An official website of the United States government
Abstract Due to the blockage of seawater, seafloor displacement cannot be directly measured by space geodesy. The combination of Global Navigation Satellite Systems‐acoustic ranging (GNSS‐A) has been used to overcome the electromagnetic barrier, so that a GNSS‐determined sea surface vessel's coordinates can be transformed to seafloor benchmarks in a global reference frame. Due to the high cost and science priorities, previous GNSS‐A studies mainly targeted relatively deep water and a minimum of three transponders were used to form an array, equivalent to a precision geodetic station. With recent developments in unmanned autonomous surface vessels, low cost GNSS‐A surveys are poised to become practical. Here we demonstrate that with a carefully designed surveying trajectory, Wave Glider‐based GNSS‐A surveying of a single transponder in shallow water can provide centimeter‐level accuracy on horizontal seafloor positioning, even if the sound speed model deviates from the actual value by a few meters per second. Results from a nine‐month experiment conducted at ∼54 m water depth show that the repeatability of the seafloor horizontal positioning is better than 2 cm. When conditions allow, the acoustic observations should be collected symmetrically about the transponder and data redundancies are recommended to reduce the error associated with time‐dependent variations in sound speed.
more »
« less
Normal mode dispersion and time warping in the coastal ocean
Simple, analytically solvable models of normal mode propagation in the coastal ocean are developed and applied to study the effect of the seafloor bathymetry on modal travel times. Within the adiabatic approximation, horizontal inhomogeneity of the waveguide is found to change the modal dispersion curves in a way that helps separation of the modal components of the acoustic field using the time-warping transform. It is shown that moderate seafloor slopes can lead to surprisingly large errors in retrieved geoacoustic parameters and cause a positive bias in bottom sound speed estimates if horizontal refraction is ignored.
more »
« less
- Award ID(s):
- 1657430
- PAR ID:
- 10593693
- Publisher / Repository:
- Acoustical Society of America (ASA)
- Date Published:
- Journal Name:
- The Journal of the Acoustical Society of America
- Volume:
- 146
- Issue:
- 3
- ISSN:
- 0001-4966
- Format(s):
- Medium: X Size: p. EL205-EL211
- Size(s):
- p. EL205-EL211
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract Modal analysis of freestanding rock formations is crucial for evaluating their vibrational response to external stimuli, aiding accurate assessment of associated geohazards. Whereas conventional seismometers can be used to measure the translational components of normal modes, recent advances in rotational seismometer technology now allow direct measurement of the rotational components. We deployed a portable, three-component rotational seismometer for a short-duration experiment on a 36 m high sandstone tower located near Moab, Utah, in addition to conducting modal analysis using conventional seismic data and numerical modeling. Spectral analysis of rotation rate data resolved the first three natural frequencies of the tower (2.1, 3.1, and 5.9 Hz), and polarization analysis revealed the orientations of the rotation axes. Modal rotations were the strongest for the first two eigenmodes, which are mutually perpendicular, full-height bending modes with horizontal axes of rotation. The third mode is torsional with rotation about a subvertical axis. Measured natural frequencies and the orientations of displacements and rotation axes match our numerical models closely for these first three modes. In situ measurements of modal rotations are valuable at remote field sites with limited access, and contribute to an improved understanding of modal deformation, material properties, and landform response to vibration stimuli.more » « less
-
Tsunami wave observations far from the coast remain challenging due tothe logistics and cost of deploying and operating offshoreinstrumentation on a long-term basis with sufficient spatial coverageand density. Distributed Acoustic Sensing (DAS) on submarine fiber opticcables now enables real-time seafloor strain observations over distancesexceeding 100 km at a relatively low cost. Here, we evaluate thepotential contribution of DAS to tsunami warning by assessingtheoretically the sensitivity required by a DAS instrument to recordtsunami waves. Our analysis includes signals due to two effects induced by thehydrostatic pressure perturbations arising from tsunami waves: thePoisson’s effect of the submarine cable and the compliance effect of