Parabolic Equation Modeling of Scholte Waves and Other Effects Along Sloping Fluid-Solid Interfaces

Several methods for handling sloping fluid–solid interfaces with the elastic parabolic equation are tested. A single-scattering approach that is modified for the fluid–solid case is accurate for some problems but breaks down when the contrast across the interface is sufficiently large and when there is a Scholte wave. An approximate condition for conserving energy breaks down when a Scholte wave propagates along a sloping interface but otherwise performs well for a large class of problems involving gradual slopes, a wide range of sediment parameters, and ice cover. An approach based on treating part of the fluid layer as a solid with low shear speed is developed and found to handle Scholte waves and a wide range of sediment parameters accurately, but this approach needs further development. The variable rotated parabolic equation is not effective for problems involving frequent or continuous changes in slope, but it provides a high level of accuracy for most of the test cases, which have regions of constant slope. Approaches based on a coordinate mapping and on using a film of solid material with low shear speed on the rises of the stair steps that approximate a sloping interface are also tested and found to produce accurate results for some cases.

Utilizing distributed acoustic sensing and ocean bottom fiber optic cables for submarine structural characterization

The sparsity of permanent seismic instrumentation in marine environments often limits the availability of subsea information on geohazards, including active fault systems, in both time and space. One sensing resource that provides observational access to the seafloor environment are existing networks of ocean bottom fiber optic cables; these cables, coupled to modern distributed acoustic sensing (DAS) systems, can provide dense arrays of broadband seismic observations capable of recording both seismic events and the ambient noise wavefield. Here, we report a marine DAS application which demonstrates the strength and limitation of this new technique on submarine structural characterization. Based on ambient noise DAS records on a 20 km section of a fiber optic cable offshore of Moss Landing, CA, in Monterey Bay, we extract Scholte waves from DAS ambient noise records using interferometry techniques and invert the resulting multimodal dispersion curves to recover a high resolution 2D shear-wave velocity image of the near seafloor sediments. We show for the first time that the migration of coherently scattered Scholte waves observed on DAS records can provide an approach for resolving sharp lateral contrasts in subsurface properties, particularly shallow faults and depositional features near the seafloor. Our results provide improved constraints on shallow submarine features in Monterey Bay, including fault zones and paleo-channel deposits, thus highlighting one of many possible geophysical uses of the marine cable network.

IDENTIFICATION OF NORMAL WAVES FORMING A SOUND FIELD IN A SHALLOW SEA AT THE INFRASONIC FREQUENCY RANGE. Part II

Анализируются результаты экспериментальных исследований звукового поля, зарегистрированного комбинированными приемниками, образующими вертикально ориентированную двухэлементную антенну. Звуковое поле формировалось дискретными составляющими вально-лопастного звукоряда шумового сигнала НИС «Юрий Молоков» в инфразвуковом диапазоне частот 2–20 Гц, а также буксируемым низкочастотным излучателем полигармонического сигнала в диапазоне частот 30–60 Гц. Глубина моря и рабочий диапазон частот 2–20 Гц исключали возможность возбуждения нормальных волн дискретного спектра в модельном волноводе Пекериса в этом диапазоне частот. По результатам спектрального анализа шумового сигнала получена оценка потенциальной помехоустойчивости комбинированного приемника при использовании полного набора информативных параметров, характеризующих энергетическую структуру звукового поля. По результатам анализа вертикальной структуры звукового поля в инфразвуковом диапазоне частот был сделан вывод о том, что звуковое поле сформировано неоднородными нормальными волнами Шолте, регулярной и обобщенной (гибридной). В дальней зоне источника доминирует регулярная волна Шолте, локализованная на границы раздела вода – морское дно. В ближней зоне источника возрастает роль обобщенной волны Шолте, локализованной на горизонте источника, а звуковое поле формируется парой волн Шолте, регулярной и обобщенной. The results of experimental studies of the sound field recorded by combined receivers forming a vertically oriented two-element antenna are analyzed. The sound field was formed by discrete components of the vane-blade scale of the noise signal of the science ship «Yuri Molokov» in the infrasonic frequency range of 2–20 Hz, as well as by a towed low-frequency emitter of a polyharmonic signal in the frequency range 30–60 Hz. The depth of the sea and the operating frequency range of 2–20 Hz excluded the possibility of exciting normal waves of the discrete spectrum in the model Pekeris waveguide in this frequency range. Based on the results of spectral analysis of the noise signal, an estimate of the potential noise immunity of the combined receiver was obtained using a full set of informative parameters characterizing the energy structure of the sound field. Based on the results of the analysis of the vertical structure of the sound field in the infrasonic frequency range, it was concluded that the sound field is formed by inhomogeneous normal Scholte waves, regular and generalized (hybrid). In the far zone of the source, a regular Scholte wave dominates, localized at the water – seabed interface. In the near-field zone of the source, the role of the generalized Scholte wave localized at the source horizon increases, and the sound field is formed by a pair of Scholte waves, regular and generalized.

