Ocean Bottom Sensors Research Articles

Hazard and Risk-Based Tsunami Early Warning Algorithms for Ocean Bottom Sensor S-Net System in Tohoku, Japan, Using Sequential Multiple Linear Regression

This study presents robust algorithms for tsunami early warning using synthetic tsunami wave data at ocean bottom sensor (OBS) arrays with sequential multiple linear regression. The study focuses on the Tohoku region of Japan, where an S-net OBS system (150 pressure sensors) has been deployed. To calibrate the tsunami early warning system using realistic tsunami wave profiles at the S-net stations, 4000 stochastic tsunami simulations are employed. Forecasting models are built using multiple linear regression together with sequential feature selection based on Akaike Information Criterion and knee-point method to identify sensors that improve the accuracy most significantly. The study considers tsunami wave amplitude at a nearshore location and regional tsunami loss for buildings to develop hazard-based and risk-based tsunami warning algorithms. The models identify an optimal configuration of OBS stations and waiting time for issuing tsunami warnings. The model performance is compared against a base model, which only uses the earthquake magnitude and epicenter location. The result indicates that estimating the tsunami amplitude and loss via S-net improves accuracy. For the hazard-based forecasting, adding six sensors from the S-net improves the accuracy of the estimation most significantly with an optimal waiting time of 3 min. For the risk-based forecasting, a longer waiting time between 5 and 10 min is suitable.

Active-Source Interferometry in Marine and Terrestrial Environments: Importance of Directionality and Stationary Phase

ABSTRACT We utilize active-source seismic interferometry with dense seismic arrays both offshore and onland to explore the utility of this method to create virtual sources and reveal body-wave reflections in these two different environments. We first utilize data from an ocean-bottom cable (OBC) array in the Gulf of Mexico with equal numbers of sources (160 airgun shots) and receivers (160 ocean-bottom four-component sensors). We next use data from a geophone array across the Bighorn Mountains of Wyoming with many receivers (1300 vertical-component geophones) but a small number of sources (14 borehole active-source shots). We find that the OBC virtual source results, which produce strong reflections from sub-seafloor structures, are far superior to the onland results which lack usable reflections, and we explore reasons for these differences through a set of selective stacking approaches. We present techniques to account for the direction the seismic waves travel (directionality) and stationary phase and show that improvements can be made when incorporating these corrections. Although interferometric methods are based on assumptions of large numbers of widely distributed actual sources, we find that selective exclusion of potentially problematic source–receiver pairs can yield improved results. These geometric adjustments to active-source interferometry methods have utility for dense-nodal-array surveys that are now common in academic studies, but that often suffer from sparse source geometry.

Near-field tsunami inundation forecast method assimilating ocean bottom pressure data: A synthetic test for the 2011 Tohoku-oki tsunami

An approach for forecasting near-field tsunami inundation was developed by combining two methods. The first method computes tsunami by assimilating pressure data observed at numerous ocean bottom sensors without tsunami source information, and the second method forecasts near-field tsunami inundation by selecting a site-specific scenario from a precomputed tsunami inundation database. In order to evaluate the validity of this combined method, we performed a synthetic forecast test for the 2011 Tohoku-oki tsunami along the Pacific coast in Japan. A tsunami computation test performed using the assimilation of synthetic pressure data reveals that the method reproduced well the tsunami field for the 2011 Tohoku-oki tsunami. A synthetic near-field tsunami inundation forecast at four sites, Kamaishi, Rikuzentakata, Minamisanriku, and the Sendai Plain for the 2011 Tohoku-oki tsunami also worked. The results indicate that an accurate tsunami inundation forecast method by this combined approach using pressure data from numerous ocean bottom sensors is now available.

Submarine fiber optic earthquake detection

Optical Seismology![Figure][1] A subsea cable coiled on board a ship, ready for deployment CREDIT: RUN STUDIOS Seismic networks detect earthquakes and are common on continents, where they are easy to install. However, most of Earth's surface is under the oceans, where placing seismometers is difficult. Marra et al. now find that ordinary submarine telecommunication cables can be used to detect earthquakes. Small strain changes associated with the passage of seismic waves were detected with laser light sent through in-use fiber optic cables by ultrastable lasers. This strategy could turn intercontinental fiber optic cables into ocean-bottom strain sensors, dramatically improving our ability to record earthquakes. Science , this issue p. [486][2] [1]: pending:yes [2]: /lookup/doi/10.1126/science.aat4458

Joint inversion of the first overtone and fundamental mode for deep imaging at the Valhall oil field using ambient noise

