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The Contributions of Abyssal Hill Morphology and Noise to Altimetric Gravity Fabric

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The Contributions of Abyssal Hill Morphology and Noise to Altimetric Gravity Fabric Special Issue—Bathymetry from Space The Contributions of Abyssal Hill Morphology and Noise to Altimetric Gravity Fabric John A. Goff This article has been published in Oceanography, Volume 1 Volume This article in Oceanography, has been published University of Texas Institute for Geophysics • Austin, Texas USA Rockville, MD 20851-2326, USA. Road, 5912 LeMay The Oceanography machine, reposting, or other means without prior authorization of portion photocopy of this articleof any by Walter H.F. Smith and Karen M. Marks NOAA Laboratory for Satellite Altimetry • Silver Spring, Maryland USA Introduction Earth’s deep seafloor is, for the most part, a vast, (e.g., Smith, 1998; Goff and Smith, 2003) and will be unexplored terrain. Only a miniscule fraction has ever shown here, geological information is contained in the been observed directly, using deep submersibles and small-scale fabric of altimetric gravity data. This fabric, remotely operated vehicles. Most of our understanding which has the appearance of the skin of an orange peel, of seafloor physiography is instead derived from or the surface of a football, exists everywhere in the remotely sensed data, such as sonar or satellite altime- altimetry data, filling the otherwise uninteresting 7 try. Sonar data must be collected by ships or underwa- reaches between larger-scaled, recognizable features , Number ter vehicles, which is an expensive and time-consum- such as mid-ocean ridges, fracture zones and ing process. Technological advances over the past two seamounts. Although noise is an important factor, 1 , a quarterly journal of The Oceanography Society. Copyright 2003 by The Oceanography Society. All rights reserved.Reproduction Society. The Oceanography 2003 by Copyright Society. The Oceanography journal, a quarterly of decades have enabled sonar devices to collect detailed small-scale gravity fabric, and rms roughness in partic- “swath” coverage (Figures 1b and 2b), but only a few ular, varies regionally in ways that are suggestive of a percent of the seafloor has been mapped in this fash- tectonic origin; that is, related to mid-ocean ridge ion, typically in areas of prominent seafloor structures spreading rates or ridge-axis morphology at the time of such as mid ocean ridges, fracture zones and trenches. seafloor creation. Such variations must, in some way, be More generally, sonar coverage of the ocean floor is related to the small-scale (i.e., < ~20 km full-wave- limited to profile coverage along disparate ship tracks, length) morphology of the seafloor. well concentrated in shipping lanes and very sparse In this study we examine the gravity roughness elsewhere. characteristics in two regions, the South Atlantic Ocean Over the past decade or so, satellite altimetry data (Figure 1) and southeast Pacific Ocean (Figure 2), where have been employed to fill in the gaps left by incom- changes in spreading rate over time have been observed plete sonar coverage. The marine geoid and the gravi- in the magnetic anomaly record. We employ a statistical ty field derived from it are dominated by the seafloor characterization method developed by Goff and Smith topography signal at scales less than a few hundreds of (2003) that provides estimates of root-means-square prohibited. Send all correspondence or Society is strictly to: [email protected], kilometers (Smith and Sandwell, 1997; Smith, 1998). (rms) roughness, characteristic length and width, and Major seafloor features are easily detected in the alti- strike azimuth to identify fabric. Our samples avoid metric gravity data (Figures 1a and 2a), and their major physiographic features, such as fracture zones, so bathymetry can be predicted based on geophysical for- that our expectation is that the measured gravity fabric mulations constrained by existing bathymetric cover- is related to the primary fabric of the seafloor: abyssal age. However, this predictive ability is severely limited hills, which constitute the most widespread geomor- at smaller scales. The altimetric gravity field is, in phology on Earth. Abyssal hills are related to the nature essence, a filtered and somewhat noisy expression of of the mid-ocean ridges at which they form (Goff, 1991; bathymetry (Smith, 1998). Depending on water depth, Goff, 1992; Goff et al., 1995; Goff et al., 1997). The that filter is ~18-20 km wide (full-wavelength); we can- seafloor roughness represented by abyssal hills is also of not expect a one-to-one correspondence of gravity and interest to the physical oceanographic community seafloor features below this scale. Furthermore, noise because of the critical role it plays in ocean circulation would be translated in