Drivers of Strategic Contestation in South America

The politics of contestation on the part of secondary regional powers such as Argentina, Chile, Colombia and Venezuela towards Brazil as the regional leader oscillate between competition and cooperation, inasmuch as the South American region has one regional power and is a zone of negative peace without aggressive rivalries. The secondary powers use different tactics, which constitute their respective foreign policy strategies, to soft balance Brazil. These tactics include alliance building, entangling diplomacy, binding, and omnienmeshment. This paper identifies, first, the specific drivers of contestation towards Brazil and, second, why the secondary powers' foreign policy strategies vary in how they directly or indirectly contest the rise of Brazil at the regional and international levels. The paper demonstrates that in a regional order such as that of South America, which is characterized by relative stability, domestic drivers of contestation are key to explaining secondary powers' varied strategic responses to the regional power

Bell nonlocality

Bell's 1964 theorem, which states that the predictions of quantum theory cannot be accounted for by any local theory, represents one of the most profound developments in the foundations of physics. In the last two decades, Bell's theorem has been a central theme of research from a variety of perspectives, mainly motivated by quantum information science, where the nonlocality of quantum theory underpins many of the advantages afforded by a quantum processing of information. The focus of this review is to a large extent oriented by these later developments. We review the main concepts and tools which have been developed to describe and study the nonlocality of quantum theory, and which have raised this topic to the status of a full sub-field of quantum information science.Comment: 65 pages, 7 figures. Final versio

Experimental Studies of the Acoustic Signal generated by Marine Seismic Sources

Marine seismic acquisition is the most commonly used geophysical technique to image the subsurface for different purposes, e.g. exploration of reservoirs or investigation of subduction zones as areas where earthquakes are generated. The acquisition consists of a sound source emitting the signal into the subsurface and dedicated sensors which receive the reflected signals from the subsurface structures of interest. The most common sound source used in marine seismics over the last 50 years is the airgun. The airgun is a mechanical device releasing high pressurized air from a chamber into the surrounding water within a few milliseconds. The acoustic source signal consists of a sharp peak when the pressurized air is released followed by an oscillation corresponding to the expanding and contracting air bubble in water. The main advantages of the airgun are its signal repeatability and long lifetime. However, the emitted frequency bandwidth is limited by the oscillation period and the destructive interference of the direct downgoing wavefield with the first reflection from the sea surface, referred to as the ghost. In recent years there has been increasing interest in improving two major aspects of the sound source in marine seismic acquisition. The first aspect is the enhancement of emitted low frequency signals ( 100-200 Hz) which can have an impact on marine life, e.g. masking communication or behavioural disturbances. Therefore, alternative source types, e.g. marine vibrators, are developed and investigations are conducted on how to tune the signal emitted by airguns. The thesis consists of several, individual manuscripts where the acoustic signal generated by single marine seismic sources, especially from airguns, is experimentally investigated. Several experiments are conducted to study the mechanisms that have an impact on the low frequency content of the emitted sound signal. First, the signal generated by the rising airgun bubble is investigated separately from the main peak and oscillating bubble by dedicated experiments with rising buoys and the results are compared to measured airgun signatures. Secondly, the impact of the spherical wave front on the ghost reflection is studied for different source-interface distances in a tank experiment. Third, the impact of the interaction between the airgun bubble and sea surface on the source signature is investigated for very shallow source depths and the variations of the oscillating bubble and ghost reflection are discussed. In addition, the signal emitted into air is analyzed for these shallow seismic sources. Another study deals with the reduction of high frequency emission from seismic airguns. The impact of bubble curtains surrounding an airgun on the source signature and high frequency emission are investigated for different bubble curtain configurations. The results from the rising airgun bubble indicate a low frequency signal emitted by this movement. However, the signal strength is low and seems to have no significant impact in marine seismic acquisition. The results for the spherical wave front indicate that the low frequency signal (< 1-2 Hz) could be enhanced with decreasing source depth. However, the noise level at this frequency band is usually strong and it needs to be further investigated how large the impact could be in field applications. The results from the source-interface interaction indicate that the low frequency signal (< 5 Hz) is strongly enhanced when the airgun bubble bursts directly at the water surface and no oscillations occur. This effect seems to be promising to enhance the low frequency signal in field applications. The results from the bubble curtain surrounding the airgun indicate a gradually decreasing frequency content above 50 Hz with increasing air injection into the bubble curtain. Hence, bubble curtains could potentially be considered to reduce high frequency emissions from airguns

Cecil Rhodes distorted politics in South Africa long before apartheid

Cecil Rhodes’ policy reforms disenfranchised up to 15,000 mostly black and mixed-race voters in South Africa. This voter suppression created an unequal political environment that favoured white men 50 years before apartheid, write Daniel de Kadt and Joachim Wehner

Tube-wave monitoring as a method to detect shear modulus changes around boreholes: A case study