Surface waves derived from ambient noise data are composed of fundamental and higher modes. The first overtone is sensitive to structure from the surface down to a greater depth than the fundamental mode. We use 6.5 hr of continuous recording of noise on 2320 ocean bottom cable sensors from the Valhall Life of Field Seismic and we compute the intersensor cross-correlation functions for the vertical and radial components. We observe that the vertical component is dominated by the fundamental mode whereas on the radial component, the first overtone is stronger than the fundamental mode. Forward modelling demonstrates that a few hundred metres of low velocity sediments along with the water layer plays an important role for the generation of stronger first overtone signal on radial component. When we invert only the fundamental mode phase velocity data, the S-wave velocity model has vertical resolution down to 600 m depth. Combining the fundamental mode and the first overtone enables to image deeper structure down to 1 km depth, highlighting the presence of a low velocity zone.

Measurement of seismometer orientation using the tangential P-wave receiver function based on harmonic decomposition

Accurate determination of the seismometer orientation is a prerequisite for seismic studies including, but not limited to seismic anisotropy. While borehole seismometers on land produce seismic waveform data somewhat free of human-induced noise, they might have a drawback of an uncertain orientation. This study calculates a harmonic decomposition of teleseismic receiver functions from the P and PP phases and determines the orientation of a seismometer by minimizing a constant term in a harmonic expansion of tangential receiver functions in backazimuth near and at 0 s. This method normalizes the effect of seismic sources and determines the orientation of a seismometer without having to assume for an isotropic medium. Compared to the method of minimizing the amplitudes of a mean of the tangential receiver functions near and at 0 s, the method yields more accurate orientations in cases where the backazimuthal coverage of earthquake sources (even in the case of ocean bottom seismometers) is uneven and incomplete. We apply this method to data from the Korean seismic network (52 broad-band velocity seismometers, 30 of which are borehole sensors) to estimate the sensor orientation in the period of 2005−2016. We also track temporal changes in the sensor orientation through the change in the polarity and the amplitude of the tangential receiver function. Six borehole stations are confirmed to experience a significant orientation change (10°−180°) over the period of 10 yr. We demonstrate the usefulness of our method by estimating the orientation of ocean bottom sensors, which are known to have high noise level during the relatively short deployment period.

Introduction to this special section: Impact of compressive sensing on seismic data acquisition and processing

The last 30 years have seen ever-increasing amounts of seismic data being acquired. It is now common to see 12 or more streamers deployed with a single seismic vessel, ocean-bottom sensors deployed by the thousands, or land surveys with channel counts into the many tens of thousands. The volume of data has been defined not only by the sensor density but also by the number of components recorded at each location. Radial data coverage has grown from narrow azimuth to multiazimuth to wide azimuth to full azimuth. A higher sensor count has only been a part of the data explosion story. Source effort has evolved as well, with continuous recording and a variety of simultaneous-source schemes being utilized. Data density and field effort have grown, and continue to grow, with the imaging and interpretational demands of the industry.

Orienting ocean-bottom seismometers fromP-wave and Rayleigh wave polarizations

We present two independent, automated methods for estimating the horizontal misorientations of ocean bottom seismic sensors from their recorded data. The techniques measure the three-dimensional directions of particle motion of (1) P-waves and (2) Rayleigh waves of earthquake recordings. For P-waves, we used a principal component analysis to determine the directions of particle motions (polarisations) in multiple frequency passbands. Since P-wave polarisation may deviate from the direction of propagation due to seismic anisotropy and dipping discontinuities beneath stations, we further corrected for these deviations using a simple fit equation which yields sensor orientations of significantly increased accuracy. For Rayleigh waves, we evaluated the degree of elliptical polarisation in the vertical plane in the time and frequency domain. We apply both methods to 57 three-component broadband and wideband seismometers that were installed for 13 months on the ocean floor around La Reunion hotspot as part of the RHUM-RUM project (http://www.rhum-rum.net/). For P-wave polarisations we obtained misorientation estimates for 31 out of 44 functioning ocean-bottom seismometers (OBS), with an average uncertainty (95% confidence interval) of 11° per station. At 7 of these OBS, data coverage was sufficient to correct polarisation measurements for underlying seismic anisotropy and dipping discontinuities which improved the average orientation uncertainty to 6° per station. For Rayleigh-wave polarisations we obtained misorientation estimates for 40 out of 44 functioning OBS, with an average uncertainty (95% confidence interval) of 16° per station. Results obtained from the two methods are fully consistent within their respective error bars; the orientation angles differ on average by 3.1° and 3.7° for circular mean and median statistics, respectively. The good agreement between the two methods also makes them useful for reliably detecting possible misorientations of terrestrial seismic stations.