theseafloor. It also includes the effect of seafloor shear stresses andtemperature transients induced by the horizontal fluid flow associatedwith tsunami waves. The analysis is supported by fully coupled 3-Dphysics-based simulations of earthquake rupture, seismo-acoustic wavesand tsunami wave propagation. The strains from seismo-acoustic waves andstatic deformation near the earthquake source are orders of magnitudelarger than the tsunami strain signal. We illustrate a data processingprocedure to discern the tsunami signal. With enhanced low-frequencysensitivity on DAS interrogators (strain sensitivity ≈2×10 at mHz frequencies), we find that, on seafloorcables located above or near the earthquake source area, tsunamis areexpected to be observable with a sufficient signal-to-noise ratio withina few minutes of the earthquake onset. These encouraging results pavethe way towards faster tsunami warning enabled by seafloor DAS.more » « less
-
null (Ed.)Abstract The erroneous flipping of polarity in seismic records of ocean-bottom seismometers (OBSs) could go unnoticed and undiagnosed because it is coupled with unknown horizontal orientation of OBS instruments on the seafloor. In this study, we present detailed approaches to first identify potential errors in the flipping polarity of individual OBS instruments, and then determine the correct orientation of OBS instruments on the seafloor. We first conduct a series of tests by artificially flipping the polarity of seismic records of the Global Seismographic Network stations to determine the effects on orientation estimates, utilizing polarization characteristics of teleseismic P and Rayleigh waves, respectively. The tests demonstrate that erroneous polarity reversal in seismic recording could cause false estimates and reverse radial (R) and tangential (T) components. We determine the sensor orientations through comparing the observed waveforms to the synthetic waveforms, which could solve the ambiguity of R and T directions caused by potential erroneous polarity reversal of OBS data. We then apply the approaches to an OBS data set collected in the southern Mariana subduction zone to obtain the correct orientation for 9 out of 12 OBS instruments.more » « less
-
Abstract Surface and upper-ocean measurements of mesoscale eddies have revealed the central role they play in ocean transport, but their interior and deep ocean characteristics remain undersampled and underexplored. In this study, mooring arrays, sampling with high vertical resolution, and a high-resolution global atmosphere–ocean coupled simulation are used to characterize full-depth mesoscale eddy vertical structure. The vertical structure of eddy kinetic energy, e.g., partitioning of barotropic to baroclinic eddy kinetic energy or vertical modal structure, is shown to depend partly on bathymetric slope and roughness. This influence is contextualized alongside additional factors, such as latitude and vertical density stratification, to present a global landscape of vertical structure. The results generally reveal eddy vertical structure to decay with increasing depth, consistent with theoretical expectations relating to the roles of surface-intensified stratification and buoyancy anomalies. However, at high latitudes and where the seafloor is markedly flat and smooth (approximately 20% of the ocean’s area), mesoscale eddy vertical structures are significantly more barotropic by an approximate factor of 2–5. From a climate modeling perspective, these results can inform the construction, implementation, and improvement of energetic parameterizations that account for the underrepresentation of mesoscale eddies and their effects. They also offer expectation as to a landscape of eddy vertical structure to be used in inferring vertical structure from surface measurements. Significance StatementThis work addresses the question of how do ocean seafloor features (bathymetry) affect the vertical structure of ocean currents and eddies? Seafloor features modify eddies in complex ways not often accounted for in global ocean simulations. We analyze high-resolution velocity observations, find diverse structures at four mooring sites, and consider how sloping and rough bathymetry change distributions of eddy kinetic energy throughout the water column. Comparison to theory and model output reveals a relationship between vertical structure and bathymetry. These results show that vertical structures vary significantly with bathymetry, density stratification, and latitude and contribute to model development efforts to reproduce the effects of eddy turbulence without explicit representation. These results also enhance interpretations of more numerous surface observations.more » « less