Monitoring the shear modulus of formations around boreholes is of interest for various applications, ranging from near-surface investigation to reservoir monitoring. Downhole logging tools and borehole seismic are common techniques applied to measure and characterize formation properties. These methods rely on transmitted and reflected waves to retrieve the rock properties. Wave modes traveling along the interface between the well and the formation, such as tube waves, are often considered as noise. However, tube waves are less attenuated than body waves and contain information about the shear modulus of the formation surrounding the well. Hence, a potential use of this interface wave is of interest. Because tube-wave properties depend on several parameters, e.g., well geometry, highly accurate measurements should be performed for use in inferring rock properties. We have studied the feasibility of tube-wave measurements as a monitoring method. Different experiments are conducted using a hydrophone array in two boreholes, with depths of 30 and 95 m. The experiments are used to investigate how accurately the tube-wave velocity can be measured and which parameters have the most impact on the measurements. Our results suggest that it is hard to estimate the absolute shear modulus of the geologic formation using tube-wave velocities only. However, it seems feasible to use them to monitor changes in the shear modulus, depending on the borehole setup and the geologic formation. Tube-wave monitoring can be used as a first step to determine the depth along the well where changes occur before more accurate measurements are performed in a second step.publishedVersio

On low frequencies emitted by air guns at very shallow depths - An experimental study

In marine seismic acquisition, the enhancement of frequency amplitudes below 5 Hz is of special interest because it improves imaging of the subsurface. The frequency content of the air gun, the most commonly used marine seismic source, is mainly controlled by its depth and the volume. Although the depth dependency on frequencies greater than 5 Hz has been thoroughly investigated, for frequencies less than 5 Hz it is less understood. However, recent results suggest that sources fired very close to the sea surface might enhance these very low frequencies. Therefore, we conduct dedicated tank experiments to investigate the changes of the source signal for very shallow sources in more detail. A small-volume air gun is fired at different distances from the water-air interface, including depths for which the air bubble bursts directly into the surrounding air. The variations of the oscillating bubble and surface disturbances, which can cause changes of the reflected signal from the sea surface, are explored to determine whether an increased frequency signal below 5 Hz can be achieved from very shallow air guns. The results are compared with field measurements of a large-volume air gun fired close to the sea surface. The results reveal an increased signal for frequencies below 5 Hz of up to 10 and 20 dB for the tank and field experiments, respectively, for the source depth at which the air gun bubble bursts directly into the surrounding air. For large-volume air guns, an increased low-frequency signal might also be achieved for sources that are slightly deeper than this bursting depth. From these observations, new design considerations in the geometry of air-gun arrays in marine seismic acquisition are suggested.acceptedVersionThis is the authors' accepted and refereed manuscript to the article. Use is subject to SEG terms of use and conditions

TrueOcean: How cloud and AI technologies are revolutionizing hydrography

TrueOcean, developed by north.io GmbH, is an ocean data platform designed to address the complexities of managing, sharing, and analyzing marine data. The platform consolidates diverse sensor data formats into a unified, cloud-optimized format, leveraging scalable cloud infrastructure for efficient storage, processing, and analysis. Key features include the use of the Apache Parquet format for data unification and H3 indexing for efficient geospatial data management. The platform supports collaboration and data sharing through standard protocols like SFTP, RSYNC, WMS, and WFS. Additionally, the Geodata Processing Engine integrates Apache Spark and Kubernetes to enable large-scale data processing. By combining flexible data management, scalable analysis, and robust collaboration tools, TrueOcean serves as a comprehensive solution for marine data stakeholders, facilitating the extraction of actionable insights across various use cases

Acoustic signals in air and water generated by very shallow marine seismic sources: An experimental study

When a marine seismic source, like an airgun, is fired close to the water surface the oscillating bubble interacts with the water–air interface. The main interest for seismic applications is how this effect impacts the acoustic signal propagating into the water. It is known that the sound transmission into air is abnormally strong when the sound source is very close to the sea surface relative to the emitted wavelength. Detailed insight into how the acoustic signal changes when the source depth is changed is useful in seismic data analysis and processing. Two experiments are conducted in a water tank with two different types of seismic sources. In experiment A the source is a small cavity that is sufficiently far away from the water–air interface so that it can be assumed that no interaction between the cavity and water surface occurs. In experiment B the source is a larger air bubble that is very close to the water–air interface, and hence interaction between the bubble and water surface occurs. The effects on the water surface, oscillating bubble, and emitted acoustic pressure into air are discussed. It is demonstrated that the moving surface contributes significantly to the acoustic signal measured in air.publishedVersionVC 2020 Acoustical Society of Americ

Characteristics of tropical cyclones in high-resolution models in the present climate

The global characteristics of tropical cyclones (TCs) simulated by several climate models are analyzed and compared with observations. The global climate models were forced by the same sea surface temperature (SST) fields in two types of experiments, using climatological SST and interannually varying SST. TC tracks and intensities are derived from each model's output fields by the group who ran that model, using their own preferred tracking scheme; the study considers the combination of model and tracking scheme as a single modeling system, and compares the properties derived from the different systems. Overall, the observed geographic distribution of global TC frequency was reasonably well reproduced. As expected, with the exception of one model, intensities of the simulated TC were lower than in observations, to a degree that varies considerably across models