Seafloor Height Research Articles

Calibrated Absolute Seafloor Pressure Measurements for Geodesy in Cascadia

AbstractThe boundary between the overriding and subducting plates is locked along some portions of the Cascadia subduction zone. The extent and location of locking affects the potential size and frequency of great earthquakes in the region. Because much of the boundary is offshore, measurements on land are incapable of completely defining a locked zone in the up‐dip region. Deformation models indicate that a record of seafloor height changes on the accretionary prism can reveal the extent of locking. To detect such changes, we have initiated a series of calibrated pressure measurements using an absolute self‐calibrating pressure recorder. A piston‐gauge calibrator under careful metrological considerations produces an absolutely known reference pressure to correct seafloor pressure observations to an absolute value. We report an accuracy of about 25 ppm of the water depth, or 0.02 kPa (0.2 cm equivalent) at 100 m to 0.8 kPa (8 cm equivalent) at 3,000 m. These campaign survey‐style absolute pressure measurements on seven offshore benchmarks in a line extending 100 km westward from Newport, Oregon from 2014 to 2017 establish a long‐term, sensor‐independent time series that can, over decades, reveal the extent of vertical deformation and thus the extent of plate locking and place initial limits on rates of subsidence or uplift. Continued surveys spanning years could serve as calibration values for co‐located or nearby continuous pressure records and provide useful information on possible crustal deformation rates, while epoch measurements spanning decades would provide further limits and additional insights on deformation.

A Self-Calibrating Pressure Recorder for Detecting Seafloor Height Change

One method to detect vertical crustal deformation of the seafloor, where Global Positioning System (GPS) surveys are not possible, is to monitor changes in the ambient seawater pressure, whose value is governed primarily by depth. Modern pressure sensors based on quartz strain gauge technology can detect the pressure shift associated with subsidence or uplift of the seafloor by as little as 1 cm. Such signals can be caused by tectonic or volcanic activity, or by hydrocarbon production from an offshore reservoir. However, most gauges undergo a slow drift having unpredictable sign and magnitude, which can be misinterpreted as real seafloor height change. To circumvent this problem, we have developed an instrument that calibrates the pressure gauges in place on the seafloor. In this autonomous system, a pair of quartz pressure gauges recording ambient seawater pressure are periodically connected to a piston gauge calibrator. In a 104 day test off the California coast at 664-m depth, the contribution to the uncertainty in depth variation from gauge drift was 1.3 cm based on calibrations occurring for 20 min every ten days.

Relative Height Estimation by Cross-Correlating Ground-Range Synthetic Aperture Sonar Images

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> The relative height of the seafloor can be estimated by using two vertically displaced receivers. In this paper, we propose techniques to improve the accuracy of the estimated height. Our results are based on the use of synthetic aperture sonar (SAS) imaging, which implies coherent addition of complex images acquired from a moving platform. The SAS processing improves the along-track (or azimuth) resolution, as well as the signal-to-noise ratio (SNR), which in turn improves the estimated height accuracy. We show that the shift of the <emphasis emphasistype="boldital">effective</emphasis> center frequency induced by coherent, frequency-dependent scattering affect the time-delay estimates from complex cross correlations, and we propose a correction technique for broadband signals with uneven magnitude spectra. To reduce the effect of geometrical decorrelation and increase the coherence between the images, we beamform the sonar images onto an <emphasis emphasistype="bold"><emphasis>a priori</emphasis></emphasis> estimate of the seafloor height before correlating. We develop a mathematical model for the imaging geometry. Finally, we demonstrate our proposed estimators by providing relative seafloor height estimates from real aperture and SAS images, obtained during the InSAS-2000 experiment at Elba Island in Italy. In particular, we demonstrate that the SAS image quality is significantly improved by inclusion of the height estimates as <emphasis emphasistype="bold"><emphasis>a priori</emphasis></emphasis> information. </para>

Spatial and temporal variation of large- and small-scale seafloor roughness

Seafloor microtopography, measured at subcentimeter resolution over scales up to a meter or more by underwater stereo photogrammetry, was periodically characterized during the SAX99 high-frequency acoustic backscattering experiment. Analog 35-mm stereo photographs of seafloor roughness were digitized at 2- to 5-mm intervals from film transparencies with a stereo comparator capable of 1-mm-scale lateral precision and submillimeter-scale vertical accuracy. Relative seafloor height profiles were used to estimate roughness power spectra, which are parametrized as slopes and intercepts of regressions fit to the roughness power spectra. The composite roughness model developed by Applied Physics Laboratory–University of Washington requires these two spectral parameters to determine the contribution of the seafloor roughness to the overall backscattering strength. Data from two locations over 31 days in the experimental area off Ft. Walton Beach, FL in 19-m water depth are presented in order to demonstrate the relationship between seafloor roughness morphology at subcentimeter to meter scales and model parameters predicting high-frequency acoustic scattering. Model predictions are generated from the measured geoacoustic properties of the rippled, medium sand seafloor and the range of roughness parameters that varied temporally and spatially during the experiment. [Work supported by ONR.]