this predictive scheme into and mixing (e.g., Mauritzen et al., 2002). large bathymetric artifacts. Hence, little effort has gone Through synthetic experiments we demonstrate into trying to interpret small-scale features in the grav- theoretically the relationship between gravity fabric ity field. and abyssal hills at a variety of sizes and shapes. However, as has been demonstrated previously We explore as well the effect of noise on our ability Oceanography • Vol. 17 • No. 1/2004 24 Figure 1. Satellite altimeters can map marine gravity anomalies associated with seafloor structure. Altimeter data collected to date (top image, a) display the main features of classical plate tectonics (mid-ocean ridges, “R,” and fracture zones, “FZ"), seamounts (“SM”), and some finer-scale features associated with transient or unstable aspects of the seafloor spreading process (propagator pseudofaults, “PPF,” and axial segment boundaries, “SB”). At still smaller scales, the altimetric gravity image displays a bumpy texture like the surface of an orange peel. Though this texture seems random, it does change from place to place, often along sharp boundaries (heavy dashed line). In this paper we show that this bumpy texture can in fact be correlated with very small-scale seafloor topographic features known as abyssal hills, which are traditionally visible only in multibeam acoustic swath bathymetry (bottom image, b; hills at “AH”). The area shown here is in the South Atlantic Ocean; the bathymetry image at bottom zooms in on the boxed region shown in the top image. Note the gaps in bathymetric cover- age; this area is extremely well covered by global bathymetric standards. A profile along the heavy yellow line is shown in Figure 4. (Altimetric gravity from Sandwell and Smith, 1997. Bathymetry from the Ridge Multi-Beam Synthesis Project http://ocean-ridge.ldeo.columbia.edu). Oceanography • Vol. 17 • No. 1/2004 25 to resolve abyssal hill-related gravity fabric, and hills, are best quantified statistically; here, the individ- investigate the possible improvements that may be ual feature contains insufficient information, because achieved by a new altimetry mission that significantly the same mid-ocean ridge process can produce a range reduces the level of noise in the data set. of sizes and shapes. Goff and Jordan (1988) developed a method for quantitative characterization of abyssal hill morpholo- Seafloor Morphology and Fabric Estimation gy based on swath bathymetric coverage. The method The morphology of the deep seafloor is composed employs a least-squares inversion of the sample two- of a variety of features that exist at a range of scales dimensional covariance function to estimate parame- (Figures 1 and 2). Apart from seamounts, which are ters of a functional form: the “von Kármán” model typically isolated structures formed off axis, most of (von Kármán, 1948), which is a band-limited fractal. these features are derived from the mid-ocean ridges, The following statistical parameters are derived from where new oceanic crust is formed as tectonic plates this model: spread apart. Fracture zones are the largest and most The rms height, Hb, measures the overall bathy- prominent of such structures. Created by the transform metric roughness. ␭ faults that link ridge offsets, fracture The characteristic scale, b, meas- zones form ribbon-like trench and ridge ures the horizontal scale of the most structures oriented orthogonal to the Very little of the seafloor prominent feature size, and is defined ridge axis at the time of crustal accre- has been mapped as twice the second moment of the tion. Fracture zones are commonly covariance function (Goff and Jordan, with swath sonar methods, ␭ observed in satellite altimetry data 1988); bn, the characteristic width, is (Figures 1a and 2a). Akin to fracture and only a small fraction measured normal to the strike direc- zones are segment boundaries, small ␭ will be mapped tion, and bs, the characteristic length, non-transform offsets in the ridge axis is measured along the strike direction. that produce disturbances in the crust in such detail for the The ratio between characteristic length ␭ ␭ and, if large enough, evidence in the foreseeable future. and width, ab = bs/ bn, provides a altimetry data (Figure 1a). Segment measure of fabric. ␪ boundaries that are not stable can The strike azimuth, b, measures migrate up and down the ridge axis, creating diagonal the orientation of abyssal hill lineaments. lineaments called “propagator psuedofaults” (Figures The fractal dimension, D, measures the level of 1a and 2a). fine-scale roughness compared to large-scale rough- Between the fracture zones and segment bound- ness. In practice, this parameter is difficult to estimate aries, the seafloor is composed primarily of abyssal hill because swath bathymetry does not provide a large- fabric: ridge-parallel lineaments
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