scholarly journals A Method for Predicting Radiated Acoustic Field in Shallow Sea Based on Wave Superposition and Ray

2020 ◽  
Vol 10 (3) ◽  
pp. 917 ◽  
Author(s):  
Chao Zhang ◽  
Yihao Liu ◽  
Dejiang Shang ◽  
Imran Ullah Khan
Keyword(s):  
Shallow Water ◽  
Half Space ◽  
Acoustic Field ◽  
High Frequency ◽  
Free Space ◽  
Ray Theory ◽  
Analysis Method ◽  
Shallow Sea ◽  
Wave Superposition ◽  
Complex Channel

The traditional free-space and half-space analysis method ignore the reflection of the upper and lower boundaries of shallow sea and are not suitable for analyzing shallow sea problems especially at high frequency. Hence, a method combining ray theory and wave superposition theory is proposed in this paper to predict the high-frequency radiated acoustic field in shallow water. The proposed method takes into account the effect of channel boundaries on the acoustic field and has good adaptability to complex channel environments and accuracy of the calculated acoustic field.

Dielectric permittivity and resistivity mapping using high‐frequency, helicopter‐borne EM data

Geophysics ◽  
2002 ◽  
Vol 67 (3) ◽  
pp. 727-738 ◽  
Author(s):  
Haoping Huang ◽  
Douglas C. Fraser
Keyword(s):  
Dielectric Permittivity ◽  
Half Space ◽  
High Frequency ◽  
Free Space ◽  
Apparent Resistivity ◽  
Phase Component ◽  
High Frequencies ◽  
Space Model ◽  
Homogeneous Half Space ◽  
Displacement Currents

The interpretation of helicopter‐borne electromagnetic (EM) data is commonly based on the transformation of the data to the apparent resistivity under the assumption that the dielectric permittivity is that of free space and so displacement currents may be ignored. While this is an acceptable approach for many applications, it may not yield a reliable value for the apparent resistivity in resistive areas at the high frequencies now available commercially for some helicopter EM systems. We analyze the feasibility of mapping spatial variations in the dielectric permittivity and resistivity using a high‐frequency helicopter‐borne EM system. The effect of the dielectric permittivity on the EM data is to decrease the in‐phase component and increase the quadrature component. This results in an unwarranted increase in the apparent resistivity (when permittivity is neglected) for the pseudolayer half‐space model, or a decrease in the apparent resistivity for the homogeneous half‐space model. To avoid this problem, we use the in‐phase and quadrature responses at the highest frequency to estimate the apparent dielectric permittivity because this maximizes the response of displacement currents. Having an estimate of the apparent dielectric permittivity then allows the apparent resistivity to be computed for all frequencies. A field example shows that the permittivity can be well resolved in a resistive environment when using high‐frequency helicopter EM data.

scholarly journals Theoretical Model of Acoustic Wave Propagation in Shallow Water

2017 ◽  
Vol 24 (2) ◽  
pp. 48-55 ◽  
Author(s):  
Eugeniusz Kozaczka ◽  
Grażyna Grelowska
Keyword(s):  
Shallow Water ◽  
Acoustic Field ◽  
Acoustic Waves ◽  
Sound Propagation ◽  
Low Frequency ◽  
Water Layer ◽  
Surface Shape ◽  
Shallow Sea ◽  
Multiple Reflections ◽  
Indirect Determination

Abstract The work is devoted to the propagation of low frequency waves in a shallow sea. As a source of acoustic waves, underwater disturbances generated by ships were adopted. A specific feature of the propagation of acoustic waves in shallow water is the proximity of boundaries of the limiting media characterised by different impedance properties, which affects the acoustic field coming from a source situated in the water layer “deformed” by different phenomena. The acoustic field distribution in the real shallow sea is affected not only by multiple reflections, but also by stochastic changes in the free surface shape, and statistical changes in the seabed shape and impedance. The paper discusses fundamental problems of modal sound propagation in the water layer over different types of bottom sediments. The basic task in this case was to determine the acoustic pressure level as a function of distance and depth. The results of the conducted investigation can be useful in indirect determination of the type of bottom.

Geo-Acoustic Parameters Inversion from Vertical Coherence Characteristic of Ambient Noise

2013 ◽  
Vol 385-386 ◽  
pp. 567-570
Author(s):  
Chun Xia Meng ◽  
Gui Juan Li ◽  
Liang Zhang
Keyword(s):  
Shallow Water ◽  
Acoustic Field ◽  
Relative Stability ◽  
Ambient Noise ◽  
Frequency Range ◽  
Shallow Sea ◽  
Acoustic Parameters ◽  
Different Seasons ◽  
Noise Data ◽  
Parameters Inversion

In shallow sea the wave of ambient noise contains the information of environment acoustic field. In theory, vertical coherence, which is one of the numerous ambient noise field characters, can be used to invert geo-acoustic parameters because of its relative stability. The ambient noise data were collected in two different seasons using two vertical-laid hydrophones in shallow water. Then acoustic parameters of semi-infinite space seabed are inversed from the vertical coherence of measured ambient noise in the frequency range of 100Hz~3 kHz. The predicted vertical coherence curve of ambient noise, which is computed using inversed parameters, agrees with the measured curve by comparison. The experiment results show that geo-acoustic parameters can be inversed effectively by the vertical coherence of ambient noise.

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