Underwater ambient noise measurements were carried out in a shallow (15-20 m) brackish water in the archipelago of the Gulf of Finland for period of 1 year. The absence of traffic noise made it possible to study wind driven effects in ambient noise at lower frequencies. The ambient noise comes mostly from local sources and the propagation effects are shown to be negligible. The ambient noise develops bubble type spectral features above 100 Hz as wind speed increases. Sharp spectral declines are observed below 500 Hz, which are most likely due to resonances from oscillating bubble clouds created by breaking waves. The low frequency range of the observed declines may partly be attributed to the larger bubble size in fresh and brackish waters. In the present study the wind speed dependence factor was approximately 2.4 at 200 Hz, which is significantly higher than the typical factor of approximately 1.5 for the ocean environment. The average high-frequency spectral slope was -4.9 dB/octave which is approximately 1 dB/octave less than for typical deep water slopes. No significant seasonal effects were found in any parameter calculated from the ambient noise spectra.
Ambient noise measurements were carried out in shallow brackish water within a frequency range extending up to 70 kHz. The high-frequency spectral slopes become steeper above 10 kHz at intermediate and high wind speeds. This is because the start of the wind speed dependence shifts rapidly to higher wind speeds at frequencies above 13 kHz. A physical explanation for this observation may be the low proportion of bubbles in brackish water that are small enough to radiate sound above 10 kHz. Such bubbles apparently do not begin to develop in brackish water until high wind speeds are attained.
Numerical models employed in ground VLF modeling use a normally incident (homogeneous) plane wave as a primary field. We show that these models are not directly applicable to modeling the impedance and wavetilt in the air, quantities needed in the interpretation of airborne VLF resistivity measurements. Instead, the primary field must be replaced by an inhomogeneous plane wave incident on the ground at an angle close to 90 degrees in order to provide the correct behavior of the apparent resistivities in the air. VLF magnetic polarization parameters, however, can be modeled in the air using the normally incident plane wave as a primary field. We also show that the plane‐wave analysis provides the same attenuation characteristics for the wavetilt in the air that is predicted by the Norton’s surface wave obtained by using the vertical electric dipole as a source. Use of the inhomogeneous plane wave introduces the vertical component of the electric field in the model. A 2‐D modeling technique based on the network solution is used to demonstrate the effects of the vertical electric field in the H‐polarization case. The vertical electric field generates charge distributions on the horizontal boundaries of conductors. In the case of a vertical sheet‐like conductor, these charges cause a slight asymmetry in apparent‐resistivity anomalies. Attenuation characteristics of various VLF anomalies with altitude are also presented. The H‐polarization anomalies attenuate much more rapidly in the air than those for E‐polarization due to the difference in the dominating source of EM fields in each polarization.
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