Effect Of Sea Surface Roughness Research Articles

Assessment of offshore wind power including effects of sea surface roughness using observed wind statistics

The paper addresses how the offshore wind power can be assessed based on observed wind statistics at an offshore location. Results are exemplified for a Pierson-Moskowitz wave amplitude spectrum together with mean wind speed statistics from 11 sites in the North Sea and the North Atlantic. The ambient turbulence level, for example relevant for design of the wind turbine blades, is also estimated by adopting a wind gust spectrum. The sea surface roughness is specified using a formula given in terms of significant wave height and spectral wave steepness. The present method should be useful for making initial assessments of available wind power and ambient turbulence level at offshore sites based on wind statistics.

How rough sea affects marine seismic data and deghosting procedures

ABSTRACTMost seismic processing algorithms generally consider the sea surface as a flat reflector. However, acquisition of marine seismic data often takes place in weather conditions where this approximation is inaccurate. The distortion in the seismic wavelet introduced by the rough sea may influence (for example) deghosting results, as deghosting operators are typically recursive and sensitive to the changes in the seismic signal. In this paper, we study the effect of sea surface roughness on conventional (5–160 Hz) and ultra‐high‐resolution (200–3500 Hz) single‐component towed‐streamer data. To this end, we numerically simulate reflections from a rough sea surface using the Kirchhoff approximation. Our modelling demonstrates that for conventional seismic frequency band sea roughness can distort results of standard one‐dimensional and two‐dimensional deterministic deghosting. To mitigate this effect, we introduce regularisation and optimisation based on the minimum‐energy criterion and show that this improves the processing output significantly. Analysis of ultra‐high‐resolution field data in conjunction with modelling shows that even relatively calm sea state (i.e., 15 cm wave height) introduces significant changes in the seismic signal for ultra‐high‐frequency band. These changes in amplitude and arrival time may degrade the results of deghosting. Using the field dataset, we show how the minimum‐energy optimisation of deghosting parameters improves the processing result.

Effects of sea surface roughness on the mid-frequency acoustic pulse decay in shallow water

Recent and ongoing efforts to characterize sea bed parameters from measured acoustic pulse decay have neglected the effects of sea surface roughness. In this paper, these effects are investigated using a rough surface version of RAMPE, RAMSURF, and random rough surface realizations, calculated from a 2D JONSWAP sea surface spectrum with directional spreading. Azimuthal dependence is investigated for sandy bottoms and found that the rate of pulse decay increases when the surface wave fronts are perpendicular to the path of acoustic propagation and higher significant wave height results in higher decay rates. Additionally, the effects from sea surface roughness are found to vary with different waveguide parameters including but not limited to sound speed profile, water depth, and seabed properties. Of particular interest are the combined effects of sea bed properties and rough sea surfaces. It is shown that when clay like sediments are present, higher-order modes are strongly attenuated and effects due to interaction with the rough sea surface are less pronounced. Finally, possible influences of sea-state and 3D out-of-plane propagation effects on the seabed characterization efforts will be discussed. [Work supported by ONR.]

The Effects of Surface Gravity Waves on High-Frequency Acoustic Propagation in Shallow Water

A realistic 2-D time-evolving ocean surface model has been combined with an existing acoustic ray-based model to simulate the effects of sea surface roughness on acoustic wave propagation in coastal regions. Rough sea surface realizations are generated and used as sea surface boundaries in the acoustic model. An approach to achieve high resolution and accurate results while maintaining computational efficiency of a ray-based model is applied. The results are then compared against a unique set of experimental data collected in 15-m water depth in Delaware Bay. These data include simultaneous environment and acoustic propagation (1-18 kHz) measurements. Modeled arrival time-angle fluctuations compare well with data and suggest that there are physical processes which need to be included to improve the model, such as bubbles and turbulence as well as 3-D scattering effects.

