navy workshops in 2003 and 2005, and articles in the 2004 “Bathymetry from Space” issue of Oceanography (http://www.tos.org/oceanography/issues/issue_archive/17_1.h tml). Further discussion of the mission concept, resolving power, and applications to tsunami and seismic hazard, ocean circulation, ocean mixing, climate modeling, and the UN Law of the Sea can be found in peer-reviewed articles in Oceanography volume 17, number 1. A website (http://topex.ucsd.edu/concept) holds additional supporting materials, including animated demonstrations of the effect of bathymetry in tsunami propagation and of the recovery of mesoscale eddies from a non-repeat orbit (http://www.tos.org/oceanography/issues/issue_archive/17_1.h tml). The views, opinions and findings contained in this document are those of the authors and should not be construed as an official U.S. Government position, policy or decision. REFERE NCES [1] M. J. Carron, P. R. Vogt, and W.-Y. Jung, “A proposed international long-term project to systematically map the world’s ocean floors from beach to trench: GOMaP (Global Ocean Mapping Program)”, Intl. Hydr. Rev. 2(3), 49-55, 2001. [2] W. H. F. Smith and D. T. Sandwell, “Bathymetric prediction from dense satellite altimetry and sparse shipboard bathymetry”, J. Geophys. Res., vol. 99 (B11), pp. 21803-21824, November, 1994. [3] W. H. F. Smith, “Seafloor tectonic fabric from satellite altimetry”, Annu. Rev. Earth Planet. Sci., vol. 26, pp. 697738, 1998. [4] W. H. F. Smith and D. T. Sandwell, “Global seafloor topography from satellite altimetry and ship depth View publication stats [5] [6] [7] [8] soundings”, Science, vol. 277, pp. 1956-1962, 26 September 1997. J. A. Goff, W. H. F. Smith, and K. M. Marks, “The contributions of abyssal hill morphology and noise to altimetric gravity fabric”, Oceanography, vol. 17 (1), pp. 24-37, March, 2004. R. K. Raney, “The delay Doppler radar altimeter”, IEEE Transactions on Geoscience and Remote Sensing 36 (5), 1578-1588, 1998. http://topex.ucsd.edu/concept R. K. Raney, W. H. F. Smith, and D. T. Sandwell, “Abyss-Lite: A high-resolution gravimetric and bathymetric mission (AIAA-2004-6006)”, in Proceedings, Space-2004, AIAA, San Diego, CA, 2004.
