Sea Surface Reflection Coefficient Research Articles

Characteristics of the far-field source signature and its ghost effect of a marine single air gun

The estimation of the air gun source signature is a crucial topic in marine seismic. Typical approaches such as numerical modelling or near-field measurement-based inversion normally work in practice, but suffer from lack of knowledge on the varying sea surface reflection coefficient, the signature directional consistency and the dynamically changed source geometry. For better understanding of the above uncertainties, real far-field air gun source signatures were acquired. A customized approach combined with sparse f-k transformation and adaptive subtraction is applied to deblend the source signature from the sea-floor reflections. Characteristic of the ghost notch distribution and bubble oscillation period indicate an overall good match to the simulated results, except the latter part due to the unexpected air-leakage. Amplitude decay with incidence-angle increase is observed, indicating that the point-source spherical theory for directional signature estimation may be questionable. The sea-surface reflection coefficient appears time-variant and frequency-dependent which challenges the assumption of flat sea surface in signature estimation. An innovative notional source inversion workflow is adapted to the frequency-dependent reflection coefficient. The notional signature inversed through the adapted approach results in less ghost residuals and weaker spectral notch effect comparing with the normal inversion result, i.e., based on a constant reflection coefficient.

Simultaneous receiver-side deghosting and denoising method based on the sparsity constraint

In marine seismic exploration, the sea surface ghost causes frequency notches and low-frequency loss, which affects the signal-to-noise ratio (SNR) and resolution of seismic records. This paper presents a simultaneous receiver-side deghosting and denoising method based on the sparsity constraint. First, considering the influence of propagation direction and sea surface reflection coefficient, the ghost time delay is calculated accurately, and then the accurate ghost operator is constructed in the frequency—slowness domain. Finally, the ghost-free data are obtained using the sparse constraint algorithm that can effectively suppress the ghost along with the noise energy. This method can remove the ghost and noise simultaneously, achieving quick convergence and with few iterations. It is applied to synthetic data and actual streamer field data. Test results prove that the ghost and notches are suppressed effectively, the SNR is improved, and the band is well broadened.

Effects of wind-generated bubbles layer on sound propagation underneath rough sea surface in shallow water

The reflection coefficient of the flat sea surface in the ideal condition to the incident sound wave is –1. The perfect reflection effect does not introduce reflection loss. However, the sea surface is usually rough due to the wind. The wind-generated rough sea surface has not only reflection effect, but also scattering effect on the sound wave. At the same time, the wind-generated bubbles layer also has significant effect on the sound propagation. On the one hand, the bubbles layer can change the sound speed profile and result in the refraction of the incident sound wave. On the other hand, the bubbles layer has scattering effect and absorption effect on the incident sound wave and leads to the sound wave to attenuate. In fact, the rough sea surface and the bubbles layer are two main factors affecting the sound propagation in the windy weather at sea. Many researchers have paid much attention to the effect of the wind-generated rough sea surface on the sound propagation, but few of them have considered the effect of the wind-generated bubbles layer on the sound propagation. Based on the Ramsurf sound propagation model under the rough sea surface, the effects of wind-generated bubbles layer underneath rough sea surface on reflection loss and sound propagation with different wind speeds are analyzed. Based on the Hall-Novarini bubbles population model, the sound speed profile in the bubbles layer is modified and the attenuation coefficient due to scattering and absorption of the bubbles layer is calculated. The simulation results shows that when the wind speed is 10 m/s, the effect of the bubbles layer is significant on reflection loss with the frequencies higher than 2 kHz. In the given underwater acoustic environment, for a frequency of 3 kHz, when the source depth and the receiver depth are both 7 m, the enhancement of the transmission loss due to the bubbles layer is 2.6 dB for a wind speed of 13 m/s, and the enhancement is 8.1 dB for a wind speed of 16 m/s. And when the source depth and the receiver depth are both 18 m, the enhancement of the transmission loss due to the bubbles layer is 2.5 dB for a wind speed of 13 m/s and the enhancement is 4 dB for a wind speed of 16 m/s.