Robust of Doppler Centroid for Mapping Sea Surface Current by Using Radar Satellite Data

Problem statement: Sea surface current retrieving from Synthetic Apert ure Radar (SAR) is required standard methods due to the complexity of sea surface ocean imaging in SAR data. In this context, various analytical models have been develo ped which describe overall effects of sea surface roughness on the Doppler signal mechanisms. Nevertheless, such models are limited in the complexity of the sea surface current estimation that can be u sed. In fact, the resolution of the sea surface Dop pler velocity in azimuth direction is typically coarser as compared to the normalized radar cross section image. Approach: This study introduced a new method to retrieve sea surface current from RADARSAT-1 SAR Standard beam mode (S2) data. The method was based on the utilization of the Wavelength Diversity Ambiguity Resolving (WDAR) and Multi Look beat Frequency (MLBF) algorithms to remove Doppler centroid (f DC ) ambiguity. Results: The result showed that the proposed methods are able to correct Doppler centroid (f DC ) ambiguity and produced fine spatial sea surface current variations in S2 mode data. The current vel ocities were ranged between 0.18 and 0.78 m sec -1 with standard error of 0.11 m sec -1 . Conclusion: In conclusion, RADARSAT-1 SAR standard beam mode (S2) data can be utilized to retrieve real tim e sea surface current. Both WDAR and MLBF algorithms are able to provide accurately informati on on Doppler Centroid (fDC ) in which accurately real time sea surface current can be retrieved from SAR data.

The effect of sea-surface roughness on shallow water waveguide propagation: A coherent approach

Standard propagation models in underwater acoustics (e.g., normal modes, PE) are deterministic in nature, i.e., they deal with a single realization of the environment. Additionally, for mathematical reasons, they typically treat the sea surface as a flat pressure-release surface. Effects of sea surface and bottom roughness are incorporated through a loss mechanism. This is accomplished by including an additional attenuation factor based on coherent loss of the surface-interacting component of the propagating field. This type of correction presents a mathematically inconsistent model, since usually results from stochastic scattering models are applied to results from single realizations of the stochastic medium. Moreover, scattering kernels are generally derived assuming a homogeneous medium underlying the sea surface, an assumption incompatible with a realistic environment. Using a numerical model [Norton et al., J. Acoust. Soc. Am. 97, 2173–2180 (1995)] that combines a high fidelity Parabolic Equation propagation model with the conformal mapping technique developed by Dozier [L. B. Dozier, J. Acoust. Soc. Am. 75, 1415–1432 (1984)] to handle surface roughness in a marching algorithm, forward propagation with a single realization of a rough sea surface overlying a complex ocean environment can be modeled in a mathematically consistent way. This technique is applied to the problem of shallow water propagation with a rough sea surface. For the adopted environment and range of rms roughnesses considered (0.28<kh<2.33, where k is the acoustic wave number and h is the rms surface height) the inclusion of the rough surface introduces a significant increase in the transmission loss and vertical-angle distribution of the propagating field. Results for this environment also show that, on average, the transmission loss is close to that calculated using the roughness-induced attenuation factor approximation. The transmission loss calculated with the roughness-induced attenuation factor exhibits a smoother range dependence, indicating strong attenuation of energy at steeper propagating angles. However, when strong upward refraction conditions exist, coherent effects and energy redistribution cannot be ignored and results from the procedure which incorporates the roughness through a loss mechanism are in error.

STUDIES ON EFFECTS OF NONLINEAR FEATURES OF CAPILLARY-GRAVITY WAVE SPECTRA ON MICROWAVE EMISSIVITY OF SEA SURFACE

Based on the results of oceanic microwave remote sensing and the dynamics of ocean waves as well as in situ measurements, the effects of nonlinear features of slope spectra of the sea surface at the low sea state on microwave emissivity are studied. It is revealed that the Bragg scattering effect of capillary-gravity waves of the sea surface on microwave influences not only the scattering measurements dara, but also the microwave emission. The effects reflect that there is a trough on the curve of changes in microwave emissivity of vertical polarization with frictional velocity, u*, on the sea surface. The measurement results also indicate changes in microwave emissivity with the wind direction. This fact also proves that the directionality of capillary-gravity waves influences not only the scattering measurements, but also emission. Based on these results, the possibility of measuring the spectra of capillary-gravity waves using a multifrequeney microwave radiometer is presented. It is pointed out that when the physical parameters besides the wind field are measured with microwave radiometry, it is neocessary to correct the effects of sea surface roughness or choose the measurement circumstances to keep away from the influence of the sea state.