A single equation for predicting required ocean and seismic acoustic fiber optic sensor response

In the past decade, requirements have evolved for systems of ocean bottom and seismic downhole acoustic sensors with high dynamic range, wider bandwidths, and which will operate reliably for a decade or more in high-pressure-high-temperature environments. As seismic and acoustic survey spatial resolution, operating depth, and range requirements have increased, higher sensor string counts have been required to provide the needed improvements in target definition. Fiber optic sensors and telemetry systems are being developed which allow HPHT operation, without vulnerable electrical telemetry in the “wet” end of the system, and which can reliably transmit high-sensor count string outputs which are 10's of km from the processor. This paper introduces a modified version of the SONAR equation which allows straight forward estimation of the required fiber optic acoustic sensor response, early in the system design stage, and allows the system engineer to define and optimize sensor response requirements and help minimize system performance risks.

Retrieval of Ocean Bottom and Downhole Seismic sensors orientation using integrated MEMS gyroscope and direct rotation measurements

Abstract. The absolute orientation of the horizontal components of ocean bottom or downhole seismic sensors are generally unknown. Almost all the methods proposed to overcome this issue are based on the post-processing of the acquired signals and so the results are strongly dependent on the nature, quantity and quality of the acquired data. We have carried out several test to evaluate the ability of retrieve sensor orientation using integrated low cost MEMS gyroscope. Our tests have shown that the tested MEMS gyroscope (the model 1044_0–3/3/3 Phidget Spatial Precision High Resolution) can be used to measure angular displacement and therefore to retrieve the absolute orientation of the horizontal components of a sensor that has been subjected to rotation in the horizontal plane. A correct processing of the acquired signals permit to retrieve, for rotation at angular rate between 0 and 180° s−1, angular displacement with error less 2°.

Constraining shallow slip and tsunami excitation in megathrust ruptures using seismic and ocean acoustic waves recorded on ocean-bottom sensor networks

Great earthquakes along subduction-zone plate boundaries, like the 2011 magnitude 9.0 Tohoku-Oki, Japan, event, deform the seafloor to generate massive tsunamis. Tsunami wave heights near shore are greatest when excitation occurs far offshore near the trench, where water depths are greatest and fault slip is shallow. The Tohoku event, featuring over 30 m of slip near the trench, exemplifies this hazard. Unfortunately the rupture process that far offshore is poorly constrained with land-based geodetic and even most seafloor deformation measurements, and seismic inferences of shallow slip are often nonunique. Here we demonstrate, through dynamic rupture simulations of the Tohoku event, that long-period guided waves in the ocean (specifically, leaking oceanic P-wave modes known as PL waves) can resolve the shallow rupture process and tsunami excitation near the trench. With predicted pressure changes of ∼0.1–1 MPa along most of the seafloor landward of the trench, and periods of several seconds, these PL waves should be observable with ocean-bottom pressure sensors and/or seismometers. With cabled sensor networks like those being deployed offshore Japan and in other subduction zones, these waves could be used to rapidly quantify shallow slip and near-trench seafloor uplift and improve local tsunami early warning systems.

Improved models of the piezomagnetic field for the 2011 Mw 9.0 Tohoku-oki earthquake

To assess the feasibility of observing changes in the magnetic field produced by the piezomagnetic effect, an improved model of the piezomagnetic field corresponding to the Mw 9.0 Tohoku-oki earthquake is presented. In contrast to an earlier study, the proposed model explicitly considers the spatial distribution of slip on the seismic fault, and the results from this new model differ significantly from those of the previous model where slip distributions were ignored. Quantitative aspects of the piezomagnetic effect are discussed through comparisons of data and models. One feature clarified is that, because the fault rupture is so far offshore, the expected amplitudes are quite small at onshore existing observation sites; consequently, there would have been little chance of observing sizable piezomagnetic signals at inland sites during the Tohoku-oki earthquake. Nevertheless, piezomagnetic signals were reportedly detected at a few sites, possibly indicating that the stress sensitivity or the initial magnetization was larger (by several factors) than assumed. On the other hand, relatively large variations in the magnetic field of up to 10nT may have occurred offshore. This means that if ocean-bottom sensors had been installed, larger piezomagnetic signals would have been detected. Moreover, the piezomagnetic field in offshore areas is sensitive to the detailed slip distribution, suggesting that observations of the magnetic field at ocean-bottom sites might provide important constraints on determination of slip models.