Integrated Geophysical Analyses of Shallow-Water Seismic Imaging With Scholte Wave Inversion: The Northern Adriatic Sea Case Study

The integrated analysis using different seismic wave types in a record is a very efficient approach for a comprehensive characterization of marine sediments, especially in shallow water conditions. The proposed integrated method to analyze seismic data in post-critical conditions consists of: 1) the inversion of Scholte waves to obtain a reliable Vs distribution of the near seafloor; 2) pre-processing of seismic data; 3) construction of the P-wave velocity field by using all available information, including available well data; and 4) the application of the wave equation datuming and post-processing, such as pre-stack time migration. We demonstrate how this approach could be successfully applied on seismic datasets characterized by post-critical conditions and the occurrence of the Scholte waves, which may be exploited to provide fundamental information instead of being only an unwanted effect. The integrated analysis of seismic events can thus help, together with data processing, by providing better seismic imaging, which is a priority for a reliable seismostratigraphic interpretation.

In-situ microseism noise generation measured from distributed acoustic sensing on seafloor optical cable

<p>Measuring seismic and acoustic signals on seafloor telecom cables has proven recently its very high potential for earthquake monitoring but also for beter understanding the interaction between the oceans and the solid earth. A consequence of these interactions is the generation of the primary and secondary microseismic noise on coastal regions and in the deep ocean respectively. These seismic noises that propagate across continents are central to a large fraction of todays' seismic imagery and monitoring campaigns. Compared to previous studies and instrumentation setups, acoustic sensing over oceanic telecom cables offer the unique ability to measure in a very dense manner waves that are generated on the seafloor. We analyse a week long record of ambient noise measurements on the 41.5 km-long seafloor telecom cable offshore Toulon, south of France. At shallow depth, close to the coast, we measure the pressure changes caused by the oceanic gravity waves. The bottom pressure is then compared to an oceanographic buoy located a few kilometers away from the cable. The amplitude and frequency of the pressure are modulated by the gravity waves height and dominant periods. This observation opens the way for a distributed measurement of the oceanic waves characteristics over several kilometers. At depth larger than a 1km, we observe Scholte waves at the ocean-solid earth interface produced by the non-linear interaction of gravity waves. These waves have the double frequency of the gravity waves seen at the coast. We find that the amplitude and frequency change over time, as do the gravity waves observed near the coast. The frequency-wave number decomposition of the signal reveals that the apparent velocity of the Scholte waves does not depend of the azimuth of the fiber. These observations confirm that these deep Scholte waves are secondary microseismic noise, generated locally from the interaction of landward gravity waves with oceanward gravity wave reflected on the coast. Spatially distributed monitoring of the ambient noise wave field at the ocean-solid earth interface provides a better understanding of the noise generation and therefore will allow a better modeling of the ambient noise in the future.</p>

On the Dispersive Characteristics of Shallow Water Seismic Waves Excited by Air Gun Sound Source in Shallow Water

In order to study the dispersion characteristics of seabed seismic waves excited by acoustic sources in shallow water, the sound field at liquid-solid interface is derived based on the wave equation firstly, which results demonstrate that the longitudinal wave and transversal wave are not dispersive, while the Scholte wave is normal dispersive. Then, the numerical simulation is carried out on the sound field at the liquid-solid interface excited by the air gun sound source based on high-order staggered grid finite difference method, which results demonstrate that the dispersion phenomenon of Scholte waves is observed and the theoretical analysis results were verified. Finally, the data of experiment in the sea are analyzed based on the transformation, and the dispersion curve of Scholte wave is finely extracted.