| Summary: | Passive recording systems have become increasingly popular and important in various marine acoustic and geophysics branches. Data from these systems provide valuable information and can be used in various applications, e.g., to characterize whales, earthquakes, or, more generally, the subsurface. These applications contain different frequency ranges, and various recording systems have been developed to capture them all. For example, strainmeters are sensitive to the lowest frequencies, like those generated by Earth’s tide, while hydrophones, geophones and broadband seismometers can pick up higher frequencies, such as those produced by whales and small-magnitude earthquakes. Hence, the choice of receiving system depends on the application. These systems contain pointsensors, therefore making it challenging to sufficiently instrument the whole world. This is mainly due to the cost of installation and maintenance, hence leaving the world sparsely instrumented. In recent years, new technologies have been developed to increase instrumentation. One example is Permanent Reservoir Monitoring (PRM) systems, which have been installed near oil and gas fields to improve recovery. Such a system typically contains 1000s of 4-component sensors per PRM system and is placed near interesting features in the subsurface. Another example is Distributed Acoustic Sensing (DAS), which has, in recent years, emerged as a new technology to help increase the density of sensors worldwide. The receiver unit, the DAS interrogator, is connected to an optical fiber within a fiber telecommunication cable. This can re-purpose the fiber to a distributed sensor, sensing acoustic signal every meter for up to 150 km, thus creating a far-reaching, easily accessible receiver unit. This thesis consists of several manuscripts investigating how dense passive receiver arrays can extract information from the water column through whale songs and the subsurface through different seismic waves. Most of the work has focused on DAS data from three datasets. The ...
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