Directions in ocean-bottom surveying

The benefits of data acquired at the ocean bottom have repeatedly been demonstrated in various areas and under a wide range of conditions. Significant successes have for instance been booked over obstructed fields, in structurally complex areas, and in areas with shallow gas charging. Other promising fields of application, such as direct hydrocarbon indication (DHI) from combined S- and P- information, are maturing. The added value in these applications stems partly from the more favorable geometries of ocean-bottom system (OBS) surveying, such as wider ranges of offset and azimuth, and partly from the ability to record true vector fields of both P- and S-waves. In spite of the advantages, the share of OBS data in the seismic market has remained relatively small. Apart from the successes in areas with shallow gas charging, the potential of S-wave data has been left largely unexploited. The majority of the recent OBS campaigns aimed at the acquisition of superior P-wave data. Key obstacles to the more general application of OBS are the high cost of acquisition and the suboptimal quality of the data, particularly of the shear-wave component. Optimal data quality can be warranted by planting the sensors onto the seabed, so as to establish firm coupling to the subsurface. Unfortunately, planting of ocean-bottom sensors is currently time-consuming, and increases the survey duration and costs. There is thus a need for systems and methods that offer both high efficiency and consistent data quality. This paper first reviews the historic developments of OBS, follows with an analysis of the relevant quality and efficiency issues, and finishes with possible directions and solutions.

The ups and downs of ocean-bottom seismic processing: Applications of wavefield separation and up-down deconvolution

A key advantage of acquiring multicomponent data using ocean-bottom sensors, whether using cables or nodes, is the ability to separate the wavefield into up- and downgoing parts. This opens up a host of attractive possibilities such as mirror imaging using the downgoing wave, attenuating receiver-side multiples using the upgoing wave only, or combining both up- and downgoing waves to completely remove the free-surface effect using up-down deconvolution. We focus here on the latter.

The GITEWS ocean bottom sensor packages

Abstract. The German-Indonesian Tsunami Early Warning System (GITEWS) aims at reducing the risks posed by events such as the 26 December 2004 Indian Ocean tsunami. To minimize the lead time for tsunami alerts, to avoid false alarms, and to accurately predict tsunami wave heights, real-time observations of ocean bottom pressure from the deep ocean are required. As part of the GITEWS infrastructure, the parallel development of two ocean bottom sensor packages, PACT (Pressure based Acoustically Coupled Tsunameter) and OBU (Ocean Bottom Unit), was initiated. The sensor package requirements included bidirectional acoustic links between the bottom sensor packages and the hosting surface buoys, which are moored nearby. Furthermore, compatibility between these sensor systems and the overall GITEWS data-flow structure and command hierarchy was mandatory. While PACT aims at providing highly reliable, long term bottom pressure data only, OBU is based on ocean bottom seismometers to concurrently record sea-floor motion, necessitating highest data rates. This paper presents the technical design of PACT, OBU and the HydroAcoustic Modem (HAM.node) which is used by both systems, along with first results from instrument deployments off Indonesia.

Uma implementação do método da refletividade para modelagem eficiente de dados sísmicos marítimos e de OBC

Neste trabalho é demonstrado como o método da refletividade pode ser eficientemente implementado para simular aquisições sísmicas marítimas. Isto inclui o registro de pressão no caso da fonte e dos receptores (hidrofones) estarem localizados na camada de água, o deslocamento (ou velocidade de vibração) vertical e horizontal no caso dos receptores estarem plantados no leito do oceano, simulando aquisições multicomponentes tipo OBC (Ocean Bottom Cable) ou OBS (Ocean Bottom Sensor). Uma das vantagens do presente método é a facilidade de seleção dos eventos que farão parte da resposta, tais como múltiplas internas, múltiplas associadas à superfície livre e ondas convertidas no caso P-Sv. Os fenômenos da atenuação e da dispersão podem ser levados em conta facilmente considerando as velocidades complexas, já que o método é desenvolvido no domínio ω-p. O algoritmo proposto possui custo computacional bem menor quando comparado com métodos puramente numéricos como diferenças finitas ou elementos finitos. Sendo assim, no caso onde um modelo de camadas planas horizontais seja suficiente para descrever a subsuperfície, este torna-se uma ferramenta prática para tarefas como prever com bastante precisão e rigor a sensibilidade sísmica de um determinado reservatório com relação à substituição de fluidos ou prever como uma determinada mudança de propriedade petrofísica pode influenciar a resposta de AVO multicomponente.

Why bury ocean bottom seismometers?