The effects of sea surface roughness on the transmission of sound through an air-ocean interface

This is a review of recent experimental and theoretical studies of the influences of the sea surface on the transmission of airborne sound into the ocean. Most applications will depend upon conditions where the refracted grazing angle is greater than approximately 30° since the loss is very large for smaller angles. For grazing angles larger than 30°, two parameters serve fairly well to characterize the transmission loss through the sea surface. The first is a roughness parameter R which is equal to the sea wave height measured in airborne sound-speed wavelengths. The second is a diffraction parameter DL which is equal to the Fresnel zone diameter measured in correlation lengths of the sea surface slopes. When R is less than unity and DL is less than 3, there is usually no effect due to surface roughness. When either condition is not satisfied, there is usually more transmission loss than there would be through a flat surface. Exceptions to this depend upon the refracted grazing angle and upon the angle between the wave crests and the incident sound.

Erratum: "An experimental implementation of interferometric techniques for sea level variation measurements and reflection coefficient phase determination"

A remote interferometric method to measure sea level variations in coastal regions is discussed and results of numerous experimental tests are summarized. A bistatic scheme of measurements is employed with horizontal separation of transmitter-receiver terminals ranging from 400 m to 2700 m. To account for the impact of sea surface roughness, the method incorporates a reflection coefficient phase measuring procedure. The phase of the coherent reflection coefficient measured in the experiments is presented as a function of sea state. The experiments were performed with electronic distance measuring equipment operating at 10-cm wavelength. Preliminary estimates of the measurement accuracy show that errors in sea level variation determination were approximately between 3 cm and 9 cm. These observed errors exceed the theoretical predictions because of various causes, such as technical limitations of the instrumentation, effect of sea surface roughness, etc. More work needs to be done, especially in the area of signal processing, in order to sort out these various effects and reduce their impact on measurement accuracy.

An experimental implementation of interferometric techniques for sea level variation measurements and reflection coefficient phase determination

A remote interferometric method to measure sea level variations in coastal regions is discussed and results of numerous experimental tests are summarized. A bistatic scheme of measurements is employed with horizontal separation of transmitter-receiver terminals ranging from 400 m to 2700 m. To account for the impact of sea surface roughness, the method incorporates a reflection coefficient phase measuring procedure. The phase of the coherent reflection coefficient measured in the experiments is presented as a function of sea state. The experiments were performed with electronic distance measuring equipment operating at 10-cm wavelength. Preliminary estimates of the measurement accuracy show that errors in sea level variation determination were approximately between 3 cm and 9 cm. These observed errors exceed the theoretical predictions because of various causes, such as technical limitations of the instrumentation, effect of sea surface roughness, etc. More work needs to be done, especially in the area of signal processing, in order to sort out these various effects and reduce their impact on measurement accuracy.

Effect of sea-surface roughness on sound transmission in the ocean mixed layer

A series of 20 experiments on sound propagation in the mixed layer has been conducted in deep ocean areas around Australia. The results of the first eight experiments were reported at the 89th Meeting of the Acoustical Society [J. Acoust. Soc. Am. 57, S63(A) (1975)]. The results of the remaining 12 experiments are presented in this paper. This second set of experiments differs from the first set in that acoustic backscattering experiments were also conducted (in order that the scattering strength of the sea surface could be determined), and the sea surface roughness was monitored with a wave-rider buoy (in addition to visual estimates). These experiments have encompassed a wide range of oceanographic conditions: the mixed-layer thickness ranged from 20 m in one experiment, to 90 m in another; the swell height varied between 0.0 and 1.5 m; and the wind speed from 0.5 to 18 knots. The sound sources were small, shallow explosives, and the results have been analysed at frequencies of 4, 8, and 16 kHz. Results for propagation loss for receivers both in and below the mixed layer have been obtained, for horizontal ranges from 0.5 to 6 km. These results are compared with the normal-mode curves applicable to each experiment, in order to show directly the effect of any roughness in the sea surface. In addition, comparisons are made between the experimental results and curves obtained from a first-order scattering theory based on ray acoustics.