The influence of millimeter-scale seafloor roughness on measured and predicted high-frequency acoustic backscattering strength

Seafloor microtopography, as measured by underwater stereo photogrammetry, is systematically characterized concurrently with other sediment properties as model parameters to be used for predicting high-frequency acoustic backscattering from the seafloor as part of field experiments. Typically, seafloor roughness is digitized at 2–5 mm intervals from 35- or 70-mm film transparencies with a stereocomparator capable of 1-mm-scale lateral precision and sub-millimeter-scale vertical accuracy. Relative seafloor height profiles are used to estimate roughness power spectra, which are parameterized as slopes and intercepts of the line fit to the roughness variance regressed on the spatial frequency. According to the composite roughness model developed by APL-UW, the decay of the roughness spectra with increasing spatial frequency (slope) and the spectral strength (intercept) can be used to determine the contribution of the seafloor roughness to the overall backscattering strength. Data from field experiments are presented in order to demonstrate the relationship between seafloor roughness at sub-centimeter scales and high-frequency acoustic scattering. Experimental acoustic data and model predictions are presented from a coarse, shelly sea floor off Jacksonville, Florida, a rippled fine sand in the Quinault Range off Washington, and a very fine sand with subtle seafloor roughness off Tirrenia, Italy. [Work supported by ONR.]

Dispersive effects of zonally varying topography on quasigeostrophic Rossby waves

Abstract Dispersion of linear quasigeostrophic plane waves in a stratified ocean with bottom topography is discussed. Particular emphasis is given to cases for which zonal gradients in the sea floor height are important. As such, the relative importance of the topographic and planetary β-effects is strongly dependent on wave vector orientation. The magnitude of the topographic slopes considered is chosen such that these two effects (topographic and planetary β) are of comparable importance. In the interest of simplicity, stratification is taken to be independent of depth. The eigenvalue problem which must be solved to find the free modes of oscillation has already been treated in the literature (in fact, Charney and Flierl (1981) have treated the effects of a more realistic stratification). The aim of this note is to more fully expose, primarily by example, several dispersive properties of these free wave modes which have been largely overlooked.

Temporal variation in bed configuration and one-dimensional bottom roughness at the mid-shelf STRESS site

Time-lapse stereophotographs were taken over a 90-day period from mid-November 1990 to late-February 1991 at a 90-m silt-bottom site on the central California shelf as part of the STRESS (Sediment Transport Events on Shelves and Slopes) project. Five distinct bed configurations were observed, in order of decreasing abundance, these are: (1) bioturbated bed; (2) smoothed bed; (3) current-rippled bed; (4) scour-pitted bed; and (5) wave-rippled bed. Concurrent measurements of the flow field implicate along-shelf currents, rather than waves, as the primary agent forming the physical bed configurations. The presence of a wave-induced cross-shelf gradient in near-bottom suspended sediment during storm events and the redistribution of this sediment by upwelling or downwelling currents is postulated to control the appearance of depositional current-ripples (northwest poleward flow, downwelling) and erosional scour-pits (southeast equatorward flow, upwelling). All physical bed forms are destroyed by bioturbation processes in periods of hours to days. Analytical photogrammetric techniques were used to extract high-resolution sea floor height data from the stereophotographs. Results indicate maximal relief over a 0.25-m 2 area at this site never exceeded 5 cm. Root-mean-square (rms) height varied by a factor of 3 (3.2–9.2 mm) and is a weak function of bed configuration. Current ripples have the largest rms-height, smoothed and scour-pitted beds the smallest. Rms-heights of bioturbated beds are variable and appear to depend on the previously produced physical bed configuration. Changes in rms-height can be abrupt with factor of 2 changes observed over a 12-h period. Horizontal descriptors of roughness such as peak spacing or peak width cannot separate bed configurations. Results from surface slope distributions are broadly coherent with the rms-height data, in that surfaces with large rms-heights have broad slope distributions and vice versa. Slope distribution data also indicate that all bed configurations except the current-rippled bed are isotropic. These preliminary data suggest that time series information is needed to adequately resolve both the micro-scale roughness of the sea floor on continental shelves and the presence of short lived, but potentially flow-diagnostic bed configurations.

Scattering from a stochastically modeled seafloor approximated by a collection of discrete point scatterers

The ability to simulate a continuous, three-dimensional seafloor with a discrete point distribution could potentially allow the modeling of the scattered field generated by complex, realistic seafloor topography. In order to evaluate this approach, a one-dimensional seafloor profile was modeled as a stationary, zero-mean, Gaussian process. It was generated using the spectral method whereby the equation of seafloor height is derived as a function of the second-order statistics and the correlation length, under the assumption of Gaussian spectral roughness. In addition, this simulated seafloor was designed to be locally flat so that the Kirchhoff approximation can be safely invoked. A discrete version of this profile was simulated with point scatterers, using the simulation program REVGEN/SST. Low-frequency (150-Hz) synthetic reverberation was created by insonifying the collection of scatterers. Measurements were made in the near-specular direction and care was taken to avoid shadowing effects. The mean pressure of the scattered field was used to obtain an estimate of the mean-squared (m.s.) roughness of the seafloor according to a well-known result of Kirchhoff theory. The success of the simulation is measured in terms of the proximity of this estimate to the true m.s. roughness, which is known a priori. [Work supported by ONR.]

Stochastic modeling of seafloor morphology; Resolution of topographic parameters by sea beam data

The authors explore the resolving power of an inversion algorithm which estimates five parameters of the seafloor covariance function from a single swath of multibeam echosounding data. The resolving power is evaluated as a function of the swath length, the orientation of ship track with respect to topographic grain, and the response width of the sounding system. The analysis is conducted by inverting sets of synthetic data with known statistics. The mean and standard deviation of the inverted parameters can be directly compared with the input parameters and the standard errors output from the inversion. Experiments show that resolution of the covariance parameters is strongly dependent on the number of characteristic lengths which are sampled. Root-mean-square seafloor height can be estimated to within approximately 15%, and anisotropic orientation to within approximately 5% (for a strong lineation), using track lengths as short as three characteristic lengths.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>