Prediction of signatures of airgun arrays using dual near‐field hydrophones

ABSTRACTSuccessful estimation of airgun‐array signatures from near‐field measurements depends on the accuracy of poorly controlled model parameters such as the effective sea surface reflection coefficient and source depth. We propose a method for prediction of robust source signatures, which are insensitive to fluctuations of the latter parameters. The method uses vertical pairs of near‐field hydrophones to measure near‐field pressure and its vertical gradient, combination of which eliminates sea surface reflections from the near‐field data. This excludes the uncertainty related to the fluctuating sea state and source depth from the process of inversion of the near‐field data for source signature. The method explicitly separates the recorded near‐field pressure into its up and down going components, which allows one to measure the effective frequency‐ and angle‐dependent sea surface reflection coefficient right at the source, as well as to estimate the actual source depth. Tests on synthetic and field data demonstrate robust performance of the method.

Estimating source signatures from source-over-spread marine seismic data

Recent developments in marine seismic acquisition include deploying a source vessel above a towed-streamer spread. We have developed an inversion algorithm to estimate source signatures for such acquisition configurations, by minimizing the difference between the recorded and a modeled direct wave. The forward modeling is based upon a physical modeling of the air bubble created by each air gun in the source array, and a damped Gauss-Newton approach is used for the optimization. Typical inversion parameters are empirical damping factors for the bubble oscillations and firing time delays for each air gun. Variations in streamer depth are taken into account, and a constant sea-surface reflection coefficient is also estimated as a by-product of the inversion. For data acquired in shallow waters, we have developed an extension of the forward modeling to include reflections from the water bottom to stabilize the inversion. The algorithm is tested on synthetic- and field-data examples, and the estimated source signature for the field-data example is used in a designature processing flow.

Radiometric microwave field measurements of the complex dielectric constant of the water surface

The paper describes the new remote method of radiometric in situ measurements of the real and imaginary parts of the dielectric constant (permittivity) of the underlying sea surface, which is an important parameter for remote sensing of the Earth. The method does not require absolute calibration of a radiometer, but uses the black body with the underlying surface temperature, which simplifies the measurement and allows one to measure the effective reflection coefficient at specified polarizations. It is shown that, for a fixed value of the reflection coefficient in sensing to the nadir, R(θ = 0°) = const, the reflection coefficient at the horizontal polarization is almost independent of the of dielectric loss angle. The reflection coefficient at the vertical polarization is most sensitive to the loss angle in the vicinity of the Brewster angle Rv(θ = θBr). The paper presents the results of measurements of the sea surface reflection coefficient using the radiometer operating at frequency of 37.5 GHz. The reflection coefficients at vertical and horizontal polarizations are calculated taking into account the antenna pattern and the distribution of slopes of a weakly roughened sea surface. The paper presents the example of determining the real and imaginary parts of the permittivity based on experimentally measured values of reflection coefficients at incidence angles close to the nadir and the reflection coefficient Rv(θ = θBr) at the Brewster angle. The obtained experimental values correspond to the theory within the limits of diversity of existing models.

Receiver deghosting in the t-x domain based on super-Gaussianity

Deghosting methods in the time-space (t-x) domain have attracted a lot of attention because of their flexibility for various source/receiver configurations. Based on the well-known knowledge that the seismic signal has a super-Gaussian distribution, we present a Super-Gaussianity based Receiver Deghosting (SRD) method in the t-x domain. In our method, we denote the upgoing wave and its ghost (downgoing wave) as a single seismic signal, and express the relationship between the upgoing wave and its ghost using two ghost parameters: the sea surface reflection coefficient and the time-shift between the upgoing wave and its ghost. For a single seismic signal, we estimate these two parameters by maximizing the super-Gaussianity of the deghosted output, which is achieved by a 2D grid search method using an adaptively predefined discrete solution space. Since usually a large number of seismic signals are mixed together in a seismic trace, in the proposed method we divide the seismic trace into overlapping frames using a sliding time window with a step of one time sample, and consider each frame as a replacement for a single seismic signal. For a 2D seismic gather, we obtain two 2D maps of the ghost parameters. By assuming that these two parameters vary slowly in the t-x domain, we apply a 2D average filter to these maps, to improve their reliability further. Finally, these deghosted outputs are merged to form the final deghosted result. To demonstrate the flexibility of the proposed method for arbitrary variable depths of the receivers, we apply it to several synthetic and field seismic datasets acquired by variable depth streamer.