Theory, testing, and use of ocean bottom seismic sensors show that the fidelity of data, particularly measurements of horizontal motion, can be severely distorted and noisy because of mechanical interactions between the sensor package, the ocean bottom, and the ocean water. This lack of fidelity is common, particularly for instruments with large cross sections in the water. These problems should be minimized by burial of the seismic package below the ocean‐sediment interface. To test this hypothesis, two identical seismic packages were operated on the shallow ocean floor within meters of each other with a real‐time connection to land. The packages were configured in a variety of ways, with both packages buried in the sediment, one buried and one on the bottom, and one on a circular 1‐m‐diameter plate and the other buried or on the ocean floor. The resulting data are compared with data from both packages resting on a stable cement slab in the frequency range from 0.2 Hz to above 20 Hz. Data show convincingly that burial of seismic sensors in soft sediment greatly increases data fidelity over sensors placed on the bottom or on a plate on the bottom. While vertical sensors provide reasonable fidelity in most situations, the horizontal data are consistently reliable only when the sensor packages are buried.

Multiparameter studies of surf infrasound

Infrasound stations on islands or near coastlines routinely detect signals associated with breaking ocean waves. Although the source mechanisms of these infrasonic surf signals are not well understood, they might provide useful insight into coastal processes and permit an assessment of wave energy distribution in the littoral zone. Near-shore infrasonic arrays, ocean bottom sensors, an infrared imager, and a video camera were deployed on rocky and sandy coastlines in Hawaii during the 2004–2005 Winter swell season, with the aim of establishing a relationship between the directional ocean swell height and the infrasonic source distribution, intensity, and spectral content. The cameras targeted acoustically active regions for selected time periods to associate the timing of the infrasonic signal arrivals with the breaking and dissipation of ocean wave sets. A wide range of sea states and weather conditions were captured during two separate deployments. The results of our multiparameter analyses and preliminary comparisons of our observations with mesoscale ocean wave model predictions will be presented. [Work supported by the Defense Advanced Research Projects Agency. Support of the State of Hawaii is acknowledged.]

Challenges for efficient communication in underwater acoustic sensor networks

Ocean bottom sensor nodes can be used for oceanographic data collection, pollution monitoring, offshore exploration and tactical surveillance applications. Moreover, Unmanned or Autonomous Underwater Vehicles (UUVs, AUVs), equipped with sensors, will find application in exploration of natural undersea resources and gathering of scientific data in collaborative monitoring missions. Underwater acoustic networking is the enabling technology for these applications. Underwater Networks consist of a variable number of sensors and vehicles that are deployed to perform collaborative monitoring tasks over a given area.In this paper, several fundamental key aspects of underwater acoustic communications are investigated. Different architectures for two-dimensional and three-dimensional underwater sensor networks are discussed, and the underwater channel is characterized. The main challenges for the development of efficient networking solutions posed by the underwater environment are detailed at all layers of the protocol stack. Furthermore, open research issues are discussed and possible solution approaches are outlined.

Multicomponent seismic technology for imaging deep gas prospects

Across the Gulf of Mexico, operators are targeting deeper and deeper drilling objectives. For deep targets to be evaluated, seismic data require relatively long source-receiver offsets. Most shallow-water operators in the gulf consider 30 000 ft (9 km) to be the deepest target depth that will be drilled for the next several years. For geology at depths of 9 km to be imaged, seismic reflection data must be acquired with offsets of at least 9 km. We suggest in this paper that modern 4-C OBC data can provide good quality P-SV data to such depths and should be integrated into prospect evaluations. Long-offset surveys are difficult to achieve using towed-cable seismic technology in areas congested with production facilities, typical for many shallow-water blocks across the northern Gulf of Mexico shelf. Ocean-bottom-cable (OBC) and ocean-bottom-sensor (OBS) technologies are logical options for long-offset data acquisition in congested production ar-eas because ocean-floor sensors are immobile once deployed and can be positioned quite close to platforms, well heads, or other obstructions that interfere with towed-cable operations. An example illustrating the deployment of ocean-floor sensors through a congested platform complex in part of the area of study is illustrated in Figure 1. A 10-km diameter circle is positioned atop this map of production facilities to illustrate the difficulty of towing a 10-km cable across the area in any azimuth direction. In contrast, OBC lines AA, BB, and CC (actual profiles used in this study) pass within a few meters of several production platforms. Figure 1. 4-C OBC data acquisition across congested areas. An additional appeal of OBC seismic technology is that 4-C data can be acquired, allowing targeted reservoir intervals to be imaged with P-SV wavefields, as well as P-P wavefields. Once 4-C seafloor receivers are deployed, source boats can maneuver along a receiver line to …