Adaptive f-k deghosting method based on non-Gaussianity

For conventional horizontal towed streamer data, the f-k deghosting method is widely used to remove receiver ghosts. In the traditional f-k deghosting method, the depth of the streamer and the sea surface reflection coefficient are two key ghost parameters. In general, for one seismic line, these two parameters are fixed for all shot gathers and given by the users. In practice, these two parameters often vary during acquisition because of the rough sea condition. This paper proposes an automatic method to adaptively obtain these two ghost parameters for every shot gather. Since the proposed method is based on the non-Gaussianity of the deghosting result, it is important to choose a proper non-Gaussian criterion to ensure high accuracy of the parameter estimation. We evaluate six non-Gaussian criteria by synthetic experiment. The conclusion of our experiment is expected to provide a reference for choosing the most appropriate criterion. We apply the proposed method on a 2D real field example. Experimental results show that the optimal parameters vary among shot gathers and validate effectiveness of the parameter estimation process. Moreover, despite that this method ignores the parameter variation within one shot, the adaptive deghosting results show improvements when compared with the deghosting results obtained by using constant parameters for the whole line.

Deghosting towed streamer data in τ/p domain based on rough sea surface reflectivity

Currently, the deghosting of towed streamer seismic data assumes a flat sea level and a sea-surface reflection coefficient of −1; this decreases the precision of deghosting. A new method that considers the rough sea surface is proposed to suppress ghost reflections. The proposed deghosting method obtains the rough sea surface reflection coefficient using Gaussian statistics, and calculates the optimized deghosting operator in the τ/p domain. The proposed method is closer to the actual sea conditions, offers an improved deghosting operator, removes the ghost reflections from marine towed seismic data, widens the bandwidth and restores the low-frequency information, and finally improves the signal-tonoise ratio and resolution of the seismic data.

Sea Surface Reflection and Power Attenuation Analysis of Radio Wave in UHF Satellite Communications

Ultra High Frequency (UHF) band is increasingly widely used in military maritime satellite communications due to its good communication performance. Electromagnetic wave emitted by satellite is reflected when reaching the sea surface, and multipath fading is very serious which causes interference on the received signal, especially when satellite elevation angle is low. In this paper, the UHF band multipath effects of sea surface were studied following the general principles of modeling in multipath effects. The numerical simulation was made based on Rayleigh criterion and the judgment basis was obtained which determines whether the specula reflected component or diffuse reflected component dominates in the reflected wave, and then the reflection coefficient of sea surface in different polarization modes were calculated. Finally, the power attenuation of reflected wave relative to direct wave for different wind speeds were simulated and analyzed. Numerical simulation results show that the greater the wind speed is, the greater the power attenuation of reflected wave will become and the more serious multipath fading will be. In addition, diffuse reflection attenuation is more serious than specular reflection. DOI : http://dx.doi.org/10.11591/telkomnika.v12i4.4787

On seismic deghosting by spatial deconvolution

Receiver-side deghosting can be derived and implemented in the frequency domain as spatial deterministic deconvolution of marine pressure recordings. The deghosting/deconvolution operator is found analytically as the inverse Fourier transform of the wavenumber-domain wave equation deghosting function. For a sea surface reflection coefficient of [Formula: see text], the wavenumber-domain deghosting function has well-known poles at fundamental frequencies equal to an integer multiple of a function of the receiver depth and the plane-wave dip angle relative to the depth axis. The first singularity is always at 0 Hz. The spatial deghosting operator has singularities at fundamental frequencies equal to an integer multiple of a function of the receiver depth, independent of its lateral coordinates. The first singularity is again at 0 Hz. In addition, the deghosting operator that is applied to 3D data has singularity when its lateral coordinate is zero. A simple numerical example demonstrates the method.

Repeatability issues of high-frequency signals emitted by air-gun arrays

Recent field measurements of the acoustic signals generated by marine seismic air-gun arrays showed that the amount of high-frequency signals (above 10 kHz) increased with the size and total volume of the gun array. We found that for frequencies between 10 and 20 kHz, a strong signal is observed 7–14 ms after the main peak of the source signal. We believe that this signal was generated by ghost cavitation. We observed that this signal was significantly stronger than the high-frequency signal generated at the same time as the peak signal occurs within the bandwidth between 10 and 20 kHz. We found that this high-frequency signal was fairly repeatable from one shot to another. By “fairly,” we mean that individual high-frequency events were not repeatable; however, the envelope energy of this cascade of events was repeatable from one shot to another. The typical feature of the envelope of the high-frequency signal was that it lasted for approximately 6–7 ms and showed a monotonic increase in amplitude for the first 5–6 ms, followed by a sudden drop. The sea surface reflection coefficient for these high-frequency events seemed to decrease in magnitude as the frequency increased.

Multipath Effect Analysis of Sea Surface in Maritime Satellite Communications

Ultra High Frequency (UHF) band is increasingly widely used in military maritime satellite communications due to its good communication performance. When the electromagnetic wave reaches the sea surface, it is reflected. The reflected wave and direct wave enter the receiver in the form of a superimposed vector, which will inevitably produces multipath fading at the receiving end. This paper followed the general principles of modeling in multipath effects, and the UHF band multipath effects of sea surface were studied. The judgment basis was obtained which determines whether the specula reflected wave dominates in the reflected wave based on Rayleigh criterion, and then the reflection coefficient of sea surface were calculated. Finally, the power attenuation of reflected wave relative to direct wave for different wind speeds were simulated and analyzed.

Source signature determination by inversion

A new method for estimating the pressure wavefield generated by a marine air‐gun array is presented. It is assumed that data is acquired at a ministreamer located below the source array. Effective source signatures for each air gun are estimated by an inversion algorithm. The forward modeling scheme used in the inversion algorithm is based upon a physical modeling of the air bubble generated by each air gun. This means that typical inversion parameters are: gun depths, empirical damping coefficients, and reflection coefficient of the sea surface. Variations in streamer depth are also taken into account by the inversion scheme. The algorithm has been successfully tested on examples with unknown streamer positions, gun parameters, reflection coefficient of sea surface, and ministreamer data contaminated with white noise.

Remote sensing of the sea-state using the 0.8-1.1 μm spectral band

Abstract Previous authors have reported on sea-state local variations detected by satellite in the 0.8-1.1 μm channel (Landsat MSS 7). We first demonstrate that in this channel the observed change in reflectance is mainly due to sea-state. We use the Cox and Munk model to compute the sea-surface reflection coefficient for various geometrical conditions, and it is found that this model fits previous observations. The model is then, applied to cases of Landsat and SPOT sensors, and some useful recommendations are underlined.

Propagation characteristics on line-of-sight over-sea paths in Japan

The results of line-of-sight over-sea propagation measurements over 37.2- and 35.5-km paths near Tokyo, Japan, are presented. Measurements of the fading (0.25, 3.99, 4.15, 6.72, and 18.0 GHz) of the cross polarization distortion (linear polarized wave at 4.15 GHz and circular polarized wave at 6.72 GHz) of the space diversity effect at 4.15 GHz, of the height-gain patterns at 6.72 GHz, and of the frequency-sweep patterns from 18 GHz to 22 GHz are included. The following statistical results are presented: the frequency, seasonal, and path dependence of fading; the effect of different fading occurrence mechanisms, such as K-type, duct-type, and distorted raindrops, for the cross polarization distortion; the diversity improvement factor; the effective reflection coefficient of sea surface; and the path-length difference between direct and sea reflected waves.